Help Diagnosing carb vs ignition problem, 5.7 mercruiser

[peakay]

Hi there,

We bought a 1990 Tiara 2700 with the original 5.7 twin Mercruisers last year, which now has 1200 hours. Our STBD engine has developed a problem where once it runs for 20-30 minutes idling through the harbor it starts bucking at idle rpm in the marina and then stalls. Engine starts fine and will run fine for until this happens. Seems to be getting worse and yesterday after this happening I couldn't get the engine to stay running at all. When i shut the engine down, it did some "dieseling" type overrun with knocking. I've run a fuel treatment through it and also new gas with no change.

I am not sure if it is a carb issue (quadrajets, appear original) or ignition.

Last season we had a problem on the same engine where the coil was arcing to one of the posts, resulting in bucking at low rpms. I replaced that coil (Sierra due to time and low availability) and the engine ran fine thereafter. Put 120 hours on last season. We've recently done maintenance changing the fuel filters and the mechanic suggested new caps/rotors based on inspection and we replaced those the mercruiser oem parts.

Any ideas? @Bondo . I don't want to start throwing parts at it - a carb rebuild has been suggested, but given the issue with the coil last season I am not sure if that is it. As a next step, I am gogin to try to swap coils and see what happens in case I got a bad one.

Thank you.

 

[alldodge]

Dieseling is the sign of running rich. Pull some plugs and see if there not real black.

 

[Mcfltfyter]

Has base timing and advance been verified? I would get that engine off idle every 5-10 minutes, and if it still happens after 20-30 minutes, its probably not the carb.

 

[Scott Danforth]

its normally never the coil

dieseling is either running rich or improper timing

being a 1990, start with a fresh carb rebuild bet you find lots of gunk in the bottom of the carb bowls

 

[Lou C]

I’d start with dumping out the contents of the fuel filter into a Mason jar and let it sit. See what the gas looks like (debris/water layer on the bottom).
I’ve had to do carb rebuilds about every 6 years on my Quadrajet. It’s all in the cleaning. Usually the needle valve starts to leak and then you get flooding also the air bleeds can get clogged and then you have rich running. There are better quality rebuild kits out there for Qjets (Cliffs High Performance Quadrajets) with better quality parts and ethanol resistant parts like accelerator pumps...

 

[peakay]

Thanks guys. Seems like it could be the carb, but with having had that coil problem a while back and the fact that is seems to do it after a period of time/possibly from heat, I just started coming back to the coil or possibly an ignition module.

One possibly important bit of info I left out. When it gets into fail mode, I can get the engine back started sometimes, but the tach darts around the dial as the engine misses and goes hard to zero at times even though the engine is spinning. Was chalking it up to a weird side effect, but I've also read that tach wiring can cause problems too. not sure.

@Mcfltfyter I'll try that today as we are supposed to take some people out on a harbor cruise (two engines leaving, then one as the motor starts glitching later). I can only get it off idle in neutral that way though -- that's what I normall do when driving to clean it up.

@Lou C - good feedback. I lack the time and confidence right now to do this, but have surveyed people for who the good rebuilder is in the area and planned to take it there. I'm thinking do just the bad one first and see if that is actually the problem vs doing both at once.

Thanks guys. any other thoughts are welcomed.

 

[kenny nunez]

One thing to look at, after both engines are up tp temperature and are idling remove the flame arrestors and look at the Venturi boosters. Look to see if fuel is bubbling from the boosters on the engine that is giving the trouble.
If so then it is gradually loading up with fuel.
How does the engine run at cruising speed? Does it turn up the same at full throttle as the other engine?
Sometimes fuel or ignition problems will mimic each other.
With the amount of hours you have it might be worth sending your carburetor to some who specializes in Quadrajets only. It could be something like metering rods that are worn to the point that are causing an over rich condition.

 

[Lou C]

Also the idle air air tubes are prone to clogging and may cause the nozzle drip that Kenny was referring to. I had this on mine before I really cleaned it out.

 

[peakay]

One thing to look at, after both engines are up tp temperature and are idling remove the flame arrestors and look at the Venturi boosters. Look to see if fuel is bubbling from the boosters on the engine that is giving the trouble.
If so then it is gradually loading up with fuel.
How does the engine run at cruising speed? Does it turn up the same at full throttle as the other engine?
Sometimes fuel or ignition problems will mimic each other.
With the amount of hours you have it might be worth sending your carburetor to some who specializes in Quadrajets only. It could be something like metering rods that are worn to the point that are causing an over rich condition.

