ok vintage Mikuni Gurus....

D

dirtkatt

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Jurassic Park (78 440 SRX with some mods) bogs a little on take off, and seems to run out of steam after 5 seconds of wide open throttle. I'm gonna attribute the off the line bog to the belt being a little worn, (lays an 1/8 in low in the secondary at rest) the upper end lay down is either a vac leak, (replacing every hose in it later) weak fuel pump, etc or main jet sizing or adjustment.

I was thinkin a little carb adjustment might be in order. well got to lookin, (as I am wildly unfamiliar with Mikunis, knowin just enough to screw something up big time)

at the bottom of the pot, there's a white knob with a hose that runs up and around to the center of the carb, withan arrow that says 'rich' (pic)

kind of a head scratcher for me.what does this do?

20110113183254.jpg


Assuming this one is the idle air bleed:

20110113183310.jpg


and we have the idle mix adjustment on the side of it.


Anyone shed any lite on the big white knob?
 

The big white knob actually adjusts the main jet. Its good for about a jet size give or take a little bit. clean the carbs, make sure to replace the o-ring between the tube and the float bowl, as it will barely run if that o-ring is bad. What is in it for jets? how do the plugs look?
 
phazerguy said:
The big white knob actually adjusts the main jet. Its good for about a jet size give or take a little bit. clean the carbs, make sure to replace the o-ring between the tube and the float bowl, as it will barely run if that o-ring is bad. What is in it for jets? how do the plugs look?
Click to expand...


plugs are fair....if you attempt to 'trail ride it' yeah 9's are gonna get changed it's a fer sure....

also, i'm reolacting the fuel supply lines. this sled is running dual K&N's....i did the vac and from pump to case, and from the pumps to carbs, and i am getting some aeration in the tubes that clears up so I have a bad seal somehwere on them.....

thing not to do? twist the air bleeds out, (idle gets too high) or the side bleeds in, too rich....

guessin that on the big white knobs, a little goes a long way......
 
Those carbs need the stock airbox to work correctly, the power jet system wont work well with open carbs or k&n's. The white main adjuster knobs from full rich to full lean is about the equivalant of changing one main jet size. If the sled has stock jets, your probably safe to turn that toward the lean side.
 
FIRST OFF YOUR SRX IS A 79. SECOND THAT KNOB IS A ENRICHNER AND IS EQUAL TO A JET SIZE THAT FEEDS THE POWER JET,THESE ARE FACTORY POWERJET CARBS. I HAVE A HUNCH FOR YOU, RUN IT RICHER, ALL THE WAY. NOW 99.9% OF YOUR BOG IS THE WAY THAT BELT IS SITTING IN THAT SECONDARY. THAT SMALL SCREW YOUR POINTING AT IS YOUR FUEL/AIR SCREW. 3:16 (yammie tony)
 
fro, experiamnce if you want to run kn uni filters the trick is to try and run a piece of radiator hose from a car inbetween the filter and the carb to get the ramming affect that powerjet needs to meter that fuel you can also use pvc pipe and a t fitting from one carb to the other may make or break a ride from running rich or you chasing your tail sometimes a few inches of hose is all you need to start correcting a problem
thanxz
 
YAMMIEGOD3:16 said:
FIRST OFF YOUR SRX IS A 79. SECOND THAT KNOB IS A ENRICHNER AND IS EQUAL TO A JET SIZE THAT FEEDS THE POWER JET,THESE ARE FACTORY POWERJET CARBS. I HAVE A HUNCH FOR YOU, RUN IT RICHER, ALL THE WAY. NOW 99.9% OF YOUR BOG IS THE WAY THAT BELT IS SITTING IN THAT SECONDARY. THAT SMALL SCREW YOUR POINTING AT IS YOUR FUEL/AIR SCREW. 3:16 (yammie tony)
Click to expand...


Tks, Tony. tits on with assement of the belt, We had pulled the secondary apart, and wound it one rotation tighter, Replaced the belt, Went From NGK 9's to 8's, 3/4's of a turn out on the mid screw, (main was out but way too fat) tickled the air bleed. bog is almost non exitstant....it hits full song in about 6 feet.....idle's nice, plugs perfect. (have 7's in the box in case....)
 
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IMO, it's never a good idea to try and tune a carb problem with spark plug heat range.
If you need to change heat range, the carb is not setup. The $2 spark plug fix may cost ya $500+ with pistons and cyl re-nic
 
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