850 mikuni jetting

I'm working with an 850 and a single Mikuni 34. It's currently using the standard 750 jetting #3 slide, 40 pilot, 6dh3 needle, P-0 needle jet and has run well for the last 10 years. The owner is very happy with it overall. There is something that occurs in the 2000 - 2800 range that I would like to improve. Whether rising or falling in RPM there is a shaking that occurs at this point. At 3000 there is the usual "light switch" phenomena where the motor suddenly goes very smooth and is a pleasure to ride, but that shaking is something I would like to cure. Does anyone have some different jetting specs that work well for them? The motor starts instantly and runs very well. And the #3 slide keeps the plugs running clean for a long time. I could try a # 2.5 slide and/or a P-2 needle jet.

When was the last time the isolastics were lubricated , inspected for wear and adjusted to spec ?

My Mikuni notes show :
VM34 850 c.c.
35 Pilot , adjuster at 3 1/2 turns out .
6DH3 Needle , 2 nd. notch from the bottom .
Throttle Slide 3 .
159-P5 Needle Jet.
Main Jet 250 or 260 .
Spark Plugs NGK BP6.
Air Jet 1.
( as supplied most Mikunis come with a #2 air jet . It's a common overlook by Norton owners . )

Peashooter pipes fitted .
Un-4229ST Unifilter airfilter fitted .

Good luck .

seattle##gs said:

I'm working with an 850 and a single Mikuni 34. It's currently using the standard 750 jetting #3 slide, 40 pilot, 6dh3 needle, P-0 needle jet and has run well for the last 10 years. The owner is very happy with it overall. There is something that occurs in the 2000 - 2800 range that I would like to improve. Whether rising or falling in RPM there is a shaking that occurs at this point. At 3000 there is the usual "light switch" phenomena where the motor suddenly goes very smooth and is a pleasure to ride, but that shaking is something I would like to cure. Does anyone have some different jetting specs that work well for them? The motor starts instantly and runs very well. And the #3 slide keeps the plugs running clean for a long time. I could try a # 2.5 slide and/or a P-2 needle jet.

Click to expand...

Forget about the revs unless it's an electrical issue
And concentrate on the throttle position
Mark up the twist grip to find out what circuit is causing the issue on the carburettor
It's nothing to do with the revs

Sounds like isolastics to me.

The front isolastic has been done but the rear, as usual, is 61 years old. I've recommended replacing them (I can fit a .014 feeler gauge) but I want to exhaust everything else before I take that step. The carb is jetted for a 750, I don't know if it is the same for an 850.
The above jetting by Torontonian sounds very rich with a P-5 needle jet and rich needle position The pilot air screw is turned out 3.5 turns to compensate

Isolastics die in a couple of different ways.
The end clearances get larger, witnessed with a feeler gauge.
Less obvious, internally the rubber deforms. It can go egg shaped which means the load bearing side get denser and the non load bearing side stretches. This can affect the harmonics and the vibration.

It does not sound as though it is jetting or electrical. If jetting is too lean, or there is an electrical problem, the motor will usually miss.

MAP says the jetting is the same for an 850 as a 750 except for the main jet. I've convinced the owner to replace the rear iso in a week or two. I will post the results.

seattle##gs said:

The front isolastic has been done but the rear, as usual, is 61 years old. I've recommended replacing them (I can fit a .014 feeler gauge) but I want to exhaust everything else before I take that step. The carb is jetted for a 750, I don't know if it is the same for an 850.
The above jetting by Torontonian sounds very rich with a P-5 needle jet and rich needle position The pilot air screw is turned out 3.5 turns to compensate

Click to expand...

If the rear isolastic really is 61 years old, it predates the bike by around 10 years! Either way, it's most likely past it's useful life and sensible of the owner to have it changed.

seattle##gs said:

The front isolastic has been done but the rear, as usual, is 61 years old. I've recommended replacing them (I can fit a .014 feeler gauge) but I want to exhaust everything else before I take that step. The carb is jetted for a 750, I don't know if it is the same for an 850.
The above jetting by Torontonian sounds very rich with a P-5 needle jet and rich needle position The pilot air screw is turned out 3.5 turns to compensate

Click to expand...

Agreed, P4 sounds nearer, P2 a little weak, P0 sounds way too weak and #40 pilot too rich!

Sort the isolastics first.

The needle jets need to be lean enough to bring the five notches in the top of the needles into adjustment range. If you cannot cause the motor to miss by lowering the needles, you are too rich. But never continue to ride the bike, if the motor develops a miss - always stop and raise the needles. Main jets do not do anything until you are near full throttle, unless they are too small - so err on the rich side with them. Any change you make to the exhaust system, ignition timing or compression ratio, will require a jetting adjustment. The shape of the needles caters for loss of vacuum - it affects how quickly to can open the throttle - for more acceleration lean needles are better but require more control. Rich main jets will not slow the bike if it is lean enough at smaller throttle openings. Changes to cam timings do not usually change jetting requirements.

I suggest the jetting that is suggested for Commando engines using petrol probably comes in two sizes -too big and too small. The best is in between.