Sorting out possible pilot jet problems

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Joe Schlaberdowski

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Once started and warmed up my '74 850 runs fine. I set idle at no more than 1000. I have adjeysted the 34 Mikuni pilot jet adjusttment for a smooth tick over at that rpm. When warm it starts fine with the enrichment circuit off. One kick, maybe two. Cutting it off as it sits there idling at 1000 the plugs a a medium to light chocolate brown. But starting it cold is impossible unless, after a few kicks, you take out the plugs and clean them. It won't start off the enrichment circuit when cold and with the enrichment circuit on, the plugs foul -- black and slightly wet. I've thought of two possibilities: 1) the pilot jet is too large even though I can adjust the pilot screw for good idle(can that affect the enrichment circuit?) and 2) could the plugs bee getting oil fouled by oil leaking past the guides frroom sitting overnight?
 
I use Mikunis and like them very much. I have noticed that when the motor has cooled a bit it becomes harder to start. What works is to give the idle adjuster one or two turns of the wrist and then it usually starts right up. Why this works when turning the throttle doesn't is one of those mysteries. Then turn it back to the idle you like. I have found that most people are very reluctant to turn this screw, I do it many times a day and consider it another one of the good features of a single carburetor.
 
"I have found that most people are very reluctant to turn this screw, I do it many times a day and consider it another one of the good features of a single carburetor."

"good features?" :rolleyes:

Is that a typo where you meant to say "PITA?" ;)
 
www.accessnorton.com

74 850 very hard starting

Just got a long put off '74 850 running. Don't know the bikes history or even if it was newly rebuilt and not run in. That info is not available. It is pretty hard to kick over though. Once running it idles ok on a single Mikuni with lever down when cold, then ok on lever up when warm. And...
www.accessnorton.com www.accessnorton.com
 
“ Cutting it off as it sits there idling at 1000 the plugs a a medium to light chocolate brown.”

Are you running AVGAS? Racing fuel?
Unleaded pump gas does not color the plugs like yesteryear (leaded fuel) until it’s WAY TOO RICH. Plug chops at idle are useless as well.

The Mikuni enrichener relies on the throttle slide being CLOSED to properly provide the cold starting mixture.
Resist the urge to crack open the throttle while kicking.
When you hear the first piston or two begin to fire, ok to crack open the throttle and the engine should pick up speed.
 
Triton Thrasher said:
Try doing your cold start with enrichment on and the throttle very slightly open.
Click to expand...
What does it indicate if it does start better?
I only ask as my t160 with mikunis will only start with full enrichener and a bit of throttle
If you don't open the throttle it will not fire at all
 
baz said:
What does it indicate if it does start better?
I only ask as my t160 with mikunis will only start with full enrichener and a bit of throttle
If you don't open the throttle it will not fire at all
Click to expand...

My theory is that it means the cold start jet is too big.

But- on my Amal Mk 1.5s, I found that a cold start jet that allowed easy starting by proper technique with closed throttle on a very cold morning was then too rich on a slightly cool morning.
 
Triton Thrasher said:
E


The cold start mixture it is providing may be too rich.

Opening the throttle a very little, for kickstarting is a cheap and easy test.

It worked for me.
Click to expand...
I understand.
But, in my experience, going from a cold, “dry” engine, most of the engines go through a brief moment when the mixture is just right, on the way to flooded. Catching that moment defines a rider from an athlete doing kickstart exercises.

JMWE
 
concours said:
I understand.
But, in my experience, going from a cold, “dry” engine, most of the engines go through a brief moment when the mixture is just right, on the way to flooded. Catching that moment defines a rider from an athlete doing kickstart exercises.

JMWE
Click to expand...

Yep. It worked for me on my bike, but not for you on your bike.
 
Question, I assume that the Jap carb has a pilot circuit like or similar to the Amals type; ie there is a small bore hole going from the bellmouth that is drilled to the top of the pilot jet body which comes out past/ downwind of the throttle slide?
If so, where they clear?
 
Bernhard said:
Question, I assume that the Jap carb has a pilot circuit like or similar to the Amals type; ie there is a small bore hole going from the bellmouth that is drilled to the top of the pilot jet body which comes out past/ downwind of the throttle slide?
If so, where they clear?
Click to expand...
This is a reply to all actually. A question. I have probably tried every combination of throttle cracked, closed, enrichment circuit in, enrichment circuit off, that can be tried. With no seeming difference. What makes a difference (and only after kicking the hell out of it) is removing the plugs (now sooty and wet) and brushing them off. Then it starts right up. The plugs are dry and not sooty when it has been running and then shut off (of course). I will check results when shutting off, say overnight. And then after about two kicks. But an earlier question remains: could a wrong size pilot jet have this effect?
 
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