Another Dreaded Mikuni Thread

[domiracer66]

Hi all,
I'm having somewhat of a frustrating time trying to dial in the set of Mikuni carbs installed on my 750. The engine and the bike for that matter have been completely gone through and the carbs are the only thing standing in the way of a long overdue ride. The bike only runs for a few minutes at idle before fouling out the plugs. The motor has been essentially built to Combat spec, 10:1 comp, 32mm intake ports, hotter cam, drilled reverse megs, and, Uni filters. I have done several searches on the forum and there seems to no real consensus as to base settings. I've seen everything from 20 through 40 on the pilot, 2.0 through 3.0 on the slide, and 200 through 240 on the mains. Has anyone had success running a set of VM 32s on their 750? If so could give you give me some base line settings? Any help would be HUGELY appreciated! I tried contacting Stan @ Rocky Point Cycle who I bought the carbs from but his number seems to be disconnected.

Current VM 32 settings:
Pilots 30
Mains 210
Slides 3.0
Pilot screw 1.5 out
Idle screw 1.5 out
Slides synced @ 3/16" opening

Engine:
750
Cam PW3
Compression 10:1
Intakes 32mm
Exhaust swept back low restriction megs
Uni filters
Ignition Boyer
Battery fully charged @ 13v

 

[Jeandr]

Contact Kenny Cummings (Homeslice), he has a set of Mikunis on his 750 racer, I am sure he will share some of his secrets if you promise not to tell his competition :mrgreen:

Jean

 

[DaveK]

I have a set of mikuni carbs on my bike, I have to do some work on the bike this week so I will have a look what I am running....I have 3.0 cut outs also. Air screw - 1 turn / idle screw - who knows, I adjust them constantly depending on what I am doing...I dont idle that often anyway. Main jets sound right, Pilot.....I can't remember. You should only be using your Pilot and air screw at idle anyway...I would check the Choke to make sure it is not being held open......I'll give you more specs later.
Also what needle are you running?

 

[domiracer66]

Thanks Jean but I'm guessing a race bike would be all together a different animal from mine, probably large bore matched carbs for easy breathing at high RPM?

Hi Dave,
I'll double check the choke and get the details on the needle posted tomorrow. The weather here in Wisconsin is finally getting warmer and I'm afraid my procrastinating ways have caught up with me once again. Thanks again for your help.

 

[Gino Rondelli]

I know this may sound dumb but check the slides are in the right way round, i.e the throttle cutaway to the rear of the carb.
Don't ask how I know

 

[zotz]

I'm running a single Mikuni on mine but found that I had to learn a warmup procedure as far as choke use is concerned. I need to use the choke for cold starts but need to shut it off after about a minute or it will foul up the plugs. Usually I need to increase revs a bit as I flick off the choke and then slowly ease off throttle as the engine clears itself.

 

[domiracer66]

Dave,
I checked the needle last night and it's marked 6DH3. The choke seems to operating correctly without sticking.

Gino,
I did checked to make sure the throttle slides were in correctly with the cut away facing towards the rear of the carb.. Something I double check for now having put them in the wrong way before one of those "ohh duh!" moments we all have from time to time.

Zotz,
I've warmed the engine up with and without the choke on and seem to get the same results (fouled plugs) so I don't think it's the choke.

Thanks for the input guys...

 

[ML]

Checked the floats are round the rightway and not sticking on the pins? Also is the top hat the right way down between main and needle jets?

Mick

 

[DaveK]

Stupid question it is fouled rich mixture right? not oil fouled.

 

[Ron L]

Stan has relocated from Xenia (Beavercreek) Ohio to Morgantown, W. Va. I would send him an e-mail at sales@rockypointcycle.com. I run dual 34 Mikunis on an 850 with flowed, big valve head, so I don't think any of my settings will help much.

 

[Diablouph]

Wouldn't a leakdown test be in order at this point?

 

[DaveK]

Ok, I had a look at my carbs. I am running 750 powermax (domed) pistons on a combat head BUT I have a D+ Grind cam

However the carb settings seemed ok running standard pistons with a "standard" head with a regular cam for the last few years too.

Needle 6DH2/ Main Jet 220 / Pilot Jet 25 / Needle jet 176 P8.

......It makes me wonder ......If it was working before.....what's changed??

If you find a sweet setup let me know, my carbs run just a hair too rich for me, not dark enough to worry about but not tan enough to make me happy type of thing.

 

[domiracer66]

ML,
If memory serves me correctly I don't believe you aren't able to install the symmetrical Mikuni floats incorrectly, meaning they are reversible from side to side and the same proportions front to rear.

Diablouph,
The engine is rebuilt and the valves and clearances were all checked after torquing so I don't think a compression test would be necessary at this point. Although I am curious to see what kind of pressure the cylinders are at after skimming the head.

Ron L,
Thanks for the contact info! I sent Stan an email this morning. Hopefully his move is going smoothly and the transition is an easy one. Although I've never been to WV I hear it's a beautiful part of the country with lots of twisting mountain roads.

Dave K,
Thank you very much for taking the time to check your settings and posting them here. The plugs are definitely gas fouled, sooty flat black, as apposed to oil fouled, shinny black. As soon as I hear back from Stan I will post his recommendations so that we can compare notes. When I get a chance, most likely towards the end of the week, I'll try your settings and let you know what my results are.... "stay tuned"... :roll:

Thanks again!
Kris

 

[splatt]

what plugs are you using?

 

[Diablouph]

I was thinking of DaveK's comment.

"by DaveK » Mon May 17, 2010 4:42 pm

Stupid question it is fouled rich mixture right? not oil fouled."

 

[domiracer66]

Splatt,
I'm using the BP7ES which I believe are the recommended heat range.

Diablouph,
The plugs are definitely gas fouled, sooty flat black, as apposed to oil fouled, shinny black.

 

[DaveK]

Could it be that the carbs are overflowing with gas and then pouring down the intake ? Unlikely but might be worth checking.....or maybe we are going the wrong way and should be looking at an ignition problem.........not sure I'm still thinking about it.