Bronco sputter and stalls

[77broncobilly]

HERE IS WHAT WE ARE DEALING WITH:
New 351 Cleveland with aussie heads
Alum 4 bbl Intake
New holley 770 carb
Shorty headers with thrush daul exhaust out to back
Electric holley fuel pump
Electric fan
PROBLEM IS:
Starts and runs fine
At 2000 RPM Bronco sputters or stalls, I cankeep it running after 2000 and runs ok until rpm get back around 2000 then sputters again
Called Holley they suggest exchange 770 with 670 carb
Did that and same response with 670 carb
So I am looking for ideas or suggestions
I am thinking maybe its timing or electric fuel pump
Anyone know timing degrees on 351 C I am assuming 6
HELP email me directly b77broncobilly@hotmail.com

 

[broncnaz]

What ignition timing you running now? You should be able to run between 6-12 easily. Might want to check your spark plugs what color if white its way to lean.
Do you know what comp ratio your running? Aussie heads should bump up the ratio about 1 1/2 point over stock. how about camshaft? What grade of fuel are you running? Might have to run premium as you might be getting detonation and thats why its basically falling on its face at 2000 rpm.
While I wouldnt rule out the fuel pump 2000 rpm is not much even a weak pump should be able to keep up.
Lastly I would suspect a vacuum leak between the heads and intake.

 

[77broncobilly]

I will check

Thank you for your input, I will run a check list on your suggestions.
I purchased ths bronco 3 months ago and do not have all details on cam
I do no it is a crated stroked 351C with MSD ignition

 

[77broncobilly]

completed test

OK
Test completed today
12 to 13 in-hg vac
6 psi fuel pressure
Timing at 12 degrees, did change timing to 9 and became worse
Fuel tank vent holes unpluged
MSM mechancial cap off checked ok and new, plug wires new
Spark plug pulled and found black with carbon from back firing

So we have a sputter, couf, backfire and will stall if I keep my foot into it
or if I back off engine will keep runnning

This is happening 1500 rpm to 2000
I do have another new carb on its way from holley a 670 but almost do not think this is problem
Waiting for the bronco gods to scratch there heads and help.
b77broncobilly@hotmail.com

 

[broncnaz]

Any idea on what the specs are on your camshaft? 12-13 vac is a little low. I'm still thinking vacuum leak.
Did you check all the spark plugs or just one?

 

[77broncobilly]

test

Thank you for your reply
I only checked the front drivers side spark plug
I did spray soapy water around intake checking for vac leak
No idea on camshaft but I think the fellow I purchased it from said it had
a mild rv cam it does not have that rumble a race car have.

 

[Wyflyer]

have you checked to see if the accelerator pump in the carb is squirting fuel correctly?

 

[broncnaz]

I think you have a vacuum leak somewhere. Could be to the inside of the engine. A RV type cam should pull close to 20 in of vaccuum I would expect between 15-19 easily esepcailly if you dont have any lope at idle.

Doubt its a accelerator pump issue he's had the issue with 2 carbs. But it is something to check.

 

[NC-Fordguy]

x2 on a vacume leak. Get some carb cleaner in a spay can and spray anywhere where a vacume line exists and at the base of the carb and listen for a change in rms. I actually had a vacume leak once at the power brake booster. Took a while to find that one

 

[77broncobilly]

More testing

Will do more testing for vac leaks
I have a new carb coming from Holley this week, they are sending a 670-2
When I install new carb I will spray carb base and vac lines checking for leaks
Thanks Bronco gods I will reply back with test results

 

[canrel]

You say it idles fine and as you increase the rpm to 1500-2000 it goes to crapping out, but above 2000 it is ok?

Are you doing this parked in nuetral or driving down the road?

What dizzy is it. You say msd, is it an add on MSD-6. Is it points or electronic ign?
Sounds like possible spark scattering in the dizzy. Points good? Or electronic pick-up clean?

A vacuum leak will pull, suck, in what ever you spray. So soapy water does not work for this.
When spraying carb cleaner at and around any gasket, port, shaft, hose....while the engine is running. Listen for a change in rpm, might even kill the engine.

Be very careful while doing this. the vapors of the carb cleaner are VERY VERY dangerouse. You should do this outside in well ventilated area. A bad plug wire can ignite the vapors. Spray a small area 3-4 inches of the intake or base gasket at a time.