engine dies when braking hard

[zrayr]

here's my situation, I'm using a Holley 650 dp on a Hi-Po 289 GT-350 with a Paxton. With or without the Paxton the engine dies on heavy braking. Setting the float level lower or higher on either bowl has no effect on my problem. I've ruled out any ignition related cause. I have a wideband a/f gauge installed and can see the mixture get richer & richer as I'm braking 'till it hits 10:1 & the engine starts to die. With the Paxton disconnected I can attach hose extensions to the vent tubes, and am reasonably certain no fuel is coming up the hoses. However, I can see that the tops of the secondary butterflies have gas on them. Where could this be coming from? I didn't hit the gas hard enough for the secondaries to be moving, so I'm thinking it could not be coming from the secondary accelerator pump shooters.

any ideas ???

thanks,

Z. Ray

[garry viohl]

Its coming from the secondary vent tube. Float level too high, lower level of gas in bowl to almost the bottom. Same thing happened to me. Do you have vent whistles installed?

[zrayr]

Originally Posted by garry viohl

Its coming from the secondary vent tube. Float level too high, lower level of gas in bowl to almost the bottom. Same thing happened to me. Do you have vent whistles installed?

I have the whistles, they came already installed when I bought the carb new a few months ago. I don't think any fuel is coming from the vent tube. I have a 3 1/2 " long hose attached to it (for checking purposes) & it appears to be completely dry inside.

But I will keep lowering the sec. float level as you suggest, and see what develops.

Is there any other source for gas to come from, beside the vent tube & shooter nozzles, to end up on top of the sec. butterflies?


Z. Ray

[Jerry Clayton]

it can come thru the jets

What do you have for a fuel pump, what fuel pressure are you running and do you use a regulator that is referanced to the manifold pressure?

what do you use to balance the differant pressure issues??

[zrayr]

Originally Posted by Jerry Clayton

it can come thru the jets

What do you have for a fuel pump, what fuel pressure are you running and do you use a regulator that is referanced to the manifold pressure?

what do you use to balance the differant pressure issues??

I'm using a mechanical Carter fuel pump (M 60454 )

http://www.summitracing.com/parts/CRT-M60454/

the fuel pump has been adapted to be directly referenced by the boost via a port on the Paxton. No other regulator needed with the vintage Paxton's.

It put's out 6 1/2 lbs. without the Paxton hooked up. With the Paxton the fuel pressure can range from 6.5 lbs. to 12 lbs. depending on the boost. Most of the time it is around 6.5 - 7.5 lbs.

where would it be "coming from the jets" ? though the boster venturies ?


Z.

[Bob Malmen]

Good to hear from you Jerry. I'm having almost the same issue upon hard braking. Also when I come to a stop under normal braking the idle hangs around 1100 and then will come down to 900 after about 30 to 60 seconds.

After your comments my fuel pressure maybe to high at 9.5 psi? I have a holley with a return setup. Any thoughts?

[zrayr]

Originally Posted by Bob Malmen

Good to hear from you Jerry. I'm having almost the same issue upon hard braking. Also when I come to a stop under normal braking the idle hangs around 1100 and then will come down to 900 after about 30 to 60 seconds.

After your comments my fuel pressure maybe to high at 9.5 psi? I have a holley with a return setup. Any thoughts?

I have seen 8 lbs as the upper limit for stock Holley needle & seat (non-supercharged application)

Z.

[Ron61]

Originally Posted by Bob Malmen

Good to hear from you Jerry. I'm having almost the same issue upon hard braking. Also when I come to a stop under normal braking the idle hangs around 1100 and then will come down to 900 after about 30 to 60 seconds.

After your comments my fuel pressure maybe to high at 9.5 psi? I have a holley with a return setup. Any thoughts?

I had much the same problem as you do except my idle returned to the normal immediately. After looking at my fuel pressure I saw it was running over 8 pounds and Holly told me to keep it around 6 1/2 to 7 so I set it at 7 and never had another problem after that. Your fast idle would almost sound as if you are still getting fuel for a few seconds before the feed shuts down to idle.

Ron

[Jerry Clayton]

Bob

check your power brake unit----do you pump the brakes gently on a normal stop????vacume issues probably

I don't like over 5 psi for carbs

[trularin]

If someone has not suggested this, older Holleys had a slosh problem and many carbs needed slosh tubes installed to prevent the gas from traveling up the vent(s) into the intake killing the engine.

It was a common problem in the 70s and early 80s.

Hope this helps.

[zrayr]

Originally Posted by Jerry Clayton

Bob

check your power brake unit----do you pump the brakes gently on a normal stop????vacume issues probably

I don't like over 5 psi for carbs

no power brakes.

thanks,


Z.

