stalling under WOT

[Tscott38]

Bought a brmnad new Holley 3310-4 750 cfm back in '98.  Drove the car very little - then it sat in storage for 12 years, occasionally starting to make sure it still ran.  Idled fine all those years.  LAst year, I decided to start work on the car again.  Rebuilt the Carb with a holley kit.  I have messed up rebuilds in the past so I thought I was extra careful this time.  Afterwards, it seemed to start up and drive at low rpms just fine.  I adjusted the idle mixture to get the highest vaccuum reading  - 9inches at about 900 rpms (from the factory tach).  Accelerator pump seems to deliver a reasonable shot of fuel.  The linkage there is adjusted properly, or close to properly I think - this is a tough one to do it seems. 

Take the car out and drive a slow speeds up to 45 mph it seems to cruise along fine.  Stomp on the gas and it accelerates for a couple seconds and dies.  No pump shot visible after looking in the carb.  I figure its the float level.  I adjust that in my garage on a level floor, and i've got them so the gas just barely dribbles out under the vibration of the motor. Take it out again and it does the same thing.  Funny thing though, it ran good and accelerated just fine at least one time in my half doxzen or so half mile trial runs.  I'm now afraid to step on th egas too hard. 

The car has a new gas tank, new screen and pickup/gage assembly, new fuel lines, new Carter mechanical fuel pump, and of course a basically brand new holley 3310 with a rebuild by a conciencious amateur.  The floats are new nitrophyl and I didn't bend them in any way - installed right out of the box.   The only componnet that is old is the vapor seperator that I put on back in 98 - its run less than 10 miles on it.  I'm planning on bypassing this next.  Any other suggestions?  Thanks . 

[Chryco Psycho]

What size cam & squirter for the accelerator pump , it maybe enough fuel  atlow RPM but lacking under full throttle .
the second possibility is the secondary spring may be too light allowing the secondaries to open too fast .

[Tscott38]

The motor is a 67 440.  Whoever owned it before me put this in it, as the car is a 68 r/t.  They did other go-fast upgrades such as a DANA 60, disc brakes, so did they upgrade the motor while they were at it? The idle is a little lopey, but not much - so my guess is its stock, but I don't know.

As far as the pump squirter - I don't know.  Its right our of the box holley sent me.  The pump shot is decent, but I have seen better on some Q-jets years ago when I was into Chevys. 

I have yet to verify the secondaries are opening.  I was going to try tying a twist tie to the shaft to see if it gets bent out of shape when it opens, if it opens.  If there is another way to verify they open I'd like to know. 

It just seems odd that 12 years ago, this problem didn't exist.  Now with my rebuild it does - could the vapor separator be a clog in the line? 

[Chryco Psycho]

The vapor seperator could be restricting flow but the carb should not stumble 2 seconds into WOT , it would take longer to empty the bowls & have a fuel line restriction cause a stumble .
What about the power valve , you need to be at least 1.5 inches lower than the engine vacuum reading at idle In gear , so if you have 6" you need a 4.5 power valve

[Tscott38]

thanks for the information.  I'll look into the power valve and bypass the vapor separator for now.  I have 9inches now at idle - is that a typical vacuum reading?  I really have no knowledge of what is "normal" for a stock engine.   

This isnt just a stumble, this is a total loss of power and stall a few seconds later, coasting to the side of the road and when I check the squirter - nothing or very little pump shot coming out.  I have to refill the bowls to get it to start.  Thanks again.  I will report back my results after this weekend. 

[Chryco Psycho]

Sounds like you have a fuel supply or low float level problem , you can check the pressure at the pump & check float levels . The fuel pump should move approx a pint of fuel in 15 seconds cranking the engine over , there could ba a problem with the pump or the pushrod could be worn down limiting the travel of the pump arm .
9" of vacuum sounds low for a true stock engine , cam choice will dictate vacuum , in gear it will be lower than 9" for sure , a stock engine should be 16-18" at idle .

[Tscott38]

Bypassed the fuel vapor separator this weekend - no changes there.  But, in checking fuel supply, I just realized now that I used 15 gallons of fuel in maybe 8 miles of driving and about 1 hour of idling time.  The exhaust has always smelled of fuel and it burns your eyes.  Could that be a blown or wrong power valve? 

