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Yet another carb thread - but my first one!

dardoonk

dardoonk

Forum Mentor
Bike: 77 GS750B - "my beloved electrolux"
Okay, I am passed all the basics:
They are completely rebuilt VM26 carbs, with new Mikuni components and all o-rings replaced.
No, the boots don't leak. There are no leaks
Yes, they are bench synced
No, I haven't vaccum sync'd them yet.
Yes, I know this needs done

Settings:
Fuel screws: 3/4
Pilot Jets: 17.5
Main Jets: 117.5
Stock needle, middle clip
Air screws: started at 1.5 turns (but see below)
Bike has a Jardine 4-1 with Uni-pods

When cold starting: I need to use the choke to start and can push it in after a short time. It will idle smoothly at 1000rpm and accelerates smoothly with a smooth twist all the way to full throttle. At this point I wanted to begin plug chops, but have one ridability issue to solve beforehand. I think I know the cause, but want to check with the gurus.

If I accellerate to say 1/2 throttle and begin to shift, the revs do not drop. Well, they drop, but it takes like 30 seconds or more when the clutch is pulled in. I checked the cables (they are new) and there are no kinks and is enough play in them. The throttle grip returns fine, so I am sure the problem is in the carbs.

When idling in neutral, if I rev it to 2500+ rpms, it will hang there just as I describe below. Slowly, slowly, it returns to idle.

I suspect I've got the float levels too high, I set them to 26mm and did not expect that to be an issue.

Using the highest rpm method, I tried to adjust the air screws, but get only a negligable change in rpm turning them from 1/2 to 3 turns out. I may be incorrect, but I thought this furthered my theory of the floats being too high.

Guru's - give me your thoughts. By the way, advice on jetting is appriciated if you have any thoughts on my initial setup.This is my first run tuning VM's, so I selected these jets based on my own analysis of what I have read.
 
Bigger mains. 127.5 was what i was told. Dunno how it will run yet, but the 117s are too lean. If i recall correctly hanging idle is also a lean condition? id have to go back and re-read Keith's suggestions. I went with them, but havent had it running yet.
 
I can not say anything about all the other settings, but a hanging throttle usually indicates an air leak between the carbs and the head.

When you say "all the o-rings" are you including the ones on the intake tubes?
Are the intake tubes in good condition and clamped snugly?


.
 
Yup, air leak. Intake boot O-rings most likely.

Misadjusted/blocked mixture screws can cause a hanging idle as well but not normally to the extent noted.
 
Lean mixture... I had it before I tweaked the screws out a bit more. Having the float level high would probably help it if anything, it's probably why you are running good on the smaller mains (a float 1.5mm higher is equal to about 5 on the main jet on the Mikuni scale I believe).

Dan :)
 
Steve said:
When you say "all the o-rings" are you including the ones on the intake tubes?
Are the intake tubes in good condition and clamped snugly?.
Click to expand...

I was referring the carb o-rings. The intake rubbers are in good condition and the o-rings weren't cracked or anything. I bolted them back on and check for a leak by spraying some WD-40 around them and listening for a change in rpm. I got nothing, so figured they were okay.

Since two of you have said the same thing, I just ordered a new set and will see if they improve the situation.

Out of curiosity, how does the leak keep the rpms up? I don't understand, from a technical aspect, what causes this.
 
26mm is STOCK on the 77 VM set up. There is no "range" according to my manuals, like the BSs have. The 77 VM carbs also are designed differently than that of the 78 and 79. There are no "spacers" on the needle, just the clip, and the needle fits into the bottom of the slider arm, and there are no brass plates or washers. dunno if that makes a difference, but they are different, as i found out when i took mine apart and thought parts were missing as i compared it to the ones featured in the VM cleaning tutorial in the Garage. At any rate, I'm still of the opinion, seeing as he has pretty much the same set up I do, except a Jardine pipe instead of the VH pipe, that he's lean on the main, based on what has been explained and suggested to me by Keith.
 
dardoonk said:
I was referring the carb o-rings. The intake rubbers are in good condition and the o-rings weren't cracked or anything. I bolted them back on and check for a leak by spraying some WD-40 around them and listening for a change in rpm. I got nothing, so figured they were okay.

