Can't get the tick over down
Discussion
Having a nightmare here and need help!!
Car's fitted with a Holly race style carb and I can't get the tick-over down.
I've checked for air leaks, none. The butterflies are closing fully (extra spring required on the secondaries!) I've played with the "Idle air" screws with no joy. The tickover screw is right out (both sides) i.e. the butterflies wont shut any more, yet the car still refuses to idle below 1500rpm :mad:
It's almost as if there's too much air bleeding at low revs. Any help would be much appreciated.
Ta
Andy
Car's fitted with a Holly race style carb and I can't get the tick-over down.
I've checked for air leaks, none. The butterflies are closing fully (extra spring required on the secondaries!) I've played with the "Idle air" screws with no joy. The tickover screw is right out (both sides) i.e. the butterflies wont shut any more, yet the car still refuses to idle below 1500rpm :mad:
It's almost as if there's too much air bleeding at low revs. Any help would be much appreciated.
Ta
Andy
Andy, are you sure about the air leaks? Is the vac pipe in the base plate blanked off or routed to the dizzy? If it's not plugged you will have issues.
Is it a new carb or old carb, check the seals on the throttle shafts aren't damaged in case air's getting in there. Also look in side at the chokes, at the bottom will be some vertical grooves, 1 per choke. These are transition slots for when coming off idle. Make sure the blades sit at the bottom of those when idling otherwise to much are bypasses the blade. I assume you have a good gasket between the carb and base plate. Is the exhaust sooty? Maybe the powervalve has failed. It's been a while since I tuned a holley so I could be wrong on that one.
Boosted.
Is it a new carb or old carb, check the seals on the throttle shafts aren't damaged in case air's getting in there. Also look in side at the chokes, at the bottom will be some vertical grooves, 1 per choke. These are transition slots for when coming off idle. Make sure the blades sit at the bottom of those when idling otherwise to much are bypasses the blade. I assume you have a good gasket between the carb and base plate. Is the exhaust sooty? Maybe the powervalve has failed. It's been a while since I tuned a holley so I could be wrong on that one.
Boosted.
Carb's brand new, well seated on a new gasket with a smidgeon of blue. There's no carb vac advance takeoff so this has been drilled and tapped into the manifold with proper fittings.
She seems to be using alarming amounts of fuel as well. You mention the "powervalve", second time this has cropped up, where is it and what does it do? Makes me larf operating the accelerator pump, everytime I do it I think of America and the Kyoto agreement
She seems to be using alarming amounts of fuel as well. You mention the "powervalve", second time this has cropped up, where is it and what does it do? Makes me larf operating the accelerator pump, everytime I do it I think of America and the Kyoto agreement
Don't have the book to hand but seem to remember there was some form of additional butterfly adjustment on my Holly which you could only adjust from the base plate of the carb i.e. with the carb removed and upside down. This was i believe to set an initial slight opening of the secondary butterfly.
Steve
Steve
It's a 4150HP 650cfm race carb.
Original was an Elderbrock but we couldn't stop the engine dying whilst braking hard!!
The underside adjuster is fully out as well. The butterflies were sticking slightly on this side but I cured it with a suplememtary return spring.
Any ideas MUCH appreciated
Original was an Elderbrock but we couldn't stop the engine dying whilst braking hard!!
The underside adjuster is fully out as well. The butterflies were sticking slightly on this side but I cured it with a suplememtary return spring.
Any ideas MUCH appreciated
The powervalve is screwed into the underside of the carb. To get to it you have to remove the base plate. The powervalve opens at wot and lets fuel flow from the fuel bowl without going via the main jets. The pv has drilled calibrated restrictions to limit fuel flowed. Lack of vacuum opens the power valve so, it could open if you have a very low idle vacuum (lumpy cam) or it could be damaged. Either way you would probably be flooding the engine if the pv's at fault. Hence my asking about the exhaust.
Boosted.
Boosted.
Have you tried setting the tickover with the throttle cable disconnected! Mine used to speed up the tickover when hot due to the expansion of the outer cable.
Sorry to contradict the other post but the power valve is in the front metering block just above the main jets.(Metering block is the bit in between float chamber and the main body)
Sorry to contradict the other post but the power valve is in the front metering block just above the main jets.(Metering block is the bit in between float chamber and the main body)
Have you checked the idle mixture screws? How many turns are they out (all 4 the same?)
If you have a Holley race carb, it should have removable air bleeds. What is the size of the idle bleeds (= the 4 outer ones, the number is stamped at the side of the bleed). Don´t think that is the cause of the problem, but can be checked easily.
If you have a Holley race carb, it should have removable air bleeds. What is the size of the idle bleeds (= the 4 outer ones, the number is stamped at the side of the bleed). Don´t think that is the cause of the problem, but can be checked easily.
B1 ECC said:
Sorry to contradict the other post but the power valve is in the front metering block just above the main jets.(Metering block is the bit in between float chamber and the main body)
Just to add here: if I am not wrong, I think on the Holley race carb there should be two power valves, one for the primary and one for the secondary side, i.e. in both metering blocks.
B1 ECC said:
Sorry to contradict the other post but the power valve is in the front metering block just above the main jets.(Metering block is the bit in between float chamber and the main body)
No offence taken. I've not worked on a holley for years so forgot the finer details
Andy, does your carb have bypass holes in the blades, if so are they to big? You could plug them or change blades. Or change the base plate assembly.
Boosted.
Fantastic idea "Block the holes in the blades" Perfect size for small pop rivets. Seems to work a treat. I now have to adjust the tickover screw to make it run. Much better because you're coming down against a positive stop. Can you set the idle air without vac gauges? Small adjustments seems to really effect the low end mixture. Seems to go from lean to black smoke in no more than half a turn each. I have to say it runs fine high up with no apparent flat spots on the way there
mrloudly said:
Small adjustments seems to really effect the low end mixture. Seems to go from lean to black smoke in no more than half a turn each.
That shouldn't happen? This carb sounds to big for your motah
or the idle jets are 2 big. Does your metering block allow for jet changes? Is it 4 corner idle or just 2 corner? Boosted.
This may be irrelevant to the discussion, since I no next to nothing about carbs... but I'd be curious to know if this problem is at all related to what happened with me - a small bit of debris that got stuck in the float valve seat on the secondary. In my case, gas was pouring into the engine, even at idle, and out of places gas is certainly not supposed to come from! Could it be that a smaller piece of debris would result in extra fuel flow and raise the RPM, but not be as dramatic as my problem (which caused the car to sputter, stall and then be unstartable due to chronic flooding). Or would extra fuel just cause a rich burn, and not impact RPM?
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