... and the backfiring continues...

This has been going on for several months now, and I am really starting to bang my head against the wall. My starboard Crusader RH 454 (Model 350, circa 1989-ish?) has been backfiring out the carb under load around 2500 RPM, or just about any RPM if the throttle is blipped quickly in neutral. The engine purrs like a kitten at idle, and runs smooth as silk up to about 2500 RPM's where it will start to backfire and fall flat on it's face.

Here is the run-down of what we have done so far. What am I missing? Any more ideas?

Checked timing... slightly off, but not enough to backfire. Adjusted anyway.
Replaced fuel/water seperator and inline carb filter... no change.
Checked fuel pressure at carb inlet... 6 PSI continuous, even while backfiring.
Swapped carbs from port <---> starboard... problem stayed with starboard.
Swapped ignition coils... problem stayed with starboard.
Swapped complete distributors (cap, rotor, wires... the whole deal)... problem stayed with starboard.
Replaced plugs... no change
Ripped some of my hair out... no change
Jumped coil's ignition wire straight to battery (bad ign wire? Low volt?)... no change.
Compression test results:
Cyl 1 - 150 Cyl 2 - 150
Cyl 3 - 150 Cyl 4 - 148
Cyl 5 - 150 Cyl 6 - 150
Cyl 7 - 148 Cyl 8 - 149
Vacuum at idle is perfectly-steady 17.5 in/hg...
Completely out of ideas... sticking valve? ...
Pulled the heads and took to machine shop. Heads were tanked, magna-fluxed, disassembled and completely inspected. Spring pressures were measured, valve seats were "not perfect, but wouldn't cause a popping" so he did a light grind and lap. Guides and stems looked perfect... (couldn't go wrong for only $100 in labor, $110 in gaskets, and a saturday's work).
While the heads and intake were off, put a dial indicator on the cam and measured every lobe. Lift values from #8 exhaust down to #1 intake:
0.258"
0.266"
0.248"
0.274"
0.255"
0.267"
0.267"
0.277"
0.273"
0.275"
0.275"
0.275"
0.278"
0.274"
0.269"
0.278"
Granted some are a little low/worn, but would that cause a backfire? I wouldn't think so.
Reassembled with lots of lube, started, warmed up, adjusted valve lash while idling (1 turn past lash), installed valve covers, and low and behold...
STILL BACKFIRING!!!!!!!

I am completely out of ideas here. I have ruled out ignition. I have ruled out fuel. I have ruled out a valvetrain problem. Compression is perfect...

Any thoughts on cam timing possibly being off? Chain maybe skipped a tooth? Is this a common problem on these? Any easy way to check without pulling the front cover?

EDIT: This all started happening when we put some "rebuilt" carbs on the boat. One of them was dumping raw fuel like the float was stuck down or something. We were literally spitting raw fuel out the exhaust. When the backfiring started, it would occur at 3k RPM. When we got back, there was literally a gallon of fuel in the oil. I flushed the oil out three times. Next time we took the boat out (old carbs put back on), it was backfiring at 2700 RPM. Next trip was moving the boat from Long Beach to Dana Point, and it was then backfiring at 2500 RPM. Not sure if that is pertinent information or not, but it seems to have gotten worse a couple times, now it is 2500 RPM every trip out.

I work on pretty much nothing but OBDII Hondas, so this Carb'ed BBC stuff is new to me. I would appreciate any help/ideas. Case of beer to the winning suggestion, hehe.

Thanks!!
Eric

I'll bet you a six pack (though I don't drink--yet) a couple of wires are crossed. My Mopar 360 did the exact thing, and that was the cause.

Try this: Using insulated pliers and gloves, remove one wire at a time from the distributor cap, then replace it. (Motor thoroughly warmed up and idling at 1,000 rpms.) Note the loss then gain of rpms as the wire is removed and replaced. All eight cylinders MUST react the same. If not, leave that wire out and keep going around the cap; and if you find two wires that don't react, swap the two on the cap.

If only one wire reacts that way, replace it and the plug it goes to.

Jeff

PS: A marine engine--having solid mounts--does not rattle and shake like a car engine when it's idling crappy. My Mopar 360 started and idled 'normally' with two wires crossed. I was stunned to discover the marina guys (doing a compression test for the insurance company--long story) goofed like that.

Perhaps it's time to EFI those lumps.

fastjeff said:

I'll bet you a six pack (though I don't drink--yet) a couple of wires are crossed. My Mopar 360 did the exact thing, and that was the cause.

