Carb stumble lean spike???? - Team Camaro Tech
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post #1 of 6 (permalink) Old Apr 27th, 11, 08:21 AM Thread Starter
Travis
 
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Location: Fleetwood Pa
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Carb stumble lean spike????

385 stroker with single plane high rise, crower flat tappet with 104lsa very similar to the mutha thumper cam. Have a quick fuel 750 dp mech. Sec. Carb with 76 primaries and 84 secondaries. 31 squirters and 30cc acc. Pump with pink cam. I have a lean spike and sometimes back fires on initial touch of gas. When I pull out normal or am light on the gas. Tried moving up float level adjusting both prime and sec. Idles and air/fuel adj. Can't get rid of it?????timing is 13 at idle and 34 by 3000rpm. Have air/fuel gauge that tells me lean. Also what is good idle air/fuel and cruise. Right now idle is at about 13.1 and cruise is fat at about 12.1. Wot is 12.5 to 1. Help?????
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post #2 of 6 (permalink) Old Apr 27th, 11, 02:43 PM
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Re: Carb stumble lean spike????

Before you change anything make sure your pump arm is adjusted correctly. With the throttle resting on the idle speed screw tighten the nut and bolt to compress the spring until there is play between the pump cam and the arm. Then loosen it just enough to eliminate the play. If that doesn't work try an orange cam then green. Use the number 1 hole on both the cam and the throttle. Remember to adjust the pump arm after each cam change. If it still stumbles put a 35 shooter and the pink cam back on then orange then green until the stumble goes away. Don't use the shooters with the little tubes. Never had much luck with those on a street car.
A 104 LSA cam is gonna have a lot of overlap. Tuning the air/fuel at idle will probably be easier with a vaccuum gauge. See what vaccuum you have at idle and make sure your power valve in the carb is 1 or 2 in/HG lower. Most carbs come with a 6.5 power valve so if you have 6 inches of vaccuum at idle it's already opening. Tuning is a science that requires patience. Just make one change at a time. You'll get it eventually.

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Last edited by 12run; Apr 27th, 11 at 03:05 PM.
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post #3 of 6 (permalink) Old Apr 27th, 11, 04:48 PM
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Michael Gekko
 
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Re: Carb stumble lean spike????

Check to see if the PV is fractured. The float level is correct when you take the sight screw out and nudge the fender if a bit of fuel dribbles out when rocking the car. If you have an electric pump it needs to be on while doing this, if you have a mechanical pump the car needs to be running at idle and take precautions so you don't catch it on fire. You can also install clear sight plugs for this excercise to eliminate fuel spills but I wouldn't use them as a replacement for the metal ones longterm.

Have you tried turning down the timing?

Also putting bigger squirters is a cheap experiment too.

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post #4 of 6 (permalink) Old Apr 28th, 11, 05:28 AM Thread Starter
Travis
 
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Re: Carb stumble lean spike????

tried 37 squirter thats all i had and no diffrence.still stumbles. i have clear sight glass for fuel level. had it at the base at first then moved it half way up to see if it helps. might of helped a little but still there. the poervalve is new but is only a 2.5 because my vacuem is so low 6-8". should i try a larger power valve? i have a 3.5 and i think a 5.5. ive been told to eliminate the power valve and just square up the jets???any comments on that?
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post #5 of 6 (permalink) Old Apr 28th, 11, 05:23 PM
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Re: Carb stumble lean spike????

Quote:
Also what is good idle air/fuel and cruise. Right now idle is at about 13.1 and cruise is fat at about 12.1. Wot is 12.5 to 1. Help?????
When using a O2 data logger..caution.
Make sure all your HT leads , plugs etc are in top condition and everything is firing 100%.
A slight , undetectable misfire will come up as a lean condition, because of increase in O2 levels...a miss fire maybe under load (plug gaps) only.
This is where a data logger using O2, a MAP (vac) , advance, and maybe throttle postion...then reveiw logs later will pick up stuff like that.
Not being 100% sure all is correct can get u and the engine into a lot of trouble.
Do not rely just on a O2 gauge for tuning
A vac leak will generally give incorrect readings right across the board.

Ideal AFR is 14.7.
One goes for either ecomony mixtures or power mixtures
Economy:
Idle around 15 to 15.5...a leaner mixture u can run more idle advance...and smoother idle.
Cruise 30 mph same as idle
Cruise 60 mph will drop due to slightly heavier loads (wind etc) 14.7 to 15.0
Light load up a slope 14.5 to 15.0
WoT 13 to 13.5
Power:
Idle basically as above, 15 to 15.5
30 mph cruise 14.6 to 15.0
60 mph cruise 14.4 to 14.7
WoT 11.5 to 12.5.

Be 100% sure the O2 sensor calibration is correct.....in fresh air should read 20.95%
If the O2 sensor has been positioned correct, not in a low spot of the exhaust, over night check will show about 20.9% as a quick check....If not located in a good spot, HC gases are heavier, tend to hang around and the O2 will read lower than 20.9%

Use a logger for tuning, do not use a gauge...they are not quick enough response to give accurate reading for tuning purposes.

If you data is faulty, everything falls apart....Caution.
If correct, the results are dramatic.

My Spelling is not incorrect...it is creative

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post #6 of 6 (permalink) Old Apr 28th, 11, 06:50 PM
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Todd
 
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Re: Carb stumble lean spike????

Quote:
Originally Posted by 12run View Post
Before you change anything make sure your pump arm is adjusted correctly. With the throttle resting on the idle speed screw tighten the nut and bolt to compress the spring until there is play between the pump cam and the arm. Then loosen it just enough to eliminate the play. If that doesn't work try an orange cam then green. Use the number 1 hole on both the cam and the throttle. Remember to adjust the pump arm after each cam change. If it still stumbles put a 35 shooter and the pink cam back on then orange then green until the stumble goes away. Don't use the shooters with the little tubes. Never had much luck with those on a street car.
A 104 LSA cam is gonna have a lot of overlap. Tuning the air/fuel at idle will probably be easier with a vaccuum gauge. See what vaccuum you have at idle and make sure your power valve in the carb is 1 or 2 in/HG lower. Most carbs come with a 6.5 power valve so if you have 6 inches of vaccuum at idle it's already opening. Tuning is a science that requires patience. Just make one change at a time. You'll get it eventually.
Read what this guy said again......... I agree 100%. I'd go right to the accel cam. That pink cam is to small for most performance applications. Took me longer than I'd like to admit to figure that out.
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