5.0LX surging/stumbling

[keviwe]

5.0LX 230HP 4bbl Rochester carb recently started stumbling, then surging (or running normal). Starts normal, Idles fine. Sometimes accelerates normal, sometimes not normal (less power). Sometimes runs normal at 3,000 rpm sometimes it falters. Doesn't feel like a spark plug misfire (not a hard misfire, more like a soft falter). I have newer genuine Mercruiser plug wires, dist. cap, rotor and AC Delco spark plugs. I have just changed the fuel filter (thinking it was water in the fuel (or alcohol separation), added several different fuel additives (in different tank fill ups) to remove water from gas, but nothing has helped.
Sometimes it runs good, sometimes it doesn't.
Carb has never been touched, so I will rebuild carb.
Any other ideas?
Can an ignition coil get "weak" or "weak intermittently"?
Thanks for your suggestions!




Reply

 

[Scott06]

5.0LX 230HP 4bbl Rochester carb recently started stumbling, then surging (or running normal). Starts normal, Idles fine. Sometimes accelerates normal, sometimes not normal (less power). Sometimes runs normal at 3,000 rpm sometimes it falters. Doesn't feel like a spark plug misfire (not a hard misfire, more like a soft falter). I have newer genuine Mercruiser plug wires, dist. cap, rotor and AC Delco spark plugs. I have just changed the fuel filter (thinking it was water in the fuel (or alcohol separation), added several different fuel additives (in different tank fill ups) to remove water from gas, but nothing has helped.
Sometimes it runs good, sometimes it doesn't.
Carb has never been touched, so I will rebuild carb.
Any other ideas?
Can an ignition coil get "weak" or "weak intermittently"?
Thanks for your suggestions!




Reply

I would also lean towards carb. to rule out ignition test with spark gap tester when its running poorly. Should jump a good 3/8" gap nice blue spark

 

[keviwe]

Too hard to test with spark gap tester because it only acts-up while in gear and accelerating or cruising. I'm going to replace the ignition coil first (since it's an easy quick swap). If that doesn't cure it, I'll rebuild the carb

 

[Scott06]

Too hard to test with spark gap tester because it only acts-up while in gear and accelerating or cruising. I'm going to replace the ignition coil first (since it's an easy quick swap). If that doesn't cure it, I'll rebuild the carb

you can ohm test the coil

 

[keviwe]

Now that I'm home, I did quite a bit of research on "weak" coils and have learned that the coil is a possibility for faltering. I also occasionally have a back-fire through the carb as well, while cruising at 3,000 RPM. I store my old 1989 boat out of state and only use it a few times a year. I am far away from my boat right now and will be using it again in September with family, so I think I will do a real quick coil replacement (the coil is a 1988 original), to not keep family waiting because if I ohm test it and it's bad, I'll have a day delay going to buy a new coil. I'll pull the carb if the coil doesn't fix it and rebuild the carb at home over the winter months.
Am I correct in thinking that it would not be a fuel pump, since sometimes it runs fine, (assuming a bad fuel pump (hole in diaphragm) would not be intermittent?
Thanks!

 

[Scott06]

Now that I'm home, I did quite a bit of research on "weak" coils and have learned that the coil is a possibility for faltering. I also occasionally have a back-fire through the carb as well, while cruising at 3,000 RPM. I store my old 1989 boat out of state and only use it a few times a year. I am far away from my boat right now and will be using it again in September with family, so I think I will do a real quick coil replacement (the coil is a 1988 original), to not keep family waiting because if I ohm test it and it's bad, I'll have a day delay going to buy a new coil. I'll pull the carb if the coil doesn't fix it and rebuild the carb at home over the winter months.
Am I correct in thinking that it would not be a fuel pump, since sometimes it runs fine, (assuming a bad fuel pump (hole in diaphragm) would not be intermittent?
Thanks!

yes coil can cause it but coils rarely fail unless the sensor inside dist is leaking voltage when car is off heating up coil ( had that on a VW back in the day)

more times than not on TB if it is actually an intermittent ignition issue it will be the sensor inside distributor or ignition module. these two situations can be tough to diagnose.

The intermittent backfire could also be a lean sneeze from carb being plugged.

Dont know the history of the boat but if I were coming into it with no prior knowledge would be looking to verify the following

Compression test - just a sanity check to baseline the engine is in good shape mechanically

Ignition timing verify initial and total advance , double check firing order

Spark gap test to see if you have a weaker spark even when it is running well

Ohm test the old coil.

Take fuel line off of carb , check filter at carb inlet if there is one, crank engine with fuel line directed into container or better yet test fuel pump pressure (4-7 psi is good)

When it gets goofy open fuel filler cap to see if it is under vacuum could have a plugged vent line.

