Stumbling at take off

My 73 850 Interstate with points and single 34mm Makuni stumbles badly between approximately 1200 and 2500 RPM. It always did do this a little but now it is getting worse and to the point of making an unpleasant ride. Fiberglass tank has been sloshed (two coats) with Caswell's epoxy stuff so it is not that. At idle it is absolutely fine and from 2500 up it runs just great. I had a mishap on a ride lately where I couldn't shift much and it seemed like if I buried the throttle, the stumble slowly went away. At idle the carb seems to be a little on the rich side. It starts first kick or two when cold but the motor won't run at all with the choke on. Since it won't run with the choke on, it is slow to warm up. Even though it starts easy when cold, it can be a bear to start after it is warm but other times, no problem at all. As far as the carb, I have only cleaned it. The carb specs are

Main 220
Pilot 35
Needle 6DH2
Slide 2.5
Needle Jet P-2 (I believe)

Thanks for any help on this.

Mine does that if I've just washed it and the 'waterproof' tickler has let water in again. Symptoms are consistent with jet blocking.

The other option is a Boyer with a dodgy dry contact or broken wire somewhere but this usually happens with a few more revs.

I starting blaming electrics for everything. you could have a bad ground making intermitent contact, could the main engine to frame ground,
could be your negative battery ground, could be you need new points, or you mechanical auto advance it shot, could be non charging batter.

90% of what are thought of are carburation problems end by by being solved with an electrical solution.
Check all of above, then if that doesn't make it right,check it all again?

Start with the easiest, condensers can cause this problem.

Flo said:

Start with the easiest, condensers can cause this problem.

Click to expand...

Where are the condensers located? Can't find them in the parts book.

Believe they huddle up by the coils. Attached to the same bracket. Mine had a Boyer when I got it so I am not speaking from experience.

I am looking at the Parts Book Basic Electrical Components all in the area of the coils. I see what is called a capacitor #54420128 (not the blue can) but no condenser.

Nested between the coils, look to see if there is a black rubber covered box about 2.5 or 3 inches long with two spade connectors/wires leading from it. This is the condensor pack, if you take the rubber off the individual condensors are exposed.

They are under the long rectangular black boot between the coils. On American autos they're called condensers. They store the low voltage current to the coils between plug firings.



could be worth a try. I've had the soldered "L" shaped lead (that sticks through the rubber boot) detach from the end of the condenser, giving a really bad connection.

OK, that is what the Parts Book calls a capacitor. Is it possible that this could be the problem if it only lacks responsiveness between 1200 and 2500 and everything from 2500 RPM up is just fine with plenty of power and responsiveness?

Also, is it correct that the condensers would go if I changed from points to electronic ignition?

The condensors are only needed for points, electronic ignition does not use them.

I'd go back to your carb. Mikunis can be a pain to set up but once done they're pretty good. Get a tuning manual from Victory Library - http://victorylibrary.com/MIK-BT.htm. Your settings seem a fair bit removed from the starting settings they recommend. This is what they have for a starting point for an 850 with a VM34 (and please remember it's only a starting point).

pilot jet- 30-35
slide- 3.0
Needle Jet Type- 159
Needle Jet Size- P4\P6
Needle- 6DH3\4
Main Jet- 260
Air Jet- 1.0

I had similar problems on my 750 and found I was leaving the choke on too long at warm up and fouling the plugs.

I had the same problem. On mine, it just creeped up on me, next thing I knew was stalling at every light.

I advanced the timing, somehow it had gotten too retarded. Fixed it instantly. I just did it by ear, but a light would be a safer bet.
make sure you mark it once you set it.

"
90% of what are thought of are carburation problems end by by being solved with an electrical solution."

Actually, I thought the old adage was the opposite and usually, but not always, if a problem exists at a fixed RPM it is usually a carburetion problem as ignition problems are usually evident at all RPMs. To complicate matters, running rich sometimes can disguise ignition problems.