Mikuni VM 26SS

[Baz1954]

This a follow up from problems I was having running my GS1000. Discovering it was contaminated fuel I stripped the carbs again going further this time. 

So I find I have 115 main jets (up from original 95). Because memory fades I couldn't remember doing changes when I fitted a 4 into 1 exhaust and the K & N's so thinking I dropped it into a shop for tuning.

what I found;

Cyl *1 - Fuel Mix 3/16 of turn out. Air screw 14/16 turn out.

Cyl *2        "         8/16          "              "          12/16       "

Cyl *3        "         10/16        "               "          14/16      "

Cyl*4         "         3/16         "               "           13/16      "

 

So conundrum, I have original exhausts with muffler but not packed and still have K & N's. Waiting for a set of vacuum gauges and will check timing first.

Do I set the screws to these positions again? I used 1/16 as lowest denominator but it's the disparity for 1 & 2 and 3 & 4 fuel mixing that's got me baffled. Before I abandoned the bike to a lonely garden shed on the far reaches of my property it had run just fine.

 

 

[BigT]

No

Stock setting for fuel is 3/4 out and air 1 1/2 out

You should go to 1 turn out on fuel and 2 turns out on air. Tune to high idle with the air screw

double check that the tips of the fuel screws are not broken off in the holes

If you have access to a Colortune, use that to tune. Remember, these screws are for idle mixture

115 mains sound about right for your set up

Plug chops are in your future

[Baz1954]

Took them all out. Cyl *4 fuel screw  shorter and rounded (broken?). Fuel passes through easily enough so not sure if tip is down there. Also there are no part numbers for the fuel screw shown on parts diagram so that's a major problem.

Thanks for response.

[BigT]

Shine a torch in the carb bore while you're looking down the pilot hole to see if it's clear

There's no part # for the screw as it's "preset,nonadjustable" from the factory

There's another, earlier motorcycle of 903cc that uses another version of these carbs that has the screw available

Here in the States, screws are available from Z1 Enterprises

[Baz1954]

Thanks for the replies Big T. Yes, I shone a torch through the bore and thought I glimpsed brass which would be usual as you seat brass on brass. I will confirm it's clear although I confirmed flow amount / stream through the orifice with another carb. Anyway I found the needles in Belgium and ordered 4. There are also full kits (as in everything except tube) available from a company in S. Africa but they don't appear to ship overseas. Thumb twiddling until they arrive.

[Baz1954]

Got the carburfuckingettors back together and set Fuel mix at 1/2 turn out and air at 1 1/2 turns out. After using choke and a bit of idle adjustment screw got her idling at about 2000 rpm. The vacuum gauges all read 5 inhg. Blip the throttle and no change other than revs hanging before dropping and then engine cut out. Then it ran steadily at 2000rpm still showing only 5 inhg. at this reading on the vacuum gauges it says "Late valve timing or intake manifold leak"

I checked (again) my shimming and all between 0.001 to 0.0025" clearance and timing chain correctly set.

The bowls would randomly overflow fuel. So now I have to wait (it is Oz FFS) for one of those right angle carburfuckingettor screw adjusting screwdrivers.

A couple of plug chops, nothing outrageous, mildly wet-ish to dry and dark.

It is mostly the vacuum readings that got me as there was no change with occasional blip to 4000 rpm. The gauge snubbers / damping control were set correctly.

Any help appreciated.

[Gixer1460]

If it don't idle down - you've got a vac. or intake leak somewhere! I'd expect a reading up towards 9 - 10 at proper idle speed that drops on blip, goes high on release and settles at above level. Random overflows . . . . . float valves not fully seating (stating the obvious but they rarely leak from other reasons or you've crashed in your garage!) 0.001 - 0.0025" clearances don't sound right - even on a hot engine!

[Baz1954]

I'm back with problems again. So I bought another set of carbs from Milwaukee or somewhere. They were in very good nick for the money. Anyway I had broken the tip off no.4 fuel mix needle due to not "feeling" when it is snug. Clown move.

Stripped and cleaned them noting the settings then I stripped my original internals (115 mains and pilot). I could feel the air and fuel mix needles when seated snug ( the internal springs were "talking" to me) and set them in original positions with the exception of air which I set to 1 and 1/4 turns out.

Warmed her up and all vacuum gauges read 5in Hg at 1500 revs set. Rev her 3K revs and she's hanging.  Moved No. 1 air needle in stages down to 5/8 open and vacuum went to 10 inches. Tried same with cylinders 2,3 & 4 nothing happening. Swapped lines from each cylinder to No. 1 gauge thinking could be faulty. No readings stayed same. Bought really expensive vacuum gauge and did same test no change bank of gauges all read correct.

Ok, so what are chances of air leaks on carb / cylinder head boots? Sprayed with carb cleaner no rise in revs.

No. 3 header was running cooler than the rest, plug showed wet so eased fuel needle  1/4 turn in, plug chop and it's tan and dry with header running somewhat hotter. Vacuum rose to 6 inches.

So after this long diatribe and I'm still left with three cylinders that no matter where I position the air needle ( from fully seated to all the way out) I can't raise the vacuum. The bike appears to run fine it's just the revs take a couple seconds to drop from 3K to 1200 idle.

Hope one of you knowledgeable chaps read this and pass me the holy grail of cures.

Thanks

 

[gs7_11]

Why are you adjusting vacuum with the fuel/air screws?

That's what the synchronising screws are for under the carb tops.

Set the screws to the generally accepted base settings, then move on.

Have you changed the o-ring under the carb/head rubber? Classic spot for an air leak.

Edited May 10 by gs7_11