C9H14N

Help

I have a 351 cleveland that has developed a backfire or popping through the carb at around 5000rpm. I have checked the dizzy and it is advancing enough (mechanically). I have checked the fuel pressure and it is maintaining about 5psi at full throttle. The motor is pulling hard up until the popping and I can't find out what is going wrong. The jets have not been changed as they were running fine previously. Any suggestions.

brenx

Have you replaced the plugs? Or even checked plug color. It should be a creamy color.

[edit] also the points and condenser. If a points dizzy?

Brenden

__________________
XB Fairmont (street car) 11.07@123.02mph.

C9H14N

Brenden.

Thanks for the reply. I have changed the plugs and leads. The plugs are running at a light brown to cream and are clean. The dizzy is a Mallory that has been converted the crane fireball electronic some time ago. I am quietly thinking that this may be whats holding the engine back but can't get another one to try at the moment.

useless

Your poping sound is typical to a car that is not getting enough fuel at that speed .I would recheck fuel pressure under load on a dyno .It is a lean pop or the carb is not letting enough fuel in.With the holley carbie I am pretty sure that the float adjustment helped this problem.

brenx

Checking fuel pressure on a dyno's a good thing but a dyno won't simulate the g-forces of a launch. I had that issue with mine. It was low on the dyno but still didn't completely drop. On the track/road it completely dropped.

[edit]On a strong motor a mech pump usually can't keep up around the 5000-5800rpm range.

Brenden

__________________
XB Fairmont (street car) 11.07@123.02mph.

C9H14N

I am in Darwin and don't have access to a chassis dyno so I have to rely on the seat of my pants at this stage. I am running a Holley red pump which appears to be keeping up. I have tested the on road pressure by running a fuel line from a t-peice onto a guage that I have in the car previosly and only found about a 2psi drop at WOT. I may have to check this again.

useless

It sounds like your fuel bowl is emptying out at 5000rpm and you are getting the lean misfire or pop sound.I cant remember how to fiw this.But if you adjust the float level a little bit you can always put it back after.It could not hurt that much.If it fixes the problem then thats a bonus.If you go too big on the jets it may stuff low down mixtures ..

xbhoon

Hi
Did You Check The Phasing Of The Dizzy When Installing The Crane Gear Cause It Sounds Like At High Revs The Spark Discharge May Be Going To The Wrong Cyl , & That May Be Due To The Dizzy Not Being Correctly Phased

Cheers Pete

C9H14N

Pete

Thanks, but you have lost me. Can you tell me more about the phasing of the Dizzy. I have not heard of this before.

xbhoon

hi again
type in phasing the dizzy in google search engine & read all about it , the quick version is, the timming between when the dizzy fires & the position the rotor button is in , This is not normally a problem but since the dizzy conversion was done this just may be your problem . I found out aboout phassing when i efi'd my clevo & had to shorten my dizzy by a inch & made it solid drive , no advance as the computer does the advancing
steep learning curve, But i found out about it on the net & also found out how to check it
hope this helps, let me know how u go
pete65@iprimus.com.au

cheers pete