Carb Tuning help
12-30-2004, 03:38 PM
#1
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Join Date: Oct 2002
Location: Middeltown NY
Posts: 3,959
Carb Tuning help
slight problem with my big block...Iff im driving around and i crack the throttle its like it chokes it self for a second likr kinda bogs down the pcisk up and goes.... if ur very lite on the pedal it will climb thro the rpms no promblem....i checked the timign i runing 10 degrees at idel (1000) rpms. any ideas maybe change plugs car ran really rich for awhile till i jeted it down... still runs a lil rich.....not much tho
12-31-2004, 08:32 AM
#2
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Location: Buffalo, New York
Posts: 10,745
Re: Carb Tuning help
That may mean it is going lean. You need to increase the squirter size. You also don't specify what kind of carb it is. I am assuming it's a Holley, the only kind I know anything about. If it's a vacuum secondary carb you may need to try a stiffer spring on the diaphragm that controls the secondaries.
Rich
Rich
Last edited by rskrause; 12-31-2004 at 08:57 AM.
12-31-2004, 09:10 AM
#3
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Join Date: Dec 1969
Location: Phila., PA
Posts: 1,150
Re: Carb Tuning help
If it's a Holley there are many adjustments you can make to the accelerator pump. Which is, by the way, what is causing your bog unless you jetted it WAY down. I assume you only went 4-6 jet sizes leaner than stock at most?
If the carb USED TO "hit" clean when you nailed it and suddenly one day it doesn't or it went downhill over a modest period of time then you just need to clean and rebuild the carb. The accelerator pump check valve (especially the rubber umbrella-type) often get distored and stop sealing. The anti-reversion needle vavle up under the squirter can get dirty and not seal up. Etc., etc. All this leads to a reduced or delayed pump shot and a bog. Holleys require rebuilds from time to time. Nature of the beast.
Now if this is something that it has ALWAYS done (or it's a brand new carb) then you should begin tuning the accelerator pump squirter system. By the way, you didn't mention if this is a vac sec carb or a double pumper. If it's a vac sec carb you only have an accelerator pump squirter on the primary side. If it's a double pumper you have them on both primary and secondary side.
Before you make any changes to the squirters check the adjustment on the pump arm. There's a spring-and-nut looking thing out on the end of the arm which is used to make the adjustment. Have a friend hold the throttle fully open. There should be JUST A SMIDGE of travel left in the accelerator pump lever on the bottom of the fuel bowl before it bottoms out and won't go any further (.015" or thereabouts). If it's already bottomed out tighten up the nut until it's as described above. If it's got plenty of travel left in it, loosen the nut to take up some of the room. Let the throttle close back to idle and check one thing ABOVE ALL ELSE.... from idle when you just barely open the throttle fuel should IMMEDIATELY begin to discharge from the squirter. In other words, there should be NO slack at all in the acc. pump linkage. As soon as the throttle begins to move fuel should squirt. No exceptions. Do this for both the primary and secondary side acc. pumps (if applicable).
NOW you can begin playing with the accelerator pump tuning.....
The main thing to play with is the squirter discharge size. Holley makes interchangable squirters with different sized discharge holes. Primary side for a street/strip application you'll probably want to be around a #31 for starters, but it varies from carb to carb. (Seconary side can be all over the map, but is usually pretty close to the primary side.) If you get up where you're 8+ sizes bigger than stock and you still have a bog then the problem is elsewhere in the carb. Holleys are set up well enough that you should never have to make MASSIVE changes to any part of the calibration.
The other thing you can play with is the accelerator pump cam which determines the VOLUME of the pump shot. Stock cams are usually adequate as-is. If you don't have enough volume you'll get a bog, but slightly different from what you describe. It will take off real well for a split second, then bog, and then recover again. It won't be an instantaneous bog like too small a squirter will give you.
If the carb USED TO "hit" clean when you nailed it and suddenly one day it doesn't or it went downhill over a modest period of time then you just need to clean and rebuild the carb. The accelerator pump check valve (especially the rubber umbrella-type) often get distored and stop sealing. The anti-reversion needle vavle up under the squirter can get dirty and not seal up. Etc., etc. All this leads to a reduced or delayed pump shot and a bog. Holleys require rebuilds from time to time. Nature of the beast.
Now if this is something that it has ALWAYS done (or it's a brand new carb) then you should begin tuning the accelerator pump squirter system. By the way, you didn't mention if this is a vac sec carb or a double pumper. If it's a vac sec carb you only have an accelerator pump squirter on the primary side. If it's a double pumper you have them on both primary and secondary side.
Before you make any changes to the squirters check the adjustment on the pump arm. There's a spring-and-nut looking thing out on the end of the arm which is used to make the adjustment. Have a friend hold the throttle fully open. There should be JUST A SMIDGE of travel left in the accelerator pump lever on the bottom of the fuel bowl before it bottoms out and won't go any further (.015" or thereabouts). If it's already bottomed out tighten up the nut until it's as described above. If it's got plenty of travel left in it, loosen the nut to take up some of the room. Let the throttle close back to idle and check one thing ABOVE ALL ELSE.... from idle when you just barely open the throttle fuel should IMMEDIATELY begin to discharge from the squirter. In other words, there should be NO slack at all in the acc. pump linkage. As soon as the throttle begins to move fuel should squirt. No exceptions. Do this for both the primary and secondary side acc. pumps (if applicable).
NOW you can begin playing with the accelerator pump tuning.....
The main thing to play with is the squirter discharge size. Holley makes interchangable squirters with different sized discharge holes. Primary side for a street/strip application you'll probably want to be around a #31 for starters, but it varies from carb to carb. (Seconary side can be all over the map, but is usually pretty close to the primary side.) If you get up where you're 8+ sizes bigger than stock and you still have a bog then the problem is elsewhere in the carb. Holleys are set up well enough that you should never have to make MASSIVE changes to any part of the calibration.
The other thing you can play with is the accelerator pump cam which determines the VOLUME of the pump shot. Stock cams are usually adequate as-is. If you don't have enough volume you'll get a bog, but slightly different from what you describe. It will take off real well for a split second, then bog, and then recover again. It won't be an instantaneous bog like too small a squirter will give you.
01-02-2005, 10:08 PM
#11
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Join Date: Nov 2004
Location: La Porte, TX
Posts: 377
Re: Carb Tuning help
Unless your carb is jetted dangerously lean I would leave your jets alone till you get your drivability issues settled. Your car revs like a beast in neutral because there is no load on the engine and it needs less of a initial pump shot. Your carb is also new which means that it has a baseline calibration that may not be perfect for your car. Get a good book on Holley's and you'll see that Damon didn't just make that all up. Try what he said and I'll almost guarantee your problem goes away. Before you buy every squirter size and try each one, get a stock size squirter and go up one size at a time with a hand drill till you fix the problem. I went up 3 sizes and fixed all of my throttle tip in problems.
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