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Alpha One 4.3LX Won't Start & Backfires

J

jkm3093

Member
Hi all, new here and kindly need some advise.

This boat is a 1994 that I've owned for about 5 years, mostly without problems until earlier this season it began starting hard and some hesitation during running. It got worse to the point it won't start at all. Initially I determined it wasn't getting spark & focused there. Now it still won't start & backfires both through the intake and exhaust. I've replaced lots of things trying to get it right, so far to no avail. Summary so far:

1. Thunderbolt IV Ignition:
Replaced spark plugs, wires, distributor cap, rotor, ignition sensor, ignition module, coil, both battery cables, new 550 cca marine battery. I bought a timing light and during cranking best I could tell it appears to be correct at 8 deg. BTDC. After replacing battery cables, cranking speed is good & voltage at coil primary went up from 9.5V to 10.8V during cranking.

​
2. Fuel System:
I could see fuel squirting in when opening the throttle (Weber 4 bbl carb.), so initially did not focus there. I changed & double checked the fuel water separator - no visible water. I used the electric fuel pump to totally empty the tank (concerned about bad / stale fuel) & put in fresh. Fuel pump works OK - pumps about 2.5 liters/min. I have not yet opened up the carb to clean out the inlet filters, but fuel seems to flow fine.

​
3. Compression:
It is supposed to be tested with a warm engine, but testing cold I read consistently about 90-95 psig across all cylinders. I know this is low, but being a cold engine - does this explain these readings? I squirted oil into a couple of cylinders, but the readings didn't change much. I haven't done a leak down test yet. I would think that bent or burnt valves would show either zero compression or at least a big variation.

​
So that's where I am - dreading the worst that I have to tear into the engine, at least the top end. I certainly don't want to do that until I am convinced this is the real issue. Does it make sense to rebuild the carburetor in case fuel is too lean or too rich thus causing the backfiring? Maybe play around with the timing? Any thoughts or ideas are appreciated!

Thanks, Jim
 
Now it still won't start & backfires both through the intake and exhaust.
Click to expand...

Ayuh,... Recheck yer Firing Order,...

I think ya screwed it up....

Btw,... 90psi, crankin' a cold motor, is still a worn out motor...
 
Thanks for the response. I sure wish it was only wrong wire locations. You wouldn't believe how many times I re-checked the firing order - 1,6,5,4,3,2 and right wire locations. Looking at the distributor from the front, No. 1 is about 5:30 position and I double checked by manually rotating the crank until the timing mark lined up with TDC (per service manual). Funny thing is that originally (I think) the wires were all off one pin clockwise at the cap. Putting them back that way made it worse of course.

Also, I forgot to mention I did the compression check at WOT, so the low readings weren't due to that. I am not a pro at compression testing, but I am afraid you're right about a worn out motor. That's why I am looking for other opinions before going further. This motor doesn't have a lot of hours on it, but can't say how its been maintained its whole life.
 
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Chris / Bondo,

Thanks for responding. I guess I needed to hear it from others to get past denial. Bummer - no easy fix here. So, what do you think I should do - remove the heads with the motor in place with the hope the lower end is OK? It would be something I can manage myself and send the heads out for a valve job. Backfiring usually is indicative of valve problems, right? Also since squirting oil didn't bring the compression up can I hope the rings / cylinder walls aren't trash?

Or would you think the whole motor needs work and pull the whole thing out now? That is beyond my home garage set up.
 
Master mechanic said:
What sort of compression tester did you use ? Was it a screw in type ?
You now have spark at the plugs ? Your post was unclear on that point ?
Click to expand...

Yes, the tester was a screw-in type loaner from AutoZone - it seemed to seal well at the spark plug hole. My only verification I actually have spark is my inductive timing light flashes showing No. 1 plug at 8 deg. BTDC. I looked again at the wiring diagram - the shift cut-out switch only interupts ignition? I thought it prevented the starter from cranking if not in neutral. Is there hope yet?

If I had no spark, would that cause backfire?
 
jkm3093 said:
Chris / Bondo,

Thanks for responding. I guess I needed to hear it from others to get past denial. Bummer - no easy fix here. So, what do you think I should do - remove the heads with the motor in place with the hope the lower end is OK? It would be something I can manage myself and send the heads out for a valve job. Backfiring usually is indicative of valve problems, right? Also since squirting oil didn't bring the compression up can I hope the rings / cylinder walls aren't trash?

