Lowish power and rough under acceleration - Team Camaro Tech
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post #1 of 7 (permalink) Old Oct 27th, 09, 09:28 PM Thread Starter
Mark
 
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Lowish power and rough under acceleration

Well with the rain we are having here I can't tinker with this issue so I thought I might post here to see if I get any ideas on where to start. My engine starts great, idles great, revs great while parked and if you drive it like my grandmother probably would seems to have no problems. However under part and full throttle acceleration it runs rough and seems like it wants to really go for a second and then not the next over and over creating an almost jerky feeling but too minor to really be called jerky, hard to describe so hopefully that makes sense.

I have a 350 in the car with a factory roller cam that I had laying around when I built the engine so nothing radical there, running about 21 vac. A Holley 4160, 64 main jets and 6.5 PV that appears to be as it left Holley with no real changes. MSD ready to run Distriutor and coil that are only a couple of months old with a new fuel pump installed at the same time verified then that it was running around 6-7 PSI.

The engine has never run as well as I think it should but then I used the old fuel pump and ignition which I know was not in the best of shape so I always assumed that was the reason. Seemed like it improved some after the igntion and fuel pump replacement but now has gotten worse.

Looked at the timing the other night and it seems to be good now, was a little off prior to that.

Thanks
Mark
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post #2 of 7 (permalink) Old Oct 28th, 09, 04:43 AM
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Re: Lowish power and rough under acceleration

The interesting symptom you mention, give it gas, over half throttle, and runs for a second then kindda dies. Plus the fact, you can s-l-o-w-l-y bring it up to speed without a hitch/stumble.

This tells me the accelerator pump circuit is good, no immediate stumble, no bog, and runs for the duration of the pump shot. After the pump shot is dispensed, it dies.

I would wager if you have a vacuum gauge next to you while driving, I would say your half throttle would decrease vacuum to about 10". Carb/engine stumbles because lack of fuel. Since it has a 6.5" power valve, exchange it for an earlier opening one, say around 12"-13". Power enrichment circuit is alittle lean.

The engine should run smooth through the throttle opening regardless of the speed of the pedal is depressed.

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post #3 of 7 (permalink) Old Oct 28th, 09, 07:56 AM Thread Starter
Mark
 
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Re: Lowish power and rough under acceleration

Thanks for the reply, pv makes sense to me.I will try stopping by speed unlimited today and see if they have one in stock.
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post #4 of 7 (permalink) Old Oct 28th, 09, 03:59 PM Thread Starter
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Re: Lowish power and rough under acceleration

Ok, I replaced the PV with a 10.5 and this feels like a step in the right direction but still not quite there definately seems less jerky and to have more power.

I plan to check a couple of other things but have a question. I hooked a fuel pressure gauge back up to it and the pressure constantly jumps, very quickly, back and forth between say 3.5psi and 7.5psi. Those are rough numbers though as it jumps so fast it's hard to tell exactly what the high and low points are. Also the jumping increases in speed while accelerating the engine, mechanical fuel pump by the way, I can also feel the pump working while holding/touching the fp gauge.

Anyway my question is whether or not this is normal behavior for the gauge or if this might indicate an issue with the fuel pump?
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post #5 of 7 (permalink) Old Oct 28th, 09, 05:45 PM
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Re: Lowish power and rough under acceleration

Its normal because the mech pump has one 'power' stroke.
Naturally, you've set the float(s) alittle higher just to see if any improvement.
I would up the main jets from 64 to 68.

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post #6 of 7 (permalink) Old Oct 28th, 09, 06:47 PM Thread Starter
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Re: Lowish power and rough under acceleration

Actually I lowered the floats as they were way high, I will double check them again though to be sure I didn't go too far.

It also seems that the secondaries aren't opening much at all based on testing with a paperclip on the rod so that is probably contributing on the low power. I don't think the secondaries should cause the rough/jerky acceleration though.

Thanks for the help, I will try the jets.
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post #7 of 7 (permalink) Old Oct 29th, 09, 04:28 AM
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Re: Lowish power and rough under acceleration

The secondaries only open when the demand is needed, maybe after 5K RPM, higher rpm, more air needed.

True, secondaries would not cause a jerky motion. Maybe fuel sloshing out the vent tube and into the venturis might under forward motion. A vent tube extension might work.

http://www.summitracing.com/parts/HLY-26-89/

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