engine bogs, carb tuning - Chevelle Tech
Troubleshooting Diagnosing problems done here.

 
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post #1 of 11 (permalink) Old Feb 9th, 02, 11:05 AM Thread Starter
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I really need your guys help. I recently built a 400hp 350 sb. I topped the engine with a Peformer RPM intake and 1407 Performer Carb. My problem has two parts. First, the engine would not idle with the choke plate closed- so I took the choke off the carb. From there the car would idle, and I then proceeded to set the idle mixture. With the engine warmed up and running I turn the screws in until I hear the motor begin to stumble and I back them out 1 1/4 turns. After doing boths screws I set the idle. The engine has an Isky cam with 280 degree of running duration that produces approx 7 inches of vacuum at idle. Heres my problem. I went out to test drive it today and the engine bogs and stumbles when I accelerate from idle. The car runs fine above 2500 rpm, but below that, it stumbles and even backfires sometimes. And if I floor it off idle, the motor almost stalls for a second then quickly jumps back to life. Also, if I take a quick U- turn then the car almost stalls. I dont know where to go from here. I need to know if I should richen or lean the mixture, change rods or springs, or change the accelerator pumps squirt, or adjust the float level. Please help. I need to get my Chevelle on the road, its my daily driver.

Greg

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70 Chevelle 350ci/th350 10bolt Go Earnhardt Jr.!
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post #2 of 11 (permalink) Old Feb 9th, 02, 11:34 AM
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Greg, check the float levels. After turning the idle mixture screws in, only turn out about a 1/4 turn. You can also set them by reading a vacuum gauge ( turn out to highest vacuum reading) or a tach. I have a stumble on my Performer carb also. Its not new and I'm still working on it. Try adjusting the linkage on the accelerator pump. Try changing the timing a little. You might have to rejet. Good luck.

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KJ
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post #3 of 11 (permalink) Old Feb 10th, 02, 12:18 AM
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Are you absolutely sure there are no vac leaks on the carb area anywhere? Did you possibly bump the timing while working on the carb problem? 1. problem sounds like the timing isn't right. That also sounds like a decent size cam. Are you running a stall convertor and some good rear gears? Or is it stock convertor with 2:73 gears? The motor can't rev with the lack of the gears and convertor causing it to stall and spit from being hit with too much gas right off the bat.
Give some more info on your complete car set up and we'll go from there.
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post #4 of 11 (permalink) Old Feb 10th, 02, 11:27 AM
 
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Check fuel pressure. too much pressure will cause these concerns.Max should be 6PSI

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post #5 of 11 (permalink) Old Feb 10th, 02, 5:58 PM Thread Starter
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kerry- Ive tried adjusting the accelerator pump, one bottom hole made the stumble even worse, while the other caused a huge flat spot.
Randy- Iam positive there are no vacuum leaks. I just assembled it like many times before. Ive had problems with vacuum leaks before, this isnt it. Timing is the same at 12 deg initial and apprrox 38 total. YES IT HAS 273 gears with a stock 1800 stall convertor. How would that cause it to bog? It may cause the engine to work harder, but not almost stall....
Mello- the pump Im using is a carter electric that will only pump fuel at a max psi of 6.



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post #6 of 11 (permalink) Old Feb 10th, 02, 10:18 PM
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Have you got another carb around anywhere to try. Sure would help narrow it down a bit.
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post #7 of 11 (permalink) Old Feb 10th, 02, 10:34 PM
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Greg,

I had a similar problem with my 750 double pump. I visited Holley's web page and went through their tech section. They recommended decreasing the size of the accelerator pump nozzles. I went from a .28 nozzle to a .25 nozzle. The hesitation is gone. Check out the Holley website. Lots of good info.

Bubba

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post #8 of 11 (permalink) Old Feb 11th, 02, 1:01 AM Thread Starter
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I was on edelbrocks website, and they were saying that low vacuum idling cams may cause the rods to run super rich. The metering rods at idle are supposed to be in the lean or down position, but if the vacuum is too low causing the rods to climb up and run rich on idle and then bog until I floor it and let more air in.... What do you think.... in the mean time I'm going to do the leak test with carb spray tom. morning and change th carb base gasket, and also check the timing and see if my plugs look black... The accelerator pump shot may be the prob. but Ive tried moving that rod to the different holes, and either the bog got worse or a flat spot was created. So... Ill keep you posted.

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post #9 of 11 (permalink) Old Feb 11th, 02, 11:41 AM
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Greg, I also changed my distributor vacuum from ported to manifold and that helped a little. My hesitation is worse when cold and almost disappears when warmed up. I was thinking of changing my rods too. Been busy finishing my basement. Hope to get back to the Chevelle soon. Please post if you get this figured out.

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post #10 of 11 (permalink) Old Feb 11th, 02, 4:51 PM
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IS this carb a mech. secondary or vacuum secondary unit? If the secondaries open too quickly it can cause the engine to fall flat on it's face then pick back up. Make sure they are adjusted properly. I know this is real critical when setting up a Quadrajet. Isn't the Edelbrock very similar?

Troy.
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post #11 of 11 (permalink) Old Feb 11th, 02, 4:53 PM Thread Starter
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UPDATE: no vacuum leaks at intake to head or carb to intake leaks. timing okay, and vacuum is read at 12 inches at 800rpm idle, idles the smoothest at 800rpm. I got 90% of the bog to go away, and the engine doesn't seem to lope as much, and it idles a whole lot better. What I did was change the step-up spring for the two metering rods in the carb. Now for the other 10%, I think that my mixture is a little rich, BUT I'm going to leave it that way right now because its been very cold here in CA. Once it warms up then I might lean it out. I have to see if it will idle with the choke. I dont know if I will put it back on or not.... havent decided. I think the other 10% of the hestitation could be from such a long duraction cam and freeway gears coupled with a stock converter... What do you guys think?

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[This message has been edited by green70 (edited 02-11-2002).]
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