Engine dies in decelleration?

[Dangerous Doug]

My nephew has a vintage 1965 Mustang fastback with a 302. The car runs fine EXCEPT when he's decelerating (e.g approaching a stop sign at normal speed) and then the rpms drop much faster than normal and the engine wants to die.

Any ideas what to look for?

Checked the normal stuff: timing, dwell, looked for vacuum leaks.

Any suggestions?

[Gatorac]

Originally Posted by Dangerous Doug

My nephew has a vintage 1965 Mustang fastback with a 302. The car runs fine EXCEPT when he's decelerating (e.g approaching a stop sign at normal speed) and then the rpms drop much faster than normal and the engine wants to die.

Any ideas what to look for?

Checked the normal stuff: timing, dwell, looked for vacuum leaks.

Any suggestions?

Lower the float level in the carb. Under braking you'll find the car floading out.

[Ron61]

Also if he has a Holly Carb. he might want to check to see if fuel is splashing up into the vents and going into the carb. I had that problem with my Cobra and finally had a new racing carb built and they put what they call whistles, which are just plastic extensions in the bowls that extend out and up so the fuel going forward and back can't get into the vents to flood the car. The Cobra would die every time I had to brake of back off hard and after having that done, I never had another problem.

Ron

[undy]

The front bowl fuel level could be too low too. They make a slosh tube that can be added to the front bowl that helps prevent flooding/lean conditions from high "g" loading, typically made from plastic. Your purpose built road race carbs typically have them in there, possibly the HPs too.

Dave

[sharpe 1]

Sounds like you have a problem with the dash pot on the carb. If its equipped with one.

[bkozlow]

I agree with Sharpe 1, it sounds like a dash pot problem, but I guess the problem is, you are not running one. The next problem will be trying to find one.

bkozlow

[Dangerous Doug]

What in the heck is a "dash pot"?

I don't think this is happening during hard decelleration. This appears to be just rumbling up to a stop sign.

Of course, my nephew is just 21 so you never know...

[patrickt]

Originally Posted by Dangerous Doug

What in the heck is a "dash pot"?

It's usually called an "anti-stall dashpot" and it acts as a damper so your throttle doesn't close too quickly when you take your foot off the pedal.

[xlr8or]

Lower the float in the secondary bowl and get the vent whistle.

[jhv48]

Tell your nephew to put in the clutch whenever he is slamming on the brakes in order to prevent running a stop sign.

Actually, it usually is fuel sloshing out a vent tube caused by too high of a float level. Is it a Holley or Edelbrock type of carb? The Holley type you can fix easily. The Ed is much harder.

[flipscobra]

if it has a big cam he might need a vacum canister to help with the vacum on the power booster brakes

[Dangerous Doug]

Holley carb.

Cam isn't very aggressive.

Would the float get out of adjustment? I think this just started happening recently.

DD

[Pete Munroe]

My Holley 750 double pumper with center-hung floats has caused the engine to "almost stall on hard braking" since new.

The floats have been up, down, and at the recommended height. The pressure started at 7 psi and now is at ~4.5 psi. The vent whistles were installed by the factory. Purchased about 2003.

On my 327" Corvette I have a smaller, 580cfm/1850 Holley with SIDE-HUNG floats. It has NO problems braking, or cornering.

The 750 center-hung double-pumper has problems braking AND cornering...(stutters/burps in cornering, left or right).

As to the whistles, if stalling on braking is from fuel spilling out of the rear float bowl vent tube under braking how can the fuel move to the REAR of the float bowl...then simultaneously go FORWARD through the whistle and up and out of the vent?

Some off-road Holleys use a tube that connects the front and rear vents and has vent holes drilled in the top center...think it is meant more for the extreme angles of rock-climbing than braking.

Hope someone has the answer, either that IS what a center-hung carb will do or whatever the fix is.

[xlr8or]

My Holley double pumper with center hung floats and vent whistles had the same problem until I lowered the floats. The vent whistles don't totally seal it off.

[jhv48]

Two solutions now that we know it's a Holley.

1. Lower float level to bottom of sight plug. If already there, lower a tiny bit farther.

2. Connect the two vent tubes with a rubber tube and cut a couple of holes in the top of the tube so air can still vent.

One or both will cure the problem.

And tell the kid it is OK not to lay down rubber when he stops!

[Dangerous Doug]

Knowing my nephew, the rubber streaks lead away from the stop sign, not toward it.

Thanks for the input. Time to tinker and test...

DD

[Dangerous Doug]

Resolution from my nephew:

As for my problem with it stalling, I seemed to have fixed it. I'm not sure exactly what I did - I just took apart the rear float, played around with it, and put it back together. I'm not aware of really changing anything. But it seems to be running beautifully now, especially after I tweaked the timing again. I guess sometimes that's all you have to do - just take it apart and put it back together again.

Thanks everyone for all the help. Uncle Doug, reading the replies on the thread you created on the Cobra forum really helped a lot. Thanks so much for doing that and keeping up with it all. That was extremely helpful. I'll let y'all know if anything changes, but for the time-being the Mustang is running like a charm. Thanks again!

DD