Holley 1000 HP stumble - Chevelle Tech
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  #1  
Old Apr 27th, 12, 7:25 AM
ChevelleOZ ChevelleOZ is offline
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Chris
 
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Default Holley 1000 HP stumble

Hi All

I am having an issue with my 1000 HP, around 1700rpm I get a slight hesitation, generally happens around corners and the higher gear the worse it is.

I have tried bigger squirter and different pump cams, the larger the worse it gets, I think it could be to do with the transfer slot on the secondarys, however when I set my secondary slot to 20 thou the car idles around 1100rpm with the primary wound right off.

with the picture below what is the small whole no1. above the transfer slot? when I set the secondaries totally closed I get the stumble. Picture isnt the best put I think you can see what I mean.



Would I be better increasing the idle mixture screws or playing with the secondary transfer slot, I get about 5" of vacuum in gear at idle.

Holley is set up with the following

Jets 84,92
PV 3.5 primary Secondary blocked
Green primary pump cam
35 front squirter/37 rear

Engine is a 540
Solid cam
iron heads
10.5:1 comp
Turbo 400 with 2000 stall

The engine drops from 1000rpm to around 650rpm when in gear at idle. I think this is to do with the tight converter 3.73 rear gears.

Any help/suggestions would be greatly appreciated. at WOT the car runs awesome
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  #2  
Old Apr 27th, 12, 10:16 AM
planetek7 planetek7 is offline
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Default Re: Holley 1000 HP stumble

Before I blame certain parts on the carb, I go back to basics. Engine timing, float levels set correctly, then mixture. Not sure what you have already checked. Sure, that's quite a bit of cubic inches you have but, that is a massive carb meant really more for racing IMO. I still use a vacuum gauge to help with mixture settings, getting the highest reading possible, then tune from there. Also, that converter seems to me to be way to small for your combination. Sorry...a series of sentences with more opinions than help.
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  #3  
Old Apr 27th, 12, 10:34 AM
Highhat Highhat is offline
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Default Re: Holley 1000 HP stumble

I'm no carb expert, but here is how I set it up on mine, and some ideas:

Go back to stock jet numbers and squirters to start. I went smaller squirter to rid a stumble.
If you didnt buy it new, check whether the air bleeds are stock numbers
Set the secondary transfer slot to where it looks like a square
Check/reset your timing
Check your float levels
If used, make sure there are jet extensions in the rear float

Then adjust the idle mixture screws, and then set your idle speed

It also looks like you have holes in the throttle blades. That may be causing the high idle speed if the idle speed screw is out all of the way. If you have high idle after doing the above, you might want to try putting in no-hole blades. A quick check - put your fingers over the holes while idleing and see if that reduces the idle. If it does, then I would try them. You will need to re-adjust your idle mixture screws after that.

Anyone, please jump in with corrections or suggestions.
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  #4  
Old Apr 27th, 12, 10:36 AM
CDN SS CDN SS is offline
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Default Re: Holley 1000 HP stumble

I run the same carb and a have played with idle circuit it alot I think your on right track working the idle/ transistion .....I found the this carb very sensitive to float levels .......dont go too high .....idle mix setting..... give carb what it wants .setting with vacuum guage for highest number is fine for a mild cam but not for a perf cam ...pbly like a bit of a rich idle mix ......... have you tried a 4.5 or 5.0 PV sounds like this happens on throttle tip in ...... I dont recall those holes above transistion slot sori
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  #5  
Old Apr 27th, 12, 10:38 AM
ChevelleOZ ChevelleOZ is offline
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Default

Thanks guys

Have checked timing 24 initial 36 all in. Floats are set correctly.

Carby has stock air bleeds, I think it truly is to do with the transfer slot in the secondaries, hence the question of the small hole above the transfer slot so I am setting it correctly. When I have the slot at 20 thou it idles at 1200. Mixture screws are out 11/4 turns all round.

It has the jet extensions and I had the miss with the previous stock jetting hence the changes as with the squirter in the primaries.
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  #6  
Old Apr 27th, 12, 10:48 AM
ChevelleOZ ChevelleOZ is offline
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Thanks Bill, the highest vacuum I have achieved with a vacuum gauge is the 5" my understanding with the power valve is to have it 2" lower that lowest curb idle so the power valve doesn't open when it's idling it that the correct theory?

I have been reading a lot on the Internet and we know how dangerous that can be
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  #7  
Old Apr 27th, 12, 10:53 AM
Andy's Auto Repair Andy's Auto Repair is offline
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Default Re: Holley 1000 HP stumble

Quote:
Originally Posted by ChevelleOZ View Post
Thanks Bill, the highest vacuum I have achieved with a vacuum gauge is the 5" my understanding with the power valve is to have it 2" lower that lowest curb idle so the power valve doesn't open when it's idling it that the correct theory?

