Reply With QuoteBefore doing anything what size powervalve are you running?If this engine has a big cam you are not drawing vacum which could be causing the powervalve to dump fuel at idle.or low rpm
(This topic is also posted under Gearheads)
I seen a number of posts regarding the drilling of air bleeds in the butterflies of Holleys.
Engine is a 502 with a vac sec 750 Holley. Legend pump.
Rich idle, soots up transom.
Another curious behavior is that after cruising around 3-4K RPM and sidesteping foot pedal, the engine simply dies. RPM goes to 0. Restarts OK. Other than this, no other symptoms.
Pulled carb off and throttle plate showing .08-.09 of transfer slot. Idle mixture screw set at 1/2 turn out.
Readjusted throttle plates to .03-.04 slot showing and 1.5 turns on mixture screw.
Restarted engine and won't idle, as the idle speed is too low. I think I need more air.
I am looking for specific info:
1. Where to drill, near the edge or mid-way on the butterfly, or near the shaft?
2. Diameter to start with. .093, .125, or ??
3. Any other recommendations.
thx for your help.
Before doing anything what size powervalve are you running?If this engine has a big cam you are not drawing vacum which could be causing the powervalve to dump fuel at idle.or low rpm
Before drilling any throttle plates open the secondary throttle stop screw a turn. If it improves some try another 1/2 turn.
[This message has been edited by 058 (edited March 23, 2002).]
PV was a 10.5, replaced by a 6.5 with no change in idle sooting condition. Cam is standard for 415hp motor - not too big.
I really like the secondary throttle stop screw adjustment. It is a souce of bleed air which may well solve the supply problem.
I'll try this first before any drilling on the primaries.
Before you changed and replaced the power valve did you check the Vacuum at idle first? A 10.5 sounds really big with any engine that has a big cam, if yours does. Most stock Holleys run a 6.5 PV out of the box, and they recommend that you size the PV 1.5 less than the idle vacuum. You might check around and ask others with similar engines and cams run for a PV in their combo. Still sounds like low vacuum is opening the PV at idle. Maybe you need a 4.5 or a 3.5 PV.
I would think a stock cam would have a plenty strong vacuum signal and would not need the throttle plates drilled. Try screwing the idle mixture screws all the way in, if the motor does not die you might have the floats set to high or a leaking needle and seat causing unmetered fuel to drip into the intake.
Can someone help me out here? What does the power valve have to do with idle quality? I can see if the diaphram is ruptured it will dump raw fuel into the manifold but if the p/v is good how does this effect idle, open or closed?
Go to the Holley site and read.
Too large of a PV and at idle it could open because there might not be enough vacuum to hold it closed, hence it enrichens the main fuel system. This could be the same as a blown PV that is also leaking the additional enrichment to the main circuit. The power valve is a key component of the power enrichment system. The power enrichment system supplies additional fuel to the main system during heavy load or full power situations. It is not supposed to supply additional fuel during idle conditions, which could occur with the wrong PV.
Holleys utilize a vacuum operated power enrichment system and a selection of power valves is available to "time" this system's operation to your specific needs.
http://www.holley.com/HiOctn/TechSer...o/Trblsht.html
http://www.holley.com/HiOctn/TechSer...o/FMCTech.html
This is from Holley.
QUESTION I have a camshaft with a long duration and a high lift. It will burn your eyes at an idle. What can I do to fix this?
ANSWER The most common cause is an improperly rated power valve. You will first need to choose the correct power valve for your application. This is covered in another section of the Technical Information Library. When you have a camshaft of this configuration it requires more air and fuel for it to operate efficiently. This in turn requires you to raise your curb idle screw to obtain the same given rpm that you had with a stock camshaft. If you are having a problem with the air/fuel mixture screws not making a difference then what happens is with turning the curb idle screw in farther this will sometimes cause the throttle plates to open too far and they will uncover the idle slots. This will allow it to bypass the idle circuit and start pulling fuel from the transfer slot. To correct this problem you can raise the secondary throttle plates a little with the secondary adjusting screw allowing more airflow into the engine. This will allow you to lower the primary throttle plates so you will be able to get back into the idle circuit. Another method would be to drill a 1/64" - 1/8" hole in each of the primary throttle plates on the side closest to the transfer slot. This will allow extra airflow into the carburetor also alowing you lower the primary throttle plates.
[This message has been edited by gnarley (edited March 25, 2002).]
The curb idle mix screws were set at the factory (Marine Power) and I have never touched them. Once I found that they were 1/2 out, I knew there was a problem, especially with the fine info shared from this and other threads.
I reset the curb idle to 1.5 turns open and set the primary butterflys to about .03-.04 and set the secondary butterflys to about .02 visible.
Fired the engine and it idled, but needed more air. I added more of the primary and a touch of secondary. Definitely on the right path.
I am going to remove the carb again, using these settings as a baseline and increase both sides to a common opening level, as they are probably not equal at this point.
I spoke to Holley tech services and this is exactly what they suggested. They advised against drilling - in their opinion this is not required. Just keep working airflow with both sets of barrels. If not successful, then drill.
FYI - power valve, needle/seat, and float levels are spot on - not just my opinion, but that of my dealer as well.
To all - thank you for the invaluable info you have shared. I've worked with Holleys for over 30 years and thought I knew a whole lot about them. Not enough obviously.
Share and share alike! I can always learn more, so please share what you found when it does idle right, maybe it will help me in the future.
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