80 750 ca

[Souske]

Hello GS community. I recently became the proud new owner of a 750GS


The problem is with the carbs. The bike came with two banks This is the one claimed to be original and a bike:


It looks like the P.O. snapped off an EZ out in the #4 mixture screw
It also came with another bank of carbs he claimed were off a 1000


The second set appears to be gone through, and has #125 jets, (Bike already has Jardine exhaust, 4-1 header, and K&N Pods taped off about 1/2) It also came with a big box of extra jets, gaskets, o-rings etc.

Cleaned the carb bank 2, removed everything i could, sprayed out all passages, set float level, and.... #3 still pees everywhere. The needle and seat looked good upon dissasembly, are my eyes playing tricks on me? The seats were shiny new brass, maybe a defective one? Would it be advisable to juggle #3,4 and see if the problem follows? It looks like the P/O was aware of this, as the float tabs were bent so as to hold the needle closed under all circumstances.

Also, these appear to be the same carb to me with the exception of the phenolic caps, and different bowl design. On the engine side of the carb the original measure 32mm, where as the newer ones are 31.97 Is this a nominal measurement, or will this slight difference mess things up? Do I need to order parts differently?

Also In my research it was said to use only oem needles and seats, however $39 each sounds a little pricy, is there somewhere else i should be shopping?

I will be going to get a carb dip can, and begin the complete teardown as outlined. Just want it to work on the next re-assembly. Thanks!

[BassCliff]

Greetings and Salutations!!

Hi Mr. Souske,

I'm pretty sure all of the '80 and newer 1000cc models use BS34SS carbs, whereas your 750 uses BS32SS carbs. So I can only guess as to where the second rack of carbs came from. But it sounds like you are off to a good start with dipping the carbs and all. Yes, OEM float needles and seats are expensive, but they don't leak. The parts included in aftermarket "carb rebuild kits" are of lesser quality and are problematic a lot of times.

Let's keep the terminology consistent. There isn't a "pilot air screw" in the CV type carbs. I think you are referring to the "idle mixture screw". It meters the amount of fuel in the idle circuit.



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Thank you for your indulgence,

BassCliff

[Souske]

Ok so I swapped needles and seats between #3 and #4, now fuel is rushing out at a greater rate. On #1, there is fuel shooting out of the pilot air jet and the breathers.

Totally confused

How do I tell the BS34SS from a BS32SS?? I did not see any markings like this when I took them apart the first time

[chef1366]

Measure the ID on the intake side of the carbs. If they are off a 1000 they will be 34mm and the 750 carbs ard 32mm.
The black carbs appear to be off an 83 GS750 with the 80-81 choke assembly on it.
Did you change out the o-rings on the inlet valves? (float seats)

[Souske]

Ok so #3,4 are dipped, and re-assembeled

Checked #3 and found the float was sticking, turns out it the tang on the fuel inlet retainer was bent just enough that the float arm would hang up on it in the fully down position. Bent it back and it seems to be working great now.

Waiting on #2 to come out so #1 can go swimming.

Thanks GSresource!

[Souske]

Alright carbs are re-assembeled, inlet needles and seats replaced on #1,2 along with new o-rings for everything.

Bike runs great at 0-1/4 throttle, any more than that and it bogs and dies.

Stock jets 42.50 main and 170 air, with the exception of 115 pilots substituted for 112.5 fuel float level set.

Idle screws set at 3.5 as per this chart: http://members.dslextreme.com/users/...at_height.html

There was no listing for an 80, so i used the setting listed for an 83 750, is this a wrong assumption?

Will adjust the valves and report back

[Nessism]

Your numbers for the main and pilot jets are reversed.

I didn't look up the stock jetting numbers for your carbs but please understand, changes to the airbox and/or exhaust system will alter the jetting requirements, particularly if the stock airbox is not installed. The inability for the engine to accept throttle is a classic example of a missing airbox, or installation of pods without changing the jets to match.

[Big T]

Hmm

42.5 main jet?

Are the pods installed?

