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850 Brawler carb tuning

Posted: Wed Jul 21, 2021 9:16 pm
by runawy9
Need some help fellas.
850 qft brawler 75 main jet 85 secondaryjet
383 sbc
11.7 compression
Motown single plane intake
Lunati voodoo solid roller .605/.605 254/258@.050
Afr 210 heads
Msd ignition locked out at 36
Full throttle and cruise it seems fine. If I hold the throttle right at half or just before the back barrels kick in it goes flat and start breaking up until I let up or mash it down. Ive changed the hsab from stock (.032) to .036 with no change. Could it need to be fattened up?

Re: 850 Brawler carb tuning

Posted: Wed Jul 21, 2021 9:23 pm
by 1980RS
My buddy had one of those QF 850's on his car and had to drop his rear jets by 5-6 to get it straightened out. Only thing to do is fatten it up or lean it out to see what it does. Just keep an eye on the plugs when tuning.

Re: 850 Brawler carb tuning

Posted: Wed Jul 21, 2021 10:02 pm
by runawy9
Could it be the power valve maybe?

Re: 850 Brawler carb tuning

Posted: Wed Jul 21, 2021 10:20 pm
by ClassAct
The bigger MAB is delaying the mains activating. You went the wrong way. Also set your power valve opening by cruise vacuum, not idle vacuum.

Re: 850 Brawler carb tuning

Posted: Wed Jul 21, 2021 11:17 pm
by rustbucket79
The bigger bleeds leaned the mixture, the way you describe it sounds like it’s lean so you could try .005” smaller bleeds than stock and try it.

You could try tuning with the power valve, but I would plumb in a vacuum gauge and try it first. If you’re pulling 13” of vacuum in that “unhappy” area, a power valve won’t fix it.

If I had encountered that problem I would have started with a pair of 77’s in the primary side as my first change.

Re: 850 Brawler carb tuning

Posted: Wed Jul 21, 2021 11:41 pm
by 1972ho
Seems like one thing you did when you half throttle it the squirters have already been kicked in so you get no fuel from them and the power valve restriction may need a bigger restrictor.Have you tried doing the same thing from a dead stop to see if it does the same thing