I have been reading a lot on the Internet and we know how dangerous that can be
You gotta stop reading the Intranets, half of what you read on there is BS, the other half, well, lies.

5" of mercury? How big is the cam? Or more so, what is the overlap? You seem to be asking the car the right questions so far but my first thought after the read is too much accelerator pump. Next step I would take is a smaller pump cam to see how she reacts.
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  #8  
Old Apr 27th, 12, 10:59 AM
ChevelleOZ ChevelleOZ is offline
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Default

Cheers Andy, I will give the smaller pump cam a go tomorrow see how she goes.

The cam is a custom comp cam, the specs I have as I didn't build the motor are 306 adv duration .632 lift int and exh thats all I have I'm afraid.

Should I put the stock squirter back in as well?
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  #9  
Old Apr 27th, 12, 11:57 AM
CDN SS CDN SS is offline
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Default Re: Holley 1000 HP stumble

........PV selection ......that 1/2 idle vacuum theory is incorrect IMO see what "Mark " has to say about the that ....... you may know he is a very very well respected carb guy ........in the end you may want to contact him re your tunning issue


jmarkaudio
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--------------------------------------------------------------------------------

Basing the PV selection on idle vacuum is BAD advise that has floated around for a long time. If you take CRUISE vacuum, divide it in half and add about 2" you will be close. You want the PV to be closed while running a steady cruise, and open when the load is enough to require more fuel. Idle has nothing to do with it, even if the PV is open at idle the idle feed restriction is so much smaller the open PV will have no significant effect. Much like drinking through a coffee stirrer, it don't matter whether it's in a cup or a gallon jug, you will only flow so much through it.


FYI my car idles at 1200 rpm with 7hg vaccuum I run a 6.5 pv at track 4.5 on street but even when I leave the 6.5 in on street it fine at idle just cruising it a bit rich .....idle mix screws out 2 turns on some and some 2.25 .......I see your at 1.25 ....... assume if you open up more too rich ......you may want to consider going larger on the idle air bleeds leaner).010 that should give you more adjustability on idle mix screws with out going rich early .......
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  #10  
Old Apr 28th, 12, 4:32 AM
ChevelleOZ ChevelleOZ is offline
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Chris
 
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Default Re: Holley 1000 HP stumble

Cheers Bill, do you think the incorrect PV could be causing the stumble? I mean in theory I am running 5hg so a 2.5 would be Holleys recommendation. cruise vacuum is about 15-18hg

thanks for everyones input so far.
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  #11  
Old Apr 28th, 12, 4:33 AM
ChevelleOZ ChevelleOZ is offline
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Default Re: Holley 1000 HP stumble

oh and its raining here so havent been able to test the smaller pump cam and squirter as yet
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  #12  
Old Apr 28th, 12, 11:08 PM
jmarkaudio jmarkaudio is offline
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Default Re: Holley 1000 HP stumble

The above post is copied from another here and is correct, even on a race engine with little idle vacuum you want at least a 6.5 PV. The PV open or closed has little if any influence on fuel to the idle circuit. If you have the vacuum rating too low it will however run it too lean on part throttle loads, and may be an issue if you are at the point where the mainwells are active, but at 1700 RPM it's not likely. It very well may need more initial timing, larger cams with low idle vacuum usually respond to little timing curve or even locked timing. If the fuel won't tolerate more timing you can try going down .002-.003 on the idle air bleed, and readjust the idle mixture as needed. Richer mixtures are easier to ignite, but also waste a little more fuel.
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  #13  
Old Apr 29th, 12, 6:57 AM
ChevelleOZ ChevelleOZ is offline
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Chris
 
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Default Re: Holley 1000 HP stumble

ok put the 6.5PV in today and put the original pump and squirter back in and the stumble has gone I also reset the transfer slots while I had the carby off.

I changed the PV a while back and as it was what Holley recommends on their website I didnt even think it could be the issue.

Thanks again for everyones input and the PMs I got also.

Cheers
Chris.
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  #14  
Old Apr 29th, 12, 5:13 PM
CDN SS CDN SS is offline
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Default Re: Holley 1000 HP stumble

Great glad Mark jumped in ,FWIW I run a 6.5 pv and 7hg idle vacuum with locked out timing ........I did have to play with the bleeds, screws around 2 turns out to get a clean idle on my HP 1000
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  #15  
Old Apr 29th, 12, 6:27 PM
Rokker Rokker is offline
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Default Re: Holley 1000 HP stumble

Idk if you've already mentioned it, are you running any vacuum advance?
I would try to adjust the idle screws a little and see if it improves the situation or not.
Is it worse when cold or better ? That way you'd know if it is a lean or rich condition.
If it's a lean stumble some more timing could help. Maybe a softer spring in the distributor?
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