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350 cui SBC
09-17-2016, 02:58 PM
Post: #1
350 cui SBC
I have a 350 SBC that I did a compression test on. It ran #1-85,#2-85,#3-65,#4-80,#5-50,#6-80,#7-85,#8-90 psi. I realize these are low numbers and am wondering if this would explain the engine missing on 2 cylinders periodically. I am at my witts end. After overheating, I have put new plugs in, new distributor, and cap , rotor cap, new hei coil, changed carb., new manual fuel-pump, new in-line electric fuel-pump, new inline filters before and after pumps, new poly tank, and still have issues with engine missing occationally and/ or even dying and after sitting, it starts and runs excellent until the next episode happens. I am thinking of changing motors and starting anew again. Any suggestions?Sad
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09-17-2016, 03:12 PM
Post: #2
RE: 350 cui SBC
You say it runs well but those numbers tell a different story. If I were in your shoes the first thing would be to borrow a known good compression tester to confirm the base health of that engine. Also do a wet compression test to see if it's rings or valves.

As for the miss, if it's intermittent then likely you need to fine tune your trouble shooting to point gap, timing, or possibly a sticky valve. While poking around you should also check the distributor shaft for excess side play and see if the fly-weights for the advance are doing their job. Same goes for the vac advance. If the breaker plate doesn't stay flat as it moves then things get funny. If thing are "walking around" that could explain lots.
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02-21-2017, 04:47 PM (This post was last modified: 02-21-2017 04:59 PM by ivan.lord.1953@gmail.com.)
Post: #3
RE: 350 cui SBC
(09-17-2016 03:12 PM)Rupe Wrote:  You say it runs well but those numbers tell a different story. If I were in your shoes the first thing would be to borrow a known good compression tester to confirm the base health of that engine. Also do a wet compression test to see if it's rings or valves.

As for the miss, if it's intermittent then likely you need to fine tune your trouble shooting to point gap, timing, or possibly a sticky valve. While poking around you should also check the distributor shaft for excess side play and see if the fly-weights for the advance are doing their job. Same goes for the vac advance. If the breaker plate doesn't stay flat as it moves then things get funny. If thing are "walking around" that could explain lots.

I apreciate the advice. I have an H E I distributer. I Bought from O'Reilleys. with a new vaccum advance, coil, and ignition module. Brand new wires and plugs. I double checked the wiring, free play, did a compression test (145-150 lbs per cyl.}.rechecked everything I can think of and still have 2 cyl that is gas fouling. Black sooty looking plugs. I am thinking of changing from the Edelbrock 4bl to a 2 bl. to see if that will help.
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02-22-2017, 12:43 AM
Post: #4
RE: 350 cui SBC
Took 5 months for a reply so I take it this is a project vehicle, and likely low priority, right?

Ok, HEI distributor tells me this is a pre 1980 engine, so something more than 35 years old. Sill in all, the rules are the same. We need to look at diagnosing what's causing the plug fouling. Might be a carb issue but let's back up a bit since it sounds like NON - OEM intake.

Short story time: Back in 1974 I was doing a carb overhaul on my (future) wife's car. I was pretty sharp on this stuff even over 40 years ago. Started it up and it just ran like crap. Pulled my hair (had more back then) and scratched my head, made a few phone calls to friends, and an old guy (my future father in-law) said to double check all of the old parts on the bench. IOW, re-trace my steps because it ran better before I screwed with it. I'll never forget picking up the carb base gasket and comparing it to the one I took out. I'll be dipped! (in you know what) I had installed the wrong base gasket and it didn't have a place for the PCV to flow through, causing a really rich mixture.

My point here is to look carefully at the pieces to this puzzle. You are the eyes and ears at your end and we can only suggest what *MIGHT* be wrong. It sounds like you have a mix of OEM and aftermarket parts, so some may be right and some may be just the wrong combination for what you intended. What I am driving at is changing the carb to a two barrel may or may NOT be a fix depending on what else you stumble across. Doing that will require a manifold swap and what else can go wrong there? Are the gaskets compatible with the EGR, the PCV, and that carb / manifold combination? Is it a used manifold and possibly the EGR passage is burned through or clogged? Somewhere along these lines is probably the real answer. Another possibility is you have too much fuel pressure with both pumps and that carb is just flooding. You really have to look at the big picture one piece at a time.