[m5extc]

Just a suggestion,

I had a simular problem. I readjusted the throttle linkage. I suspected all of the same things, then thought that the linkage on mine is hard, not a cable so maybe it's an issue. The motor mounts flex just enough on hard braking to push the throttle closed a little more, shoking the engine. My problem isn't completely cured but is 100% better now after adjusting the rod differently.

Andrei

[Argess]

Very interesting m5extc......good thought.

I have a smiliar problem. Doesn't stall, but idle goes very low. Happens after hard braking, but comes back to normal in a second or two. BUT, it also happens after I return to idle from a WOT hard run and then it takes 15 to 20 seconds to come back on its own. In each case, and instead of waiting, a quick stab of throttle clears it out and it will idle perfectly immediately after that.

I have a 2x4 arrangment with the Holleys rotated 180 from a single Holley installation. Most of the vent whistles, baffles, etc, are meant for normal carb orientation, so may not help much for me. Personally I think the float levels need to be a tiny bit lower. I've got a couple of Holley sight glasses ordered as my previous attempt at this spilled a lot of fuel....chuckle.

Anyway, this is of interest and may also be the problem:

Here's a fuel slosh mod....check out figures 5 and 6 on pages 6 and 7:

http://www.holley.com/data/Products/...99R10268-4.pdf

And heres a site on doing the mod yourself:

http://classicbroncos.com/tech/offro...4-barrel-carbs

I have to try the slosh mod myself....after I try lower float levels first.

[zrayr]

I also fabricated a vent tube like the one Holley uses on the off road carb. It didn't help my problem, but I enjoyed making it .

Z.

[zrayr]

Originally Posted by Argess

..................
Anyway, this is of interest and may also be the problem:

Here's a fuel slosh mod....check out figures 5 and 6 on pages 6 and 7:

[url]http://www.holley.com/data/Products/...99R10268-4.pd ...........

couldn't get to page(s) you noted, any details on what is there ?

Z.

[Argess]

Oh my gosh....I just found and posted those links a few days ago and now they appear "gone".

What it is, is a tube that connects the front and back fuel bowl vents. The extension tube has a number of vent holes along the top. The idea is that if fuel is sloshing out the vent tubes,, it merely travels to the opposite float bowl. The hoels are needed to maintain fuel bowl venting.

Sorry about the confusion.

EDIT: I think the cut-and-paste messed up. Here, try again:

http://www.holley.com/data/Products/...99R10268-4.pdf

http://classicbroncos.com/tech/offro...4-barrel-carbs

[Argess]

Oh, by the way, you mentioned you are using a Carter pump? So am I. I wonder if the pressure is a bit high on those. I have a little fuel pressure guage on the inlet line to the carb and it sometimes reads 9.5 psi, whihc apparetnly is high. But I don't trust the guage as it also can read 6 at times, Guess I ought to replace it sometime.

[zrayr]

Originally Posted by Argess

Oh my gosh....I just found and posted those links a few days ago and now they appear "gone".

What it is, is a tube that connects the front and back fuel bowl vents. The extension tube has a number of vent holes along the top. The idea is that if fuel is sloshing out the vent tubes,, it merely travels to the opposite float bowl. The hoels are needed to maintain fuel bowl venting.

Sorry about the confusion.

EDIT: I think the cut-and-paste messed up. Here, try again:

http://www.holley.com/data/Products/...99R10268-4.pdf

http://classicbroncos.com/tech/offro...4-barrel-carbs

that's what I thought you were talking about. I saw the photo of the Truck off-road carb and made one similar. Mine wasn't quite as pretty. lol I also had to make mine a bit shorter to fit under the Paxton bonnet.

Z.

[zrayr]

Originally Posted by Argess

Oh, by the way, you mentioned you are using a Carter pump? So am I. I wonder if the pressure is a bit high on those. I have a little fuel pressure guage on the inlet line to the carb and it sometimes reads 9.5 psi, whihc apparetnly is high. But I don't trust the guage as it also can read 6 at times, Guess I ought to replace it sometime.

the pressure starts out at 6.5 lbs and goes up from there depending on how much boost is being produced. but the fuel pressure is never more than 5 or 6 lbs. more than the boosted atmosphere pressure. So the fuel isn't, or shouldn't be, blowing the float needle open.

Z.

[Argess]

Your pressure sounds fine. I wasn't thinking it forced the float needle open....I was thinking of excess fuel entering when the float was low due to fuel up aganst the front of the bowl when you brake. Then once the fuel settles down, the level is now too high. But it was a long shot I suppose.