Since two of you have said the same thing, I just ordered a new set and will see if they improve the situation.

Out of curiosity, how does the leak keep the rpms up? I don't understand, from a technical aspect, what causes this.
Click to expand...
Were they flattened and dry, or shiney? Mine werent cracked either, but two were leaking. I sprayed carb cleaner, got bog. Dunno if WD makes a difference either, never tried it.
 
Had exactly the same issue on my GS550B. Bought a colortune, set the idle to the correct color and then richened it a tad on the air screws and it performs beautifully now. It was a lean condition running on the idle screw that caused the problem in my case. It is definitely a lean condition.
 
My needle has 5 clip positions. #5F21-3 (3 being the needle position). I am out of town right now and don't recall which of my manuals I referenced, but it did specify a range of a couple mm for the float heights.
 
Neither of mine did. The OEM shop manual, or my Clymer. Third possition is stock, so that's right, but again as per Keith's suggestions, i moved mine to the fourth possition. And again, i have yet to start the bike with the jets or needles. Never got them vaccum synced before the weather started to turn crap. SO this could turn out to be entirely wrong. However, i had a hanging idle AFTER i fixed my carb rings, and changed the orings in the carbs, so i CAN say that the intake rings may NOT be your problem.
 
Most of my experience is with Keihin CV carbs, so I've got some learnin to do with the VM's. My theory of the high float level is because I figured the pilot jet pulled too much fuel thru when the slides snapped shut.

On my Triumph with CV carbs, deceleration popping thru the pipes is typically a lean indicator, but oddly enough, dyno results show the setup as spot on and actually a bit rich when at idle and roll-on. Even so, on decel, the engine rpms pull vaccum against the carb and results in a lean pop. It turned into a balancing act, and the decel only occurs for about 500rpm and under no load, there is no popping. Correcting it would have resulted in a performance loss.

Using that experience, I assumed the opposite for a hanging throttle with no load. Sounds like that may not be a cause, so advancing the idea of a lean condition, and assuming the problem remains after I replace the o-rings, then what carb adjustments need made?

Even with less than a 1/4 throttle turn, the VM carbs will hang, so I do not feel the needle clip position has come into play, otherwise I could raise the needle one notch. With the 17.5 pilots, (15 being stock) I'm not thinking it could be that either. Hopefully the 0-rings solve this, but the theory behind how this can be carb settings related in intriguing to me.
 
dardoonk said:
Bike: 77 GS750B - "my beloved electrolux"
Okay, I am passed all the basics:
They are completely rebuilt VM26 carbs, with new Mikuni components and all o-rings replaced.
No, the boots don't leak. There are no leaks
Yes, they are bench synced
No, I haven't vaccum sync'd them yet.
Yes, I know this needs done

Settings:
Fuel screws: 3/4
Pilot Jets: 17.5
Main Jets: 117.5
Stock needle, middle clip
Air screws: started at 1.5 turns (but see below)
Bike has a Jardine 4-1 with Uni-pods

When cold starting: I need to use the choke to start and can push it in after a short time. It will idle smoothly at 1000rpm and accelerates smoothly with a smooth twist all the way to full throttle. At this point I wanted to begin plug chops, but have one ridability issue to solve beforehand. I think I know the cause, but want to check with the gurus.

If I accellerate to say 1/2 throttle and begin to shift, the revs do not drop. Well, they drop, but it takes like 30 seconds or more when the clutch is pulled in. I checked the cables (they are new) and there are no kinks and is enough play in them. The throttle grip returns fine, so I am sure the problem is in the carbs.

When idling in neutral, if I rev it to 2500+ rpms, it will hang there just as I describe below. Slowly, slowly, it returns to idle.