Or would you think the whole motor needs work and pull the whole thing out now? That is beyond my home garage set up.
Click to expand...

Ayuh,.... Ya might try loosenin', 'n re-setting the valve adjustments...

I've seen alota valves adjusted by the Non-knowin' to be Way to Tight, 'n not closin' properly...
 
ok if your getting a spark signal to your timing light your getting spark. secondly really people telling this guy he just needs a new motor, really come on i have seen 4.3 ltr engines run fine right down to where all cylinders are running about 60 psi. yes his engine is worn but definately not trash yet. have you checked your valve timming, is the chain loose? am i to understand that the problem only started this season? did you change plugs to start the season? if so may sound stupid but if by some chance you put short reach plugs in a long reach plug hole it will run great for a while but soon as it starts to foul up the plug and your spark falters slightly you will have conciderable trouble getting the engine to run right, and start. you will also get conciderable backfiring through both the exhaust and the intake. i would do some more checking your low compression hasnt changed that fast( eg you didnt have 165 psi compression at the beginning of this season and all of a sudden its down to 90 psi this has happened over a period of years)start with plug reach check. then work through the valve timing, the leak down test would be something to consider. dont feel beat down yet there is still hope for another season or 2 out of that motor.
 
bondo said:
Ayuh,.... Ya might try loosenin', 'n re-setting the valve adjustments...

I've seen alota valves adjusted by the Non-knowin' to be Way to Tight, 'n not closin' properly...
Click to expand...

Valve adjustment is an idea, thanks. I've done it in cars before - got to check if the valve spring compression tool I have will work here. One thing I can say with certianty is that the valve covers have never been off the motor - I'm the second owner and know the original owner never did ANYTHING to it for 10 years - but then again it was only in the water less than a dozen times when I bought it.
 
fish1967 said:
ok if your getting a spark signal to your timing light your getting spark. secondly really people telling this guy he just needs a new motor, really come on i have seen 4.3 ltr engines run fine right down to where all cylinders are running about 60 psi. yes his engine is worn but definately not trash yet. have you checked your valve timming, is the chain loose? am i to understand that the problem only started this season? did you change plugs to start the season? if so may sound stupid but if by some chance you put short reach plugs in a long reach plug hole it will run great for a while but soon as it starts to foul up the plug and your spark falters slightly you will have conciderable trouble getting the engine to run right, and start. you will also get conciderable backfiring through both the exhaust and the intake. i would do some more checking your low compression hasnt changed that fast( eg you didnt have 165 psi compression at the beginning of this season and all of a sudden its down to 90 psi this has happened over a period of years)start with plug reach check. then work through the valve timing, the leak down test would be something to consider. dont feel beat down yet there is still hope for another season or 2 out of that motor.
Click to expand...

Fish1967,

Thanks for the alternate opinion.

No, I don't have comparitive compresssion readings. It's been 2 years since I changed plugs before now so I doubt the plug change is the reason but will double check heat ratings, etc. I'm pretty sure I used the specified Champion RV15YCA but will confirm.

The problem progressed this season (maybe 10 launches). You're right the compression drop is not sudden - like I said I don't know what compression readings I should see with a cold motor, nor comparison for this motor.

I have not torn into the motor yet, so I don't know about timing chain slack, etc. I'm still looking for simple answers like throttle position switch, etc. I sure hope the answer is short of a new motor and will keep checking.

Thanks to all for your help & ideas.
 
...read consistently about 90-95 psig across all cylinders.
Click to expand...
That is a good sign even though it's low.

I squirted oil into a couple of cylinders, but the readings didn't change much.
Click to expand...
Another good sign.

Concerning the 90 PSI compression. Using loaner tools can be beneficial. However they can also be out of calibration from being mishandled by a multitude of users. Buy a quality compression/vacuum gauge and use it before you tear into your engine. Don't do anything else until you recheck compression w/a good gauge.

If the compression readings are 120 PSI and above continue w/trouble shooting. Your engine calls for AC Delco MR43T spark plugs. Stick w/the original plugs that came w/the engine.
Does it make sense to rebuild the carburetor in case fuel is too lean or too rich thus causing the backfiring? Maybe play around with the timing?
Click to expand...

