Author Topic: engine dies when braking hard  (Read 4492 times)

zray

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engine dies when braking hard
« on: August 23, 2009, 11:13:10 AM »
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

Soaring

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Re: engine dies when braking hard
« Reply #1 on: August 23, 2009, 03:11:24 PM »
You can pretty much narrow it down to the carburetor.  Maybe a bad power valve?   Or the needle is not seating right?  At any rate, I would  consider overhauling  the carb. 

FASTBACK65GT

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Re: engine dies when braking hard
« Reply #2 on: August 23, 2009, 08:10:31 PM »
I think its the carb too.  I recently had a similar problem with my Demon 625 cfm >:(.  It would die at hard brake even as slow as 5 mph.   I had the pwr valve changed, the jets changed, even checked the slosh tubes (you might check them to see that they are in place) and still the problem never went away.  The power valve changing from 65 to 35 helped somewhat, but the problem was still there.  It finally had all kinds of flooding problems so I yanked it and sent it back to Demon for warranty repair.
I stuck on an old junkyard rusted Holley 750 80508-S vac.secondary - AND ALL MY CARB PROBLEMS WENT AWAY!!!! No more dieing at hard braking.  No flooding. Nothing but a great running engine.
If you are into tinkering, change your power valve to something smaller, you most likely have a 65.  While you are in there, check your slosh tubes.  You might also adjust your timing.  Demon said to try 14, but that didn't help.  Mine is set at 40.
Has this problem started since you've had this carb?  Or did it run fine, then the issue started later?  It could be that you are running a DP instead of a vac.secondary carb.
I am now trying a new Holley 670 cfm Street avenger from Summit.  I was about to buy a 650 dp and the cust. support talked me out of it and swayed me into a vac. secondary carb.    I know the 750 on my engine is a vac.secondary and I have no dieing issues anymore.
I hope to put the new carb on this week and I'll let you know how it goes with dieing.  I should also get my old Demon back and hopefully they will have a detailed list of the repairs they did.  I'm not putting it back on as I fear continued problems. 
Anyone want a nice warranty repaired 625cfm Road Demon?
I hope it helps! :-\

Tim175

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Re: engine dies when braking hard
« Reply #3 on: August 23, 2009, 08:29:16 PM »
The 670 cfm street avenger is an excellent carb. ive been using it for almost a year now and never had a problem with it. holley is the best in carbs in my opinion. I think you;ll like it a lot.

zray

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Re: engine dies when braking hard
« Reply #4 on: August 27, 2009, 08:30:52 AM »
I just have determined the problem lies in the accelerator pumps circuit. I had them adjusted per Holley's instructions, with a little pre-load, so the action is immediate. The problem was, when the brakes are on hard, the throttle linkage is tweaked enough to give a squirt of gas into the closed carb venturies. I tightened up on the acc. pump screw slightly, which gives a little clearance before the acc. pump is activated. This one about drove me off the deep end.
thanks for all the help,

Z. Ray

Soaring

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Re: engine dies when braking hard
« Reply #5 on: August 27, 2009, 06:51:03 PM »
I love it when a problem is solved.  Now, go chasing women. ;D

 

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