Carburetor problem?

[zerro]

I am chasing what I think is a carburetor/fuel delivery problem and would like some input from others. Here's what happens:

During qualifying (sometimes hot laps, too), I drive the car hard into the corner; I set the car with the brakes and then immediately get back on the throttle hard (to the floor). The car stumbles and hesitates until it gets about half-way through the turn and then "clears out" and moves on. The carburetor is a 750HP Holley 80528. It was new out of the box this year. The problem has gotten progressively worse as the year has gone on. I run a Holley 130 GPH fuel pump at 7 psi. The problem doesn't occur during the race, which may be because I don't get back on the gas so quick and hard because the track generally is dry and slicker.

Another thing the car has never really done well is to rev-up. When I blip the throttle, there is always a slight delay before the fuel seems to get delivered and the motor revs up. It is not crisp and quick as I would think it should be. I have increased the size of the squirters from the stock size of 31 up to 37, but that really hasn't helped. I am running the stock size jets and power valves.

Any thoughts on this would be greatly appreciated.

[mbaker76]

On the track it almost sounds like its getting too much fuel?
Have you checked float levels, to make sure they arent to high and spilling over on entry?

[zerro]

Float levels are even with the sight holes. I don't have the entended vent tubes, and have considered going with longer tubes, or connecting them with a rubber hose with a cut in it.

It's been suggested that it might be too lean - running out of fuel at the end of the straight and not having time to refill the bowls when I get back on it. Also, stomping the throttle with a lean condition - too small jets, causing it to stumble.

[billetbirdcage]

Try a angled spacer under the carb, if the car is rolled up hard it can push the fuel in the bowls to the right side and uncover the jets and suck air instead of fuel. Doesn't have to be a high dollar one even angle milling a STD 1" one .150 will help. Just mill one so it leans the carb to the left.

[zerro]

Could the jet extenders help if that's the case?

One other thing the car does is to "flood" after it sits for a few minutes - could be a clue to the real problem?

[billetbirdcage]

NO!

Picture this looking at the carb with the bowl off (straight at the front metering block). This looks like a rectangle so the fuel in the bowl is half way up this rectangle.

(Sitting not racing - looking from front of car)
-------------------- top of bowl

--*************-- fuel level

*****x****x*****jets

--------------------- bottom of bowl


During high cornering and that the left side of the carb is higher then the right from the car being hiked up (can't draw the carb at an angle)

(Car with high G forces compounded by the left > side of carb higher then right <)

-------------------- top of bowl
--- fuel on right side


*****x****x
**************-- fuel on left
---------------------bottom of bowl

Ignor the *'s just there to get spacing right

see how it can uncover the left jet and suck air.

If the car is really hiked up and tilting the carb from the car being hiked up on the left side will compound the problem. This may not be it but was common in LM before we have gotten the LF to stay down more and not tilt the carb up on the left side as in years past.

[zerro]

OK, this may lead to another question and issue - my left front IS coming up much more than it has in the past. What's the best way to keep it down? I have a '02 GRT 4-bar limited car. I really haven't changed anything, but the track is tackier lately.

[Chris Steele]

Is the bar under your left axle tube straight or curved down? Some people cut the straight one out and installed a curved one. I just cut my curved one out and reinstalled a straight one. I still run a limiter chain to keep the rearend from hitting the bar...keep it about 1/8" off the bar. Jack the car up by the frame rail right under the driver seat and have someone push down on the right side of the rear bumper to try to imitate cornering. Adjust your chain accordingly. This will keep your LF down. May not be the best way, but you can keep your bar angles the same as GRT calls for.

[Rocket Bonehead]

OK, this may lead to another question and issue - my left front IS coming up much more than it has in the past. What's the best way to keep it down? I have a '02 GRT 4-bar limited car. I really haven't changed anything, but the track is tackier lately.


General answers (most effective):

- Stiffen the RR spring
- Add RRT bar angle
- Take angle out of the PH Bar (pinion is biggest change, frame is a smaller change)
- Lower Ballast height



RB

[LM14]

Has anyone watching your car stated they saw a puff of black or dark smoke out of the car when it is stumbling, right before it clears up and runs right? If so, your squirters are probably a little too big or coming in too soon.

Try dropping 4 to 6 sizes in the pump squirter and seeing if that helps. If it makes it worse, go the other way.

When you shut the motor off and it is warm, look down the venturi a few minutes. Is there fuel being drawn into the venturi with the motor off and creating a white cloud inside the carb and intake plenum? If so, your floats are too high. That could also be your flooding problem. Drop them a half turn at a time until that problem goes away. The sight holes aren't perfect for every situation. When you are checking them, is your motor running and you have to shake the car a little to get some gas to run out? If it trickles out just idling and not rocking the car a little, they are set too high.

Check power valves. If they are blown you will not be getting the extra shot of fuel when you pick up the throttle you need. These cover the time lapse between the accelerator pump shot and power circuit kicking in.

Also check to see that your accelerator pumps are adjusted correctly. Holding the linkage at wide open throttle, I like to see .015 free play between the pump arm and the accelerator pump shaft at full stroke.

Hope these help,
SPark

[zerro]

Has anyone watching your car stated they saw a puff of black or dark smoke out of the car when it is stumbling, right before it clears up and runs right? If so, your squirters are probably a little too big or coming in too soon.

Try dropping 4 to 6 sizes in the pump squirter and seeing if that helps. If it makes it worse, go the other way.

When you shut the motor off and it is warm, look down the venturi a few minutes. Is there fuel being drawn into the venturi with the motor off and creating a white cloud inside the carb and intake plenum? If so, your floats are too high. That could also be your flooding problem. Drop them a half turn at a time until that problem goes away. The sight holes aren't perfect for every situation. When you are checking them, is your motor running and you have to shake the car a little to get some gas to run out? If it trickles out just idling and not rocking the car a little, they are set too high.

