1979 GS750L Carb jetting help?????

[Albert257]

I recently acquired a 1979 GS750L which I plan to eventually convert to a cafe racer. But, before I do all the body and frame work, I want to get the engine running well. I plan to get rid of the stock airbox and run with 4 separate air filter cone pods, and also get rid of the stock exhaust and run a homemade 4-1 exhuast. My carbs need cleaned badly, and I assume they will need re-jetted as well with the air filter and exhaust mods. Has anyone done this before? What adjustments do I need to make to my carbs, and what size jets should I run with for this set up?
Also, any other advise or performance mods that you guys recommend?? This is my first motorcycle project. I need any and all advise. Thanks!!!

[KEITH KRAUSE]

The big question is how the home made exhaust will flow. If not well or it flows incorrectly, the jetting will be off. Only testing will tell.
Just be sure the bike is serviced and ready for a re-jet. Adjust valve clearances, good spark/timing/NGK B8ES plugs needed, clean carbs with fresh inner o-rings from Robert Barr, manifold o-rings replaced and manifolds inspected for any possible intake leaks, etc.
If the pipe flows similar to a quality pipe and you use K&N ovals, try the following:
130 Mikuni mains...
Jet needle e-clips in the bottom (5th) position (if stock needles) with factory plastic spacers in factory installed order...
Float levels at .94"...
Stock #15 pilot jets with richer pilot fuel screw (underneath) adjustments, about 1 1/2 turns out from lightly seated for starters. Try up to 2 1/2 turns out if needed. If no joy, try 17.5 pilot jets and start the pilot fuel screws at 1 turn out. Fine tuning will be necessary...
Set the side air screws initially at 1 1/2 turns out...
Bench synch the slides as best you can...
REMOVE the two floatbowl vent lines at carbs 2 and 4. Leave the ports open...
After starting and fully warming up, adjust the side air screws using the highest rpm method found at this site, search...
Vacuum synch...
Test the mains at full throttle, the needles at 1/3 throttle and the pilot circuit at minimal throttle/low speed cruising. Do what the plugs/performance say.

Best additional mod's for the bike if not done yet, in my opinion:
Dyna S ignition and green coils, NGK 5 ohm plug caps, Electrex or similar regulator/rectifier unit, Dynojet jet kit (if the above jetting isn't working well).
Note: the stock jet needles often can't be set rich enough. If that's the case with your bike, I suggest the DJ jet kit. It's up to you. If you buy the kit, try their needles in the 4th position from the top and try the 138 mains. Remember, that kit was designed for use with K&N filters and a quality pipe.
Good luck!

[Albert257]

Thank you very much for your advice. I will give all that you suggested a shot and let you know how she turns out!!

[Albert257]

2 more questions:
Before I found this site, I found a strictly Mikuni Carb forum. I asked my same carb recommendation question there and it was suggested that I use:
-#110 main jets
-#17.5 pilot jets
-reclip jet needle to the 4th slot
-air screw out 1 3/8 turns
-pilot mixture screw out 1.5 turns
Is this similar to the recommendation you gave me?? Why this big difference in main jet size recommendation?

Second question: Is there a thread, post or sticky in the forum that explains the 'highest rpm method'???? Thanks!

[KEITH KRAUSE]

2 more questions:
Before I found this site, I found a strictly Mikuni Carb forum. I asked my same carb recommendation question there and it was suggested that I use:
-#110 main jets
-#17.5 pilot jets
-reclip jet needle to the 4th slot
-air screw out 1 3/8 turns
-pilot mixture screw out 1.5 turns
Is this similar to the recommendation you gave me?? Why this big difference in main jet size recommendation?

Second question: Is there a thread, post or sticky in the forum that explains the 'highest rpm method'???? Thanks!

I guess everyone at the "Mikuni carb" forum likes to run their bikes lean.:roll: As in real lean at the main and jet needle circuits.
Your stock mains are 102.5 Going up to 110's isn't nearly enough. 127.5 or 130 is my guess. Still depends on that home made pipe of yours. To back me up on the main jet choice, consider this: Dynojet supplies a 138 main jet in their stage 3 kit. DJ and Mikuni use different methods to come up with their main size (Mikuni = cc's that flow through in a minute and DJ = the actual size of the hole in mm's). A 138 DJ is nearly identical to a Mikuni 130. Check the web for a comparison chart if you wish.
I have no argument over their pilot jet choice. I see no reason NOT to give the stock 15 jet a try if you richen the pilot fuel screws. If it doesn't work, get the 17.5's. Many times the 15 does work. Also, DJ suggests keeping the pilot circuit stock when using their kits. Why buy jets if you don't need them?
As for their jet needle position choice, your stock position is #3. Any decent pipe/pod mod will require you to richen the needle more than 1 position. Try the bottom. As I said earlier, the stock needle, even at the bottom (2 positions richer) may still not work and you'll need a DJ kit.
As for their air screws setting, there IS no "setting". Each bike can be a little different. You set them using the highest rpm method. They may end up 1 1/2 turns out, or maybe 2 turns out or somewhere in between.
A search would show you but here it is: set the screws to 1 1/2, adjust throttle cables correctly, warm up the bike completely and place on centerstand, set idle to 1,000 rpm's with the idle adjuster knob.
Starting at any carb, slowly turn a screw in either direction until you hear the rpm's max. Fine tune it. Now lower the rpm's to 1,000 by adjusting the idle adjuster knob. Repeat this to all other carbs.
As for their pilot fuel screw setting choice, I said 1 1/2 was a good starting point. If the pilot circuit isn't the correct mixture by the time you have these 2 1/2 turns out, go to the 17.5 and reset the screws to 1 turn out and test.
I'm not sure why you're asking me if their set up is similar to my suggestions. You can see for yourself. I typed a long and detailed message trying to help you and to avoid confusion. Maybe my messages are too long and people skip over some of it?
If you want to trust this Mikuni forum over my suggestions, that's fine. I know what works. I've jetted many bikes over the internet and most of them at this site. Many members post back to say the bike runs great and on the first try. We all know getting the jetting right on the first try is very hard to do or just plain lucky. I keep notes about previous jetting set ups and that helps. I admit that your home made pipe may add a twist to your jetting and it may not. I don't have much jetting experience with home made pipes. Mostly V&H, Yosh, Kerker, Jardine, Mac...

