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Old 06-23-2003, 04:55 PM
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Holley carb & powervalve info

I was running a Holley 770cfm Street Avenger carb. Had a slight hesitation when taking off from a stop and never was able to get the air/idle screws to change the vacuum or RPM (except to kill the engine when they were being closed all the way). Other than that the carb worked excellent. I decided that I probably had too much carb so I got a Holley 670cfm Street Avenger that supposedly has been modified to elliminate the hesitation. It has 64 primary jets and 65 secondary jets, a 6.5 power valve, and a 31 shooter. The accelerator pump cam is orange. Another thing that puzzles me is that there is a jet (didn't check the size) screwed into the bottom of the carb base centered between the primary and secondary plates. What is this for? The 770cfm has nothing in this area.

The 670cfm did get rid of the hesitation and seems to perform great. However, I still am unable to get a vacuum or RPM change when adjusting the air/idle screws. Is this because of too high a rating of the power valve? I have 10" of vacuum from the full vacuum port at the base of the carb. The RPM is 700 in gear. The reading is the same when in Park except the RPM is about 850.

I have tried opening the secondaries slightly to lower the primaries with both carbs. Still no change when attempting to adjust. Fuel pressure is 6 psi. Float level is just at the bottom of the sight window. I am unable to find any vacuum leaks. Timing is set at 36 degrees in at 3000 RPM. Intake is an Edelbrock RPM Air Gap. Heads are AFR 190cc Street aluminum. Comp Cam 280h .480/.480. 350th trans with a B&M transpak (street mod.). 3.80-1 rear gears.

Anyone out there that can help? Thanks in advance.

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Old 06-23-2003, 05:48 PM
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For automobiles with wild cams and low vacuum readings (10" is pretty low) I try to get the idle as low as possible (under 600 rpm if possible) and then adjust the mixture screws. If it gets better and rpm/vacuum goes up I lower the idle even lower and repeat. The lower you go in rpm the more responsive the mixture screws become. Keep doing that and you might find your idle vacuum go up 3-6 inches. Reducing your initial timing can help also, raise it after your done. I use a vacuum gauge to set idle mixture since it more sensitive.

As for the jet under your carb I can only imagine it is a replaceable air bleed...don't take my word on it though. Have you consulted the Holley tech page?
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Old 06-24-2003, 04:11 AM
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Thanks for the reply. Yes, I've been to the Holley tech pages and printed out everything they have. Read and re-read everything. Their drawings are generic (sort of) and are not specific to the Street Avenger series of carbs. The Street Avenger carbs do not appear to be listed in the carb numerical listings page either. I also talked to one of the "Tech" folks on the phone. Not any help there. He told me to get a 31 squirter for the 770cfm to correct the hesitation. That's what it comes with. Some tech help. He didn't even know that.

Concerning lowering the idle RPM to be able to adjust the air screws: Assuming that works as described, what happens when I re-adjust the idle back up to 700 RPM? Doesn't this defeat any adjustment made?

Either carb works great when driving around at all speeds (cruising) and under WOT runs once I am above about 1500 RPM. The 770cfm will break the rear tires loose from that RPM; but has the hesitation when going to WOT off idle. The 670cfm does not have the hesitation.

My concerns may be somewhat unfounded because the carbs do run good. I just felt that I should be able to get a change when trying to adjust the air screws at idle.
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Old 06-24-2003, 07:07 AM
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Aparently you mite have missed this page.
The amount of fuel that can be delivered by one accelerator pump stroke is determined by the pump's capacity and the profile of the pump cam. The period of time that it will take for this pre-determined amount of fuel to be delivered is affected by the pump nozzle size. A larger pump nozzle will allow this fuel to be delivered much sooner than a smaller pump nozzle. If you need more pump shot sooner, then a larger pump nozzle size is required. During acceleration tests, if you notice that the car first hesitates and then picks up, it's a sure bet that the pump nozzle size should be increased. A backfire (lean condition) on acceleration also calls for a step up in pump nozzle size. Conversely, if off-idle acceleration does not feel crisp or clean, then the pump nozzle size may already be too large. In this case a smaller size is required.
Try a larger nozzle 2 sizes larger, this should resolve your problem.
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Old 06-24-2003, 07:36 AM
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I had the same problem with my 950cfm 3-bbl when I first started my engine. Finially traced the hesitation to the accelerator pump actuating arm. It was set too loose, i.e., the throttle shaft moved before the arm contacted the pump diaphragm. A simple turn of the adjusting screw so the squirters squirted the instant the throttle shaft even thought about wiggling and my hesitation was gone.
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Old 06-24-2003, 11:03 AM
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what happens when I re-adjust the idle back up to 700 RPM? Doesn't this defeat any adjustment made?
No, all you have done is increased the engines reliance on the mixture screws by going lower in idle speed, reversion can mask idle mixture problems so lowering the idle helps reduce it. After you do that I would try a cam kit and stick the brown cam on (if you have the 50cc pump, if not get the 50cc kit) and try hole #1. After that I would try a shooter size change if you have no luck, I have never had to change a shooter except on large cube radical cammed alchohol circle track engines. Another thing to consider is if your power valve is "tipping" in, rapidly fluctuating vacuum readings can cause it to flutter and richen the mixture even at idle. Wouldn't hurt to throw a 5.0 in there to see what happens at idle if you don't have the power valve testing tool. I have seen 6.5's open at 10, like a thermostat they are hardly precision devices.


Everyone has there own way of tuning, this is just the way I do it.
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Old 06-24-2003, 05:26 PM
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Thank you 4 Jaw for the info. I will give your suggestions a try and post back the results.
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Old 06-24-2003, 07:43 PM
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Don't forget what Willy's recommended, having the linkage adjusted with little clearance (Holley recommends 0.015" but I have run 0.002" before to solve pump shot problems) is essential to proper operation of the pump, do this after your idle speed is set since throttle angle affects this adjustment through the cam.

Pump response must be instantaneous.

Good luck and let us know how you get along.
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Old 06-24-2003, 09:24 PM
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I adjust mine to 0.000"!
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Old 06-25-2003, 01:47 AM
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im with willy on this one.quarter turn to loose will make my fore head bounce off dash board mine runs best adjusted @ -000002
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