Who can give advice on Mikuni VM29ss?

[Hans]

Hullo, me's got a problem.

Bike is an 87 gsxr, motor has been rebuilt, valve clearances done, exhausts seal well.
The carbs are Mikuni VM29SS, the airbox is no longer with me, instead I have individual filters.

The bike: Runs rich from just past idle to about 6000rpm, the runs lean.
Anyone know
a) Someone who knows their shit and would be willing to lend a hand

b) Some good literature to read before I tackle it myself, doesn't have to be a book, can be online, I've looked and haven't found much.

Cheers

[tychver]

I don't know if it covers the VM29SS but there's a generic Mikuni VM tuning manual here :)

http://www.mikuni.com/pdf/vmmanual.pdf

[CookMySock]

I took the airbox off my bike to see what it sounded like, and the damn thing wouldn't rev past 6,000rpm. If these are constant-depression carbs then they will play merry hell with you with no airbox fitted.

If that's the problem, I am keen to see your solution.

Steve

[t3mp0r4ry nzr]

how do you know its rich at low rpm and lean at high rpm?
also, you havent said what the problem is. does it rev? does it bog? when and where?

generally speaking. remove airbox and expect to change to much larger main jets.

http://www.dynojet.com/pdf/3116.pdf

checkout this link. stage 3 (individual filters) require slide drilling and large main jets. Easy option is to puchase this kit.

[Hans]

how do you know its rich at low rpm and lean at high rpm?
also, you havent said what the problem is. does it rev? does it bog? when and where?

generally speaking. remove airbox and expect to change to much larger main jets.

http://www.dynojet.com/pdf/3116.pdf

checkout this link. stage 3 (individual filters) require slide drilling and large main jets. Easy option is to puchase this kit.

It's rich at low revs, 'cause there's black smoke coming out the exhaust, worse with no load.
It's lean at high revs, 'cause it loses power at high revs with the throttle open past about 1/2. And after a hard run the plugs show lean mixture as well. Cheers for the link. Oh yeah, and it's not a constant depression carb as the needle is operated directly by the throttle position and not by vacuum.

[t3mp0r4ry nzr]

It sounds as though you have a good grasp of at least the basics, and Im not sure how much of the following is going to help you but, here it is anyway. the hardest thing is diagnosing rich or lean. once you understand the different functions of the metering mechanisms in the carbs, you should find jetting not too difficult.

try leaning the pilot system by turning the fuel screw in. should have the affect of leaning mixture from 0 to 1/8-1/4 turn.

Could try raising the needles (lowering the clips) to enrich the needle zone (1/4 - 3-4 turn).

may still need larger main jets though to enrichen from 1-2 - full throttle.

this is just a rough guide only however.

search for "factory pro". they have a jetting guide that includes guidelines for setting float heights as well. have a good search for jetting info. there are some useful guides out there.

also note that the dynojet guide gives their recommended jet sizes but they are measured differently to Mikuni jets, so they cant be compared.

any specific questions just ask. Im still learning everyday about bikes but may be able to help you. :sunny:

[Taz]

Find an air box.

[t3mp0r4ry nzr]

Find an air box.

and give me the individual filters!

[Hans]

Find an air box.

No can do, 26l fuel tank. I'm actually thinking the carbs would have been rejetted, the bike has a long list of mods that tie into each other. It's possible they actually need a full clean before I start mucking around. I've got a fairly solid knowledge of principles because I used to be into fast cars a while ago. I'm pretty damn good with EFI, but I guess I'm a bit too young to have hands on experience with carbs. I've got a nice tub to soak all the parts, many zip-lock bags and labels, so I guess I'm ready to go. :eek5:

I've spent a lot of time pissing around with this bike, fully rebuilt the motor, checked everything along the way, crack tested, poked, prodded and bought millions worth in parts. Then I lost heart to the point of seriously considering selling it. Now I've moved into a place with a nice workshop and I'm just about ready to un-give-up.

[t3mp0r4ry nzr]

awesome bike mate!
Dont give up. The work will be worth it when its running well.

Cleaning the carbs would be a good start. it would only take 1 main jet to be plugged for it to be showing as lean. If that doesnt improve then look at larger mains. Carbs are easy to work on, but any changes require dissassembley and reassembley, so quite time consuming and also requires lots of trial and error to get the rght jetting. Just take your time and work through it. A shortcut is buying a jet kit.

[SS90]

No can do, 26l fuel tank. I'm actually thinking the carbs would have been rejetted, the bike has a long list of mods that tie into each other. It's possible they actually need a full clean before I start mucking around. I've got a fairly solid knowledge of principles because I used to be into fast cars a while ago. I'm pretty damn good with EFI, but I guess I'm a bit too young to have hands on experience with carbs. I've got a nice tub to soak all the parts, many zip-lock bags and labels, so I guess I'm ready to go. :eek5:

I've spent a lot of time pissing around with this bike, fully rebuilt the motor, checked everything along the way, crack tested, poked, prodded and bought millions worth in parts. Then I lost heart to the point of seriously considering selling it. Now I've moved into a place with a nice workshop and I'm just about ready to un-give-up.

