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ponchopwr70
09-26-2006, 05:13 PM
Sorry if this is long but here it goes,
My combo goes like this 1970 lemans, 3800lbs with me (just a guess), chevy 388, 10.8:1 static cr, ported and polished 2.02 heads, cam is 235@ .050, 507lift, 108 ls,rpm range 2200-6200, 69 z-28 intake port matched to heads. Timing is 13 initial and 34 Total anymore advanced and it will ping. Also rebuilt hei, accel module, hypertech cap, rotor, no vacuum advance. (I tried vacuum adv. and it pinged real bad, so I then retarted till no pinging was herd, ran good on lowend but poor on top.) Th350, 2400 stall, 3.55 gears 26" tire. This is a street car. First carb was an out of the box holley 750 vac. sec. Hesitation when I would get on it (seemed like a lean condition)everytime and very week topend. My engine builder (he is 52 and a very good friend) Gave me an old race carb (because hes not a fan of vac sec. carbs.) He says its and 850. Its a holley double pumper, choke tower milled off and bowls are cleaned up. It has 76jets front with 6.5pv and 93's in back no pv. 30cc pump in front with the white cam and 31 squirter. 50cc pump back with dark colored cam (aggressive) and 32 squirter. Heres the problem, when I'm cruising 1800rpms and get on it a little, it goes up to 2100rpms then hesitates and jumps to 2400 rpm, then everything is smooth. Top end is awesome, the motor revs quick there is no problems when the secondaries come in, no hesitation just revs quicker. This happens in every gear. If I deck it while I'm cruising it will sputter and almost die then kicks in. I pretty much drive around keeping the rpms up or feather it to get them up.How can I fix this? These symptoms are getting worst now that temp is dropping out side, (like 50 degrees out) engine stays around 170,180. Tonight I footbraked it to get the rpms up and wheels to spin and it sputtered, (doesn't do this when its 70 out) then the tires spun and it ran extremly well even on top. Does this carb sound to big? Should I try to improve on my 750vac sec. carb with a sec metering block, secondary springs, and jets? Sorry this is so long just wanted to give enough info. Thanks in advanced.

ponchopwr70
09-26-2006, 05:22 PM
Also forgot to mention Timing comes in really quick I think total is around 2500 rpms and vacuum at idel in drive is 11.

Tech @ BG
09-28-2006, 07:14 AM
An 850 would be extremely big on a combination like yours which is going to cause the engine to actually run lean at part throttle, and WOT. The smaller carburetor would be a better choice for your combination. It sound like you've had to lean out the carburetor too much due to your timing in order to get it to perform properly. You should try to recurve your distributor so you can get a little more initial timing, but still keep your total at 34. This should allow you to richen the idle back up on your 750 and help your drivability.

Shaker455
11-14-2006, 06:25 PM
Frank,
I can set you up with the right carb for you car,

I build/ modify custom Holley/Demon carbs and tune.

Send me an email
Jeff

Railing68
11-14-2006, 07:51 PM
I don't think you have to spend much on the carb:

My combo is similar with a 406 and better heads but I have and 9.8:1.

PV is 6.5( manual trans) 28 squirter front and 30 in rear. Vac is 12 with no load at idle.

Mine would hesitate at around 2k with a smaller PV. Hesitation from lean condition upon acceleration. What I am getting at is you might need different PV to richen earlier. You just don't want it open during cruise for obvious reasons.

I would first call dave from (daves small body heis) tell him what you have and see what he says, he is really honest and totally reasonable on the recurve. He also knows Holleys and re-bushed mine so see what he says.
http://www.davessmallbodyheis.com/

Your higher comp and combust chamber design differ but he'll know what you should do.



Good luck, SS.

ponchopwr70
11-16-2006, 05:44 AM
Thanks for the replies. I have changed out that carb and put on the 3310 750-vac sec. I put 73 jets in the front, bought a metering block for the back with 80 jets. I have the 6.5 pv and 31 squiter. Ihave the lighest spring in the secondaries. Throttle response is really good. I ran 12.9 at 106 in the quater with this carb and my airfuel ratio was between 12:1 to 13:1 on the dyno. But the top end of this carb does not compare to the 850dp. I think next year I'm going up on header size, 1.6 rockers, 2800stall, and hopefully a better carb.

Tech @ BG
11-16-2006, 05:57 AM
But the top end of this carb does not compare to the 850dp. I think next year I'm going up on header size, 1.6 rockers, 2800stall, and hopefully a better carb.


Are you still comparing a desk top dyno figure or an actual engine dyno?

ponchopwr70
11-16-2006, 02:34 PM
Actual dyno, I don't have a desktop dyno. I'm just just talking seat of the pants feel between the two. Here's my dyno chart with my current set-up.
https://static1.pt-content.com/images/noimg.gif (http://i110.photobucket.com/albums/n92/ponchopwr70/not-enoughHP.jpg)