motor is a freshly built 383 stroker, dart heads, 472 z28 high rise intake, original 4053DZ Holley, crane blazer cam. initial timing is about 12 deg with 36 deg total at 3K RPM. vaccuum advance is currently unplugged. idle mixture was set with a vaccuum gauge to the highest reading available.

now for the problem: the motor revs up fine and seems very strong if i bring the throttle on gradually. but if i snap the throttle open quickly, she stumbles kinda bad. the exhaust had a pretty strong unburnt odor, not enough to burn my eyes, but strong. i knocked the idle mixture down a 1/8 turn, but then a quick snap of the throttle produced a small backfire. that didn't seem like the right direction to go. where should i start first? timing or carb? i'm still feeling my way through tuning, so go easy on me. btw, all of this was done with car in neutral, no load was on the motor.

thanks,
marc

when you open the throttle blades the engine sees more air imediately . you need to supply extra fuel at the same instant . if you dont you will notice a bog or backfire. to remedy this youre carb must have enough fuel passing through the squirter to avoid the lean condition that occurs. check the squirter and the fuel level in the carb. first thing.

Hey I had the same problem. Check the back secondary diaphram (accelerator pump). Also, the throttle plate could be sticking. It also may have been the power valve. Good luck.


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It is better to burn out than fade away

I'll give this a shot. If you've got a large cam, you'll have to crank the idle screw (speed not mix) in quite a ways to get the engine to idle. By doing this you uncover the idle tranfer slot rendering it useless, and produce the symptoms you describe, when you tip into the throttle. If I'm correct, the fix for this is to drill a small hole in each of the primary throttle blades. This will allow you to return the idle speed screw to a more closed position, covering the transfer slot, when you set your idle.
Anyone else feel free to chime in any time!!!!


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currently restoring my '67, 327, T-10,3.73 10 bolt

[This message has been edited by whitey (edited 12-03-2001).]

I've had to drill holes before and it was the only thing that made it work!

Great info guys! i recently replaced both front and rear bowls as well as the accelerator pump diaphragm (30 cc) to cure some leaks. I am pretty sure the float height is good, but I will double-check. Squirters seem to be working fine.

On suspicion that I blew out the power valve (backfired real good a couple of times on initial startup), I replaced both of them. The new one I put in the front was a couple of numbers higher than the one I replaced. I think the new one is an 8.5.

Whitey, other than drilling the plates to see if it fixes the problem, is there a way to tell if I have uncovered the idle transfer slot? I have the idle set to (900 RPM), it's a 4 speed car. This carb was on my 302 and it idled at 900 as well.

Once again, thanks everyone for the great info.

-Marc

I'd check/change coil first. The lean condition plus the extra load on the plugs require more spark energy.

You can always use the set screw on the throttle plate on the pass side to open the secondaries .020-.030 inches. This will allow the front plates to close and will then be able to use the idle transfer slots, thus using the idle mixture screws for better control. This will eliminate drilling a holes in primary plates. The setscrew is gotten from the bottom of the carb, it has to removed, I unscrew it, and install it from the top, this way you can adjust it from the top without removing the carb.

Replace the squitter screw with a hollow one. If all else fails, replace the squitters with two sizes bigger, keep going in two step sizes, stop at .032, too big now.

Good luck....

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Everett 68/350/PG/11.90/115mph

I know that you can see the transfer slot looking at the carb upside down - the carb, not you- http://www.camaros.net/forum/wink.gif. The slot runs vertically along the primary throat or barrel. I'm not sure at what point, as far as looking at the slot, how much you'ld have to see or uncover until the slot became uneffective. Trial and error process I suppose.
Everett made an excelent suggestion. If it turns out the transfer slot IS the prob. definatly try cracking the secondaries open before getting the drill out. Let us know how you make out!

I know that you can see the transfer slot looking at the carb upside down - the carb, not you- http://www.camaros.net/forum/wink.gif. The slot runs vertically along the primary throat or barrel. I'm not sure at what point, as far as looking at the slot, how much you'ld have to see or uncover until the slot became uneffective. Trial and error process I suppose.
Everett made an excelent suggestion. If it turns out the transfer slot IS the prob. definatly try cracking the secondaries open before getting the drill out. Let us know how you make out!

I think Deimos is on to something, only it also could be the accelerator pump out of adjustment on the primary side. With the engine off open the throttle a little while viewing down the carb, as soon as the throttle moves the accelerator pump should start sending fuel through the squirters. If you feel the accelerator pump arm when the throttle is in the closed position there should be no play or clearance in it. To adjust it properly you open the throttle all the way and you should be able to press the accelerator pump arm down another .015" +/-, this will keep the pump arm from binding at WOT and also provide that initial shot of fuel as soon as you touch the throttle.

The big thing that most people have trouble with is accelerator pump cams and aquirters with the Holley carb IMO. Yes, you have to first set the idle mixture, the float levels, and make sure the accelerator pump lever is adjusted correctly. You might also want to try a fresh power valve just to be safe.

If that doesn't cure your problem, THEN you may have to change the amount of fuel in the pump shot and/or the length of time the pump shot lasts.

When you mat the gas when in gear and the car . . .

Goes for a split second, then bogs, then goes again. THEN you need to extend the amount of time the pump shot lasts. You can do this by installing a smaller squirter. They are numbered [#31 is pretty common on a stock Holley] and are available every three sizes. So, if you have a #31 switch to the next size down, a #28, and test again.

If the car immediately bogs you need to determine whether it is from way too much fuel or from too little fuel. If you can smell gas in the exhaust and see a puff of black smoke out the tail pipe than it's from too much fuel. If it pops [lean backfire out the carb] than it's from too little gas.

If you have a lean stumble, try a larger accelerator pump cam (they are colored according to their size). If this helps a little but does not cure the problem you can try a larger squirter too.

If you have a rich stumble, try a smaller cam. A smaller squirter will help this problem too.

Good luck, hope this helps.

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68 Camaro, 383 small block with TH350 trans. 11.98's at 111mph and never trailered.

PS. A good place to start with the pumps and squirters is a number 31 squirter and the orange pump cam. That's what I'm running on my 383 and it seems to like that combo.