At least clean/change the carb filters. Have you checked the choke operation? What is the timing setting on the engine label? 8 or 10 degrees BTDC? Recheck the firing order per Bondo. If it was me, before tearing the carb apart, I would dump the fuel from the separator filter and fill it up w/Seafoam. If you can get the engine to run do it on muffs for 5 mins. If not crank it for 10 seconds wide open throttle to get the Seafoam into the carb. Let the engine sit overnight and run it again until it stops smoking from the Seafoam.
 
Thanks for the ideas, Guyjg.

I'll check for correct plugs and change them again. I read the wrong spark plug type in my last post - the tune up specs calls for AC-MR43T like you said or Champion RV15YC4 or NGK BR6FS.

Interesting idea about Seafoaming the carb / motor. I've never done that before, but I'm sure the motor won't run on it until I find the gremlin.

I have to take the carb apart to get to the 2 inlet filters. Cleaning / adjusting the carb is appropriate in any case considering it is 17 years old and had gas sitting in it for long periods. I can then check if the choke is working properly. No idea at this time.

I am intriqued by the shift cutout switch being a possible reason for my problems. The wiring diagram shows the switch connected to ground on one side and the white / green wire going to the ignition module and ignition sensor on the other. So I understand that when open, it allows the ignition to fire properly? I am still confused about how / when that works. I need to come back to checking for spark when I have the throttle position lever pulled forward in throttle only mode. I think I'll disconnect that switch to check.
 
The shift interuptor switch is there for shifting OUT OF GEAR ONLY.

When trying to shift out of gear there is too much force on the clutch dog and gear (either forward or reverse) to allow it to dissengage. the force keeping them together causes resistance in the short shift cable, this resistance causes the V notch to move and activate the switch and this happens for a split second, just enough to allow the clutch dog and gear to seperate. If this did not occur you would have to shift past neutral to get neutral and actually engage which ever (forward or reverse) gear.


If this switch was your proplem you would have NO spark at all as it shorts the ignition to ground when activated or if the switch was bad it could have a internal short causing a driect connection to ground.

If you are concerned disconnect the ground wire tape it off temporarily and retest.

Also you could disconnect the grey tach lead going to the negative side of the coil and test again also, the tachometer gets its pulses from the neg side of coil and if the tach is bad it can cause ignition woes............if no change all is good.

Here is my take on your possible issue.

I agree even witrh low compression the motor should still start! Will it be a powerful motor? NO! but with 90 psi it still should start and run.

Backfire thru carb is either very lean (lack of fuel or seriuos vacuum leak) or round exhaust lobes on cam shaft or serverly burnt or worn vavles.

One quick check is to remove the valve covers and pull all the plugs out and crank the engine over. Look at each rocker arm and confirm they all move up and down ~ the same amount. If you have a bad cam lobe one or more will not move up or down very much.

One quick check to see if it will start would be to shoot a couple of squirts of either (starting fluid from a spray can) and see if it fires off at least for a couple of seconds.


You also abosolutly need to confirm your firing order and spark plug wire location!!!
 
Thanks for the cutout switch explanation - it wasn't intuitively obvious but makes sense now.

I've tried ether - no change. Firing order rechecked numerous times. I've disconnected the tach to test, but I need to recheck that.

I hope to work on it some more this afternoon/evening & will report back. My gut feel is it is still a spark issue - it's hard to confirm that working alone without someone to turn the key. First thing is to disconnect the cutout switch.

Thanks again for all the inputs.
 
Make your self one of these..........Works very nice when alone.

One on/off switch and one momentary switch and some 14 ga wires

red, purple, yellow

You can either connect the yellow to the starter or the slave either way will work.

Red goes to battery

purple goes to coil.

turn on the on/off switch this applies 12 volts to ignition

activate the momentary and this will crank the engine over.

Just be aware to turn it off you MUST torn off the on/off switch!!!!!
 
Tests for Thunderbolt Ignition:
W/ignition key on AND BILGE WELL VENTILATED OF GAS FUMES!!!

#1 - Connect your voltmeter neg. lead to the engine ground and pos. lead to the white/red wire terminal at the dist. It should read 12 volts.

