HI well here is the question is a holley 650 and a 750 have the same size metering plate will they work on bith carbs and also if i block the power vale hole off what do i use the same jets on all 4 and how much or what size jets should i use i have 71 front and 78 rear jets

I tried it 1 night ago all was good with the 750 holley on and tonight when i got home started up but backfired 1 big time and then started and when i went down the street it has a bad miss to it but idels great but i have unhooked the automatic chock wire that was on my 650 could it maybe spark and cause it too miss or what yall think

Are you talking about blocking the rear power valve? If so up the rear jets approx. 8 sizes. The metering blocks will work between a 650 and a 750.

blocking the front the rear is already blocked so if i block the front what jets should i use same as the rear or go up on both

Unless it's a tuned, all out full throttle race car, why would you want to block the power valve?

well it keeps blowing it

Fix your timing issues first - sounds like you keep backfiring, which will blow the power valves on the older Holley's.

i did that last night had it at idle and at 3000 rpm total was 36* and held it to 5000 and was 50* at that rpm it idles great backfired when i started it and has a bad miss under a load down the street maybe i unclipped a plug wire on the distrubtor i;ll check it tonight

At 3000 all your timing should be in, it shouldn't continue at 5000 rpm. Get your distributor curved have all your timing in at 3000 rpm. Time at your 36 at 3000. If tyour total is 50' at 5000 rpm whats your initial at idle? That might be your back fire problem.

Dave

i think it was 14* at idle

you are timing with the vacuum disconnected?

Dave

yes and im sorry when i timed it at idle was i think 14 * then at 3000 was 36* and when i hooked it up was 50* at 3000 to 3500 rpm the night before when i installed the carb i ran it and was great but i had not times it and just maybe after the timing i might have unpluged a wire on the cap or something i will look at it all tonight for sure its worried me all day