If your Zenith carb doesn't idle well and you have to keep the choke out to
run at idle or low speed. Pop the plug out of the idle jet seat and back out
the idle needle-valve a bit (with a small screw driver) until the idle is
smooth, and it will alleviate the problem. The early M151 carbs came with
long adjustable idle needle-valve on top at the base of the carb (where it
attaches to the intake manifold). Later with fuel emission modifications
(and in all complete overhaul kits) the idle needle-valve was shortened,
preadjusted and then capped so it could't be changed. This results in many
carbs running too lean and not idling well, requiring a bit of choke to
richen up the mixture to run smooth (especially at higher altitudes). I put
a long adjustable needle-valve on my carb (take-out from an early M151 carb)
and adjusted it for a smooth idle. I no longer have to choke it to get it
started nor run with the choke out. On newer carbs, once uncapped you can
get at the needle-valve to adjust it with a small screw driver) and its not
necessary to put the cap back in. Your carb will run a little richer and use
a little more gas but you will get a smoother running engine that doesn't
require you to run with the choke out. Better yet, take out the short
needle-valve and put in a long adjustable needle-valve (from one of the
older carbs) that can be adjusted by hand. An adjustable needle-valve is
handy for adjustments required for a smooth idle at different altitudes.
Another point is when cleaning or overhauling the M151 Zenith carb. The idle
needle-valve cap should be removed followed by removal of the needle-valve
and the idle jet and the idle flow channel should be flushed to remove and
foreign material that could be blocking fuel flow to the idle jet. Then
replace the old needle-valve with a new one from the kit and don't replace
the cap/plug, or replace the short needle-valve with a long adjustable
needle-valve from an older carb.
Though I haven't found it necessary, I know one Mutt owner who reams out the
idle jet a little to get a richer flow.
Just my two cents for what its worth.
Joe Bigelow
MVP #19770
M38
M151A1
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