#2 - If 12 volts is present, remove the coil spark wire from the distributor and connect it to a spark gap tester to ground. Remove the white/green lead from the dist. terminal. Turn the ign. key to on and strike the white/green lead to ground. If there is spark, replace the ignition sensor in the dist.

#3 - If there is no spark, substitute a new coil and repeat test #2. Now if you get spark, install a new coil. NO SPARK, replace ign. amp.

#4 - In #2, if there is No voltage present, disconnect the white/red wire and check it again for 12 volts. If 12 volts is present, replace the ignition sensor inside the dist. cap. If no 12 volts present w/it disconnected--ignition amplifier is shot and must be replaced.
 
A couple of comments just in case it helps:

There is not likely to be anything wrong with the valve lifter adjustment if the valve covers have never been off.
Since this problem gradually got worse, something may be wearing out slowly, as was said.
If this is cam or lifter wear the engine will be making metal. You could remove the oil filter and cut it apart to put this issue to rest quickly.
Or remove the pan drain plug and pull a small sample of oil and inspect it.

You did not say how many hours on on this engine, but a 4.3 will usually run well over a thousand hours with regular maintenance. They are a very trouble free engine usually.
I suspect your carb may need a rebuild. The bowl may be full of crud and your low speed jets may be completely plugged.
Just because your accelerator jets squirted gas when the throttle was moved does not mean your low speed/idle circuit is operating.
I would ceertainly check your carb inlet fuel filter and fuel pump for correct pressure. You should have about 3-4 psi at the carb.

Also be aware that the fuel pump does not run when you turn the ignition on, with these engines, at least from 1996 on.
It is only energized with the key in the start position, then there is an oil pressure switch that supplies power to it when the engine starts and oil pressure is established.
On my 4.3 I installed a pushbutton bypass switch to run the pump and prime the carb before each cold start. Then it would start easily therafter.
Before installing this switch mine was a bear to get started after being shutdwon for a week or more.

If it was me I would find out if the problem is lack of fuel by dumping a tablespoon full of gas down the carb then try to start it. If it runs for a couple of seconds, bingo, its a fuel issue. I understand though that there are lots of folks who would shy away from this idea. For sure dont be looking into the carb while cranking, and have a fire extinguisher handy.

Rod
 
OK, I took another stab at it. I checked the shift interrupter switch & it is working as it should - open in the normal postion and closes when it rides out of the V notch.

I replaced the plugs with AC-Delco R43T (I was told they are the same as MR43T) & gapped to 0.040".

I rigged up a remote on/off switch and starter button - much easier to work around the engine. I put a canopy over the boat to darken the light some. I took the coil wire off the distributor & checked with a spark tester to ground - good strong spark while cranking. I put my timing light on No.1 wire & while cranking it is firing right at 8 deg. BTDC like it should.

Same end result - only intake backfire & the motor doesn't even think about firing up. Spraying starter fluid made no difference (the trottle lever was in neutral position so the choke was pretty much closed).


So as Rod suggested & because I like to live dangerously, I poured a little gas into the carb, put the trottle to wide open, put the spark arrester back on top & the backfire lifted it up about an inch. Still no start.


It's time to go to school on rebuilding the carb. I printed out the 39 pages from the service manual on that. Does anyone know if there is a rebuild kit available for the Weber 4 bbl carbs?


BTW Rod, my fuel pump is wired the same as you said - I think 1993 thru 1996 were all the same. I like your idea to hot wire the pump with a push button to prime it when cold.

Regarding the run time on this motor - no idea except not much. I've used the boat more in the past 5-6 years since getting it than the original owner in 11 years. He stored it in the heated basement of his house so he didn't even winterize it. I thoroughly tuned it up when I got it & it ran like a top until this year. I keep up with routine maintenance, but haven't needed to do more until now (except replacing both exhaust manifolds that developed pin-hole leaks). I keep it in a garage I built for it, but it did spend one winter outdoors 2 years ago that didn't help. It's hard to imagine it is major internal engine problems.

As always thanks to all for the input.

