Day 2 - Mikuni woes?

FastFred · Mar 20, 2011

After yesterdays 6 hours sorting the front isolastic I went for a ten mile ride, no problems. The first trip on the new Mikuni VM34 carb. Came home last night and then washed the bike. Being stoopid and having a short memory I didn't cover the carb when I rinsed off with the hose pipe...

Muppet

Today I set off for a ride. After 200 yards the bike hiccuped at a road junction but I whipped the clutch in and caught the revs, thinking nothing more about it. A mile later it spat, coughed and died. Without boring you all I spent most of the day pratting with it. Blew carbs jets out etc as suspected water.

The engine is in need of a top end job and smells oily although it runs well. Bike has NGK B7ES plugs but are very dark and sooty. I limped it home today; it didn't like the steep hills. Should I drop the plug grade to a 6ES before I mess with the carburation?

What jet sizes are others running on an 850 with a VM34?

Thanks

Hortons Norton · Mar 20, 2011

It does sound as if its too rich, Be sure to only use the choke to get her started and as soon as you can release it and just keep it running by the throttle. I would clean the plugs and be sure your gap is not too big, And then give it another go. Have you had it out on the open road yet? The method of doing plug chops will let you know whats going on, Search on this site or you can go here: http://www.mikuni.com/pdf/vmmanual.pdf

FastFred · Mar 20, 2011

I backed the air out to 3 turns and the revs picked up (before lowering slide via screw). This indicates I needed more air to get the tickover fuel ratio correct so I will swop the pilot to the 32.5 supplied instead of the 35 fitted. This will effectively weaken the overall fuel ratio throughout the range (it's additive according to the VM tuning manual as the pilot constantly feeds the main inlet stack). I will then drop the main jet to a 240 from the 250 fitted. I guess I was asking if I should use a hotter spark plug as it smells oily but maybe it just smells rich? The NOC website states B6ES for 850's?

Madnorton · Mar 20, 2011

Know nothing of Mikunis, but check out the NGK website and fit Iridiums listed as the alternative to standard for the commando. When you look at them it defies belief they will actually work with that tiny centre electrode, but they are fit and forget, non-adjustable and last for miles.

Hobot · Mar 20, 2011

Strange, if ya cleaned carb of water and also the fuel supply tank, and it ran fine prior and heating up 200 yd didn't dry out electric wet shorts, and plugs show excess rich, I'd check I'd not left the Miki enricher lever in the on position.

bpatton · Mar 20, 2011

The spec's on Bushman's site are pretty close to what most people run. Do you know what jets are in there now? Generally, the bike won't run on the enricher, lever down, after more than about 30 seconds. If you're into the throttle with it on that would minimize the enrichment circuit but as soon as you close the throttle the engine would die. You can get a cable setup for the enricher that hooks up to the stock choke lever. (Much more convenient and noticable in my experience, been there

)
http://www.jba.bc.ca/Bushmans%20Carb%20 ... ml#7MIKUNI

72Combat · Mar 20, 2011

I have found on my Ducati Darmah once the plugs are fouled that they really need a good clean with petrol or replaced. I found this out on the side of the road.... :shock: now I carry a spare set just in case.

AussieCombat · Mar 20, 2011

Mikuni's can be touchy, so beware of the Air cleaner (filter), if conditions permit, tune the bike without the cleaner untill you get it close to right. It's a job that's easier in Summer. Good plugs help...AC.

slupdawg · Mar 20, 2011

I have a 750, but I run a 240 main, and thinking about putting the 250 in again. Think I'm running a 30 pilot, and it idles nice. Tried a 25 and it was too small. The plugs are nice and toast color on one of the cylinders, but can get a little dark if I leave the enricher on too long. Actually, I'm at the point I really don't use it unless the bike is being particularly fussy. Can't say enough good things about the soot-resisting qualities of the iridiums. They really helped me out. I'm going to try some multiple electrode iridiums I just got, and see what they do, to take advantage of my dynacoil, as comnoz suggested on another topic.

Hobot · Mar 20, 2011

On my first Combat that ran plug fouling rich when I got it, I put in 4 electrode platiumums and it worked a treat to run around while I learned all there is to go wrong with Combats.

FastFred · Mar 21, 2011

Enricher definitely not on. Will lean off and buy some iridiums.

RoadScholar · Mar 21, 2011

FastFred said:
I backed the air out to 3 turns and the revs picked up (before lowering slide via screw). This indicates I needed more air to get the tickover fuel ratio correct so I will swop the pilot to the 32.5 supplied instead of the 35 fitted. This will effectively weaken the overall fuel ratio throughout the range (it's additive according to the VM tuning manual as the pilot constantly feeds the main inlet stack). I will then drop the main jet to a 240 from the 250 fitted. I guess I was asking if I should use a hotter spark plug as it smells oily but maybe it just smells rich? The NOC website states B6ES for 850's?

