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Fouling plugs at speed. Why?

Sac '68 sport

Bronco Guru
Joined
Apr 9, 2004
Messages
1,176
Loc.
Orangevale
fairly new, less than 3K miles, 302, RV cam, larger exaust valves and some porting, unilite, new wires and plugs, performer 289, and a 600 cfm vac. secondary Holley that I just rebuilt. C-4, 4.10's, and 32's.
Problem is I'm carbon fouling plugs (black soot but only on the rear half of the motor) at speed, 2800-3200 RPM when going from near sea level up to the mountains around 7000 ft. Runs fine down low but might be a little rich, and It doesn't seem to foul plugs once I'm up at altitude, but it doesn't run great up higher either, sputters and stalls out when I first put on the gas, idles fine then once I'm on it it seems to be ok, not great, just ok. Seems like it's running rich.

Jets, power valve???

Anyone know how to check what power valve should be in there, and would that change with altitude?
Is there a formula to how many jet sizes you should go down per Xft. of elevation or is it a trial and error thing?
 

Skuzzlebutt

PhD, Dr. of Broncology
Joined
May 26, 2001
Messages
4,393
Loc.
Honeymoon Bay
Someone posted recently a rule of thumb to go down 2 jet sizes per 1k'.
I haven't heard anything about power valves, but there must be some similar rule.
 

broncnaz

Bronco Guru
Joined
May 22, 2003
Messages
24,341
This is more or less normal you cant expect to go from sea level to 7000 ft and not run rich one other varible is the timing it usually helps to advance it slightly (I think) at elevation. you will lose some power ar elevation which is another reason it doesnt perform well.Rule of thumb is go down 2 jet sizes for every 1000 ft rise in elevation but if you only wheel at elevation for a short period of time its not worth the effort of rejetting every time you go up or down holley does sell a kit that has externally adjustable main jets that could help if you do a lot of weekend mountain runs then back to sea level for the week. Power valves could also be swapped as engine vacuum tends to be lower at elevation but once again its a PITA to do this everytime. The power valves that come stock(6.5hg opening point normally) usually have a low enough range that elevation is not a problem.

Get the jetting as lean as you can at sea level it might help a little. Also if running in the 2800-3200 rpm range the secondaries may be opening and a lot of 600cfm carbs have very rich secondary jetting. What model number is the carb? we can look up the jetting the metering plate has. might need a different plate to lean it out. As for just the back half of the engine being rich that could be poor manifold flow hard to say.
 
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