New 250 Supercharges - old problem.
 

Ok now we've installed new 250 superchargers from The Blower Shop with Barry Grant Demon 850's on 454's Gen IV, .030 over on pistons and we are getting a pop on top end through carbs. Had this with Holley 750's before the blowers and ended up increasing jets to 88's all 4 corners; pop went away and we learned we were definitely lean on the top.

Barry Grants' came with #83's in primary and #88's in secondary. Changed the primaries to 88's since that solved the problem with the 750's before blowers.

Question is, what are some of you guys running for jets with similar blowers and set-up? Cams are Comp Cams with duration @ .050 Intake 230 and Exhaust 237. Could the 3/8 fuel lines now be a problem? Fuel pressure was noted at 5 psi when pop occurred at 4500 rpm. Also running stock 90 gph fuel pumps.

Any thoughts would be appreciated.

Re: New 250 Supercharges - old problem.
 

Somebody must have some thoughts on this.

Re: New 250 Supercharges - old problem.
 

The blown 454 in my 56 chevy had a similiar problem. The engine would backfire thru the carb at 5000 RPM. I was chasing around fuel and spark problems thinking that was the problem, but to no avail.
Eventually found out that the guy that put my heads together didn't know what he was doing.
Valvesprings were installed wrong height without seat pressure and let the valves float.
Bad thing was, bythe time I figured out the problem, the 5000 RPM backfires had knocked the blower vanes out of time causing them to rub into each other and wiped out the blower vanes. (Magnacharger twin supercharger, timing gears pressed on so vane timing easily messed up by back fires)

You had a backfire problem before the blowers, but jetting fixed it, now it's back with the blowers? Maybe the jetting masked a problem?

Just throwing it out there.
Good luck!
Dan

Re: New 250 Supercharges - old problem.
 

Thanks for the response Dan. At this point I'm open to checking anything out as I certainly don't want to trash two new blowers.

Re: New 250 Supercharges - old problem.
 

Couple things to consider.

You are now feeding two carbs per engine.
Your fuel supply should be enlarged from the pickup in the
tank right to the carbs. 90gph pump is whimpy.
you will want atleast 140gph. A claysmith mechanical
or aeromotive electric come to mind.

This test should be done with caution:
I always take the fuel supply off the carb and time the
lenth it takes to fill a gallon container.
in your application no more then 15 seconds.
make sure you have 6lbs fuel pressure at wot.

Check float levels.
Are you running powervalves?
Are they boost referenced?
If you whack the throttles at idle will it pop?
I like reading pugs NGK I find are the easiest.
A lean condition on a blower motor is not good.

I can check my 850 demons I ran on my old motors and see
what I was sporting,But alll engines are different.

Hope this helps.
Gerry