The Black smoke (indicating rich) at low RPM, then lean up top makes it a little bit harder to set up.

Because of the fact that it ran an airbox (now "pod filters") would suggest that that main jets are now too small. Personally (by no means a rule), I always started out with such conversions by simply going 10% bigger on the mains, it may not be where you end up, but I am 100% certain that you will need AT LEAST 10% bigger....

Does the bike idle correctly? (when it's warm that is)

Personally, I would be extremely surprised if they (the pilot jets) where too big (if they are the original ones fitted that is.... some quick research on the net will tell you what size they should be....something like .42....PLEASE DON'T TAKE THAT AS GOSPEL...... IT'S JUST AN ESTIMATION!), GENERALLY speaking, pilot jets on a 4 stroke of that era are quite big (no emissions testing back then...stark contrast to today, where most carbed bikes run like crap just off idle and need bigger pilots just to run!), but I can not think of a model from back then that suffered from them being too big (particularly now that you have leaned the whole mixture off by removing the airbox)

Personally, because of the age of the bike, I would suspect that the slide needles and emulsion tubes are worn.

Simply pull one needle out and look carefully at it, look carefully at the area where the needle slides up and down in the emulsion tube....if it's noticibly more shiney in one area (compared to the rest of the needle, then it is worn, and must be replaced) There is no real way of measuring the emulsion tubes (sometimes called a "needle jet") and if you see any wear on the needles, then the emulsion tube has TWICE the wear of the needle.

In my experience, this is the most common "too rich fault" on older carbed bikes.

as a test (if you are unsure), put the clips of the needles on the highest setting..(leanest)....see how it feels......if the low/mid clears up much, then that is the logical cause.

Oh, and I am sure you are right about being too lean on the mains, but as a tip, when you reach the RPM where you suspect the bike feels too lean, simply open the choke lever a small amount, that will richen it up (for sure), again, if it improves much, then that it a good bet!

Good luck,

The Slab sides are a real piece of history.... the first real "out of the crate racer" the world ever saw!

[HDTboy]

I have an airbox. I also have the PDF for the VM29s

[Hans]

Right. Disassembled everything. Whoevever worked on those carbs needs castrating before they breed. 3 matching main jets - too large/115/. One main jet of god knows what size/e7/. Needles stuffed. Needle jets stuffed. No o-rings on 3 air screws. List goes on. I am now on a mission with a sledgehammer. Not for the bike - for the ex-owner's cojones.

[mossy1200]

Right. Disassembled everything. Whoevever worked on those carbs needs castrating before they breed. 3 matching main jets - too large/115/. One main jet of god knows what size/e7/. Needles stuffed. Needle jets stuffed. No o-rings on 3 air screws. List goes on. I am now on a mission with a sledgehammer. Not for the bike - for the ex-owner's cojones.

ive just gone through the airbox remval with my fzr1000 that has cv flatslides stock and it just didnt work untill i installed stage three dynojet kit.
I ended up with an on off switch instead of throttle like big two stroke.
Slides just stayed shut then open fully with no control at all.Its all or nothing.
seems ok now but will dyno tune it next week.
twin pods work better on alot of bikes than individual pods also.

[Hans]

Chees, but cv carbs are a different story alltogether. On my carbs the throttle directly opens the slides and the needles are operated by the slides, not by vacuum.

[elevenhundred]

Check out this site for some reading

http://www.factorypro.com/tech/carbtun.html

[mossy1200]

Chees, but cv carbs are a different story alltogether. On my carbs the throttle directly opens the slides and the needles are operated by the slides, not by vacuum.

i think you may find the kits will still contain more than just jets .
If you search the kits you will get an idea what else they contain.

My kit(different for cv)contained air jet block off caps to completly remove some air jets.
new main jets,slide mod drill bits,needles that were completly different shape and adjustment instructions for mixtures etc with trouble shooting advice.

Not sure what you kit would contain be direct pull but would be worth finding out before you spend money trying to tune yours without the kit.

[Hans]

That does make good sense. I'll see how lucky i get finding a kit for carbs that were never sold in the u.s. Thanks all.

[Karitane pete]

The Kit 3116 is the wrong one for you bike you need E3X18.002
134395
dyna jet jet number do not match mikiuni numbers
and by the way you bike is wearing an 85 fairing with an 87 front guard.

[smebike]

Stock main jets are 97.5 to run with pod filters fit 115 main jets. I have the stock pilots listed as 32.5's. Float levels should be 14.2mm + or -1mm. Stock pilot screw setting .5 of a turn. Replace the emulsion tubes and needles with stock new ones if available (these must be replaced). Replace any missing orings etc, check the manifolds are sealing on the cylinder head and if there are no mechanical problems e.g. incorrect valve timing she should run real well.

[Hans]

Once again thanks for all the useful advice, you guys are great. I will also be replacing all screws with pozi-drive. I suspect I might have to go in a couple of times, and am sick to death of the phillips stripping. Ta.

[HDTboy]

Get an impact driver. The screws won't strip then