The 240 is what I ship the 34 VMs with. A 35 is the right idlle jet.

If the machine ran well before you soaked it, I'd be looking for an electrical issue, especially if you have the original wiring harness/coils.

RS

FastFred · Mar 21, 2011

Yes but it doesn't explain the sooty plugs does it? I did 3 miles home last night without a breakdown (!). I use to race a TZ350 and plug chops were done on long straights to check mixture but with a road bike I don't think you would see a major difference in colour (at different speeds) unless the setup was way off? In other words they would clean up fairly quickly if the mixture is right after an occasional misfire i.e go to a light tan? I will drop main to a 240 for sure. pilot jet is a little large but I have balanced it withing the air screw range.

Hobot · Mar 21, 2011

pilot jet is a little large but I have balanced it withing the air screw range.

Hmm, Miki's perfect idle screw out is same 1.5 as Amals and float level is how I get this basic base line set before going further. If ya can't then look into jet and needles or timing.

FastFred · Mar 21, 2011

Hmm, Miki's perfect idle screw out is same 1.5 as Amals and float level is how I get this basic base line set before going further. If ya can't then look into jet and needles or timing.

I know Hobot, that's why I'm going to fit a 32.5 pilot instead of the 35 that's in there. Then the screw will be nearer the 1&1/2 turns out. i still did balance it on the air screw. Effectively your just nearing a stoichimetric ratio even though there's too much fuel going in. A pilot jet doesn't have a great effect unless it's a mile out even though on the VM's it does add fuel thro' the whole throttle range.

RoadScholar · Mar 22, 2011

I had a sense that you have an idle air jet smaller than 2.0, if this is correct then your air jet is too small and could explain why your air screw had to be turned out 3 turns. The air screw is a "trim" adjustment, not meant to be a "supply" channel for the idle system. 2.0 is the jet in my 34mm kits.

Hobot's point about the float level is something to check out. The factory setting has the float arm parrallel with the carb body when the carb is turned upside down, but not compressing the float needle rebound spring. Do try a short test drive without the air cleaner, if you have the small pancake version it needs to have all its surface area fully functioning, if you have the MAP version it is not an oil type: soap and hot water with an overnight dry (no compressed air).

Good luck with this, it is far from rocket science. PM me and I will send you the calibrations I used to set up my 34mm kits, let me know your elevation.

RS

FastFred · Mar 22, 2011

Thanks RoadScholar. I bought the kit new at Motocarb here in the UK. He supplies the saem VM34 for 750 and 850's. Comes wiut a 250 main and 35 pilot with 240 & 260 mains spares and 32.5 and 37.5 pilot spares.

Last night I changed the main to a 240 and the pilot to a 32.5 and the air screw is now 2 turns out. Will have a test ride tonight hopefully on some new iridium plugs if the post arrives.

Thanks for that bit about the air jet 2.0, understand and will check that.

I read somewhere that the 850's normally require a 40 pilot? Anyway I guess it all relates to unknown engine tune and how good the top end is.

Will check float level. Cheers

1up3down · Mar 22, 2011

Changing from a 35 to a 32 pilot will make NO practical difference, and so changing your airscrew setting by a full turn
seems right in theory, but in reality is not the right solution and should not solve your problem.

The 35 IS the right jet, and set the airscrew 1 1/2 turns out, check to insure needle is in the middle clip.

I also do not believe changing the main jet is the either the problem or solution as it effects really only from 3/3
throttle to wide open.

Did your shoot water at the air cleaner? That would definitely make it run rich, take it off and blow it out.

How were your plugs before putting the Mikuni on? Maybe your motor is so worn out that the guides or rings are now allowing oil into the combustion chamber and fouling the plugs?

Hobot · Mar 22, 2011

Last night I changed the main to a 240 and the pilot to a 32.5 and the air screw is now 2 turns out. Will have a test ride tonight hopefully on some new iridium plugs if the post arrives.

Still on rich side of idle screw fudging, so still suspect float level to get base line that Iup3down advices works right. Water soaked air filter was a good one for me to keep in mind, but would not last very long before drys on its own.

FastFred · Mar 22, 2011

Yes I said earlier on that the motor is on the worn side although it's not smoking from the zorsts. I need to give it a run with new plugs and see. It did idle better three turns out on the 35 pilot hence why I downsized it; it was 'hunting' on the 35.Will let you know, thanks

Day 2 - Mikuni woes?

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