850 low RPM

The 850s I work with all have the same problem.......when shutting off the throttle, the motor is slow to reach an idle. This occurs from 3000 downward and actually seems like the slide is slow to return. With a 750 this doesn't happen. At around 2500 there is a "shudder" and it's become annoying passing through this point every time. Most 850s I work with have a single Mikuni, and I don't remember if the problem exists with dual amals. So what kind of carburetion do you have and do you experience this? I've tried different pilot jets and different slides with no real change. Currently I use the same jetting as the 750 which works well. The slide is a #3. I consider it slightly lean but a 2.5 will foul plugs quicker but gives better low end response...a compromise.
Is it possible that the balance factor for an 850 using isolastics produces this small problem? My next idea is perhaps the motor would benefit from a pair of JS lightweight pistons and the shudder will possibly disappear.
To sum up...is it a carb problem or a reciprocating mass problem?

I don't and never had this issue on an 850 with twin amals, which suggests its not balance factor (which is the same as the 750 anyway), the crank weight will be slightly different to compensate for the heavier pistons so likely more is taken off the 850 crank so the overall total ends up the same.

I currently have 2 ’74 850’s. One with a Mikuni and one with Amals. I had a ’75 in the past with Amals. I have never experienced
what you describe. I do know that the Mikuni throttle return spring is not nearly as strong as those in Amals. Perhaps that in
conjunction with cable routing/friction and lightweight slides contributes. I vote for a carburation issue.

A weak idling mixture can cause revs to lag a bit before getting down to idling speed, but what you’ve got there sounds more severe.

Hang throttle is common on Mikuni carburetors. Check the enrichener plunger seal, confirm idle passages are free, including the air passages, and pilot jet settings.

When I first bought mine it shuddered / vibrated so badly when decelerating down through 3k rpm that I thought something was coming loose… like the engine!

When I rebuilt it, this disappeared completely.

The trouble is, I changed so many things at once that I don’t know what fixed it !

MKIII 850 here, have not had that problem unless the carbs needed some kind of work.
Sounds like the idle mixture might be too rich, I have seen that issue before.

Was that with a Mikuni or Amals?

You could put a stronger spring in the slide that will shut it down quicker, seems it's a slow return slide or not jetted right, never had this problem with twin Amals or PWK carbs I ran for awhile, if the spring in the slide can lose its tension or the cable not running straight, but I say carb problem first before spending big bucket of money.

Ashley

Mk3 850 with a single carb, but a mk2 Amal. No throttle lag.