Try this: Using insulated pliers and gloves, remove one wire at a time from the distributor cap, then replace it. (Motor thoroughly warmed up and idling at 1,000 rpms.) Note the loss then gain of rpms as the wire is removed and replaced. All eight cylinders MUST react the same. If not, leave that wire out and keep going around the cap; and if you find two wires that don't react, swap the two on the cap.

If only one wire reacts that way, replace it and the plug it goes to.

Jeff

PS: A marine engine--having solid mounts--does not rattle and shake like a car engine when it's idling crappy. My Mopar 360 started and idled 'normally' with two wires crossed. I was stunned to discover the marina guys (doing a compression test for the insurance company--long story) goofed like that.

Click to expand...

If you do this while the engine is in gear, assuming it is in the water, it will be very noticeable if a cylinder is not firing.

I am pretty sensitive to how smooth the engine runs. In fact, when reassembling the engine, I adjusted the lash statically (engine off), and it just didn't seem to be idling right. pulling wires as you mentioned, I found cylinders 5 and 8 were weak. I re-adjusted the lash with the engine idling, and it smoothed out perfectly. I now get an equal rpm drop pulling wires. Definitely don't have wires to the wrong holes (quadruple checked), plugs are all new, and I have clips separating the wires.

Eric:

When I first saw this thread I thought the backfiring might be caused by a wiped exhaust cam lobe, and that diagnosis would be consistent with overfueling and thinned out oil, since these big block GM motors are famous for wiping out cams under the best of circumstances. I don't know how to measure lift and duration so I can't comment on your numbers. Hopefully someone with more experience here will chime in.

My starboard motor (Crusader 454) backfired right around 2200 RPMS and really started breaking down at 2500 RPMS. I replaced the cam, lifters, and pushrods. The total cost was around $400, and its not that big of a job if you have the clearence in front of the motor to slide out the cam. I did mine myself, and I'm not an expereinced mechanic by any means.

I would have someone double check your lift numbers, and if there is any doubt I'd replace the cam. You've tried just about everything else. Its also possible that you may have made a mistake somewhere or gotten confused, so look back carefully at all you have done.

Best wishes,

Tom

section 3B, page 7 or 8, in that multi-emailed manual will give you the cam lift numbers....I think you'll find some have inverse wear so your mount or technique with the dial gauge could be refined a bit. that said, I concur, you don't have enough missing metal to have the 'wiped cam lobe' pop.

There's no chain on the RH engine so it stretching or jumping a tooth is impossible. Given your description of the engine running and the test data, I'd doubt the gear drive has any major issues.

You say this all started with the 'rebuilt carbs'...which provided a flooded mode of operation. Any popping before, on the 'old' carb? (Old carb is current carb, right?).

Independent fuel tanks with each engine?

does engine maintain RPM when the backfiring starts, or does the rpm drop when the backfire starts?

Did the springs get height and pressure checks when the heads were apart?

Hey Mark,

The manual you sent came in handy for sure. Did not think to look at the cam specs though. The mount for the dial indicator is the vise-grip style with the locking, flexible cable (don't know what else to call it).

As for the carb, it is the original/current carb, and it did not backfire before the "incident". Port engine runs perfectly, and we swapped carb with no change.

Yes, independent fuel tanks, but we have a cross-over valve to run either engine off either tank. We ran the problem motor off the other tank with no change (you thinking anti-siphon valve or bad fuel?).

When the backing starts, it does not seem to drop the RPM. It just pops sort of randomly and loses power. Even if I tried to throttle more, the rpms will not increase. Oh, and to add, in neutral, it will rev to 2800 rpm, then lose power to rev any higher.

The spring heights and pressures were in fact measured. They were all good. Inner and outer springs good.

Hey Eric,

Have you checked the distributor shafts for runout.

I know you have suspected crossfire by the fact you have run the plug wires in separators, but if you have eliminated the valve seating issue it must be an ignition problem or a lean mixture problem.
Have you checked to see if the timing is stable as the engine is run up? Sounds like you are thorough and have checked almost everything, hope this helps, let us know what you figure out.
Good luck.
Rob

I think you need to find out if the valve timing is correct. Typically, done by matching up the two cam gear dots, with the timing cover off. Maybe someone here knows of a way to do this without taking off the cover. The fact that you can't exceed 2500rpm says more than one cylinder is involved, or a systemic thing is going on. You can shut down 2 cylinders totally and get over 3200 rpm in gear on a healthy engine. Before we leave mixture altogether, did you pull all the new plugs to see how they look after a run at 2200rpm, with load?

