Team Chevelle banner

Holley 1000 HP stumble

11K views 52 replies 11 participants last post by  ChevelleOZ 
#1 ·
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 :)
 
See less See more
1
#2 ·
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.
 
#3 ·
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.
 
#4 ·
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
 
#5 ·
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.
 
#6 ·
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 :)
 
#7 ·
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.:thumbsup:

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.
 
#9 ·
........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
Tech Team
Mark Whitener Join Date: Apr 2009
Location: Florida
Posts: 313

--------------------------------------------------------------------------------

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 .......
 
#12 ·
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.
 
#13 ·
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.
 
#14 ·
:thumbsup::thumbsup::thumbsup: 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
 
#15 ·
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?
 
#16 ·
Hey rokker

No I have no vacuum advance, but I went on a long drive today and it's back. Seems to rear its ugly head when it gets hot under the hood, the msd box and the msd advance box is under the hood. Also I turned the lights on when idling and my vacuum gauge dropped 2hg. So I am now thinking its the timing. Suggestions? Do I take the MSD box out of the loop or start with taking the msd advance box out then then go from there. Or just bypass the whole MSD box and advance unit. ( confused I am)

Also what is the deal with running them under the hood, would it not be better to install inside the car?

Thanks again.
 
#17 ·
I`m no expert but here`s what I would do.
Plug vacuum to full manifold.
Check timing at idle, making sure it`s idling somewhere between 30-40 deegrees advanced with vacuum but no weight`s giving advance.
Adjust the idle mixture screws to run richer. Not by too much, just a little on the rich side.

Getting worse when hot could be an indication of being a rich stumble, in that case try a leaner idle mixture. Idle mixture cirquit works together with transition cirquit at 1700rpm... if the idle mixture screw does`nt fix/bandaid it, you`re in the carburator tuning area. Have fun :)
 
#20 ·
If rpm drops when you turn the headlights on, vacuum will drop too. When headlights are on the alternator draw will be bigger and slowing the motor down some. Quite normal BUT I'm a little surprised it drops so much that you can see it on a vacuum gauge. Could just be your alternator is drawing a lot of current and working really good to maintain voltage.

Read ignition 101 here on the forum to get a good idea of why you would wan't vaccum advance can on your distributor.
A lot of good reasons, better response, more mpg, nicer plugs, colder engine, more vacuum for brakes, better idling characteristics and several more.. With really no downside I can come to think of.
 
#22 ·
Hey Guys

Sorry for the delay in posting back, my wife has just had a bone marrow transplant so things have been a little crazy, slowly getting back to normal with her prognosis very good :).

Thanks Jay I have tried opeing up the mixture screws but it doesnt seem to make any difference.

I have managed to borrow a friends 750 Street HP carby and the problem went away. So it is definitely the carby being the issue, takes out the possbility of timing issues.

Other things I have tried on the carb are smaller squirter to a 28 and then incremental size increases up to 37 which I now have 37/37 on the Carb, and no noticeable changes. I have also swapped out the various pump cams with no joy there either.

I have gone up to an 8.5 PV incrementally from a 3.5 and still no change. I notice when the issue occurs I have more than 10" of vacuum so not sure if an even higher PV would make a difference?

I also got a fuel pressure guage to make sure that was all working well. I have 7.5 psi on the guage so that all seems to be good.

I am really pulling my hair out with this as its at a point in the rev range when cruising around with my buddys its always occuring and is very annoying.

Now I have 84/92 jets in it, would increasing the jet size on the primaries possibly remedy this? or could it be some issue with the metering blocks or the main body?

or would the idle or high speed air bleeds come into effect at all with this issue?

Any other suggestions from you Gurus out there would be greatly appreciated.

Cheers
Chris
 
#23 ·
If it has screw-in bleeds try a smaller IAB, see if richening the circuit helps or hurts.

that small hole is the idle fuel discharge, that's what is adjusted by the idle mix screws.

unless you're hammering the throttle into the secs during this driving it's not going to be the secs causing the issue.
 
#25 ·
what sizes do you have? I'd go 4 sizes smaller, that should make a difference. If it doesn't there might be something else entirely going on.
 
#27 ·
Small hole #1 intersects the transfer slot/circuit. Just adds a little fuel at idle, and sort of primes the transfer circuit. The larger hole is the idle feed hole. Don't be afraid to go larger than 37 on the squirters. I've seen plenty of hp 1000's with 45's.

If you are saying you can expose .020" of transfer slot on the sec., and the primaries are completely closed, and your idle goes up to 1100; you probably don't need the holes in the throttle plates.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top