Mikuni Carbs

S.Nagy · Oct 2, 2022

I have a 1971 Norton Commando that I have recently rebuilt. I have gone with single 34mm Mikuni with a Tri Spark and single coil ignition. I am having problems with getting it to run right. The Carb came with a 270 main jet and #35 Pilot jet. When running the Spark plugs were very Black. I have downsize the Main jet to 250 then 240 then 200 and now I have a 180 main jet and # 25 Pilot jet. The plugs are a very light brown now. Bike starts first kick and idles good. I has a flat spot or it boggs if I open the throttle fast but is ok if I gradually increase my speed. The needle is at top ring. It does run better with the 180 Main Jet and #25 Pilot and Needle at the top. Timing is set for 28 degrees at 3000 RPM. I am using NGK7BPS Spark plugs. I do have some Champion Plugs ordered. Any suggestions. I don't know what slide is in the Carb. It is brand new. Also, now my Throttle is starting to stick.

seattle##gs · Oct 2, 2022

The common jets are:
37.5 pilot jet
6DH3 needle clip in middle or next richer slot
P-0 needle jet
#3 slide
240 main jet

acotrel · Oct 3, 2022

The main jet usually only affects performance at throttle openings above two thirds. The needle jet size and needle shape are more important than anything else. If you lower the needle by putting the clip in the top groove, you should get a gasp or cough as you open the throttle. Starting with the needle in the middle notch, raise and lower the needle one notch at a time, until the cough just disappears. You should be using the largest specified main jet when you do it. To set the idle, do similar with the mixture adjustment screw. Right across the whole range of throttle openings, you mixture should be just slightly richer than too lean. Too lean is when you get the cough..

acotrel · Oct 3, 2022

seattle##gs said:
The common jets are:
37.5 pilot jet
6DH3 needle clip in middle or next richer slot
P-0 needle jet
#3 slide
240 main jet

That should woth very well . Finding the right position for the clip on the needle is not difficult. You simply lower the needle until you get the cough, then raise it one. If you cannot get the cough, the needle jet is too big.

Craig · Oct 3, 2022

This topic has been covered a few times here , you can do a search and read all day , good luck

Big_Jim59 · Oct 3, 2022

Before you get heavy into jetting changes make sure your Tri Spark ignition timing is spot on. It needs to be checked with a strobe but your timing mark on the rotor needs to be checked with a degree wheel for accuracy. It is not unusual for these marks to be off by a few degrees.

Be careful of how your throttle cable is routed. The frame tube keeps the throttle from pulling smoothly. Back in the day we had a rash of sticking Mikuni carbs and they are not fun. It lead to my one and only wheelie on a Norton.

seattle##gs · Oct 3, 2022

acotrel said:
The main jet usually only affects performance at throttle openings above two thirds. The needle jet size and needle shape are more important than anything else. If you lower the needle by putting the clip in the top groove, you should get a gasp or cough as you open the throttle. Starting with the needle in the middle notch, raise and lower the needle one notch at a time, until the cough just disappears. You should be using the largest specified main jet when you do it. To set the idle, do similar with the mixture adjustment screw. Right across the whole range of throttle openings, you mixture should be just slightly richer than too lean. Too lean is when you get the cough..

I have always used the standard Sudco jetting, seems to work well. Do you favor another needle? And the needle jet that goes with it?
The Atlas is my main bike and all testing goes through it. I currently have the needle clip in the richest position (6DH3 needle and P-0 needle jet) which would make me think of going to a P-2. There is no cough. Your method says go leaner?
The monkey wrench here is that I was running the existing Atlas mufflers made with a big baffle plate. Previously I had drilled the plate with a 3/4" hole and my testing was with this size hole. The motor would not go past 5100rpm. Two days ago I ran the biggest hole saw I could fit, 1.5" through the baffle plate, no testing yet. Ideas?

acotrel · Oct 3, 2022

seattle##gs said:
I have always used the standard Sudco jetting, seems to work well. Do you favor another needle? And the needle jet that goes with it?
The Atlas is my main bike and all testing goes through it. I currently have the needle clip in the richest position (6DH3 needle and P-0 needle jet) which would make me think of going to a P-2. There is no cough. Your method says go leaner?
The monkey wrench here is that I was running the existing Atlas mufflers made with a big baffle plate. Previously I had drilled the plate with a 3/4" hole and my testing was with this size hole. The motor would not go past 5100rpm. Two days ago I ran the biggest hole saw I could fit, 1.5" through the baffle plate, no testing yet. Ideas?