The car stalling  th epast weekend was probably due to lack of gas in the tank, but it wasn't always that way, as it has been going on since I initially put 15 gallons in.  I am pretty sure my floats are adjusted right now - they weren't always there, but now i'm confident in them.  I will fill up the tank and try things again.

As for the cam - the idle doesn't sound completely stock  - it is a little lopey.  ALso, my headlight doors work really, really slow.   

[Chryco Psycho]

definatly check the power valve , did you get a vacuum reading at Idle In gear yet ?

[BSB67]

Why does it only have 9" of vacuum?  Does it have a large cam?  Do you have a vacuum leak?  Did the problem show up after you removed and reinstalled the carb?  If so, it is probably something that either you or your buddy messed up in the process.

[Tscott38]

The rebuild kit I used came with a new pwr valve - The markings on it were unreadable, even withthe biggest magnifying galss I had, and it was not blown out on examination last nite - it held vacuum. The numbers were chopped in half and there were only 2 - no the 4 the holley kit said they would have.  So I checked the old pwr valve that was in the carb when I bought it - it held vacuum fine, at least what i could tell from trying to suck on the end of it, so I reinstalled it last nite.  It is a 6.5 inch unit.  I haven't been able to drive it yet, but when I adjusted the idle mixture screws previously the highest vacuum I could get with the previous pwr valve was 9 inches.

I do not believe I have a vacuum leak - I sprayed starter fluid all around the carb and hoses while it was idling and never heard a change in the engine speed or anything.  I am planning on hooking up the vac gage and taking it for a drive soon to see what my lowest reading will be. 

Again, I do not know the cam, I just think the previous owner hot rodded the car up as the '67 motor was added, as was a dana 60, huge tires, non-stock disc brakes, etc... 

[Tscott38]

OK I 've got some news to report.  the stalling under WOT throttle issue is solved.  It was bad float adjustment at first, then an empty gas tank later.  How did I burn 15 gallons of fuel in 10 miles?  Still unknown, but I drove the car and while I get 9" vacuum at idle in park at about 900 rpm, about 11 at 1100 rpm in park (from the tic toc tach), I get about 3 inches at WOT.

I've got 5.5. psi on the fuel pressure gage at idle.  i have no wet or sooty plugs - they are normal, light brown and dry.  Throttle response seems good.  No (from what I can see in the rearview mirror) black smoke coming out the tailpipe under WOT.  the exhaust does smell of fuel though.  the holley 3310 750 has a 6.5 power valve in it now. the one that was in it when i burned thae 15 gallons is unknown - could not read the numbers.   from what i understand my power valve should be 1.5-2.0 inches below the lowest vacuum at wot.  so 3-2=1inch.  so they even make them that low? 

Again, I appreciate all the excellent replies. 

[c00nhunterjoe]

No fuel in the gas tank gurantees a stall t WOT... ...

[Chryco Psycho]

The power valve needs to be 1-2" under the vacuum reading AT IDLE IN GEAR not WOT , if it was less than WOT it would never open LOL . The lowest power valve is 1"
Your carb is probably too small , it should not be reading 3"at WOT though .

[Tscott38]

"The power valve needs to be 1-2" under the vacuum reading AT IDLE IN GEAR not WOT"


thanks for the reply.  i had about 9" at idle in gear, so with a 6.5" pwr valve, I'm not too far off then.  I guess I'll drive it around with this pwr valve and see if I go thru a tank of gas like i did before.  The carb is a 750 cfm, so I can't imagine needing any more.  What would be a reasonable vacuum at WOT? 

[Chryco Psycho]

low as possible 1" or so , if you have vacuum at WOT the engine is restricted by the carb being too small , the engine want more air then the carb will let in .

[Tscott38]

I haven't verified for sure that my secondaries are opening.  i intend to do that this weekend.   if they are not, then i guess that could be a reason for the 3" vacuum.  is that right?

[Paul G]

You can put a paper clip on the little rod that actuates the secondarys, than do some wot driving and see how far the clip moved.