1983 GS750E BS32 carbs jetting and needles questions

D

dilbone

Been Around the Block
So I picked up this bike last weekend and I'm digging into it for a cafe-ish build.
The PO had pods and an open exhaust so I figured something had to have been done or it wouldn't have run well enough to even be able to ride it without some moderate level of frustration.
I pulled the main jet out and it looked like plastic...
Here are the numbers. According to what I've found Main should be 117.5 and pilot 37.5
What I have in these is a 145 main and 37.5 pilot, with a 160 pilot air that isn't even listed on the chart I saw. I couldn't imagine jetting 9 steps up on the mains...my xs650 which has pods and headers with small baffles and I only went up 2 on the mains and 1 on the pilots which is perfect.
After seeing these I actually texted the PO to see if he knew anything about the mods to the carbs. He didn't, but said the guy he bought it off of had a couple of drag/race bikes and figures he must have made the changes.
I then asked him if it ran well and he replied with, "it ran pretty good, but I wouldn't call it perfect." I'm having a hard time imagining that.

At this point I know I need to up the pilots and reduce the mains...but the needles will likely be an issue also I assume. Is the stage 3 kit worth getting with the new needles? Or can I get needles for these like I can get the individual jets? Will shimming the needles accomplish what I'll need?

Any advice is appreciated.
 
145 is too big.
I think I used 125 or 127.5 on last one I did plus adjusted float level 1mm higher than stock
 
crazypj said:
145 is too big.
I think I used 125 or 127.5 on last one I did plus adjusted float level 1mm higher than stock
Click to expand...
I know 145 is enormous...
I was figuring 2 or 3 sizes up on the mains from stock. Did you change the pilots?
 
SupDOHC said:
the jet kit should come with a drill bit for the vacuum slide lift holes.
Click to expand...

Yeah, I finally looked up the kit number and found the list of parts in the kit...it just doesn't make any sense to me that the pilot jet would remain stock, it just seems like that would lean it out too badly at cruising. Enlarging the vacuum lift holes and plugging the main air jet passage and no change in pilot doesn't make sense to me. I'm no tuning expert, but I've never heard of going to pods and open exhaust and not making a change in pilot jet.
 
Ok, I'm still trying to figure out what I actually have in these carbs before I spend any money on jets or a kit.
The pilot jet doesn't match the pic on jets r us. They have a pilot the same shape but with holes in the shaft listed for this model bike...mine don't have holes.


The mains are on the left in this pic. They are nylon and have no makers mark. They are listed as 145 but I see that a 145 keihin jet converts to a 125 mikuni. Is that even a possibility. Who even makes nylon jets? I'm finding more questions than answers the more I look. Can anyone tell me if the pilot and main listed at jets r us for the 1983 gs750 are the correct jets?
 
dilbone said:
Yeah, I finally looked up the kit number and found the list of parts in the kit...it just doesn't make any sense to me that the pilot jet would remain stock, it just seems like that would lean it out too badly at cruising. Enlarging the vacuum lift holes and plugging the main air jet passage and no change in pilot doesn't make sense to me. I'm no tuning expert, but I've never heard of going to pods and open exhaust and not making a change in pilot jet.
Click to expand...
turning out the pilot screw will richen your idle circuit. the pilot jet is plenty big enough for that.
 
SupDOHC said:
turning out the pilot screw will richen your idle circuit. the pilot jet is plenty big enough for that.
Click to expand...
I'd still feel comfortable going up one step on it. However, at this point...I'm so unsure of what I actually have in these carbs I may just put the whole thing back together the way the carbs are and see how it runs before I change anything.

The nylon main jet is a mystery at this point. A keihin 145 is equivalent to a mikuni 125. It's possible I suppose that these mains are actually close to the right size. The pilot, based on jets R us info, isn't the correct one for bs32 carbs. I believe they show one with 6 cross holes in it and this one is solid.
The size is the stock size but according to them it's the wrong jet.

I still need to check and see if the needles have either been replaced or shimmed. If they have then I might just let it ride as is and see how badly it runs...unless I get info about the stock pilot jet or info on the nylon main jet labeled 145.
 
Here are the main and pilot jets in these carbs...
Is the pilot supposed to have cross holes?
What exactly is this main?
 
Looks like a nylon main jet.
They were popular with drag racers as they didn't corrode when using methanol fuels
The pilot jets come in various styles, some have cross drilling others do not
Carb will have drillings to add air. You'll need to check service maunual for type (parts list is actually better/more acurate than manual)
BTW, you don't change pilot jets, they arte not perating at a high load/throttle opening but can have a bad effect at idle/low rpm if t big.
Better test is the fuel/air screw at idle and just off idle..
If it needs to be more than 3-1/2 turns out to get a tgood reliable idle, go up one size. less than 1/2 turn, go down a size
 
Christ this thread is a mess.

How did it run when YOU rode it?

