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70s CHEVY 350 CARBON FOULING PLUGS


jsp282
06-13-2004, 08:21 PM
Have 77 Chevy 350 W/ Quadrapuke Carb. Keeps Carbon Fouling Plugs In Just A Few Miles. Re-built Carb, 180 Deg Stat, New Spark Plug Wires, New Cap, New Rotor, New Coil, And Oh Yeah, Several Sets Of New Plugs (various Types And Brands). Any Thoughts?

MagicRat
06-13-2004, 09:03 PM
Is it fouling due to burning oil?
I know that old Chevy valve stem seals loved to get brittle and let oil seep down the valve stems. New seals might help a lot.

jsp282
06-13-2004, 10:05 PM
Is it fouling due to burning oil?
I know that old Chevy valve stem seals loved to get brittle and let oil seep down the valve stems. New seals might help a lot.

THANKS FOR YOUR REPLY......

THE FOULING IS CARBON NOT OIL. IT'S FROM UNBRUNED GAS AND I CAN'T FIGURE OUT WHY. THE EXHAUST ISN'T BLOWING BLACK SMOKE INDICATING TOO RICH A MIXTURE, BUT THE PLUGS ARE STILL FOULING.

calgary_redneck
06-16-2004, 01:05 AM
check the timing if its in spec it will be your carb running to rich most likely adjust your idle mixure and see if it helps. Is it the original q-jet?

jsp282
06-19-2004, 09:36 PM
Redneck, thanks for the reply. Regarding the timing. It cannot be set exactly because who ever built the engine didn't put the timing flag on the engine. I've advanced the timing quite a bit. I know this because when the engine is warm, it cranks slowly. The carb is not the original. It is a rebuilt I bought and installed. The truck has had this problem with both the old and rebuilt carbs. I've adjusted the idle jets every which way and it hasn't made a difference.

Something I haven't brought up is that the truck sits and idles alot and it has a mild cam in it. I wonder if it's just loading up when it idles so much.

Thanks again for any help you can provide.

HDChvyTrk
07-04-2004, 10:41 AM
What distributor and coil are you running? How much compression does this engine have? Are you sure the plugs are the proper heat range? And did you replace the metering rods and seats when you rebuilt the Q-junk.

jsp282
07-06-2004, 09:04 PM
HDChevTrk, thanks for the interest. The distributor is stock (and who knows how old). I did put in a new stock coil. The compression ranges between 140 and 150 psi. Spark plugs are what's called for in the book. Regarding the carb, I just bought a rebuilt (had the same problem with the old carb). Any Ideas?

HDChvyTrk
07-07-2004, 07:44 PM
I think I would point the finger at the ignition system. If it is not smoking but fouling plugs quickly I would not think it was enough to rich to matter? I would think about a good coil, like an MSD Blater II and maybe an MSD 5 or 6 series ingnition box. This will help your ignition produce a stronger spark. You may, at some point, want to consider a distributor swap also. I asume when you say " the original distributor" you are refering to a single or dual point distributor. If so, and you want to try and keep this setup you should look at the points, replace and/or gap them. They control coil dwell (charge) time and could be a major part of the problem?? If you have a HEI They are famous for frying modules and you could also check the little nub that sits under the coil that transfers power to the rotor.

jsp282
07-08-2004, 06:33 PM
HD, thanks for your thoughts. This motor has HEI ignition and I agree with you, I think it's an ignition problem. I've replaced everything in/on the distributor except the module. If the module is bad, won't that keep the motor from running? Or, will it still run, but perhaps with problems? If this continues, I think I'll just buy a new high performance distributor with a high output coil. When I put fresh plugs in this thing, it runs great! Just can't keep it running for very long! Let me know what you think about that module and perhaps I'll replace that. If it's a go or no go on the module, I'll just get that new distributor. Thanks again for your interest.

