ULH Missing

[otis71]

I have a 41 ULH that misses at low speed when accelerating, mainly when cold. I’ve fixed all manifold leaks using PEEK seals and it bubble tests perfectly. Checked float level, fuel supply and adjusted carburetor as I’ve always done on overheads. Gas caps are venting properly. Replaced points and condenser as well as the coil. I’m relatively new to flatiies but can’t think of anything else. Any suggestions?

[jorrow]

Replace the plugs.

[T. Cotten]

If it idles potato-potato-potato, Otis,...

Please speed it up to where it smooths out.

Please note also that the transition from the idle circuit to the power circuit suffers when wear from the throttle disc into the bore extends to the idle bleeds.

...Cotten

[otis71]

I will check that. Is that more likely to cause missing when it’s not fully warmed?

[T. Cotten]

The idle speed issue is straight from the MOCO literature, Otis!

Somewhere...

The bore wear issue is anybody's guess, so I just eliminated it altogether.

....Cotten

[Arley]

Could it be that the venture has shrunken in the carb causing the carb sucking air during idle? Just asking Cotten

[T. Cotten]

Could it be that the venture has shrunken in the carb causing the carb sucking air during idle? Just asking Cotten

They all shrink a little, Arley!

These carbs are forgiving, and will run with a little wear here and there, and a gap or two, but when they all add up, you get inscrutable gremlins.

So my philosophy was to get everything as close to 'blueprint' as possible (or even better), and the gremlins dropped out.

Swaging venturies is a lot more art than science, but I'd better keep the best part proprietary for now.

....Cotten

[otis71]

Can you tell me what MOCO literature is? Internet search didn’t bring anything up.

[T. Cotten]

Sorry for the confusion, Otis!

MOCO is slang for 'motor company'.

It is useful only when distinquishing from the aftermarket, I 'spose.

....Cotten
PS: The idle should not be too slow for other reasons than off-idle acceleration, Folks!
If the bike shakes at idle, you are forcing the motor to run beyond its wide range of acceptable balance.
Raising the idle to where it smooths will prolong motor life.

[otis71]

I generally try to keep idle around 1000-1100 rpm. I was taught long ago by the old timers (now we’re old timers) that any less than 1000 and oil would be deficient.

[T. Cotten]

Perhaps it does hinder oiling, Otis!

Add that to a lack of load on the motor, while leaning on the jiffy stand,... Idling really isn't what it was designed for.

(I think lots of motors were worn out at stop lights though, revving for no reason other than their idle was too low.)

A "miss" usually makes me think electrical (which returns us to Jorrow's suggestion), but if it only happens cold at low speeds, then please describe it; A 'miss' would mean the motor failed to fire. .
But does it manage to cough, sneeze, or fart?

....Cotten

[otis71]

Hard to describe. Probably not technically a miss. More like its running retarded with a lack of power only during acceleration up to mid range.

[otis71]

Also if it helps; gas consumption is high. Symptoms lead me to think it’s rich but I’ve adjusted the carburetor exactly as stated in the MOCO manual and the exhaust doesn’t smoke.

[T. Cotten]

Let's talk about 'dialing it in', Otis!

If we start at the usual turn and half out on the HS needle, and three and a half on the LS, the motor should start and run.
Then when the motor is warm, the LS can be trimmed to its highest idle, and the idle screw adjusted accordingly.

Then the HS can be adjusted at a constant speed over 35mph, by turning it in slowly until performance falters (often it is detectable in the exhaust note), and backing out until power returns.

Beware that the needles overlap, so it isn't 'finely' tuned until the procedure has been repeated for both, more than once. The LS needle on a "rich" setting actually leans the HS circuit at speeds where the venturi kicks in. Thus both needles must be alternately coaxed to find their sweetspots that pleases the other. Let us hope this will not only help with any sneezes, but improve fuel mileage as well.

....Cotten
PS: Did I cut you a float?

[otis71]

I will try this procedure. I always have done it as the manual explains-turn low in until it falters and back out 5-10 clicks without issues on overheads. Evidently it’s not working on the flatty.