Rebuild the carb

Hard to tell from keyboard but your symptoms are very similar to my friends boat last year. Intermittent surging rough idle , other times ran ok. Rebuilt his carb, which at first glance did not appear dirty, soaked it in ultrasonic cleaner, never had any of the issues again

At this point you need to think of it as narrowing down the possibilities

 

[keviwe]

Scott06 - Thank YOU for your thorough reply. GM 305 CID (5.0LX) 4-bbl 230HP has run perfect since 1988.
I have new (genuine Mercruiser) distributor cap, rotor, plug wires and AC Delco spark plugs-(all carefully gaped).
Engine ran awesome (as usual) with these new parts for 1 season, so firing order and timing are good (but I haven't checked timing).

Then over winter, then spring start up, engine exhibited the issues I described. Starts fine, idles fine. I've always treated fuel (during winterizing) with Stabil, but winter before this issue I used Startron enzyme treatment. I have not changed the main fuel filter canister very often, since I have never had any occurrence where water could have entered fuel. I fuel-up at auto gas stations, since this is a "trailer boat"and way cheaper there! BUT I did change the main fuel filter canister last season and saw no change. Fuel filler cap is always under the boat cover too, and boat is stored under roof. Fuel tank vent is clear and does vent during fill-up.

I won't be able to test fuel pump pressure, but as stated, I suspect a fuel pump would not operate intermittently. Correct??

Oil has been changed every season (with only 2 weeks use/yr and never uses oil) so I would expect compression/rings to be fine.

Engine does seem to be fuel starved and the back-fire through carb kind of suggests that as well. What is ultrasonic cleaner (for carb)? All I have is "carb cleaner spray and a compressed air nozzle, and maybe some fine wire through the jets.

I'll replace coil anyway, since it's really old and only $55 and easy to swap-out.
It seems like I might have to deal with the engine performance issues in September (only using it a couple days then winterizing). I'll plan to pull carb off and take it home to rebuild it.

Thanks again for your detailed suggestions!!!

 

[Lou C]

On the Quadrajet one of the common problems is clogged or partially clogged idle air bleeds, but if you’re idling find that may not be the issue you’re having. Then some of the accel pump orings don’t hold up to E10 gas well there are better quality ones available. Lastly sometimes the secondary air valve door spring may need adjusting, if it opens too easily the secondary system can’t compensate for the inrush of the air in the huge secondaries. These are all well known Quadrajet issues that have well known solutions.

 

[keviwe]

On the Quadrajet one of the common problems is clogged or partially clogged idle air bleeds, but if you’re idling find that may not be the issue you’re having. Then some of the accel pump orings don’t hold up to E10 gas well there are better quality ones available. Lastly sometimes the secondary air valve door spring may need adjusting, if it opens too easily the secondary system can’t compensate for the inrush of the air in the huge secondaries. These are all well known Quadrajet issues that have well known solutions.

Lou, Thank You for your input. Do you know of any link to a detailed video on rebuilding a Quadrajet correctly?
Also, I need an idle mixture adjusting tool (which I'll use only one time)
Do you think this double D fitting is the correct one in this Amazon link?

https://www.amazon.com/Carburetor-Adjustment-Screwdriver-Carburator-Adjusting/dp/B093G55FPV/ref=sr_1_5?crid=TD7151X14MFN&dib=eyJ2IjoiMSJ9.o8Z9FIgsDw9onOTrkN80VR272yo6qeyd07vOlX5AEUeLkuPcdSxk1n8i_9skdw2A2f5S3E-jEir8_3dN-ou1WV5KXumhfyRwgVOkLs_6WXO0ZfCJjXOKu1EUP934LCa_KWROwYY3oJQSv5DYXHcwyYS55edYornc20ihqDg_pm4hfznqPzJ5e5opbIMCTL2GLnf218KuO3fcenK-uV5ZPcI-xRMApoo0cMzych38THs.62Sam3j2GSGvcsHGRQnFJ3B_bnqALzCbGcgti_wd5-4&dib_tag=se&keywords=carburetor+idle+screw+adjusting+tool+for+GM&qid=1724856528&sprefix=carburetor+idle+screw+adjusting+tool+for+gm%2Caps%2C114&sr=8-5

 

[Lou C]

There’s many in you tube and there is a link to a good one on the Boaters Ed web forum.

 

[Lou C]

As far as the tools given the access is not easy like a Holley 2 or 4 bbl it is easier with the style of tool that has the flex shaft with the various attachments. That’s what I use and when you do the port side one beware the spinning alternator fins & belt! The starboard side one is hard to see.