Best acceleration is usually just slightly on the rich side of too lean on the needles.. If you are too lean you will get the cough. Pistons and valves are only usually burned when the motor is being used at full throttle with the main jets too lean. If you use the largest specified main jet, you should be OK. I only ever lean off the main jets, if I want to use my bike in competition and I need more go at the ends of the straights. I don't usually do it.

You might need to check that you have the correct length needle for the size of your Mikuni carb. In 30mm carbs, I think #5 series needles are right. For 32mm and 34mm carbs, it is #6. The diameter at the parallel part of the needles is usually the same for both Amal and Mikuni needles - the tapers are different. I use 6D Mikuni needles in my 34mm Mk2 Amal carbs. I use methanol fuel, but taper I would use with petrol would not be different, - the taper you use is determined by the inlet port size.

S.Nagy · Oct 3, 2022

Big_Jim59 said:
Before you get heavy into jetting changes make sure your Tri Spark ignition timing is spot on. It needs to be checked with a strobe but your timing mark on the rotor needs to be checked with a degree wheel for accuracy. It is not unusual for these marks to be off by a few degrees.

Be careful of how your throttle cable is routed. The frame tube keeps the throttle from pulling smoothly. Back in the day we had a rash of sticking Mikuni carbs and they are not fun. It lead to my one and only wheelie on a Norton.

Thanks

seattle##gs · Oct 5, 2022

acotrel said:
The main jet usually only affects performance at throttle openings above two thirds. The needle jet size and needle shape are more important than anything else. If you lower the needle by putting the clip in the top groove, you should get a gasp or cough as you open the throttle. Starting with the needle in the middle notch, raise and lower the needle one notch at a time, until the cough just disappears. You should be using the largest specified main jet when you do it. To set the idle, do similar with the mixture adjustment screw. Right across the whole range of throttle openings, you mixture should be just slightly richer than too lean. Too lean is when you get the cough..

I would like to add another indicator.....When trying to get underway in first gear...such as at a traffic light...and there is a distinct bog which requires slipping the clutch quite a bit, then the needle jet is too small. A quick way to check is to put the needle in the full rich position and see if the condition improves.

S.Nagy · Oct 6, 2022

seattle##gs said:
I would like to add another indicator.....When trying to get underway in first gear...such as at a traffic light...and there is a distinct bog which requires slipping the clutch quite a bit, then the needle jet is too small. A quick way to check is to put the needle in the full rich position and see if the condition improves.

Thanks I will try it I will put back in a 35 needle jet and move the clip on the needle dwon one.

acotrel · Oct 6, 2022

If your carbs are jetted correctly, when the weather is very cold, you might experience the symptoms of too lean.

acotrel · Oct 6, 2022

S.Nagy said:
Thanks I will try it I will put back in a 35 needle jet and move the clip on the needle dwon one.

Only change one thing at a time. When tuning a motor, there are several variables. If you change more than one variable at a time, you will not know where you are. When your motor is tuned correctly, it should run perfectly, but lowering the needles one notch should induce a cough or a flat spot. And that is regardless of the taper on the needles. If you end up with the clip in the top groove and still don't get the cough, the needle jet is too big.