[Albert257]

Like I said, carbs are all new to me. I am still learning. I am excited to try re-jetting my carbs and getting my bike running, and obviously my goal is to get it right the first time. The reason I second guessed your recommendation, was simply because of the seemingly large difference (110 mains v.s. 130 mains) between yours and the other guys suggestions. Since I am new to this forum and new to working on carbs, I really have no way of knowing what is best for my set up. However, you seem much more knowledgeable than the other guy from the mikuni forum. I have already ordered some 130 mains and the o-ring set from Robert Barr, as per your suggestion. As soon as they arrive in the mail, I plan to basically go step by step through your detailed explanation and rebuilt my carbs as you instructed. I really do appreciate your suggestions and detailed instructions. And don't worry, as soon as I finish the carbs and put them on the bike, I will let you know how it all runs!! Heck, when that time comes I probably have more questions for ya anyway!

I am excited to try out my homemade 4-1 exhaust too!!

[KEITH KRAUSE]

I don't know if you have a factory manual or ? to help you, but there's some info you may be interested about at a thread here. I'm sorry I don't remember how to supply a link but you can just click the search button near the top of this page and check out Hoomgars thread "78 GS1K carb rejet bad fuel economy KEITH KRAUSE". It's a long thread but gives you some idea about re-jetting and the trials and errors that some go through. Believe me, many go through worse and decide to give up and return the bike to stock.
One very important post at that thread is post #36 on the 4th page. I describe how to bench synch the carbs. Just in case you need it. The bench synch makes the vacuum tool synch go much easier and quicker. A vacuum synch is very important to the jetting and judging performance and reading the plugs.
In this particular thread, Mark (Hoomgar) finds out after a lot of trial and error that he needed the Dynojet jet kit. I tried to advise him very early in the thread but we all want to try what we think is best or we want to save $ or whatever. This doesn't mean your stock jet needles and separate jets won't work but I try to explain the possibility.
I always try to remind others how important it is to have the rest of the bike ready for a re-jet. It's more work to adjust the valve clearances, check compression, check/clean/adjust the spark and timing, rebuild the carbs, etc, but my past experiences show that it saves time and frustration in the long run. Older bikes very often have been neglected and it's foolish to just clean the carbs, buy some jets, slap on some mods and expect extra power. Old bikes can have dirty/poor connections that mean poor spark no matter what you adjust. Low compression equals poor combustion. Worn parts. The list goes on. One very common area that's neglected is the manifold and the manifold o-rings. If it's been awhile (and it usually has been) replace the manifold o-rings whenever the carbs are off. They're cheap. The manifolds are not. Buy you can't tune/jet/synch if you have any intake leaks. Mixing re-jetting with trouble shotting can drive you crazy. Be sure the bike is ready.
In my advice to you, I also mention the DJ kit but I'm assuming (I may be wrong??) that because you're trying a home made pipe that you want to save $ too? That's why I suggested the earlier jetting set up instead of emphasizing the jet kit. It's your time/money/choice of course.
I put a lot of time and thought into my suggestions and I really try to save members the trouble I used to go through in tuning/jetting bikes. I don't get a thing for this except if a member comes back and says "Thanks". Some times they come back and say they still have problems. If they stay and keep posting their progress, it usually turns out their problem was related to skipping some part of the tune/re-jet procedure and they learn the hard way.
I also want to make clear that I'm just another member too. I share my suggestions. I wish I could guarantee they always work. I don't like to think someone goes out and spends time/money, based on my suggestions and the suggestions don't work. I just give it my best shot. Every re-jet has to have a starting point. If it doesn't work, we try again.
Your re-jet is no different than others, EXCEPT that home made pipe. I just hope it flows correctly and doesn't cause problems. Its flow characteristics will obviously dictate what size main will work. If you do end up with a main less than say, 125, then that pipe isn't flowing well in my opinion.
Just curious, can you decsribe the pipe? Did you model it after another pipe? What brand? What dimensions did you use, etc?
Also, can you post a link to the "Mikuni forum"? I may want to drop in sometime.
As for the large difference in main jet choice and how it can be so, I don't know how that forum (or just one member) came up with a good running bike that had 110 mains. I actually doubt it and he may be mistaking his performance and certainly isn't taking accurate plug reads. A lean mixture can be very deceiving. Drag racers actually tend toward a lean mixture because it can make more power in some conditions. A feeling of more power will make many tuners just stop tuning/testing and say "good enough". This isn't acceptable on a street bike.