HDChvyTrk
07-08-2004, 07:28 PM
The module will NOT keep the HEI from operating. It will however kill the timming advance and coil dwell time over 2500 to 3000 rpm. Have you noticed it hesitating or stumbling above 2500 rpm? If it were me I would probably replace the distributor anyway. HEI's are not the best distributor out there, expecially on modified engines.

jsp282
07-08-2004, 10:43 PM
Hey again. Interestingly enough, it stumbles and hesitates below 2500 revs. Once above that it really comes alive. The only reason I haven't bought a new distributor is I'm CHEAP! Don't want to spend the $150-$200. Think I'll try a new module and another set of plugs. I'll keep you posted. If you can think of anything else, please give me another holler. Thanks!

HDChvyTrk
07-09-2004, 03:55 PM
The, module is cheap abd should be easy to find. The low rpm stumble would make me think the coil was bad? Did you replace that little button under the col when you changed it and/or the distributor cap?

jsp282
07-09-2004, 07:04 PM
Hey HD. I did replace the coil and the little nub under it. I talked to a guy at a parts place today and described my problem to him. He said I'm sucking air somewhere. Either the carb base or the intake manifold. So, I'll get some carb cleaner and see if I do have leaks. When I replaced the carb I thought of re-torqing the intake, but just got lazy and put the carb on. I did have a leak at the carb base when I put the new carb on using the gasket they supplied with the carb. I took that off and used the old thick gasket that was on it before. I checked it then and didn't have any carb base leaks. Who knows? I'll do the leak test and let you know. I don't know who you are other than HD, but I want to thank you for all of your interest. My name is Jim and I live is South Carolina. Talk to you soon........

HDChvyTrk
07-09-2004, 08:43 PM
Hey jsp282, My name is Chris and I live in central MD. I enjoy helping, if I can, sometimes another perspective can jog the mind and come up with a fix. As for the carb leak, it may cause the stumble but ought to make the engine run lean, not rich? Something I meant to ask, do all the plugs look roughly the smae or are some worse/better then others?

jsp282
07-09-2004, 10:46 PM
Hey Chris. You know, I was thinking the same thing about being lean with a leak, but I'll try the leak test again anyway. Regarding the plugs themselves, the plugs on the drivers side front seem to foul first and look the worst. When they foul, the motor starts back firing through both the carb and exhaust. We'll get this figured out eventually huh? Talk to you again, Jim.

HDChvyTrk
07-10-2004, 04:13 PM
Do you still have an egr valve? And, if so, have you checked it's operation? If it is stuck open it could cause this problem. Interestingly, what made me think of this is the fouling you describe is the smae pattern as the intake manifold distribution?

jsp282
07-10-2004, 05:20 PM
Yea, it does seem to follow intake distribution. No, there's no EGR valve. The only smog on this engine is a PCV valve. I still haven't done the leak test. Been busy and sooooo damn hot down here last two weeks. Don't really want to go outside during the day and when it cools off, the mosquitos about carry you away. I'll get around to it eventually. Talk to you soon, and thanks, Jim.

AMCKB
08-03-2004, 09:06 AM
Where are your spark plugs gapped and what kind are you using? Sounds like you need a hotter spark!

jsp282
09-09-2004, 08:37 PM
All, thanks for your replys and help. I've finally figured out my carbon fouling problem. Well, I didn't exactly figure it out myself. I was looking around the internet when I came accross an "ask the mechanic" site. Someone with exactly the same problem I had wrote in asking for some help (see earlier in this post for a description of my problem). This mechanic said chevys use exhaust gas in the intake manifold to heat the engine faster and sometimes the wall between the exhaust and intake ports develop a hole allowing exhaust gas into the cylinder. This cuts down the amount of air in the cylinder and the result is carbon fouled plugs. This made more sense than anything else I'd tried, so I bought another intake manifold. When I took the old one off, I found a break in the intake gasket at one of the exhaust ports. I don't know if this gasket was the problem or if there was actually a problem with the manifold itself. Bottom line......I put it back together with the new manifold and gasket set and IT RUNS GREAT! No more plug fouling! Again, thanks to all of you who tried to help me, and perhaps this solution will help someone else.

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