950HP idles to fast

[Flying Bandit]

I put a Holley 950HP on my motor and it idle at about a 1000 RPM's when it warms up. I have backed off the idle butterfly screw all the way. What other options do I have to reduce my idle speed another 250 RPM's?

[gsxrken]

Check to see whether the secondary stop is open too far.
I assume you've tuned with the mixtures screws and your timing is in the ballpark.
And make sure there is no vacuum leak.

[Shaker455]

Yes check your timming and did you back off both front & rear butterfly screws?

Also, do the throttle plates have holes in them?

[Flying Bandit]

Yes check your timming and did you back off both front & rear butterfly screws?

Also, do the throttle plates have holes in them?

Set the 4 corners with a vacuum gauge. Don't believe the is any vacuum leaks. The timing is good 16 int and 36 total all in at 3000. I will try the secondary screw.

Not sure if the throttle plates have holes. What will holes in the throttle plates do?

[Shaker455]

If they are large like 1/8" per hole they will bypass too much air...HP 950's usally have bypass air holes.

What size engine you running this on?

[Flying Bandit]

If they are large like 1/8" per hole they will bypass too much air...HP 950's usally have bypass air holes.

What size engine you running this on?

It is a Pontiac 400 stroked to 490. Butler Performance race ported Edelbrock heads flowing 310 cfm with a mild roller cam 110LSA 248/254 .562/.580 cam. Never dyno'd but probably in the 500-550 hp and 550+TQ. I am hoping just the secondary screw will work. I will screw with it again when I get back in town.

I am am still getting some surging at 2000 RPM's cruising. Been told it may be a lean condition. What are you thoughts getting all the timing in by 2000. Would that help with my combination. It has a summit billet HEI distributor right out of the box.

[Vegas69]

My opinion is to leave the idle at 1000 rpm. I've read that it helps with oil sling on the camshaft.

[Shaker455]

It is a Pontiac 400 stroked to 490. Butler Performance race ported Edelbrock heads flowing 310 cfm with a mild roller cam 110LSA 248/254 .562/.580 cam. Never dyno'd but probably in the 500-550 hp and 550+TQ. I am hoping just the secondary screw will work. I will screw with it again when I get back in town.

I am am still getting some surging at 2000 RPM's cruising. Been told it may be a lean condition. What are you thoughts getting all the timing in by 2000. Would that help with my combination. It has a summit billet HEI distributor right out of the box.

Cool combo Pontiac bro!

You may have to jet it up a bit if the mix screws don't balance it out...what jets are in there?

With that combo you want your total to come in between 2500 to 2800 rpm.

What intake?

[Flying Bandit]

Cool combo Pontiac bro!

You may have to jet it up a bit if the mix screws don't balance it out...what jets are in there?

With that combo you want your total to come in between 2500 to 2800 rpm.





What intake?

I have a Performer RPM port matched.

Not sure what jets. It is a Prosystems carb. I have the spec sheet at home.

I recently bought the car. I changed the carb, intake and plugs to what I have now. Great improvement. Trap speed went from 110 to 116 mph.

Once I get the idle down to normal and iron out the slight surge at cruise I will be extremely happy with the car.

I ordered a jet kit, dist. Tuning kit and will try the screw and see where that gets me. Anything more that will be past my experience level. I'm Pushing it as it is but I like trying as long as I don't blow it up in the process.

[Shaker455]

Let me know your progress.


I've had many Pro-System 4150 carbs here for rework as they have issues with metering blocks being modified by Pat which may be the root of your problem.

[Pro Stock]

Did you do a manifold change at the same time? if so you could have a vacuum leak between the intake and heads or between the carb and intake, get a can of ether (starting fluid) and spray the areas around the gaskets, listen for changes in rpm while doing this or use a tach, if there is a change in rpm while spraying then you have a leak in the area where you are spraying. Good luck.

[Flying Bandit]

Let me know your progress.


I've had many Pro-System 4150 carbs here for rework as they have issues with metering blocks being modified by Pat which may be the root of your problem.
My pri jet is 76 and sec 86. Is the in the ballpark? How much should I lean it out for the 100 degree summer temperatures here in Pheonix?

I pulled the shaker off today and noticed the float levels were very low. I raised them to the upper part of the window and brought the fuel pressure up from 7 to 8. Also made all the four corner screws equal. The idle dropped to about 600 so I raised it to 650 and it is running quite well for now.

[Flying Bandit]

Did you do a manifold change at the same time? if so you could have a vacuum leak between the intake and heads or between the carb and intake, get a can of ether (starting fluid) and spray the areas around the gaskets, listen for changes in rpm while doing this or use a tach, if there is a change in rpm while spraying then you have a leak in the area where you are spraying. Good luck.

I tried it and there did not see any changes when I spayed around the intake.

[Shaker455]

My pri jet is 76 and sec 86. Is the in the ballpark? How much should I lean it out for the 100 degree summer temperatures here in Pheonix?

I pulled the shaker off today and noticed the float levels were very low. I raised them to the upper part of the window and brought the fuel pressure up from 7 to 8. Also made all the four corner screws equal. The idle dropped to about 600 so I raised it to 650 and it is running quite well for now.

Your fuel press needs to be 6.5.

Do you have the alum holley bowls with the site window rather than the screw in site plug?

I would not recomend jetting down at this point with a surge present.

If you have the window you should be over the half way point.

Jetting does sound in the ballpark and I believe you are about 1000 ft or so?

I have three combos running around that the cam specs are about the same and I can tell you the engine likes to idle about 950 in park and it also likes atleast 18 degrees initinal.

[harry k]

Jeff
all of the above mentioned are on the right track but its hard to diagnose stuff like this with out actually seeing the car ....
hear customers call on the phone with a complaint when actually see the car come in its different symptom,
best bet is the guy above to send his carb to you and confirm high idle on your car if its actually carb issue or mechanical/tune issue ...

http://www.hkmotorsportsdynoshop.com/

[Shaker455]

Hey Harry,
Agreed, and I don't know how many Pro-system carbs you've worked but one of the most recent here was one for Punch T-37 on PY.

His metering block mainwells were drilled so much by Pro-Systems that it actually cracked the casting.

Also one of they're standard mods is to drill a small hole in the accel passage in the metering block as a killbleed and run a 45 squiter.

You webiste is awsome! good carb tuning section too!