Holley 750 backfire.

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Ajp440 said:
It doesn't seem to matter what speed I'm going if I'm easy on the throttle it will go threw the gears and drive like normal. Wether at 5mph or 20 or 30 if I lay into it it spits and pops through the carb. But when I'm sitting in my garage I can snap the throttle and it's clean and crisp.
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Not trying to Hi-Jack the thread but I have the exact same issue right now. It started slowly and is getting worse. Part throttle and it rev's with no misses. As soon as I lean on it harder it gets very rough (missing), almost sounds like it's hitting a rev limiter until I back out of the throttle and it's fine again.
In park it rev's perfectly ! Did it with my 750 Holley (Vac Sec) so I changed to a Holley 650 DP I have and it was a bit better but sure not fixed.
Timing is about 18 at idle, Vac. is disconnected. Electronic Ign with MSD coil.
I'm starting to think it could be a grounding issue somewhere. The hunt begins.
Good luck with yours, I'll let you know if I find anything.
 
Exactally what mines doing! Yeah let me know and I'll share any progress I make too.
 
Cam timing shouldn't be a issue right? Especially if it revs in park good. ?
 
Ajp440 said:
Cam timing shouldn't be a issue right? Especially if it revs in park good. ?
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No, mine didn't do it at all at the beginning of the summer. It was a fresh, complete rebuild that was running very strong. This was a very gradual issue that just got worse.
 
It has to be something that comes into play while the motor has load on it from driving.
 
Ok do you have an adjustable timing light at what rpm does it start to advance? any chance you have a intake valve sticking?
 
As soon as I start to rev it it starts advancing. Is that normal? Yes I have an adjustable timing light.
 
Sounds like your ignition coil is starting to crap out. You are running a ballast resistor, correct?
 
I did a 440 w/285* solid and tried .509 before that, same intake as yours and 3200 stall w/4.30 gears....it liked the 750 with 28 shooters and red cams, 71 primary and 82 secondary...PV should be half or just above half of the manifold vac reading in gear foot on brake.

as for the staggard jetting, maybe he though that was the fix for a bad intake it may have once been used on.lol...or maybe he flowed the carb and found it to be out of spec...who knows.
 
No I havnt, it's brand new and I got it screwed into the firewall. When I bought it I bought 3 just to have extras. Would those usually crap out with a load in gear but act fine in park or neutral?
 
10'' manifold vac is really high for that cam, is it some redesign version? or do you have it in 8* degrees advance?

sorry, Im late and trying to catch up.lol
 
Nope the cams straight up dot to dot. Readings were with my edelbrock dual plane. If that means anything lol
 
I'm gonna buy a new coil and test it out I heard they can work ok but act funky under a load.
 
Ok I havnt touched the car in a couple days but I completely lost all spark. So after some days of google searching I seen some pick up coil testing being done so I pulled out the distributor and did some testing. I'm getting 458 resistance were I read should be around 800 but 600 to 1500 is ok. So it failed that test. Then spinning the distributor by hand I'm supposed to get atleast 1v ac reading and it giving me .568 and won't produce 1 volt. So I'm guessing the pic up coil is bad on it. Gonna buy a new one tomorrow and let u all know how it goes.
 
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