Oh, and you know the firing order is different between those two engines, right?

I was thinking fuel supply - yes.

The only way I can think to do the check Dave suggests is to get out the cam timing info and measure the lobe timing in crank degrees. Same process as degree-ing a cam, but with the iming cover in place. Being as the engine runs well up to 2500, I'd suspect the camshaft-crankshaft relation is fine.

If the distributor runs well in the other engine, and the same applies for the carb....I'd check the primary voltage at the ignition coil. (assuming these are electronic converts or you've verified the points are adjusted and clean.)

I'm thinking that as the RPM goes up, the coil needs to supply more energy and the primary system becomes inadequate ~ 2500 RPM. maybe high resistance in the ignition switch, wiring between switch and coil, (maybe ballast resistor) or something esoteric - like the wire running down to the starter solenoid is chafed and shorts to the block (vibrates) at the troublesome RPM.

How about bringing the engine up to backfiring speed and disconnecting one plug wire at a time...that will tell you what cylinder is causing the problem...a worn cam will definitely result in the condition you describe...the engine will idle fine and return perfect compression readings but will backfire under load and higher rpm. A sticking valve will also react in the same manner.

Two quick thoughts.
I don't know the filter setup on your crusaders, but with mine (79ch 350 454s) one of the secondary filter elements had been installed upside down and as soon as the engine needed a lot of fuel it would suck the filter element up to the top of the canister housing and shut off the fuel flow.
Could someone have mixed diesel with the gas?
Oh and one other thing, depending upon your transmissions the starboard motor can be the one that turns the original direction. Firing order will be 18436572.
My 2 Cents worth.
Rob

Lots of information given here. I was wondering what you have checked.

Why did you change the carbs to begin with? What else did you change when you put the new carbs on? Wires, coil ect. We need to get back to the basics here.

I will be back at the boat tomorrow morning and am going to start from square one. To answer some questions here (even though some were answered in the original post), Filters are good (replaced both inline and fuel/water seperator). Engine is RH (reverse) rotation. Cam has been measured and MakoMark will agree is okay. Valves and guides are perfect and not sticking. No diesel in the gas. As I said, we can run the engine off port tank and the problem is still there, while port engine runs flawlessly.

Dockside... we wanted to ditch the inop/broken mechanical choke with carbs that have electronic choke. Nothing else was changed at the time of carb replacement. I had wired up some relays to run the chokes, and that was the only modification other than the carbs themselves. We are now back to our original carbs and just dealing with no chokes (not a huge deal anyway).

The backfiring does not seem to be coming from a single cylinder. It is more of a random pop. Also note that revving in neutral will not exceed 2800 RPM. It just bogs out and will not go any higher. This leads me back to square one... thinking fuel or ignition. Even though I have thoroughly checked everything, I feel I must have missed something. The fact that it bogs like that says it is something that affects ALL cylinders. It is unlikely a problem with a single cylinder.

Even though we swapped carbs from port to starboard, and the problem stayed with starboard, I am tempted to rebuild one of the carbs again for gits and shiggles. The other engine will run perfectly and reach max RPM with either carb, either distributor/wires/coil set... That would tell me it is a mechanical problem, but again, cam measures fine, heads are good, bottom-end is near-perfect... I brought all my diagnostic tools home from work and will spend the day at the boat tomorrow. I will let you all know what I find.

Disconnect your exhaust. Leave the hoses connected to the riser and disconnect the other end. See if the engine revs. Had a customer with an old 78 Sea Ray that melted and collapsed his muffler and had the same symptoms you are having.

Eric,

Just another thought.

I was reading your original post and this comment sticks out.

"One of them was dumping raw fuel like the float was stuck down or something. We were literally spitting raw fuel out the exhaust. When the backfiring started, it would occur at 3k RPM. When we got back, there was literally a gallon of fuel in the oil."

An engine dumping this much fuel will flood out die. If you pulled that much fuel from the crank case, it was not from the carb. Did you check the fuel pump and have you checked the oil for fuel since?

I had a 4.3 Crusader in a Silverton. The guy spent $$$ trying to get it to run right. He went through a pile of carbs before I got the job. It was just the fuel pump filling the crank case with gas. The oil was as thin as diesel fuel and there was so much gas that it was choking the carb through the breather hoses on the spark arrester.

