850 low RPM

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seattle##gs

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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.
 
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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.
 
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.
 
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
 
Usually a lag to return to idle indicates a lean condition. Any hint of a stumble when coming off idle? That also points to lean condition. Try enriching the idle mix, how that is done on a mikuni is above my pay grade. So not overlook an air leak at carb to intake mani and/or head sealing joints.
 
What type of ignition do you have? I have seen this with some e.i. where the idle stabilisation plays up as the revs drop and the ignition is retarding to a point where it starts to advance again. This can cause the idle to remain high for a while before settling down
 
Everything is new. New Tri Spark, new carb and manifold new cable
This set up works perfectly on a 750, it's only the 850 that gives this problem. I agree that the slide is possibly on the lean side but the plugs stay clean longer with in town driving...lots of that in Seattle.
I tried stretching the spring and it might have helped a whisker but not yet sure of the results.
Next, I will install the next size richer pilot, #45, though it performs very well with the current #40. Perhaps that will take some of the leaness off the slide. It could all be in the convergence of the pilot jet, slide, needle jet and the single carb and manifold that's doing something weird together. I really need to know if the same thing happens with dual amals. When the throttle is snapped shut does it immediately return to idle or is there a slow return to idle.
 
I really need to know if the same thing happens with dual amals.
No

When the throttle is snapped shut does it immediately return to idle or is there a slow return to idle.
Yes
 
850/twin Amals/Trispark now had OEM ignition for years before that. Never experienced the problem described.
 
JS pistons and rods are not going to make any difference with the slow return to idle and lower RPM shudder unless you move to fantasy island where things like that can happen.

Could be timing, idle mixture, needle position, and/or a little air leak. Turn back the slide idle screw setting a little bit. It doesn't have to be too far toward the high idle side to cause the slow return to idle.

Lot of people make a single Mikuni work on 850's. I'm not one of them though. A single carburetor on a Norton twin engine is too much of a compromise in general for me. Plus I don't own and 850.

Twin Amals mostly just died when I came to stops, so no problem with a hanging idle. lol Twin Mikuni VM carburetors fixed that dying at stops issue. FCRs are a big step up from the Mikuni VMs, but could be kind of hard to source these days.
 

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