I suspect I've got the float levels too high, I set them to 26mm and did not expect that to be an issue.

Using the highest rpm method, I tried to adjust the air screws, but get only a negligable change in rpm turning them from 1/2 to 3 turns out. I may be incorrect, but I thought this furthered my theory of the floats being too high.

Guru's - give me your thoughts. By the way, advice on jetting is appriciated if you have any thoughts on my initial setup.This is my first run tuning VM's, so I selected these jets based on my own analysis of what I have read.
Click to expand...

You have a lean condition on your pilots or serious air leaks in the induction system. Do your jets have round or hex shaped heads?
Mikuni measure their jets two ways. Those that have hex heads are numbered according to their flow rate, while round headed jets are sized by the bore dimeter in millemeters. A round headed jet with a #120 stamped on it will have a hole 1.2mm in diameter.
I am running 0.4mm pilot jets on my 850 with VM26SS carbs, a 4-1 pipe and stock airbox.
My pilot mixtures are set at 1.5 turns, and airs screws at 1.5 turns. I would richen your mixture screws to 1.25 turns and try it.
Floats should be set between 21.4 and 23.4mm from the bowl mating surface. If you measure from the gasket surface, take off 1mm.
 
49er said:
You have a lean condition on your pilots or serious air leaks in the induction system. Do your jets have round or hex shaped heads?
Mikuni measure their jets two ways. Those that have hex heads are numbered according to their flow rate, while round headed jets are sized by the bore dimeter in millemeters. A round headed jet with a #120 stamped on it will have a hole 1.2mm in diameter.
I am running 0.4mm pilot jets on my 850 with VM26SS carbs, a 4-1 pipe and stock airbox.
My pilot mixtures are set at 1.5 turns, and airs screws at 1.5 turns. I would richen your mixture screws to 1.25 turns and try it.
Floats should be set between 21.4 and 23.4mm from the bowl mating surface. If you measure from the gasket surface, take off 1mm.
Click to expand...

I have round jets.
What # is a .4mm pilot jet
I have info for the 850 that shows a float height of 23-25mm and a height of 25-27 for the 750. Both VM26SS carbs, why would there be a difference?
 
Less fuel needed to the 750...the higher the number, the quicker the fuel valve shut off basicly.
 
dardoonk said:
I have round jets.
What # is a .4mm pilot jet
I have info for the 850 that shows a float height of 23-25mm and a height of 25-27 for the 750. Both VM26SS carbs, why would there be a difference?
Click to expand...

On a round headed jet, 0.4mm will be marked as a #40.

The float level figures are a guide and are dependant on the design of the float arms, float needles and seat assemblies. The Clymer manual for my 850 quotes the figures I gave you. I have found that my engine runs smoother through the rev range with the floats set a 24mm off the gasket.
Incidentally, most tuners neglect to check the condition of the float needle valve. As they wear, they change the float level accordingly.
 
All right, so I ordered new O-rings for the intake, but I can't see a way to get the old ones out. I ordered the small o-rings from cycleorings and it doesn't look like they're gonna fit either. Another thing, I can't find a fiche showing the gasket and I can't see how the o-ring will do much good pressing up against a gasket anyway. The pic below are the parts I have. The gaskets were on the bike when I bought it & they are a decent quality material, not something someone made.

How do I get the old o-rings out without damaging the boots.

Did I get the right O-rings? My invoice says "small" (the ones that cost $3.30), but the outside of the oring measures 38mm, so I think I received the wrong ones.

What do I do for the gasket?

intake05.jpg
 
For starters you have to dig out the oring. Dont worry, the channel that it sets in is metal, as is the entire back side of the boot. It may have some rubber over it, but not to worry. Matter of fact clean the rubber away from the oring mateing surface. The gasket, dunno where that came from. I didnt have em. I thought my rings were a bit big as well, but they fit in the channel and you need to have a lil room for them to squish when you tighten the boot down to seal the intake well.
 
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