The CV carbs required a certain level of restriction to perform. That's why the PO had 1/2 the pods taped over, to achieve restriction. You can drape a shop rag over the inlets as a stop gap for testing

Many new owners either get a stock airbox, or a Dynojet or Factory jet kit for the bike, so they can have an adjustable needle

Others rejet and raise the needle with shims and get good results.

It's generally found that the success with pods is directly proportional to how much they cost - EMGO's yuk, APE OK, K&Ns best

[Souske]

Ok sorry, having a bit of trouble with terminology. The pilot jet is 42.5, and the main jet is 115 (112.5 stock)

Running K&N pods taped off halfway.

I found instructions for a dynojet kit in the box of extra parts that came with the bike, however is there any identifying marks on the dynojet jets?

I also have some sets of larger main jets, 125 and 135

Any suggestions on needle settings or main jet sizes?

In other news: got the valves adjusted last night, sounds much smoother, could barely get the .003 feeler in there

[BassCliff]

Ok sorry, having a bit of trouble with terminology. The pilot jet is 42.5, and the main jet is 115 (112.5 stock)

Running K&N pods taped off halfway.

Why not run the K&N filters the way they were designed and then jet accordingly? Going up only one size may not be enough.

I found instructions for a dynojet kit in the box of extra parts that came with the bike, however is there any identifying marks on the dynojet jets?

Yes. There should be a manufacturer's logo and size numbers stamped on the business end of the jet.

Any suggestions on needle settings or main jet sizes?

Normally the jet sizes and needle height are determined by performing plug chops and reading the plugs. CLICK HERE for more information.

Also see THIS PAGE at the Factory Pro website for their carb jetting procedure.

In other news: got the valves adjusted last night, sounds much smoother, could barely get the .003 feeler in there

Is that .003 inches? That is the tightest end of the range (.003-.005 inches or .08-.13mm). I like running on the loose end of the spec so that the motor breathes better and the valves run a little cooler. But you're OK as long as you don't run too tight for too long and start burning valves.


Thank you for your indulgence,

BassCliff

[dueller]

MAN! That is a sharp color scheme. Blue just happens to be my favorite color. Might have to keep that in mind when I get to repainting mine....

Hello GS community. I recently became the proud new owner of a 750GS

[Souske]

valves were .003 before the adjustment, they seem much happier at .005 now

The only valves i found with any distinguishing marks besides the size was an R with a square and a smaller square in the lower left corner of the larger square, guessing these probably aren't the right ones

[Big T]

The square and square jets are the genuine Mikuni jets for your carbs

Check the other jets, if they lack the squares, then they're likely Dynojet sizes.

The DJ and Mikuni sizes are different for the same number (say, 125)

Somebody has the conversion table

The 115 jets are too small for your set up. Go to the 125 Mikunis and see what you get

[BassCliff]

Hi,

Nice work Mr. Souske.

The stock Mikuni jets are a measured flow rate, i.e. a 125 main jet would flow fuel at 125cc per minute.

The Dynojet jets are measured by actual diameter, i.e. a 130 main jet would be 1.3mm in diameter.

For a conversion table (of sorts), see: http://roadstarclinic.com/content/view/61/96/


Thank you for your indulgence,

BassCliff

[Souske]

Ok, turns out i only had 3 125 jets, but had 4 135 jets, popped them in and the bike runs great!

After looking at the tuning method starting with highest RPM and working down, I have decided to replaces some other items on the bike before a proper tuning.
Tires are three digits with the last digit being a 7. They are German made Metzelers, so from what i can gather in the tire threads they must be dangerously old. I am looking at the Shinko 712 http://www.bikebandit.com/shinko-712-motorcycle-tire
Will this be an adequate tire for a Noob rider? Or do I need to buy stickier tires to soften the learning curve?
The chain was also ready for retirement. Replaced it with a RK chain, and as luck would have it the US distributor is only about 30 mins away. Cleaned and inspected sprockets, they show virtually no wear, still nice square peaks, and no uneven wear in the valleys.
Got some smaller things done like cleaned and greased the speedo and tach cables, and glued the speedo needle back together.
I have also enrolled in a MSF Basic Rider course, doesnt start until 7/1, just want to shake as many bugs out as possible before then.