Mikuni on a Combat

Going Nuts!

I have recently rebuilt a 72 Combat and decided to install a 2 into 1 Mikuni kit. I have driven other Nortons with this set up and was impressed with the easy starting and smooth running. Unfortunatley I have been unable to set this thing up to run properly. She started on the third kick after the rebuild and idles and revs fairly smoothly but is running very rich, even with the mixture screw 3 plus turns out. The other problem is it just bogs when I attempt to drive it. I was able to get it going fast enough for the main jet to kick in and that seemed to be ok. The kit came with a 240 main jet with the needle on the second to lowest setting. I tried raising the setting (lowering the needle) but no improvement. I read somewhere on this site that it should have a 300 main jet and the needle should be on the highest setting (lowest position) but I tried that and no improvement. It appears that the biggest problem is in the take off and mid-range area which might indicate a needle setting. I would appreciate any input anyone might have on this conundrum!

Cheers,

Gordon

Mikini mixture screw is a FUEL screw, you went the wrong way (Amal is an AIR screw)

concours said:

Mikini mixture screw is a FUEL screw, you went the wrong way (Amal is an AIR screw)

Click to expand...

On a VM it's usually an air screw.: http://www.mikuni.com/pdf/vmmanual.pdf
Section 3-1

L.A.B. said:

concours said:

Mikini mixture screw is a FUEL screw, you went the wrong way (Amal is an AIR screw)

Click to expand...

On a VM it's usually an air screw.: http://www.mikuni.com/pdf/vmmanual.pdf
Section 3-1

Click to expand...

Well, sort of. Anti-clockwise richens by increasing the volume of mixed air delivered by the idle circuit resulting in a richer mixture supplied.

concours said:

Well, sort of. Anti-clockwise richens by increasing the volume of mixed air delivered by the idle circuit resulting in a richer mixture supplied.

Click to expand...

I don't think so? Check the air screw opening x fuel flow graph Fig.16,

http://www.iwt.com.au/mikunicarb.htm

The pilot air screw and pilot jet. The air screw can be located either near the back side of the carburetor or near the front of the carburetor. If the screw is located near the back, it regulates how much air enters the circuit. If the screw is turned in, it reduces the amount of air and richens the mixture. If it is turned out, it opens the passage more and allows more air into the circuit which results in a lean mixture. If the screw is located near the front, it regulated fuel. The mixture will be leaner if it is screwed in and richer if screwed out. If the air screw has to be turned more than 2 turns out for best idling, the next smaller size pilot jet will be needed.

Click to expand...

Thanks,

I took a look just to be sure and can confirm its an air screw. I will probably have to go down a few sizes on the pilot jet but will this help my midrange response? Is there anything else I should consider?

Gordon

Hi Gordon,
I too had trouble getting my Mikuni to be lean enough. I ended up changing the needle jet to a leaner size. I assume that the Mikuni kit you installed was Norton specific which means that it probably has a 6DH3 needle and probably a type 159 size P-8 needle jet. I ended up with an O-6 needle jet. They go O 1-8, P 1-8, Q 1-8 (theoretically). The different sizes can be had from Sudco company (website and downloadable catalog are available), and sell for approximately $15usd each.

GB

L.A.B. said:

concours said:

Well, sort of. Anti-clockwise richens by increasing the volume of mixed air delivered by the idle circuit resulting in a richer mixture supplied.

Click to expand...

I don't think so? Check the air screw opening x fuel flow graph Fig.16,

http://www.iwt.com.au/mikunicarb.htm

The pilot air screw and pilot jet. The air screw can be located either near the back side of the carburetor or near the front of the carburetor. If the screw is located near the back, it regulates how much air enters the circuit. If the screw is turned in, it reduces the amount of air and richens the mixture. If it is turned out, it opens the passage more and allows more air into the circuit which results in a lean mixture. If the screw is located near the front, it regulated fuel. The mixture will be leaner if it is screwed in and richer if screwed out. If the air screw has to be turned more than 2 turns out for best idling, the next smaller size pilot jet will be needed.

Click to expand...

Click to expand...

I stand corrected.