 

[Scott06]

Here’s a link to mikes carb webpage that has parts and videos for q jet rebuild

Mikes and cliffs are the two guys I would go to for parts not sure if cliff is still selling parts but I don’t think either is rebuilding carbs anymore .

Above you asked what an ultrasonic cleaner is

I got an ultrasonic bath cleaner at harbor freight fill it with carb cleaner or simple green and put the disassembled carb parts in it for cleaning. Did the tripower set up off my 65 GTO and a 2 bbl Holley from my buddies Volvo Penta 4.3

The Holley had similar symptoms to what u describe and cured them by doing a rebuild

 

[Lou C]

Yep they do now make an ultrasonic cleaner big enough to fit a 4 bbl in. Next time I do a cleaning & rebuild I’m going to get one…

 

[Scott06]

And of course I forgot the link in earlier post

Rochester Quadrajet Rebuild

www.carburetor-parts.com

this is the ultrasonic I used for larger parts you may have to run it through twice with part of it sticking out

https://www.harborfreight.com/25-liter-ultrasonic-cleaner-63256.html?utm_source=google&utm_medium=cpc&utm_campaign=12192885310&campaignid=12192885310&utm_content=115861554166&adsetid=115861554166&product=63256&store=&gad_source=1&gclid=EAIaIQobChMItYbX_66YiAMVGU1HAR3kLiHYEAQYAyABEgLRC_D_BwE

I used simple green on the holley b/c I didnt want to strip the factory VP red paint. it started to take the paint off it. Never would have though simple green would have been that good on it as a cleaner

 

[cyclops222]

Her is a funny 4 bbl event. Family car Mercury with the 430 Ci V8. Ran VERY fast off the line. But it really drained the gas tank.
Parents sent it to the dealer. They fixed the excessive gas consumption. Mechanic said the gasket at the back of the front fuel bowl had a piece missing, at the back center of it.
NO more rubber burning from a dead stop.
Not all improvements are good.

 

[Lou C]

Rochester Quadrajet Rebuild Video

I have rebuilt several quadrajets on cars awhile ago and had a little knowledge in the basic skills to do so. When I had a carb that was bad on my boat I looked around for some videos on the net and couldn't find any. I did a bunch of research and rebuilt mt first marine carb. It worked out...

www.boatered.com

finally found the one I was looking for, this is a good one, easy to follow....

 

[thedinz]

5.0LX 230HP 4bbl Rochester carb recently started stumbling, then surging (or running normal). Starts normal, Idles fine. Sometimes accelerates normal, sometimes not normal (less power). Sometimes runs normal at 3,000 rpm sometimes it falters. Doesn't feel like a spark plug misfire (not a hard misfire, more like a soft falter). I have newer genuine Mercruiser plug wires, dist. cap, rotor and AC Delco spark plugs. I have just changed the fuel filter (thinking it was water in the fuel (or alcohol separation), added several different fuel additives (in different tank fill ups) to remove water from gas, but nothing has helped.
Sometimes it runs good, sometimes it doesn't.
Carb has never been touched, so I will rebuild carb.
Any other ideas?
Can an ignition coil get "weak" or "weak intermittently"?
Thanks for your suggestions!




Reply

Was it sitting for longer than normal? Even if you treated the fuel, the quadrajet is notorious for this, have had to rebuild mine twice due to sitting a little longer then expected. I would lean towards the carb being the issue at this point, just my experience at least.

 

[thedinz]

Rochester Quadrajet Rebuild Video

I have rebuilt several quadrajets on cars awhile ago and had a little knowledge in the basic skills to do so. When I had a carb that was bad on my boat I looked around for some videos on the net and couldn't find any. I did a bunch of research and rebuilt mt first marine carb. It worked out...

www.boatered.com

finally found the one I was looking for, this is a good one, easy to follow....

I followed this one. Quality is much better which makes a hge difference when trying to actually see what they are doing.

 

[cyclops222]

NEVER ad several different fuel additives !! You are mixing up a NEW chemical that can cause engine problems. Fact
Only completely use up that tank of gas before trying a different chemical.

 

[nola mike]

I have not changed the main fuel filter canister very often, since I have never had any occurrence where water could have entered fuel. I fuel-up at auto gas stations, since this is a "trailer boat"and way cheaper there! BUT I did change the main fuel filter canister last season and saw no change. Fuel filler cap is always under the boat cover too, and boat is stored under roof.

You still haven't actually checked for water. Check instead of assuming. Do you have a water separator installed? Might try running off of a remote tank.