Engine stumbles under lateral g load. Carb issue?
 

Before I run my car again at Pocono this weekend, I want to see if anyone has an idea of what is happening to my car.
The motor is a 351W, built up fairly well. Approx 375hp. The carb is a Holley 750 double pumper.
When I ran at Pocono a few weeks ago, the engine would misfire very badly under any kind of lateral g load. We raised the primary float up and it helped a good bit, but did not totally eliminate the problem. By the way, I'm talking about with a full fuel tank. I know there is a fule pick up issue, and I'll cure that eventually by changing the pump location.
But, with a full tank and the primary float up high, why would it still stumble under a hard g load? Is this common to the Holley 750?

Steve

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How many pick ups are in your fuel tank and where are they located? Sound like you are sucking air from sloshing and trying to compensate with high float level. Not a good idea if it is a pickup problem. Drop the tank and look. For a road race non fuel cell set up you would want the 2 pickups at the lowest level in left and right corners. Fuel safe has a set-up that is second to none.

Steve,

A 750 in a 351W can be too much carb for the engine. Unless it is stroked or really hot a 650 is usually the limit. A friend of mine has a similar setup and could never get the carb to run right. Lots of stumble and spitting. He's going with a 650.

Roscoe

Roscoe
 

You are probably right but he was talking about lateral g's. It could be either or a combination of the 2.
I used a racing 750 with my 12.5:1 427.

I run 750 DP's on all stroker small blocks. You need to get different shaped floats in your bowls. Yours are probubly rectangle shaped, you need the ones with the lower corners removed so they have a trapezoidal shape, that way the needle and seat doesn't get closed when the fuel sloshes to the sides of the float bowls. You can cut and trim the Nitrophil floats, but you will need to seal the foam with epoxy where it is trimmed, it is easier to just buy floats with the shape you allready want.

Mr. Fixit,

I agree that a 750 on a stroked small block is good but Steve is running a non stroked 351W (I believe). The floats you refer to are known as 'roadrace floats'

http://www.braswell.com/assets/image...race_float.GIF

However, before I put any money into that carb I'd like to slap a 650 on it and see what it does. Check Demon selection guide.
http://www.barrygrant.com/demon/default.aspx?page=5

Roscoe

Steve,

If it was stumbling while in the corner, it is not likely to be fuel pickup as there would not be enough time for the air to make its way into the carb bowl.

Raising the float seemed to partially correct it so it is likely fuel sloshing away from the pickup in the float bowl, or the carb is a side float model.

There are aftermarket bowls designed to help slosh:

http://www.braswell.com/html/severe_bowls.html

If it is a side mount float carb you could switch to a center mounted float carb which fares better under lateral gS.

Hope this helps,

Jim

I run a 750 DP with the above mentioned floats and it runs flawlessly. My motor makes 380 RWHP. The spitting condition referred to earlier is the result of the motor going lean as the fuel is climbing up the side of the bowl.

750 should have the center hung floats from the factory.
Changing to a 650 will have no effect on this stumble while cornering, it may improve low RPM throttle response however.
Change the floats in both bowls, see what happens.

Thanks for all the feeback, guys.

The motor is not stroked, but it is bored .040 over. I would call it "mildly" worked over. It's not extreme, but it has a number of goodies in it.

I only have one fuel pump, and it is located in the tank, on the right side. That would explain some of the problems I had in the long sweeping superspeedway Turn 3. That is the Winston Cup track, almost no banking and speed around 100mph. The way we were running, it was a right turn, so all the fuel went to the left side. Here's a quick nutshell summary of how it went.
Run session one..car stumbled really bad. It was hard to drive, as it just wouldn't go at ALL under any lateral g load. Raised the float level up.
Run session two..the car actually ran much, much better. It still stumbled on occassion, but about 85% better than the first session.
Run session three..ran good for about two laps, then really started to misfire. Noticed I was well under 1/2 tank of gas. Went and filled the tank.
Run session four...with a full tank and raised primary float...the car ran quite well again. It did stumble a tiny bit. It is most noticable when going through a tight turn then having to get quickly back on the gas. It would tend to cough and pop, then take off like a shot. It ran well for the most part until, for reasons still totally unknown, the pin in the distributor drive gear (on a 2 day old distributor) sheered off and ended my day.