Chris

Docksidemarineservices said:

Disconnect your exhaust. Leave the hoses connected to the riser and disconnect the other end. See if the engine revs. Had a customer with an old 78 Sea Ray that melted and collapsed his muffler and had the same symptoms you are having.

Click to expand...

Yes, I had a car with a plugged cat converter, and although there was no backfiring, it ran slow rpms, even out of gear.

Eric:

You may owe Dockside a cold one....I went back thru your old posts and never saw mention of an overheat or any vacuum gauge readings. in lieu of disconnecting the exhaust hoses, you may want to hook up a vacuum gauge as it is much less work. A restriction in the exhaust will cause the flow thru the engine to suffer and the 'normal' vacuum readings will decline. use the other engine to establish some reference points.

Another coarse check would be to check the transom outlet. If restricted, with the engine at elevated rpm, the water/gas mixture won't 'shoot out' very well, and may just dribble. Again, use the other engine as a reference.

If it turns out to be an exhaust hose, i'd consider replacing all of it, especially if it has any signs of aging. Fiberglass tube, using hose sections to couple them, used to be cost effective. Just don't use it for the initial 18" out of the elbow.

If you're thinking the manifolds are clogged up, that might be a possibility. What, from carbon/soot build-up from running so rich? When we had them off for the head job, the ports looked pretty clear. We did not take the elbows off to see what the top-ends looked like though. As for the mufflers melting, they are fine. When we replaced the flywheel on this engine (another story), we also re-hosed both engines, Including the 3" exhaust hose. While we had it off, I was curious what the mufflers looked like inside, so I looked through and could see the flappers on my transom. Saw inside again when we did the head job.

I have also seen engines bog out like this on Honda's (Honda Tech), and we would loosen the exhaust manifold bolts and see if it would rev. If it would, it was the cat clogging. We obviously don't have cats, so the manifolds are the only think I could see clogging, but man those are some big ports to clog up. I will check today anyway; though my neighbors might not like me after, LOL.

No, there has not been any fuel in the oil since we took those carbs off. Oil has been sparkling clean and at the proper level since. It was absolutely the carb doing it. We put about 60 miles on the engine with the bad carb. Night-time out to the island at low RPM, then in the morning we noticed the black exhaust smoke and rainbow in the water and limped it back home. I can't believe I didn't bring the old carbs with us (kicking self). That engine didn't FEEL right with that carb, but out of sight, out of mind, and we were excited for the trip. Should have turned back at the first indication of a problem.

The fuel pump... what is it called... overflow tube (?) had zero fuel in it. Is there any other way of checking the fuel pump other than measuring pressure and not seeing fuel in the tube?

"The oil was as thin as diesel fuel..." Yup! Pulled 12 quarts of black, fuel-smelling, thin oil out of the engine when it holds eight quarts. Since an oil change doesn't get all of it (cooler, hoses, etc), I changed the oil three times, idling it for about ten minutes in between each.

makomark said:

Eric:

You may owe Dockside a cold one....I went back thru your old posts and never saw mention of an overheat or any vacuum gauge readings. in lieu of disconnecting the exhaust hoses, you may want to hook up a vacuum gauge as it is much less work. A restriction in the exhaust will cause the flow thru the engine to suffer and the 'normal' vacuum readings will decline. use the other engine to establish some reference points.

Another coarse check would be to check the transom outlet. If restricted, with the engine at elevated rpm, the water/gas mixture won't 'shoot out' very well, and may just dribble. Again, use the other engine as a reference.

If it turns out to be an exhaust hose, i'd consider replacing all of it, especially if it has any signs of aging. Fiberglass tube, using hose sections to couple them, used to be cost effective. Just don't use it for the initial 18" out of the elbow.

Click to expand...

Heard my email notification just before I posted that last response.

I did mention vacuum at a rock-solid 17.5 in/hg. Should I rev the engine with the gauge on to see what it does? I will try that on both engines and compare. There was no overheat either.

I have not checked the transom outlet while reving the engine, but at idle, water shoots out pretty good, and the flappers 'float' with the amount of air coming out. I am usually solo during all this troubleshooting, so maybe a neighbor will be down there to rev for me while I watch the outlet. My brother has family stuff going on today.

Our engines are on V-Drives, so the exhaust hoses are only about 18" long each. We recently replaced all four.