Jim
 
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Backfiring and poor compression sounds like valve timing.
If it did not start with the gas down the carb trick, it may not be a carb issue.
I think the next thing I would do is pull one valve cover and see what kind of valve lift you are getting.
You may have collapsed lifters, or rounded lobes, as was said.
If you do, it will be making metal, and you will find evidence of this in the oilpan, as I said earlier.
Valve lift should be in the region of 0.4 to 0.5 inches at the valve stem. Maybe someone else can chime in with the correct specs.
If not I am sure I can find it.
If that checks out I would pull the harmonic balancer and the timing chain cover and make sure the cam is timed properly. If it has slipped a cog she won't start.
Your engine will have a gear driven balance shaft just above the camshaft, driven by the camshaft.

Rod
 
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Rod, I can't argue about the valves being suspect. I ordered the carb overhaul kit in any case to rebuild it due to age if nothing else. Guy, thanks for the correct part number.

One question about the timing chain jumping a cog - Isn't the distributor driven from the cam shaft? The timing mark on the harmonic balancer (crank shaft) lines up correctly with the rotor pointing at No.1 cylinder, and the timing light shows correct 8 BTDC. I have not touched the distributor position. If it slipped would that not be the case? If the timing chain is worn allowing excess play, could that explain these symptoms?

I have the engine service manual & found the following about cam lobe height: Valve spring length: Closed = 1.70"; Open = 1.25". Therefore lobe height = 0.45". I'll pull off the port valve cover & check that and valve lash (probably not until tomorrow). I still find it hard to believe the lobes are worn down - the motor has always had good lubrication, etc.

Jim
 
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Cam wear is not always a result of poor lubrication. It is a metal on metal working area and they do wear, material issues. etc etc. Millions of cam shafts are made not uncommon for some not to last.

In the mid '90's I had a car with a chevy V8, ~ 100,000.00 miles, I purchased it with 84,000. I started using Mobile 1 at 84,000.
No reason to think it would be a cam shaft issue. Motor always had a slight miss/non smooth idle. new intake, new carb, high quality ignition components nothing made it run any smoother.

Then all of a sudden one day driving I stepped on it and POP out the carb, every time I stepped on it POP out the carb........removed valve covers and two exhaust vavles had finally gone round..........

Reconditioned heads, new push rods, lifters, cam, timing chain. When I sold the car a few years later with 194,000 on it it still ran like a raped ape....no miss at idle and lots of streetable power............

Also one thing that has started to happen is with the alcohol in the fuel, it is creating a lot of issues with valve trains so a burnt valve or some related issues is not out of the realm of possiblities.........
 
jkm3093 said:
I still find it hard to believe the lobes are worn down - the motor has always had good lubrication, etc.Jim
Click to expand...

As Kghost says, cam wear is not always caused by poor lubrication.
It is more likely though in engines that are otherwise low time; but have seen long periods of no use; ie marine engines.
I agree with you that the timing chain jumping a cog is unlikely if the spark timing has remained on spec. I was not sure if you had reset the timing or not.
Chain wear alone is not going to be enough to get the valve timing far enough off spec to cause non-starting.
Another possibility is broken valve springs such that one or more valves are hanging; not closing. This would certainly explain the backfiring.
Again, pulling one or both valve covers will tell the story here.
Also, your rockers will be a 1.5 ratio, so the 0.45 lift is measured at the valve stem; not the lifter/pushrod.

Rod
 
Thanks guys for your insights. It looks like my winter project is set. Today I drained all the coolant lines & am packing it up to take it to my storage garage at our weekend house 2 hours drive from here. I'll work on it there from here on out. I have to take all my tools with me I think I'll need. I took today & tomorrow off work and should at least get both valve covers off to take a look. I'll take the carb off too & rebuild that at home once the rebuild kit arrives.

I'm comfortable working around engines - have done timing belts, etc. but I've never torn into a GM motor. I hope I can work on the motor in place to the point of heads & camshaft removal if needed. What do you think - there's only about 10" clearance in front of the crank pulley and 2' + on either side. How tough is it to get the crank pulley off? I know in some cars I had to use a long cheater pipe with at least 200 lb-ft torque & there isn't that much room in this case. Do I need to make a tool to hold the pulley while tightening / loosening the bolt? Maybe I'm getting ahead of myself.

If the motor needs to be pulled I don't have the lifting facilities (height wise) or the alignment tools to get it all back together.

Thanks, Jim
 
Jim

regardless of what you find wrong you have a tired motor.

