Slightly Perplexed

[kainon]

1988 OMC 4.3L V6 - Points Ignition, 2bbl Rochester Carb(amature rebuilt once, pro rebuilt once), New Long Block, Wing Manifolds, New Water Separator, Checked Fuel-OK Fresh this month, clean. 4yr old plug wires (they look marine), Age of coil unknown, New Points(readjusted and checked multiple times by 3 mechs and Condensor replaced. New Cap & Rotor last year(marine-brass), age of fuel pump unknown, anti-siphon is questionable but has been checked by mouth(no comments).

So.. last run yesterday my friend and boat owner says it ran the best it has ever but still has a "rolling" miss-fire at the upper 10% of the throttle, at that point running 4200rpm @ 33mph. Gets up on plane ok though does seem to lack a bit of power. we had issues that the timing wasn't coming back to what it was set on and don't have a timing light that is adjustable to we can't see the max. I want to get in there with a spring and weight set tomorrow, change the plugs ( R44T currently ), wires and Coil, and check the coil Voltage. Previously we sprayed fuel mix into the carb underload and it reacted as if it was too rich so I'm thinking this is electrical(spark).

My Main question is obviously thoughts about the cause of the miss fire and when a coil goes, can it provide a weaker spark then normal or does it just die?

Thank you in advance for your thoughts and assistance.

 

[hystat]

the one time I had a coil going south on my old 3.0L, it was cutting out and in and out like someone was wiggling a loose connection on the key switch.

 

[crankbait]

my train of thought, is i start looking for an electrical problem when it is cutting out under load.....

 

[Master mechanic]

Wiggle the shaft on the dist.

What i think you will find is a loose shaft. If yes...

Replace the dist.

Hei maybe with a built in coil one wire in, one wire out and one wire to the tach.

If the shaft is solid no wiggle i would disconnect the tach. yea i know now your thinking the Chief is smokin some weird gonga or prairie mushrooms.

If you want hook up a known good shop tach and go for a run if it performs good then you found your problem ether a short to the tach or the tach.

Post back !

 

[kainon]

Thank you for reading and posting, it's got Points Ignition and After seeing the old plugs @ .038 to .045 I was hoping this was resolved. But it continues. I changed the Plugs to MR43Ts .035 Gap, Timing OK, Gap Ok. Fuel OK

the problem at hand happens when you either go right to full thottle it bogs or AFTER you run the boat up at 3/4 throttle then go full throttle it bogs.

My question is, about the power valve circuit. The carb was rebuilt by an amateur and then pro and the carb has a problem where if you go Full from a dead stop it bogs and will not pick up. If you nurse it using no more than 3/4 throttle, and get up to 4000rpm then push the last bit, it bogs down and miss fires in a kind of cycle, like 5 seconds, when I take the Flame Arrestor OFF it does it in like 7.5 second cycle... If I CUP my hand over the open carb at speed, I can make it worse, so I'm very sure its a carb issue,

but I'd like to know if 2 people have already worked the carb, could the circuit be damaged enough and would it be better to replace it?

 

[kainon]

We are replacing the Carb, unknown history, I'll take apart the old one before sending back as a core.

 

[makomark]

The power valve is controlled by engine vacuum...most are set to start the richen process when the vacuum is reduce to about 8" Hg...I would suspect that the vacuum on that engine has fallen below that level at the RPM where the issue is occurring.

I second the distributor shaft check as well as inspecting the advance mechanism...you don't need a "new" timing light, just measure the balancer's diameter and get a timing tape - much cheaper than an advance timing light.

I'd also suggest checking each plug wire or just replacing them...they only transfer a spark a finite number of times and then start to degrade....one mishandling during a spark plug change can damage them...

 

[kainon]

The engine seems to like the Carb lean, as it is very sensative to the new carb's choke and idle mixture settings which the new carb resolved the problem. it was the Power Valve or Jets and the power valve in the carb. thanks !!

 

[RicardoMarine]

1. New Points (readjusted and checked multiple times by 3 mechs
2. I want to get in there with a spring and weight set tomorrow,

1. Contact points must be adjusted via a dwell meter. Point gap is an estimate only of correct dwell.
   The correct Dwell Angle is rather critical with regard to ingition coil saturation duration.
2. Please do not do this!
   Changing flyweight return springs is a task for a pro, and with the distributor being run on a Sun, King or Allen machine.
   2 degrees off at the distributor, is 4 degrees off at the crankshaft, and so on.

Advance that is too late will cause a loss in power.
Advance that is too early, puts your engine at risk for detonation damage.

I second the distributor shaft check as well as inspecting the advance mechanism... you don't need a "new" timing light, just measure the balancer's diameter and get a timing tape - much cheaper than an advance timing light.

Ditto Mark.
This also removes any user or equipment error from the equation, and you'll be seeing your ignition advance in Real Time, and in Real Degrees.
See your OEM ignition specs including your TA.
Increase your RPM up to the TA point....., and compare this to the OEM specs.

The engine seems to like the Carb lean,

You do not want to operate a Marine gasser lean.


.

 

[kainon]

Completely agree on not wanting to run it lean, but when the engine is fired up in 80+ heat with a tightly closed choke, and the engine is spewing black spoke, gotta open the choke setting a bit..

We got a rebuilt carb from Flying Fish, and there is only one hole in the accel pump lever and it did give a little hesitation when dropping the hammer, but I leaned the idle mixture i think 1/2 turn on the right but shoulda only done 1/4 turn, and did a 1/4 turn on the left, and the hesitation ( or mild bog ) was gone.. so I prefer to set the carb out of the water so I can hear the tone change, and have done this a few times with great success.

Also on the dist, the timing is close but not dead on but the engine runs great and I'm going to check that today, NONE of the shops around me have timing tape, but I have another plan and have a couple other places to check at too.

any way, I took a look again at the dist, even ordered OEM Springs from here and they match up, but to do the job, the dist needs to be pulled, degreased as its all a very sticky consistency in where the weights are soo.. I'm probably not going to do that but will let the new owner know if it sells tomorrow.

if we keep the boat, I'm going to clean up the dist and replace the springs and have the dist checked for proper advance on a machine.

 

[RicardoMarine]

1. NONE of the shops around me have timing tape, but I have another plan and have a couple other places to check at too.
2. if we keep the boat, I'm going to clean up the dist and replace the springs and have the dist checked for proper advance on a machine.

1. You can mark off the balancer yourself by using some simple geometry, the known balancer diameter, a good thick piece of craft paper, a 360 wheel and an architect's divider.
   See first image below.
2. Smart move!

Examples only.

 

[makomark]

I've found its a lot easier to make a 'poor man's' timing tape using the old fashioned arc length formula...yes, it does require a little math but everybody reading this has a computer (likely a spreadsheet on it, too) so it really isn't difficult...you can print them and temporarily tape them or just mark the balancer using a fine sharpie.

You don't want the choke setting any richer than what is required to get the engine started...As far as setting the mixture 'lean', what was discussed was the idle mixture and having it a little lean isn't anywhere near as dangerous as setting the mid-range or high speed systems lean...sounds like you are dialed in...

 

[kainon]

I tried adjusting the idle mixture out of the water, but will need to redo it in the water as its still a little to rich, when we go to quickly dump the throttle there is a quick bog, though its running ALOT Better Overall. no problem

 

[makomark]

Does the accelerator pump work? if so, is it adjusted correctly?

 

[kainon]

new carb, and it does and the only adjustment is bending the arm it works fine out of the water on the trailer, in the water with the back pressure, I think the IM is still a little rich. 1/4 turn.