1979 GS750 engine questions.

[Anonymous]

Hi Guys,

I have a 1979 GS750 and I've been looking around for carb tuning tips. I've rebuilt the carbs using 4 Keister rebuild kits and adjusted the Air mixture screws to one turn out from closed, and the fuel mixture screws also to one turn out (That's what the Cymler manual said).

Anyway, The GS750 starts up fine, seems to run fine (Back fires a little bit, but I'll try adjusting the valves to see if that;s the reason). I also have a homemade exhaust installed on it so maybe I need to make some changes to the exhaust but I'm not sure yet.

I'm wondering how I dial in the air jets and if I need to make any changes to the fuel mixture screws.. I have 4 K&N air filters installed and a custom manafold/exhaust. Also, the engine is tilted up quite a bit so the float level inside the carbs is probably a little higher than normal. I'm wondering if I can get carb angle adjustment mounts(?).

If someone can help me tune this, I'd really be gratefull.. BTW, here is a picture of my 1979 GS750

I've posted more pictures of it here: http://www.psychokart.com

TIA,

Adrian

 

[KEITH KRAUSE]

Well, I'm not sure how the angle of the carbs is going to effect jetting.
The "rear" part of the bowl will have a higher fuel level than normal and the front part will be lower. Right at the middle of this point is the main jet and pilot jet. I have to believe the jets won't draw sufficient fuel from a stock float setting, which is between .90 and .98". I would think the bowl levels will have to be increased, but I can't say how much. I've never tilted the carbs like you have them. Maybe 1/8" more fuel level?? Of course, you can try it like it is and see if you get fuel starvation while on the main or anywhere else. If you do, then this has to be fixed FIRST. If you do have fuel starvation, it could be difficult to initially blame this on a low bowl level or small main jets though.
As for the jetting, I don't know how your header effects the jetting I'm used to installing, but you can try this:
Main jet, 5 full sizes up. Stock is 102.5, so try Mikuni 127.5
Jet needle position, the e-clip is in the 3rd position stock, so try the bottom (5th) position. Make sure the two stock plastic spacers go back as stock...thicker one above the e-clip, thinner one below.
The pilot fuel screws (underneath) are sensitive and often set at different points because they are the fine tuning for each cylinder. For your mod's, 1 1/4 turn out from LIGHTLY seated is a good starting point. These help fuel the pilot circuit, so check performance at minimal throttle openings and to take any plug reads. Turning them out, richens the mixture. You normally need to make only small adjustments, such as 1/4 turn or so.
Then adjust the side air screws for highest rpm. Set them initially to 1 1/2 turns out. Warm up the motor first, then adjust idle to 1,100 rpm's with the idle adjuster knob. Starting at any carb, slowly adjust the screw in either direction to find highest rpm sweet spot. This is generally 1 1/2 to 1 3/4 turns out. Now re-set the idle to 1,100 rpm with the idle adjuster knob. Repeat this to the other carbs. DO NOT change the air screw settings unless you change the pilot fuel screw settings beyond a 1/4 turn from the starting point or you increase the pilot jet size. In any case, set them as I said and as a final adjustment after you know the jetting is done.
REMOVE the two float bowl vent lines if they're still attached. Leave the ports open.
You MUST SYNCH the carbs as a part of jetting. Bench synch first, then vacuum synch.
This jetting should get you going. Of course, these are only suggestions. You have to test. Full throttle for the mains, 1/3 throttle for the jet needles and minimal throttle opening for the pilot circuit. If the pilot circuit wants to run lean, you can adjust the pilot fuel screws to about 3 turns out and still have effect. After that, you'll need to increase the pilot jet to a 17.5 if needed.
By the way, valve clearances must be correct and the ignition timing spot on BEFORE the vacuum synch. I also always suggest an o-ring check/replacement inside the carbs (Robert Barr sells a good/fair priced o-ring kit) and new manifold o-rings too. Clean/blow out the carbs while you're doing this and checking float levels. Oil the K&N's correctly with K&N oil only.
If the above, or a jetting set up similar doesn't get you going, I suggest a Dynojet jet kit.
Let me know if you need help.

 

[KEITH KRAUSE]