90 psi is not good. It should be 140-160 psi period!!! that puts you ~ 1/3 down in power and efficency.

My advice would be to do your reaserch/homework and find a marine replacement either good used or rebiult long block and swap all accessories from the old to new and start your next season with a good stout motor.

Typically the cost of a refurbished long block can be had for under $2500.00.

Removing a motor is a 45 minute job, very easy compared to automotive. remove outdrive, undo rear and front motor mounts, remove some wires, some cables and out she comes........Yes you need a alignment bar but that can be had at a reasonable cost also....

Just my thoughts on waisting too much time trying to rescue a tired motor........
 
Well kghost, you were right. As much as I was hoping there was a cheaper fix like a valve job, I will have to bite the bullet and renew the motor. This past weekend I removed the valve covers & measured valve travel. The cam shaft is worn down - in the worst case #6 intake valve is only opening 0.26". I didn't measure valve lash (I left my feeler gauges at home), but the rockers had slight play when closed like they should.

I started pricing rebuilt long blocks at $1600 to $2000 versus $2500 for a brand new motor. Shipping is extra. For that difference, I am leaning towards new. Bummer, this isn't in my fun & games budget. Time to shake the money tree.
 
Dont be to dissapointed.

You also have the choice of sending this motor out for a rebiuld. What that would cost would most likely be equal if not more than a good rebiult.

Maybe look at it this way, There is a HUGE aftermarket engine rebiuld market out there, Why? because you are not the only one who faces this challenge!!

About 8-9 years ago My friend, he has a 1997 inboard ski boat, 350 merc. Buy 2001 time frame his motor was junk! Not his fault but the motor was never right from day one. It ran but never as weel as one expect. By the end of the season the blow by on the motor was so bad there was no choice but replace it.
He was going to purchase a new boat at a cost of ~ $30,000.00 because he was so dissapointed. I talked him out of it.

I did the research and was able to get a fresh rebiult 350 roller cam replacement marine engine for $1700-1800 I forget exact.
I did the motor swap and a carb rebiuld and the motor is still running exeptionally strong as of this season. Now I cant take credit for the motor biuld but the point is he has had the SAME boat for a additional 9 years virtually trouble free and it runs stronger than the original and he got out of it for only ~$2500.00 (my friend so cheap labor........lol)

Take your time and good luck.
 
Good perspective, thanks. I'm on board with this project. This boat is worth putting the $$$ into - 1994 Bryant 18' open bow run-about in otherwise really nice shape - I only have about $6k into it; new in 94 cost $16k, and today the closest Bryant is 19' at $26.5k. I spent more than my purchase price to put up a metal garage to stow it in.

After a little more checking I found rebuilt marine long blocks for $1400 + shipping, but still feel more comfortable spending $2,500 for a new GM crate marine engine so I'll be ordering that in the next couple of days. I've swapped car motors before so this relatively looks like a piece of cake.

To pull it I just picked up a 2,000 lb. electric winch from Harbor Freight for $60 (I love that place!), will hang it from the roof beam with a temporary column & bracing mid span for additional support (yesterday I work out the stress calculations with a design engineer friend), and I'm good to go! I already have an engine hoist to move both motors around the garage, and need to pick up an engine stand either from Craigs List or it's back to Harbor Freight.

Question: Can I put a 4.3 vortec head engine in it? that would gain me about 40 HP. Do I need to change out the intake manifold from 12 bolt to 8 bolt? Will my Weber 4 bbl carb still be suitable?

Here's a pic of my boat & work area. Don't mind the mess. I'll be back there next weekend to move it along a little further.

Stay tuned . . .
Jim
 

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Question: Can I put a 4.3 vortec head engine in it? that would gain me about 40 HP. Do I need to change out the intake manifold from 12 bolt to 8 bolt? Will my Weber 4 bbl carb still be suitable?

You kinda answered your own question.

Yes I would think you can upgrade to the better motor. Yes reuse your carb, yes a model specific manifold,

The only issue of concern that I can think of is the flywheel and coupler. it may need to be changed due to a different crankshft bolt pattern but I am not sure on this.

There are a couple of members who have much more detailed expertise on the V6 motors.

Maybe start a new thread with the questions so others can offer there opinions.

1994 V6 vs vortec V6 differences..............
 
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