You rehosed the exhaust - does it have the proper 'slope', so everything runs 'downhill'? what about any elbows, y-pipes, or other connectors? usually, an overheat will cause the plies making up the hose to separate and one of the inner ones will lift up, just like a check valve....

The tygon tube between the fuel pump and the carb is called the sight tube.

The elbow could also be restricted but i can't say I've seen one, at least not for the exhaust gases. Same for the cast iron manifold.

Good news is you don't have too many more possibilites left to check. Don't forget to memasure the primary voltage on the coil - one of your earlier posts noted how impressed you were with the (then) existing wiring. One bad crimp connector in the main ignition lead (yes, the purple wire) is all it takes. Don't forget the engine-harness plug is also known to corrode given enough time.

Only drawback to these forums - can't read and type simultaneously. Remember the v-drives in the pics. Would still run the vacuum test (idle & elevated rpm) to get those data points.

Low primary voltage will drive an electronic distributor system nuts, too. another pretty quick check.

Okay, down at the boat right now and re-ran fuel pressure and vacuum checks. Vacuum is still the 17.5 in/hg I measured before. If slowly increase RPMs, it sucks up to 20 in/hg. Fuel pressure is at 8 PSI and only drops to just above 7 PSI when it starts bogging out. Definitely not fuel pressure.

Let me see if I can upload pics from my phone. Exhaust slopes down to muffler.

As for low volt on the primary ignition, while solo'ing the diag, I am jumping the purple wire straight to the battery and using a trigger on the starter. That way I can start, rev, kill engine without going up to flybridge. Eliminated coil voltage. Also, harness is solid from gauges to engines. No harness plug.

Vacuum not dropping with RPM would say no restriction, correct?

I keep leaning to this being a lean issue, but can't figure it. Any chance a carb would run fine on one engine, but lean/backfire on the other? Float too low? Running the bowl dry?

Trying to upload from phone. Doesn't show a preview. Hope I got the right picture. This should be vacuum at idle.


This one should be fuel pressure at idle.

It is NOT the carb or any other fuel related issue. I suspect cross firing of your ignition wires. Do that test I mentioned above (pulling one wire at a time) and get back to me.

Jeff

fastjeff said:

It is NOT the carb or any other fuel related issue. I suspect cross firing of your ignition wires. Do that test I mentioned above (pulling one wire at a time) and get back to me.

Jeff

Click to expand...

I replied to your post already:
"I am pretty sensitive to how smooth the engine runs. In fact, when reassembling the engine, I adjusted the lash statically (engine off), and it just didn't seem to be idling right. pulling wires as you mentioned, I found cylinders 5 and 8 were weak. I re-adjusted the lash with the engine idling, and it smoothed out perfectly. I now get an equal rpm drop pulling wires. Definitely don't have wires to the wrong holes (quadruple checked), plugs are all new, and I have clips separating the wires."

It seems at this point, the backing is a result of whatever is making the engine not rev up in neutral. If I can fix the bogging in neutral, I am sure the backing will stop. Good vacuum at idle and increased rpm would suggest mechanically, the engine is sound, and there is no exhaust restriction, correct? Fuel pressure TO the carb is good, but who knows what's going on inside the carb (though I would think having swapped port and starboard would eliminate that). Timing with the shunt on is set to 12 degrees. When removed, the timing st idle is aroun 20 degrees. Total timing advance seen is 30 degrees at around 2500 rpms. Says timing is okay, correct? Jumped ignition wire straight to battery, so primary voltage is good. Swapped coils, so coil is good.... what am I missing?

My only suggestion would be to try a Colourtune sparkplug,in different cylinders.its shaped like a spark plug,but you can see through it,actually see the combustion.you screw it in,and connect the HT lead to it,then start the engine and see what colour the combustion is.I used it once to find out why a ford 6 cylinder engine was running on 5 cylinders,with good compressions on all 6.turned out there was water being injected into the bore,just above the piston at bottom dead centre,thereby killing the combustion.If you used it in your engine at the revs you have problems with,you could see whats going on.just dont get it too hot,or they melt.worth a try.

After all the diagnostic work you have done and if the exhaust is not clogged in some way, I feel you are going to find a crack in your block. I hate to say that. Get a small compressor and loosen the rocker arms. Close off the cooling system with a factory GM auto t-stat housing and install a flex hose from the water pump to the t-stat housing. Drill and tap the housing and screw in a pressure gauge. Now fill each cylinder with air and see if you are pressurizing the coolant system. Then listen to the exhaust and the intake for an air leak.