Mikuni's on a 750

Hi Guys
Five years in the making and I finally put a fire in the beast today, started fourth kick and ran like a bucket of proverbial sh@t. The Mikuni's seem to be way too rich (belching clouds of balck smoke and very dirty plugs) although I have set them up to what most say is a good starting point. Has anyone managed to successfully get mikuni’s to run on a Commando? The carbs were not new but were in perfect condition, I completely stripped and cleaned them, then replaced the Idle Jets (22.5), Main Jets (220), Needles (6DH2) and Needle Jets (159-P4). These are mounted on a set of professionally manufactured manifolds (no brand name). The engine has been rebuilt: is "060+, has a high comp Combat head, std cam, Boyer Ignition and 1 1/2' Headers. Any help will be greatly appreciated.

Cheers
Woolant

Have you checked the float setting? The jetting seems reasonable, the Pilot jet even a bit on the small side, normally a 30 would be OK.

There is a good Mikuni VM manual in PDF that you can download from the web. These settings here work well for a 850 as a single carb, but as you have a Combat spec, you could be near these -

Needle Jet P2 or P4

Needle 6DH3 or 6DH4

Pilot Jet 30-35

Air Jet 1.0

Slide 3

You never did say if you've done anything specific to diagnose where your problem lies. You'll need to test where your plugs get fouled. Just at idle, or did you take it out for a spin and they were all cruddy? You'll have to test at specific throttle settings and be systematic about it.

For most carbs you will not need to do more than main jet and mixture screws, maybe a needle clip change.

Hi
No I have not road tested it, the over richness is at idle and worsens with throttle, I have pulled the carbs back off and are checking the float levels, they did not seem to be far enough out to cause the problem the the degree I am seeing, assumming I am doing them correctly?

Cheers
Tony

slupdawg said:

You never did say if you've done anything specific to diagnose where your problem lies. You'll need to test where your plugs get fouled. Just at idle, or did you take it out for a spin and they were all cruddy? You'll have to test at specific throttle settings and be systematic about it.

For most carbs you will not need to do more than main jet and mixture screws, maybe a needle clip change.

Click to expand...

Hi ML
Yes I am thinking along the same line!!! float levels seem to be okay assumming I am doing them correctly. I ma looking on the net to to chaeck that now.

Cheers
Tony

ML said:

Have you checked the float setting? The jetting seems reasonable, the Pilot jet even a bit on the small side, normally a 30 would be OK.

There is a good Mikuni VM manual in PDF that you can download from the web. These settings here work well for a 850 as a single carb, but as you have a Combat spec, you could be near these -

Needle Jet P2 or P4

Needle 6DH3 or 6DH4

Pilot Jet 30-35

Air Jet 1.0

Slide 3

Click to expand...

750 with mukuni

I have 750 with a single mukuni from Matt at CNW runs great here are the settings: 34mm
main jet 230
pilot jet 35
needle jet 159 PO
needle 6DH3
needle position middle
slide 3.0
air jet 2.0
air filter K&N
hope this helps...... Doxford

SINGLE and DUAL carb setup is totally different.

I don't have any input as to specifics, but you can't assume "same thing times two".

Single mukuni

your right grandpaul two carbs will be slightly different set up. I went to the single set up for ease of maintenance and tunning just wanted to let it be known that one carb does work nicely. Last summer I was sitting at a light heading out onto the highway with a good straight run posted speed 100 klicks/63mph when a guy pulls up with a yamaha road star 1200cc big grin on his face making like he's going to blow me away. The light turned and away we went. Now I have to say I was kinda wondering how this would turn out. We were side by side till I shifted into third then it was just me pulling away. I came outa third at about 75 mph and took her up to 90 and waited the rice burner finally went past but I already made my point. Now this guy got ahead and quickly waved to have me pull over for a chat. He was quite impressed he thought he was going to blow me away. He said once I shot ahead he couldn't catch up the distance was the same he just couldn't close the gap. Not until I held her 90mph then he shot past. Ha ha surprised me but I didn't let him know that lol lots of fun he he ....... Doxford

Hard to say from just what you've described. If the engine runs rich in idle I'm not surprised it would as you open the throttle, as it's cumulatively affected across the range. However, your pilot jet certainly sounds like it should be in the ballpark. Is it just that it's running rich, or maybe it's another problem besides your carbs that's causing excessive smoking and as you perhaps implied, incomplete combustion?

Hi All
Just an update to say I sorted the carbs without too much further trouble and with great results, the bike is going very well, I went back to basics and stripped individually documenting each part as I went, I a 159-P4 needle jet in one and a 166-P4 in the other, changed that, which sorted the over-richness, then progressively changed needles and settings to get the bike to run well right throught the rev range.
Thanks to everyone for their help and advice.

Cheers
Tony

Woolant said:

Hi Guys
Five years in the making and I finally put a fire in the beast today, started fourth kick and ran like a bucket of proverbial sh@t. The Mikuni's seem to be way too rich (belching clouds of balck smoke and very dirty plugs) although I have set them up to what most say is a good starting point. Has anyone managed to successfully get mikuni’s to run on a Commando? The carbs were not new but were in perfect condition, I completely stripped and cleaned them, then replaced the Idle Jets (22.5), Main Jets (220), Needles (6DH2) and Needle Jets (159-P4). These are mounted on a set of professionally manufactured manifolds (no brand name). The engine has been rebuilt: is "060+, has a high comp Combat head, std cam, Boyer Ignition and 1 1/2' Headers. Any help will be greatly appreciated.

Cheers
Woolant

Click to expand...