Well, I had people aboard for a Sunday cocktail cruise yesterday. before leaving I inspected the ignition module for any melting (all good), disconnected the plug and cleaned with contact cleaner, plus swapped coil wires between engines. this was all i could do in my limited time before people arrived.

Then we cruised and right at 20-25 minutes the engine started glitching again. I had paused it twice along the away in neutral to give it a few revs and it didn't seem like it needed cleaning out and nothing seems to go wrong until it just starts running like crap for no reason.

I had another mechanic call me and since he is too busy, he listened to the symptoms and still recommended carb rebuilds. I guess I'll do it, but it just seems so weird that it takes time and heat before it glitches. I might swap coils as a last trial and see what happens. I just hate fixing crap that isn't broken and possibly causing additional problems.

unfortunately i can't even test how it runs at high throttle as the engine glitches before I can leave the harbor and it won't run right at any throttle position once that happens.

Thank ALL for the help.

 

[Mcfltfyter]

I'm leaning towards ignition. I'm not sure what ignition system it has, but I would swap coils, modules, etc between the two engines and see what happens. Coils and modules can get hot and act up, just not as common as a total failure.

 

[kenny nunez]

If it was possible to leave the harbor on the good engine then try running the problem child once you are in open water to see how it does while planed out for 30 minutes.

 

[alldodge]

With it taking a while before it happens, next time it starts, open the gas cap for couple seconds then close up and see if it gets better

 

[peakay]

Thanks All. I ended up pulling the carb today and was going to swap them, but decided not to mess with the working engine. Will have it back on this weekend and see if that causes the problem. I'll be sure to report back -- I read so many threads with similar problems and then nobody closes the loop on what the cause was in the end.

 

[Mcfltfyter]

If we never hear from you again we will know the carb rebuild worked, or you sunk it for insurance.

 

[Paintman1960]

^lol

 

[peakay]

If we never hear from you again we will know the carb rebuild worked, or you sunk it for insurance.

Or Both!

 

[peakay]

OK boys, thanks for all the help and advice. Had the carb rebuilt, started the motor, ran great .... but started crapping out after 20 minutes or so just like it did before..

Seems like its some sort of ignition issue to me at this point where something is getting hot and failing.

I then swapped the thunderbolt ignition modules from port to starboard, restarted, problem did not follow. Issue is still on the stbd engine. I did have a failed ignition coil on the stbd engine last season and had to replace it with a west marine sierra coil due to time and availability. At this point I'm going to swap that out for a new one and try it (port engine coil did not want to come out to swap), but if that doesn't do it I'm stumped.

Engine starts and runs fine until 20-25 minutes, then idle falls, it stumbles and doesn't want to restart. It;'s like clockwork at this point. Can replicate it in motion or at the dock.

Any other ideas??

 

[Mcfltfyter]

It sounds like vapor lock, pulling a vacuum on the fuel tank, or coil to me. I doubt a marine engine gets hot enough to vapor lock, but it could be a fuel tank issue if it has independent tanks.

I'd still guess coil. Either replace it, swap them, or try to cool the coil every 5 minutes with a can of air flipped upside down and see if it runs longer.

 

[alldodge]

When it dies pull the fuel filter to see if its full

 

[nola mike]

OK boys, thanks for all the help and advice. Had the carb rebuilt, started the motor, ran great .... but started crapping out after 20 minutes or so just like it did before..

Seems like its some sort of ignition issue to me at this point where something is getting hot and failing.

I then swapped the thunderbolt ignition modules from port to starboard, restarted, problem did not follow. Issue is still on the stbd engine. I did have a failed ignition coil on the stbd engine last season and had to replace it with a west marine sierra coil due to time and availability. At this point I'm going to swap that out for a new one and try it (port engine coil did not want to come out to swap), but if that doesn't do it I'm stumped.

Engine starts and runs fine until 20-25 minutes, then idle falls, it stumbles and doesn't want to restart. It;'s like clockwork at this point. Can replicate it in motion or at the dock.

Any other ideas??

How about you confirm that it's an ignition problem? When it doesn't start throw a timing light on a lead or use a spark plug tester and see.