Check power valves. If they are blown you will not be getting the extra shot of fuel when you pick up the throttle you need. These cover the time lapse between the accelerator pump shot and power circuit kicking in.

Also check to see that your accelerator pumps are adjusted correctly. Holding the linkage at wide open throttle, I like to see .015 free play between the pump arm and the accelerator pump shaft at full stroke.

Hope these help,
SPark

I did have one guy tell me it was "gas" smoking out of the turns.

There are little puffs of smoke when I turn it off and it sometimes "floods" after it sits for a few minutes.

I checked the accelerator pump the way Holley tech told me, which was to make sure that the lash was zero at no throttle, and it wasn't. I had quite a bit of preload on the accelerator pump, and after I adjusted that out, the hesitation when revving it up from idle went away, at least in the garage.

I checked the power valves and they are 6.5's. The jets are 73's. The vacuum at idle is between 8 and 9 in Hg. The Holley tech guy said if it's below 9, use 1/2 half the value for the power valves. If it's above 10, use 6.5's. It never went above 9.

I checked the float levels and they are set right below the lip of the opening. I also reset my fuel pressure from 7 psi to 6 psi per Holley's recommendation.

Any other thoughts on this, especially the accelerator pump and the power valve would be great.

[zerro]

General answers (most effective):

- Stiffen the RR spring
- Add RRT bar angle
- Take angle out of the PH Bar (pinion is biggest change, frame is a smaller change)
- Lower Ballast height



RB

The RR spring is 225; LR is 250; shock 94; travel indicator stops about 1-1/2" above bottom,

RRT is in the top hole of the 4 holes that are there, 17 3/8" lg; RRL is in the bottom hole of 2, 15 3/8" lg,

LRT is in the top hole of 4, 17" lg; RRL is in the top hole, 15" lg; indexed to middle hole of 3 on the birdcage,

J-bar is in second hole up from bottom on pinion; 10 1/2" up from bottom of frame rail on frame, 19 1/4" long. I moved the frame end down 1 hole last weekend and it helped the lifting some,

Ballast is mostly fuel, with about 90# mounted high and to the right just behind the rear end; 52.1% left, 54.7% rear; 110 - 120 # bite.

LF 75 shock with 450 spring, RF 76 shock with 375# spring, ~ 1-1/2" above bottom on shock travel indicator.

In the past, when I have used a 250 on the RR, the car would get VERY loose during the race. Have not played much with the j-bar angle or RF spring.

[Rocket Bonehead]

Well, scrap what I said and add some L%. That will free the car up in and thru the middle, but it will tighten it up off. How much is hard to say, but in any case, the car should be much easier to drive. I'd go to between 53.5% and 54% (w/driver). Your hiking problem should decrease noticeably (sp?)



RB

[zerro]

My LS % is without driver, and I'm not sure what it is with me. I weigh about 190# in a 2400# total car.

[Rocket Bonehead]

Ok... Scrap what I said again, lol... Well, see where it is at with you in the car. You could also lower that 90lbs of ballast that is high on the RR and that will help as well. Looking back at your setup, you could also take some rake out of the PH Bar on the frame and that should help too.



RB

[zerro]

I'll work on those two items.

[all_jacked_up]

We had the same trouble at a track we run at where you run wide open to time in & sometimes in the heats also. We have a tilted spacer & a good aftermarket carb that we had no trouble with for the past 2 years until we started going to this new track. The car would start to miss at early mid turn & stop missing about halfway down the straight (running wide open). We tried changing the fuel pump, added a fuel pressure regulator, took it back off, spark plugs, distributor cap & rotor, different floats, jet extensions, lowering the floats (made it worse) & after that, raising the floats . Raising the floats fixed it. I used to set the floats to the bottom of the window with the carb level. Now I set the floats with the car level (carb out of level to the left on the tilted spacer) to the bottom of the window. It is probably 1 turn higher on the floats. The carb still works good everywhere else. I know that the higher float level pulls the mains in quicker, but it does not seem to affect anything & it works on the hooked up track.

Also note that after the track slicked off the slightest bit, the problem went away.

Hope this helps. I know how aggravating it can be.

Also, a 4.5" power valve seems to be about right for us.

[zerro]

I'm going to stick in a set of 4.5's tonight and try it with that this weekend. I'll Probably mess with the squirters a little, also.

[zerro]

Ok... Scrap what I said again, lol... Well, see where it is at with you in the car. You could also lower that 90lbs of ballast that is high on the RR and that will help as well. Looking back at your setup, you could also take some rake out of the PH Bar on the frame and that should help too.



RB

Here's a picture of the car in the turn (orange "0"). You can see that it's tight, and rolled way over. I'd much rather have the car in the position of the 63 car, which is a little loose and on all fours!

[Rocket Bonehead]

Less rake in the PH Bar and lowering your ballast should help that.



RB

[zerro]

I think you said earlier that the pinion connection is the place to start with that adjustment. Is that correct? I'm in the second hole up from the bottom - maybe go to the third up?

[Rocket Bonehead]

No, the frame is the first place I would start. The frame adjustments are much more subtle than the pinion adjustment.


RB

[zerro]

I moved the connection at the frame down a notch, and un-indexed the LR upper bar, and it helped a lot.

I found another issue after this weekends race - the RF frame rail is really dragging the track. About 2-3 feet of paint is gone from the lower frame rail almost half way up the rail. I think I should stiffen the right front because that can't be good. The car has been really erratic is qualifying, and maybe that's part of it.

Also, the silly thing cut out again in qualifying. It only does it during qualifying, mostly in turn 3 as soon as I get back on the throttle. Still a mystery that I can't seem to track down.