I have had a Mikuni VM on my 69 750 Fastback for about 12 years. Years ago I dialed it in an it ran great. About 5 years ago I put the bike aside due to health reasons. This year I cleaned the carb, used the enricher and the Fastback fired right up and idled at 2000 RPM with the enricher on. When I take the enricher off the bike will not idle and dies. It will always start up after but will not maintain an idle off the enricher. Any suggestions?
Mikuni VM idle
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Nater_Potater
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Idle jet. They're tiny, as I'm sure you've noticed. Pull it out and drop it in some good carb cleaner, followed by a good blast of compressed air. The fuel enricher adds enough fuel and air that it'll completely mask a plugged idle jet. I have to be very careful to drain the carbs on my wife's bike in the Fall, or we'll be pulling carbs the next Spring for the very same thing.50vincent said:
Nathan
Nater_Potater said:
+1
Yep, as Nathan says, no idle circuit because the pilot jet is plugged. The only thing I'll add is that the green crud that forms in the tiny orifice of the pilot jet can sometimes be difficult to remove with solvent alone. If you've got pin gauges they might come in handy to clear the orifice, or the small wire attached to common hang tags used for ID or pricing is an appropriate size for the job (< 0.015" dia?).
concours
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If you mean Pilot jet, probably not plugged (it's a relatively large hole, as compared with Amal idle passage, or the Mik drillings in the bore). The most common blockage on the Mik are the tiny drillings at the bottom of the bore. I use a wire plucked from a wire brush held in needle nose vice grips. Reach in and poke out all 2 or 3 of the holes. Verify flow with aerosol solvent. It must stream from all holes.
Also, a thorough (read that "do it once and be confident it will run") Mik cleaning includes dismantling needle jet, cleaning it's cross drillings and all features, main jet, cleaning and VERIFYING flow of ALL passages, including air from intake horn. Remove idle mixture screw and VERIFY FLOW to pilot jet well and discharge holes in the bore using spray solvent.
Also, a thorough (read that "do it once and be confident it will run") Mik cleaning includes dismantling needle jet, cleaning it's cross drillings and all features, main jet, cleaning and VERIFYING flow of ALL passages, including air from intake horn. Remove idle mixture screw and VERIFY FLOW to pilot jet well and discharge holes in the bore using spray solvent.
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Nater_Potater said:
+2
The idle/pilot/slow jet is not worth cleaning; you spray it, blow it out, ultra sonically vibrate it, but you'll really never know if it is thoroughly clean to its original size, but you'll think it is and go on to mess up something else.
When I sell a Mikuni seal kit I always sell an idle jet, they are some of the cheapest insurance available, and they rarely last more than 3-4 seasons with our drunken fuel, certainly there are exceptions, but having a few idle jets in your spares bin will save you plenty of time when your idle circuit fails.
Nater_Potater
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My 34mm runs a #32.5 pilot jet which, at .0128" diameter, is smaller than a #80 drill bit (the smallest in my set) and substantially smaller than the passage through the carb body. If the discharge hole was smaller than the pilot jet, it would be the restriction, rather than the pilot jet.concours said:
That being said, I won't discount problems arising from ethanol as far as corrosion and the like. And, in my earlier post where I suggested carb cleaner, I should have detailed the nasty carb dip that'll eat just about anything. The solvent-based "cleaners" are no good against well-cured varnish, whereas the carb dip is. Just don't throw in your plastic floats and Viton-tipped needles!
Nathan
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