Stevo · Nov 20, 2022

S.Nagy said:
Thanks

Recently had sticking throttle slide issues on my newly acquired Mk ll 850 with new Mikuni 34mm which worsened from a cold start and provided several sphincter tightening moments. Strangely, the pre purchase test ride was fine. On investigation, twist grip and cable are new and seemed fine. Removed carb and found there was a ‘sticky’ to the touch deposit on the head side of the carb bore and on that side of the slide bore also. An alcohol moistened wipe removed this and it left a faintly black stain on the wipe.

With both bores clean and smooth and the slide free to move, I followed a tip read here and added the return spring from the Amal monobloc replaced on my Matchless. There must be better way to do this other than my way, took ages and invoked much cursing!
Initial test ride shows all is good, throttle action is smooth, not overly resistant and engine revs now fall away as needed when shifting and coming to a stop.

Thanks to forum for the heads up, looking forward to learning more about this great bike and taking part in conversations.

Steve

cliffa · Nov 20, 2022

Stevo said:
Recently had sticking throttle slide issues on my newly acquired Mk ll 850 with new Mikuni 34mm which worsened from a cold start and provided several sphincter tightening moments. Strangely, the pre purchase test ride was fine. On investigation, twist grip and cable are new and seemed fine. Removed carb and found there was a ‘sticky’ to the touch deposit on the head side of the carb bore and on that side of the slide bore also. An alcohol moistened wipe removed this and it left a faintly black stain on the wipe.

With both bores clean and smooth and the slide free to move, I followed a tip read here and added the return spring from the Amal monobloc replaced on my Matchless. There must be better way to do this other than my way, took ages and invoked much cursing!
Initial test ride shows all is good, throttle action is smooth, not overly resistant and engine revs now fall away as needed when shifting and coming to a stop.

Thanks to forum for the heads up, looking forward to learning more about this great bike and taking part in conversations.

Steve

Welcome Steve!

dobba99 · Nov 20, 2022

I wrote down the jetting in the single VM34 Mikuni after i took it off to change to an SU
Throttle slide 2.5
6DH2 Needle
240 Main Jet
159 P-O Needle jet
Air jet 2.0
Pilot Jet 35

I would use the cable cold start device attachment, available for the mikuni, rather than the lever cold start that is usually supplied. It can be connected to the choke lever so gets rid of the fiddling under the tank to 'take the choke off' that is a pain with the lever one

concours · Nov 21, 2022

Stevo said:
Recently had sticking throttle slide issues on my newly acquired Mk ll 850 with new Mikuni 34mm which worsened from a cold start and provided several sphincter tightening moments. Strangely, the pre purchase test ride was fine. On investigation, twist grip and cable are new and seemed fine. Removed carb and found there was a ‘sticky’ to the touch deposit on the head side of the carb bore and on that side of the slide bore also. An alcohol moistened wipe removed this and it left a faintly black stain on the wipe.

With both bores clean and smooth and the slide free to move, I followed a tip read here and added the return spring from the Amal monobloc replaced on my Matchless. There must be better way to do this other than my way, took ages and invoked much cursing!
Initial test ride shows all is good, throttle action is smooth, not overly resistant and engine revs now fall away as needed when shifting and coming to a stop.

Thanks to forum for the heads up, looking forward to learning more about this great bike and taking part in conversations.

Steve

Welcome.
Sticky stuff is bad.
Do you have a fiberglass fuel tank?

SteveA · Nov 21, 2022

seattle##gs said:
The common jets are:
37.5 pilot jet
6DH3 needle clip in middle or next richer slot
P-0 needle jet
#3 slide
240 main jet

And the common air jet is?

...typically a 2.0 and wrong!

SteveA · Nov 21, 2022

S.Nagy said:
Thanks I will try it I will put back in a 35 needle jet and move the clip on the needle dwon one.

#35 is the pilot jet, the Needle jet mentioned is 159-P0, it is probably close, but I would probably be trying a P2 with the needle in the middle clip.

SteveA · Nov 21, 2022

S.Nagy said:
Thanks I will try it I will put back in a 35 needle jet and move the clip on the needle dwon one.

You are definitely confused, the #35 is the pilot jet.

Mikuni Carbs

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