Well, I'm not sure how the angle of the carbs is going to effect jetting.
The "rear" part of the bowl will have a higher fuel level than normal and the front part will be lower. Right at the middle of this point is the main jet and pilot jet. I have to believe the jets won't draw sufficient fuel from a stock float setting, which is between .90 and .98". I would think the bowl levels will have to be increased, but I can't say how much. I've never tilted the carbs like you have them. Maybe 1/8" more fuel level?? Of course, you can try it like it is and see if you get fuel starvation while on the main or anywhere else. If you do, then this has to be fixed FIRST. If you do have fuel starvation, it could be difficult to initially blame this on a low bowl level or small main jets though.
As for the jetting, I don't know how your header effects the jetting I'm used to installing, but you can try this:
Main jet, 5 full sizes up. Stock is 102.5, so try Mikuni 127.5
Jet needle position, the e-clip is in the 3rd position stock, so try the bottom (5th) position. Make sure the two stock plastic spacers go back as stock...thicker one above the e-clip, thinner one below.
The pilot fuel screws (underneath) are sensitive and often set at different points because they are the fine tuning for each cylinder. For your mod's, 1 1/4 turn out from LIGHTLY seated is a good starting point. These help fuel the pilot circuit, so check performance at minimal throttle openings and to take any plug reads. Turning them out, richens the mixture. You normally need to make only small adjustments, such as 1/4 turn or so.
Then adjust the side air screws for highest rpm. Set them initially to 1 1/2 turns out. Warm up the motor first, then adjust idle to 1,100 rpm's with the idle adjuster knob. Starting at any carb, slowly adjust the screw in either direction to find highest rpm sweet spot. This is generally 1 1/2 to 1 3/4 turns out. Now re-set the idle to 1,100 rpm with the idle adjuster knob. Repeat this to the other carbs. DO NOT change the air screw settings unless you change the pilot fuel screw settings beyond a 1/4 turn from the starting point or you increase the pilot jet size. In any case, set them as I said and as a final adjustment after you know the jetting is done.
REMOVE the two float bowl vent lines if they're still attached. Leave the ports open.
You MUST SYNCH the carbs as a part of jetting. Bench synch first, then vacuum synch.
This jetting should get you going. Of course, these are only suggestions. You have to test. Full throttle for the mains, 1/3 throttle for the jet needles and minimal throttle opening for the pilot circuit. If the pilot circuit wants to run lean, you can adjust the pilot fuel screws to about 3 turns out and still have effect. After that, you'll need to increase the pilot jet to a 17.5 if needed.
By the way, valve clearances must be correct and the ignition timing spot on BEFORE the vacuum synch. I also always suggest an o-ring check/replacement inside the carbs (Robert Barr sells a good/fair priced o-ring kit) and new manifold o-rings too. Clean/blow out the carbs while you're doing this and checking float levels. Oil the K&N's correctly with K&N oil only.
If the above, or a jetting set up similar doesn't get you going, I suggest a Dynojet jet kit.
Let me know if you need help.

I'm not sure why I bother. :?

 

[Anonymous]

I hear ya Keith, I am a member of an ATV forum as well and I've typed several long replies to try to help others with no reply. It is a little disheartening but let me say I fully enjoyed this post and all the others of yours I've come across. I'd hate to see you stop trying to help because of the few who don't come back and say thanks.

So let me say thank you for being one of the many extremely helpful people on this forum.

 

[KEITH KRAUSE]

I hear ya Keith, I am a member of an ATV forum as well and I've typed several long replies to try to help others with no reply. It is a little disheartening but let me say I fully enjoyed this post and all the others of yours I've come across. I'd hate to see you stop trying to help because of the few who don't come back and say thanks.

So let me say thank you for being one of the many extremely helpful people on this forum.

Well, I'm not sure why I vented like that. I was doing a search on a subject and that old topic came up. I clicked it and for some reason got irritated about it. :roll: I've seen a lot of hit and run "members" stop by and I've never said a thing before. It's just that I'm confident that my reply to him will at least get his jetting close, if not correct. I know I've saved him many hours of playing with the jetting and money too. So I took the time to help him and then get nothing back, not even an "OK, will try it" reply. But who knows, maybe he'll come back and I'll feel like a jerk.
Thanks for the compliment. Your reply is what makes me and I'll bet a few others stay here. It feels good to help, or to at least try, and a "Thanks" goes a long way!

 

[humhead]

t is a little disheartening but let me say I fully enjoyed this post and all the others of yours I've come across. I'd hate to see you stop trying to help because of the few who don't come back and say thanks.

So let me say thank you for being one of the many extremely helpful people on this forum.

There's no doubt you are one of the most helpful people on this forum. I've learned from your replies to this person & the many others you've been kind enough to help.

Thank you
Terry

 

[KEITH KRAUSE]

t is a little disheartening but let me say I fully enjoyed this post and all the others of yours I've come across. I'd hate to see you stop trying to help because of the few who don't come back and say thanks.

So let me say thank you for being one of the many extremely helpful people on this forum.

There's no doubt you are one of the most helpful people on this forum. I've learned from your replies to this person & the many others you've been kind enough to help.

Thank you
Terry

Thanks Terry. I appreciate that.
I was just venting a little. We all have to be well vented you know. :wink:

 

[jimcor]

Keith it's a dirty job but somebody has got to do it and...you got it. 8O

If it was up to me Berryman's stock would go sky high! (It does fix everything carbarberator, don't it?)
Dump in half a can and call me in a hunnert miles!

PS...they're going to double your pay!!!