holley tuning advice

holley tuning advice - 08/16/09 10:00 PM

just replaced the carb on my 440 last friday with a almost new 770 street avenger. The engine starts and idles fine but seems to want to stumble at about 2300 rpm(which just happens to be cruising speed). It also wants to cough if you try to jump on the throttle. I can opren it up slowly and it will take fuel fine but if you stand on it it just stumbles. Any body got any ideas what to look at?

Posted By: ohiodemon

Re: holley tuning advice - 08/16/09 10:26 PM

check your float levels and accelerator pump setting.

Posted By: D_C

Re: holley tuning advice - 08/16/09 10:46 PM

Agree with above. Stumble at Cruise sounds like Float level induced fuel starvation or problem with secondaries.

Stumble off start or acceleration from cruise is the Accelerator-Pump circuit (or sometimes, secondaries coming in too soon or late.) This from Holley.com:

Quote:

ANSWER: The most common cause of a stumble is not having an adequate accelerator pump shot. The first thing to do is to look at the discharge nozzle and make sure you are getting a good strong pump shot. If not then you need to inspect the pump diaphram for a hole or tear. You will also need to make sure that the pump passage is clear from any trash or debris.

First you will need to check the adjustment on the pump. To do this you will open the throttle all the way (WOT). Push the pump arm lever down and then adjust the pump override spring to obtain .015" clearance between the pump arm and lever.

If you are having a stumble and no black smoke out the tailpipe then you will need to increase the shooter size. If it stumbles and you are getting black smoke from the tailpipe then it will be necessary to decrease the shooter size.

Final fuel-level setting is made at operating temperature, engine running, sight-plug out, by adjusting the needle-seat / float height with the adjusting-nut / screw.

Holley has a good Tech Section, lots of easily understood topics, plus "Holley TV" and you can view videos from Holley on YouTube.

Holley Tech Section

Choosing the best/correct Power-Valve for your engine, plus cracking the secondary throttle plates open a bit (secondary idle set-screw) often helps get Idle speed down to "Normal" range (like 700 - 800 RPM) even with a moderately big camshaft.

Getting the Accelerator-Pump circuit (Squirter Size, Pump-Cam, Lever-Adjusted) helps with instantaneous throttle-response (no Bog from a stop or sudden acceleration.)

Invest some time in the Holley website. Doesn't take all that long to learn a lot.

Posted By: mopars4ever

Re: holley tuning advice - 08/16/09 11:18 PM

I just went thru a similar problem. Maybe this will help you with yours. First thing I did was make sure the timing was good. I pulled the bowls off and I found one of the small passages was almost covered with casting flash and I cleaned it off. Second, I found my floats were off (too low). I flipped them upside down and adjusted them until they were level with the bowl.( check again later after you get it running) Third, I checked my vacuum in gear (auto trans)and put in a power valve that was 1 1/2 below that number. My vacuum was around 7 inches so I put in a 5.5 pv. Fourth, I stepped up to the next size squirter and I made sure the pump shoots as soon as there is any movement of the throttle. Fifth, I jetted my primaries up a couple of numbers (51 to 55). Then put in new bowl gaskets and put the carbs(2) in my case back on and adjusted the mixture screws to achieve the highest vacuum or in my case the smoothest idle. My car was falling on it face when I shifted gears and with the help of some people here it finally got rid of the dead spot. PS: make sure your getting good fuel pressure and I removed the fuel filters in the carbs also.

Posted By: b body dreams

Re: holley tuning advice - 08/16/09 11:35 PM

Thanks for the tips guys. I'll have to check out the holley website. I just spoke to the friend I got the carb from and he also said I may need to convert the accelerator pump from 30cc to 50cc. Anybody ever heard of this?

Posted By: D_C

Re: holley tuning advice - 08/16/09 11:54 PM

Yes, there is a 50 cc upgrade kit.

It includes a deeper pump housing, deeper diaphragm, taller spring, larger pump-lever and pump cam.

Holley 50 cc Pump

There is a point (squirter-size) at which this upgrade is recommended by Holley.

I have 50 cc pumps on my 850 CFM Dual-Feed, Double-Pump Holley List# 4781-2.

Posted By: mopars4ever

Re: holley tuning advice - 08/17/09 12:24 AM

Before spending money on the 50cc I would check everything else out first.

Posted By: Baxter61

Re: holley tuning advice - 08/17/09 12:34 AM

Quote:

I just went thru a similar problem. Maybe this will help you with yours. First thing I did was make sure the timing was good.

This is where I would start.

Posted By: D_C

Re: holley tuning advice - 08/17/09 12:35 AM

with mopars4ever,

Adding a 50 cc pump may be unnecessary, waste fuel and hinder rather than help performance.

IIRC, it is not needed or required until you use either it was #31 or #36 size squirters. Something on that order. That, and the hollow squirter-screws.

Excessively rich (too much fuel) can hurt performance as can excessively lean.

Posted By: b body dreams

Re: holley tuning advice - 08/17/09 12:49 AM

Thanks guys I will definately start with the basics before I start spending cash. the carb is lots better than the old one I took off. Engine is much more reaponsive even sounds "crisp". Apparently the old carb way a bit on the lean side as my engine temps have dropped 15 to 20 degrees even in todays 90 degree heat.