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Discussion Starter · #1 ·
Put a new sppeed demon 850 mech on and have had trouble tuning. 489 hyd roller 242/246 540/560 cam 10:1 comp. GM rect alum heads all new msd. 8" vac @ idle. Have not checked in gear yet. Idle is good cold or warm. Squared the idle slots and had to turn the idle screws a out 1 1/2 turns to stay idle. Timing is 18 with 36 total . Primary jets were 88 Sec 90. Motor idles good but has a popping at about 3000 before the secondaries. Sounds like a lean pop but not for sure. It will go past and clear and run great in the secondaries. Backfires through the carb when throttle is hit hard. Plugs look a little dry and black. The idle mixture screws will get rough idle more than a turn out. From 1 turn out to closed and on seat the idle will go up and vaccuum will go to 10". It idles smoothest almost all the way in. Any help appreciated. Tring again in the mourning
 

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Sounds like its going lean before it starts to feed off the main jets/boosters start coming in. Any power valves in it. Front power valve cerciut may need to be worked.
 

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Discussion Starter · #4 ·
It had a 6.5 PV in it and I put a 3.5 in and no change. Found the factory main jetting on the bg site (primary 85 secondary 93)... I tried what I had Primary 86 Secondary 93 and still popping but it seemed a little earlier than the 88 90"s. Had to go to work and will try 91's in front. I may buy/install mufflers first. It has bullets on it from race car and is loud.
 

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91s seems like to big of a jet in the primaries for an 850, do you know what size the pump discharge shooters are? I had a 850 dp on a 510 BB chevy and ran 82s in the primaries and 88s in the secondaries and .035 shooters in both, pulled 10" vacuum idle in gear so I ran 5.5 power valves in both front and rear. Ran good with that set up. That's the way I tune though, I alway's run the leanest primary jet I can, and then play with the pump shooters. I don't touch the secondaries utill the primaries are perfect.
 

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My 850 mighty demon came with 80/88. I have since changed to 82/90 and it runs fine.
 
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