Thanks all,

Yes GB, the needle is a 6DH3 and I assume we probably have the same kit. I will move to a smaller needle jet. Did you also have to move to a smaller pilot jet as well? How is your bike performing in the mid range with that configuration?

Thanks again to all,

Gordon

Gordon,
In my situation, I found that I was chasing an overly rich situation across the board on my VM34. I went to a 25 pilot jet to get a normal response to air screw adjustment. In the end, I checked the float setting and found that if I lowered it I could go back to the standard jetting and needle settings and that transformed the way the bike ran. I got it from a reliable source that Mikuni's might be very sensitive to float height. Have you checked it?

Hi Bob,

Thanks for the feedback.

I have not checked the float level but will investigate that as well.

I would have thought that they come properly calibrated but never assume!

Regards,

Gordon

Assumption is the mother of all feck ups.....

step one:

take the four screws off that hold the bottom float bowl on

then take a small screwdriver and carefully unscrew the pilot, idle, jet

you should have about a number 35

anything much higher than that is likely too rich and causing the plugs to foul

regardless, either clean the fouled plugs good or get new ones

you can't set it properly with fouled plugs anyway

the mixture screw on a standard setting should be about one and quarter turns out from fully seated

check the number on the pilot jet again

while you are at it check the number on the slide, does it say 2.5?

Hi 1up,

You are correct. My pilot jet is a #40 and it is running much too rich. My slide is a 2.5.

It is running too rich at idle and cuts out on initial acceleration. I am not sure if thats because the needle jet is too lean or to rich. A gentleman on this forum was nice enough to send me some smaller pilot jets and a different needle and needle jet which I will recieve in the coming days.

I will post the results shortly.

Regards,


Gordon

Hi there,
ive got a vm36 on my std 750 commando & its never seemed as quick as when I had 30mm amals on it.
The acceleration in not as good or the top end speed. I only fitted the mikuni as the amals were worn out & the mikuni was meant to make the bike easier to start which it does seem to do.
One of the lads in my NOC branch has twin 32mm amals on his combat & I had trouble keeping up with him even though I was on my 1200 Suzuki Bandit. His bike goes like stink.
However, he has had to detune it a bit for reliability purposes.
I am sticking with the mikuni as i am only a poor brit and cant afford new amals with all the new feul lines and cables etc, but I will keep my eye on this thread to try and improve the mikuni performance.
I'm just waiting to get my refurbed forks & new dampers back from Landsdowe engineering, it should certainly be a much smoother ride.
Cheers Don

ive got a vm36 on my std 750 commando & its never seemed as quick as when I had 30mm amals on it.
The acceleration in not as good or the top end speed.

Click to expand...

correct

a properly set up twin carb will outperform a single, all things being equal, blah, blah

you never see a racing twin with a single carb, for good reason

a single carb is great only for its simplicity, don't have to fiddle with two

a twin carb set up will start just as easily, idle just as good, and because the air/fuel mixture has a straight and undisturbed path to the combustion chambers through two manifolds will make more HP

we have gone over this time and time again along with Jim's dyno tests showing the power differences

Strewth, I only repeated old stuff in case gordon did'nt know.
some people are new too the site.
sorry.

Don, My wife is also a poor Yorkshire lass :lol: ..

Don Tovey said:

Hi there,
ive got a vm36 on my std 750 commando & its never seemed as quick as when I had 30mm amals on it.
The acceleration in not as good or the top end speed. I only fitted the mikuni as the amals were worn out & the mikuni was meant to make the bike easier to start which it does seem to do.
One of the lads in my NOC branch has twin 32mm amals on his combat & I had trouble keeping up with him even though I was on my 1200 Suzuki Bandit. His bike goes like stink.
However, he has had to detune it a bit for reliability purposes.
I am sticking with the mikuni as i am only a poor brit and cant afford new amals with all the new feul lines and cables etc, but I will keep my eye on this thread to try and improve the mikuni performance.
I'm just waiting to get my refurbed forks & new dampers back from Landsdowe engineering, it should certainly be a much smoother ride.
Cheers Don

Click to expand...