Keep us posted on what you find and we'll try to help.
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02-22-2017, 06:32 AM
Post: #5
RE: 350 cui SBC
Unless I wasn't paying attention; which happens a lot, I didn't see which 2 cylinders were fouling out. I had an old 350, HEI, that kept fouling out 7 & 8. It turned out to be a large vacuum leak near the rear of the intake due to swapping out carbs and missing things in the process. So; much like Rupe said, recheck all your work. Sometimes you get to frustrated that your eyes become clouded and you miss an obvious problem. Also, if someone else helps you, double check their work. I have found that people helping me have made a mistake many times. It happens! We are human.

Please keep us posted as to what you find.

Thanks for using the forum,
Garner

Interested in Mining? Look here: Coal Mining, PA
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02-25-2017, 08:29 PM
Post: #6
RE: 350 cui SBC
I appreciate all the help and suggestions all of you have given to help me get this 350 to run right. I guess I need to retrace all my steps and use up a can of carb cleaner and hope to find a leak some where. I really appriciate your help. I'll post the solution if I find it. It might help someone else with a simular problem. I used to be able to work on a sbc's with my eyes closed, I guess it's time to open them up.
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02-25-2017, 11:48 PM (This post was last modified: 02-25-2017 11:49 PM by Rupe.)
Post: #7
RE: 350 cui SBC
Just had a thought...

I picked up where you said it seems intermittent so I have to ask if it's temp related? Does it happen cold or hot? If it only happens hot could you have a vac line that collapsing when it gets warm? (old and spongy PCV hose?)

I also have to ask why you have 2 fuel pumps and if too much fuel pressure could play into a flooding issue? Most carbs are more than happy at 2-3 PSI fuel pressure and with today's gasoline that seems to soften up all kinds of rubber parts you may have a double whammy of only flooding hot.... or the wrong base gasket that allows too much heat near the carb.

Got headers on this beast? Does the fuel line go close enough to absorb the extra heat? You can insulate the line or find / fabricate a return line to circulate the fuel back to the tank. Years ago both Jeep and Corvair used a fuel filter that had a built in return line with a restrictor so only a small amount went back to the tank. Recalling the return size was 1/8" or 3/16" so fairly small, but that's all it takes to keep cool fuel handy at the carb. The Corvair (turbo model) filter had threaded fittings and the Jeep filter was hoses with clamps. (much cheaper) If you need to fabricate a restrictor, use a short piece of copper line or steel brake line, solder the end closed, then drill a small hole in the solder. Using brake line you can assemble a T-fitting with the proper sizes for the supply and carb, then do the restrictor from the T back to the tank via another fitting you fabricate into the filler neck of the tank. You can either splice into the vent hose or remove the filler neck and fabricate something on the bench. DO NOT try to solder into the fill neck while it's connected to the tank! (can you say boom?)

I know I'm grabbing at straws but it sounds like you're looking for some suggestions.
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02-26-2017, 01:42 PM
Post: #8
RE: 350 cui SBC
1st off yes I am looking for suggestions. I removed the electric fuel pumps, and am using a stock mechanical pump. 2nd The running rich to the point of fouling #3 and #7 cyl.s, happens from the first starting of the engine and continues to normal operating temp. It is acting as though it is trying to run on raw gas. I have retested the compression and dueing a wet test is not changing the reading. I have tried re adjusting the valves(lifters) I have adjusted the 1/4 turn past zero lash so as not to set the to tight. I know they should be adjusted 1 full turn but I am trying to air on caution. All I can say is black smoke is rolling out wish tell me it is to rich.
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02-26-2017, 02:39 PM
Post: #9
RE: 350 cui SBC
The fuel is coming from someplace and I would suspect the carb float is sticking or the choke pull-off is not adjusted correctly, the choke itself, or a combination of those things. More obscure possibilities would be fuel from the carbon canister / vent system. Perhaps disconnecting all of the vac lines except the vac advance and plugging them will tell you something there? On the choke issues, you can move those by hand to get a feel for any changes. A sticking / sunk float may show up as extra fuel dripping inside one barrel of the carb. If you are careful, on most carbs you can remove the top while on the engine and take a visual of where the gas level is. Keep stroking, you'll find it!

Hint: if you have the option to park the truck on a hill nose down see if that changes anything. Usually the extra gas just rolls down hill and if you look inside the manifold you can visualize why #7 (rear cyl) is one of your issues. IOW, most engines are not mounted level, but slanted toward the rear.

BTW, if this is an auto trans with a vac modulator also disconnect that vac line.
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