If you havent rode it yet wtf are you doing trying to jet it and furthermore wtf is anyone doing making suggestions
 
I've done a bunch of GS750 and GS1000 carbs when working at Suzuki dealership in late 70's to 1990 so I feel I'm quite qualified to make suggestions :p
(plus 4 valve GSX motors) :p
 
crazypj said:
I've done a bunch of GS750 and GS1000 carbs when working at Suzuki dealership in late 70's to 1990 so I feel I'm quite qualified to make suggestions :p
(plus 4 valve GSX motors) :p
Click to expand...
B.S. cmon pj, you have no idea whats inside his engine. He said he bought it from a drag racer. If he upped the bore or the compression or both it very well needs more fuel, and also, :p
 
To be honest, it doesn't really matter whats inside the engine.
Getting it running is the first order of business.
On the street it will not need massively rich jetting as it will rarely, if ever, see max rpm under load (that's why 'all' carburettor tuning manuals say slight up hill in third gear - and dyno runs are usually performed in third gear to load motor)
First or second gear isn't going to put much load on the motor, running slightly lean won't matter for a few seconds. You rarely ever need crazy rich jetting if there is a big bore kit fitted, the numbers I put out will be OK for an 820 which is most common overbore for GS750.
High lift, long duration cams will probably cause major flow reversion at low/starting rpm so smaller than stock pilot jets may be needed (as well as dropping needles to a leaner position) Servicing stock bikes may pay the bills but it really isn't any 'fun' which is why I've been building oversize engines since 1974. I also taught this stuff at MMI for almost 12 yrs but regularly got called into office for giving extra information to students (if it ain't on the curriculum it doesn't exist ::) )
 
Never give jetting info without a ride and running condition report. End of story, any other answer is just wrong.
 
crazypj said:
Looks like a nylon main jet.
They were popular with drag racers as they didn't corrode when using methanol fuels
The pilot jets come in various styles, some have cross drilling others do not
Carb will have drillings to add air. You'll need to check service maunual for type (parts list is actually better/more acurate than manual)
BTW, you don't change pilot jets, they arte not perating at a high load/throttle opening but can have a bad effect at idle/low rpm if t big.
Better test is the fuel/air screw at idle and just off idle..
If it needs to be more than 3-1/2 turns out to get a tgood reliable idle, go up one size. less than 1/2 turn, go down a size
Click to expand...

Thanks crazypj, do you know of any way to find what a 145 in this nylon jet is comparable to in mikuni size wise or am I just better off to start with a mikuni main sized as you suggested and go from there? I will hold off on buying pilots until I get it running and see.

DohcBikes said:
Christ this thread is a mess.

How did it run when YOU rode it?

If you havent rode it yet wtf are you doing trying to jet it and furthermore wtf is anyone doing making suggestions
Click to expand...

I haven't ridden it...it's in pieces.

1. The stock jetting has been changed
2. The main jet is a "Mystery" jet that I have no way to tell what size it is in mikuni sizing
3. Returning to stock sizing would be an exercise in futility with current mods the PO's did
4. Having a place to start is kinda necessary
5. Am I expecting to have it tuned perfectly on the first start up...no
6. Do I need a decent place to start...yes

crazypj said:
I've done a bunch of GS750 and GS1000 carbs when working at Suzuki dealership in late 70's to 1990 so I feel I'm quite qualified to make suggestions :p
(plus 4 valve GSX motors) :p
Click to expand...

I appreciate you sharing your expertise

DohcBikes said:
B.S. cmon pj, you have no idea whats inside his engine. He said he bought it from a drag racer. If he upped the bore or the compression or both it very well needs more fuel, and also, :p
Click to expand...

I didn't purchase it from a drag racer...my PO said HE bought it off a guy that had a couple of race or drag bikes. I don't think either of them knew what they were doing. The mains have been changed and yet the plugs are still in place covering the air mixture screws. I'm not sure how much tuning could have been done on these carbs without making any adjustment on the air mix....

Conclusion...I think this guy probably changed the mains out, put pods and exhaust on it, and called it a race bike...

crazypj said:
To be honest, it doesn't really matter whats inside the engine.
Getting it running is the first order of business.
On the street it will not need massively rich jetting as it will rarely, if ever, see max rpm under load (that's why 'all' carburettor tuning manuals say slight up hill in third gear - and dyno runs are usually performed in third gear to load motor)
First or second gear isn't going to put much load on the motor, running slightly lean won't matter for a few seconds. You rarely ever need crazy rich jetting if there is a big bore kit fitted, the numbers I put out will be OK for an 820 which is most common overbore for GS750.
High lift, long duration cams will probably cause major flow reversion at low/starting rpm so smaller than stock pilot jets may be needed (as well as dropping needles to a leaner position) Servicing stock bikes may pay the bills but it really isn't any 'fun' which is why I've been building oversize engines since 1974. I also taught this stuff at MMI for almost 12 yrs but regularly got called into office for giving extra information to students (if it ain't on the curriculum it doesn't exist ::) )
Click to expand...

I may be asking for more advice once I get it running and playing with the tuning.

DohcBikes said:
Never give jetting info without a ride and running condition report. End of story, any other answer is just wrong.
Click to expand...

As stated earlier, I need a starting point...
 
You had a starting point.

Never attempt jetting if you dont already know how it runs. I'm not going to change my advice, it's good advice that has been proven to avoid wasting time and thought.

Keep us posted.
 
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