QuestionQUESTION: I have a 55 chevy p/u with a 383 stroker engine with .470/.480 lift cam with 700r4 automatic transmission with shift kit. My problem is I'm using a 750 Holley dual pumper carb that I'm having trouble with. I've tried to adjust my idle mixture screws, but only one set works. The primary set I'm having trouble with. Sometimes it makes a difference when I screw them in and try to adjust them and then other times I can screw them in all the way and they don't make a difference. Is there something I can do to get them to work right? I had some bad gas a good while back and I've been having trouble with them since, but can't figure out why the secondary set work fine if the other set doesn't. I haven't messed with Holley's too much, but would like to find out how to get the one set working again. The number on the airhorn is 4779-9 2887, but not for sure if that's the number I need to try to get a kit for it. Any help is appreciated.
Ron
ANSWER: If your primary idle mixture screws have no effect on idle that means that you have your throttle plates open too much at idle. What you need to do is:
1. remove the carb from the engine and drain the fuel from the bowls by turning it over on a drain pan.
2. In the primary throttle bores there will be two small slots at the bottom. If more that 1/16" of these slots are exposed with the throttle closed the mixture screwws won't work. Back off on the main idle screw until you have 1/16" slot exposed.
3. On the secondary side, at the bottom of the throttle plate,on the passenger side of the carb you will see a very small slotted screw recessed into the plate. Turn this screw clockwise until you can see the bottom of the bore slots in the secondary bores. Do not expose the slots! Adjust until the plates are right below the slots.
4. Now, flip the carb back over and lightly screw the mixture screws in until they seat. Then back them out one and a half turns.
5. Put the carb back on the engine and fire it up. The idle may be a little high so adjust it down with the main idle screw.
6. Now, slowly turn one mixture screw in a half turn at a time until it begins to stumble. Back off one quarter turn, repeat for the other screw.
7. Reset idle if necessary and check the timing.
If this advice has helped you in anyway please consider my webstore for your next performance purchase. www.lyonsperformance.com
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QUESTION: I did that and the engine idles high even without the idle screw screwed in or touching anything, but the idle mixture screws do work now. It did that before I took the carb off also. I have my timing set a little high since I'm running a .470/.480 lift cam. If I need to change my timing to get the idle down more, won't that cause me to loose horsepower? Just wondering.
Ron
ANSWER: That little screw underneath can be installed from the top of the plate so that you can adjust the idle. It sounds like you have too much initial timing. That cam is not too big so I would shoot for 12-15 BTDC with the vac canister unhooked. Have you had the distributor recurved?
Horsepower is for top end speed, its bottom end torque that gets you moving. Timing like everything else needs to be in balance with the rest of the system.
What power valve are you running and how much vacuum does the car have in gear?
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QUESTION: Not for sure what you mean about recurved. It's an MSD 8362 street fire electronic distributor. It was new when I installed it a year ago. And not for sure what you're talking about when you're talking about a power valve. On the carb, it's what ever came with the Holley carb. I'm sorry if I can't help more, but my son and I have been trying to do things by ourselves. The truck is something he had me buy when he was on his second tour in Iraq so now we're trying to get it running good for him so he can enjoy it in case he has to go back again. Thanks for the help, it's truly appreciated.
AnswerFirst off, Thank your son for protecting our way of life. His time is very much appreciated and he should be proud!
Now, the cam you have in the engine creates a lower vacuum than the stock cam. So the power valve in the primary metering block could be opening at idle and causing issues. If the carb is stock it has a 6.5" power valve. If you can put a vac gauge on the engine pulling manifold vacuum (PCV or Brake booster line) and tell me what it reads in gear I can get you the number of the valve you need.
As for the distributor, recurving changes the way the mechanical advance in the distributor works. On a performance engine we try to set the mechanical advance at 28 degrees at 3000 rpm. So if you have a dial back timing light you would rev the engine to 3000 rpm and set the timing so the dial reads 40 degrees of advance. Thats 28 in the distributor and the 12 degrees initial timing it needs to keep running. The groove in the balancer should line up with the 0 mark on the timing tab when you check it with a dial in advance timing light. If you don't have that type of light, I would get a timing tape that goes around the balancer (Mr. Gasket sells them) and you can use you standard light.
But first I would try tuning that secondary screw to bring the idle down a little. You should have 1 to 1-1/2 turns on the main idle speed screw to start with, so back off on the secondary screw and try it again.
I'm done for tonight. But I'll check my mail in the morning.
God Bless America!