My mechanic and I debated between the 650 carb and 750 carb when we put this motor in (the day before my Pocono run). We went with 750, because we felt the 650 would choke it at higher RPMs. We suspected the problem is with the float type, but wanted to get opinions from other racers who have had similar experiences. The car runs fine on the street, we even got the initial throttle tip in hiccup almost completely worked out of it. Just on the track, it is stumbling, and I go back this Saturday.

Steve

I have a 351W, 40 over, afr 185 heads, rollers etc. and am running a 650 speed demon with regular floats. Never had a stumble in a hard turn. Not saying that the 750 is too much but everyone I've seen or heard about with this setup has had problems. Fix one...another one pops up..and on and on.

Roadrace floats will probably cost you around 50 bucks.

Roscoe

You're not getting any white smoke out the sidepipes when this happens, I assume?

TT

Hey Steve!

On Saturday when your kidneys are touching in one of those turns, take a look out and see if there is smoke coming out of the pipes!! YIKES!

All kidding aside, you may want to have someone spotting for you.

Roscoe

Not to change the subject, but I have close to the same setup
(351W, trickflow alum heads, 292 cam, bored 40 over, and a
750 DP). I have been tryin to sort thru some "running rich (idle &
at speed) problems this weekend. The car runs great, just rich.
makes your eyes water in the carage. The jets in the carb are
71H (dont know what the H stands for). I stuck a set of 68s in
and it ran awful (stumbling and such). Any expert opions out there? or maybe I should live with the water eyes.
WB

WB,
Do you know someone with a 650 you can swap for a trial run?
You may find that the problem is too much carb.
Roscoe

Having a 750 instead of a 650 will have only this effect: slight loss of throttle response at lower RPM's and less chance of a few lost HP at the top of the RPM's. Stumbling, idle quality, etc will not be affected.

WB: how far turned out are your idle screws?
Is it possible you have a blown (or incorrect) power valve?
Under what conditions did your motor run awful after putting in 68's?

I was actually watching out the side of my car, looking straight at the exhaust pipes, right as I got to the apex of the corner... maybe that explains why I kept going into the grass. :p Seriously, I actually did have someone watching the car on the track, and there was no report of any smoke from the car. I have never seen any either.

Roscoe, to make it worse, I don't have my racing seats installed yet, so I'm still using those stock seats. Nothing like having to use the steering wheel to help brace yourself. :rolleyes: No lateral support. Maybe it's me that's coughing and stumbling under lateral g.

Definitely great to have this feedback. I'm going to try to get the issue resolved before the weekend, but if not, I'll just keep the tank full and my fingers crossed. It ran fairly well once we had the floats up and my tank full. One good test for this would probably be autox. Lots of left/right/left, and fuel sloshing.

If I don't get to it this week, I'll definitely do it before my next track run in October.

Steve

Roscoe:
I have a 650 on an old ford truck, but it does not
have duel fuel feeds and it would require to much
work to remove and reinstall.
Mr fixit:
I set my idle just under that that resonating sound that
hurts you ears - (cant remember the rpm) If that helps.
With the 68s in, the car was stumbling during acceleration.
But it was not running as rich. Like I said early, with the 71h(s)
in the car runs very smooth and pull so hard its scary. Maybe
this motor is not ment to idle of run slow.
On the plus side I have not fouled a plug in 600 miles.
How do you tell if your power valve is wrong or blown?
WB

Steve,

Read this http://www.nationaltbucketalliance.c...tion/carbs.asp

Roscoe

Your power valve is just a srpring and a diaphram that acts on engine vacuum to rivhen up the fuel circuit. It has two number stamped on it. If it has a 6 stamped at 12:00 and a 5 stamped at 3:00, you have a 6.5 power valve. Which is what the vacuum reading will be when it opens. Your number should be about 2-3 less than your idle vacuum. If your motor only pulls 6" of vacuum, than get a 3.5 power valve. It is blown if the diaphram has a hole in it. I would say stick with the 71's for now, check and change your power valve (I ran a 3.5 with a 292 cam in a motor I had) I also had to drill holes in the primary throttle blades next to the idle transfer slot to keep the airflow fast enough to keep the idle circuit functioning correctly. Normal starting point for idle circuit mixture screws is 1.5 turns out from all the way in.