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Old 06-11-2003, 03:55 PM
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Holley tuning questions.

I am using a Holley 670 on my 383. I have had to reduce primary jets to get it to correct for a rich plug cut. The plugs are now reading OK. However, I now have an off-idle stumble upon acceleration. I tried increasing the accelerator pump stroke, but this does not correct the situation and otherwise, the throttle response is crisp once the rpms pass the stumble.

Additionally, I have a slight surging or stumbling and recovery at 1500-2000 rpm under a static, modest load as when street cruising. So the problem is in the primary throttle system somehow as the vacuum secondaries are not opening.

I have checked my float levels and both ends are set at the bottom of the sight plug and I have no reason to question a relatively new high volume low pressure Edelbrock fuel pump. Pump pressure is about 5-6 lbs.


An additional clue: when going around a sharp corner or up a steep hill, it looses power completely and recovers quickly as though the main jets went dry. I recently had the primary float stick open, dumping fuel out the vent pipe on top.

I don't know if these are related or seperate problems. Any suggestions would be appreciated.

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Old 06-11-2003, 04:22 PM
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Maybe the shooters for accelerator pump need to be made larger or try richening idle mixture slightly. Or you could try reducing the air bleed size for the mains on primary side but I would try other stuff first.

Last edited by Greg Latvala; 06-11-2003 at 04:36 PM.
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Old 06-11-2003, 06:41 PM
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If you are having surging at cruise it is most definately too lean. I would go up two jet sizes and try it there. If you do not have a metering block kit for the secondaries you should get one. That is where you need to be making your tuning changes for wide open throttle.

Also, how are you checking the plugs. To do it right you need to make a hard run and kill it immediately and check them. Let it idle and they will show rich.

My guess is that you are running lean on primary and then making that worse when mashing the throttle and the acc. pump is not able to compensate. Once you get the secondaries where you can tune them, you can get it where both circuits work properly. Dont go to messing with air bleeds unless you really know what you are doing. A little dab will totally F you up there.

Chris
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Old 06-11-2003, 06:50 PM
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Sounds like you have went too far the wrong way, do your idle mixture screws have any effect? What size power valve? What's your vacuum reading at idle? Do you get a nice puff of grey smoke when you mash the throttle from idle rpm? or is it black? What color accelerator pump cam are you running?

Need more info.
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Old 06-12-2003, 10:50 AM
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The Carb came with 68s in the secondaries and 64's up front. It was running real rich. I bought 62's and put them up front and put the 64s in the rear. It ran well, no bad manners, but still was rich.

I then bought a pair of 58s and put them in front and it had all the problems I have described. I tried raising the float level in front and it did not help. So I lowered it back to the bottom of the sight plug. Problem still exists.

I then bought a pair of 60's and the surging is not so much an issue, but the stumble is still there, but not as bad. I also tried raising the float level here and that did not help, so I returned it to the bottom of the sight plug.

I could go back to the 62 jets in front and I believe all would be ok, but still rich.

How do I determine what size the squirters are? Is it the valve on the bottom of the front float bowl which has 4 screws that hold it together. Is the size denominated there?

Thanks guys.
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Old 06-12-2003, 11:14 AM
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While looking down the primary side open the throttle. See where fuel is squirting out for accelorator pump shot? thats it, the size is stamped on it
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Old 06-12-2003, 11:52 AM
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I agree with Turbo if you are surging it's too lean. I'd get it so it is jetted correctly and then work the rich plug reading, many other things could cause that besides jets.
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Old 06-12-2003, 11:52 AM
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Usually the stock jet sizes on a Holley are pretty close and you generally only need to go up or down 1 size. Sounds like the jetting is now too lean. The richness you're seeing might be from the power valve opening too early. Any idea what PV you're running and how much vacuum the motor has @ idle?
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Old 06-12-2003, 04:26 PM
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I solved my Holley tuning problem...I put my Edelbrock back on!
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Old 06-13-2003, 07:48 AM
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Old 06-13-2003, 08:41 AM
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Turbo had you going in the right direction, why quit now.
By the way that valve at the bottom of the primary bowl
with 4 screws is the acc. pump. The power valve screws into the metering block.

A holly is like a lot of other things, if you play with it enough it will work.

Troy;
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Old 06-13-2003, 10:56 AM
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OK guys, you are helping me. I am convinced that I have leaned it too much. Bear with me, I will take a vacuum measurement and try to get some information on the power valve and let you know what I find.
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Old 06-13-2003, 08:40 PM
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Now you are talking. If you work through the problem, you will have a better understanding of how the thing works, and will likely be much more happy with the Holley that the Edelbrock. I would never run anthing but a Holley (well maybe demon) if I do a carbed buildup. Once you get them figured out and tuned there is nothing better.

Chris
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Old 06-13-2003, 08:48 PM
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Quote:
Originally posted by TurboS10
Once you get them figured out and tuned there is nothing better.

Chris
Amen!


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Old 06-13-2003, 11:02 PM
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Only change shooters as a last resort, most engines work just fine with the included shooter size. I would look at your accel cam and buy the cam kit once you get the jetting close.

Idle mixture should not be set to cover a hole in the off idle response...the cam does that. Set your idle mixture for max vacuum and then get your jetting close...your 64 fronts should be close...maybe even going up to 66's. Leave the secondaries until you get the primary set, even take the linkage off if you like.

Your power valve generally is between 6.5 and 8.5. I prefer 6.5 for most every performance vehicle I've tuned for the street. If your cam is really radical or your idle vacuum (in gear) is low like 10 inches of Mercury then run the lower number valve. Ideally I find the power valve should just start to open under mid rpm load like accelerating to speed quickly on the highway.

Mounting a vacuum gauge in the car temporarily can be helpful during your "tuning days". Bring a friend to look for black smoke out the back during your runs. A nice light puff of grey when you mash it is what you want for max power.

Have fun.
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