Yet another carb thread - but my first one!

[TheCafeKid]

Less fuel needed to the 750...the higher the number, the quicker the fuel valve shut off basicly.

[49er]

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?

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.

[dardoonk]

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?

[TheCafeKid]

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.

[ptm]

There is no gasket http://alpha-sports.com/spst/1977%20GS750/01.htm. The o-rings do all the sealing which is why it's so critical to have them in good shape.

[Nessism]

Yup, there is not supposed to be a gasket. Peel if off and replace the o-ring underneath.

[dardoonk]

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.

I wasn't sure if the entire back of it was metal or not.. Gives me a bit more confidence in digging these things out. I'll let you know how it turns out.

[KEITH KRAUSE]

No gasket is used between the manifold and head. Makes you wonder what else could be wrong with the carbs.
The manifold o-rings should always be replaced if it's been a few years since taking off the carbs. If hard/stuck, I use a toothpick to clean out the old o-ring. I suggest buying some Allen bolts so you can torque the manifolds down to about 6ft/lb. Apply some hi-temp bearing grease to them to help them last.
Without a factory manual, I'm not sure about the float level but I'm thinking 24mm is very close.
You probably do have an intake leak but double check the throttle pulley for rubbing anything when you re-assemble.
Also, double check advance timing mechanism for correct/smooth operation. That will cause a "hanging idle" too. If by any chance you have an old Martek or similar ignition, make sure the set screw is tight on the shaft collar holding the sensor. A "spinning" collar will hang the idle.
An air leak at the manifold will allow combustion too easily/at the wrong timing and combustion will be irratic between cylinders, so you get a higher rpm.
As for jetting, I suggest getting rid of the foam pods if you can. My opinion is they are poor quality. Older ones begin to deteriorate.
Set up as is, I think your model came with 102.5 mains as stock? 3 full sizes (up to your currect 117.5) isn't enough for cheaper pods and that pipe. I don't have much experience with those foam pods but I'd go with a range from 122.5 to 130. 125/127.5 being my guess. Advance the timing 3-5 degrees too and see how it works AFTER the jetting seems close.
A minimum of one position richer for the jet needles. I'd probably initially try 1 1/2 positions richer by using the .022" thick jetting spacers on top the e-clip in the 5th position from the top. That assumes your stock needle e-clip position is 3 from the top?? Not sure, but I thought some '77's 750's (or was it '76's ?) came with the needle e-clip in the 2nd position?? Verify correct/factory position first. Whatever, I'd try 1 1/2 positions richer first. If you decide to go with some K&N's, go straight to position 5.
I think the 17.5 pilot jets will work. I hope it's the correct length pilot jet though. Mikuni makes two different lengths of pilot jet for the VM's and you have to be sure of that.
The bike should respond to side air screw adjustments. Many say they can't tell a difference and swear the carbs and bike are tuned well. I don't see how the screws won't make the bike respond as expected if that is the case. In your case, they are generally in the ballpark at 1 1/2 to 2 turns out. Try 1 3/4 and if that is also close to where you believe there's a slight rise/improvement in rpm, leave them there.
I'd initially try 1 1/4 turns out from LIGHTLY seated for the pilot fuel screws underneath.
Don't forget, if you re-use old header gaskets, they can leak and cause decel' popping and you don't want to blame the pilot circuit for that.

[dardoonk]

Well, after digging for 20 minutes and getting about 1/8 of the first old O-ring out, I figured the H3ll with it, and ordered a new set of intake rubbers. By the rubber on the inside of the boot is beginning to crack further down as I try to remove the old O-ring, so it's time to put these one's to bed.

Keith, I'll heed your advice and put a set of 125 mains & and work from your starting points. The carbs were completely torn down and rebuilt, so I am not worried about anything being wrong with them. Clip position currently remains in the stock (#3) setting.

I'll have to stick with the Uni's though.. They are brand new & frankly, are the price I pay for having "the look" I want on the bike.

Parts won't be here before Feb 5 & I am heading to Vegas for a week\\/, so I'll let you know how everything is working in a couple weeks

[KEITH KRAUSE]

OK. Good luck with it.
Just to mention...is it the 750 that uses different manifolds (2 left and 2 right)?? I may be wrong. They should have different part numbers when you ordered if this is so.

[dardoonk]

OK. Good luck with it.
Just to mention...is it the 750 that uses different manifolds (2 left and 2 right)?? I may be wrong. They should have different part numbers when you ordered if this is so.

The fiche shows only one manifold.

[robertbarr]

Just to mention...is it the 750 that uses different manifolds (2 left and 2 right)?? I may be wrong. They should have different part numbers when you ordered if this is so.

The earlier bikes (pre-80) aren't directional, Keith. It's the 80 & later you're thinking of. They also have RH & LH molded into them, for cavemen like yours truly.

[dardoonk]

All right Keith, here ya go... in anticipation of my next project, I just landed a set of 29mm smoothbores, but I figure now that I have them & the project motor is a while off, why not try them on my 750?? Will I see a benefit once they are tuned properly?

Any advice on what to expect for jetting and what intake rubbers I am going to need now? I assume the stockers are not going to fit the smoothbore carbs.