vm carb jetting questions

[wolfie]

I'm working on my boss's 1977 GS750. It has an after market free flowing 4 into 2 slash cut exhaust. I don't know who the manufacturer is. His air box is in rough shape, so he has decided to go to a set of pod filters. I need to know the details on doing a rejet on this particular model with these upgrades.
According to the parts fiche, the stock main jet is a 100. I have read on here that different jet manufacturers use different sizing numbers. Do I need to be using mikuni jets or is there another manufacturer that I should go with. Also, what size is recomended? 110?
The needles on these carbs are adjustable with a clip. It is currently set in the middle at the third clip. Can I just adjust it up, or do I need to replace it? If I can just adjust it, where should I adjust it to?
Do I need to replace the pilot jet, or can I adjust it enough with the mixture screws? As it is, the bike falls on it's face on the low end. I believe that whoever installed the pipes just replaced the mains.
Thanks in advance for your help. I need to hurry on this. It's nice out, and he has the bug real bad.

[Guest]

From all the threads I have read regarding the VM carbs, a Dynojet jet kit is the only way to go. Hoomgar tried adjusting the stock configuration with no good results until he put the jet kit in. It will include the right main jet and jet needles. You will use the stock pilot jet. Get new o-rings and the jet kit.
I took the above advice when I put K&N pods and a V&H pipe on my 1000. She fired right up, and with a little adjustment, she runs great!

[wolfie]

What all is included in the kit? $113 sounds steep if all that needs replacing is the mains and needles. Is there a place where I can buy individual needles, and if so, what size do I need? Thanks again.

[Guest]

What all is included in the kit? $113 sounds steep if all that needs replacing is the mains and needles. Is there a place where I can buy individual needles, and if so, what size do I need? Thanks again.

Two sets of main jets, and the needles with spacers. The needles seem to be the important part that make the difference

[SqDancerLynn1]

If you are just changing the jets OEM are the best & all that I would use. The Dyno jet kits are numbered different & the needles are NOT stock replacements- they are custom flowed for thr specific kit
If you are determined go up at least 4 sizes & raise the needles 2-3 grouves if they are adjustable

[wolfie]

Lynn,
Are you counting 5 or 2.5 as a size? What I mean is, if my stock size is 100 and you say to up 4 sizes, do I need to go to a 110 or a 120?
The needles are adjustable, they were on #3 of 5 settings. I'm going to try moving them to their highest setting (clip on the bottom to raise needle) and see if that will work.
Thanks.

[wolfie]

bump......... Keith Help!
thanks

[odl777]

I read some jetting mods a guy did to his 750. said he out 120 mains and put the needle to the lowest position. im gonna try it when i get home next week.
Reply on what you used. thanks

[tkent02]

Didn't think you could get a jet kit for the early 750s....

If you can who sells them?
Need one for my '78

[KEITH KRAUSE]

Hard to guess, not knowing what brand pods and pipe.
I'll assume the pods are Emgo quality. I have no idea about the pipes flow, but will assume it flows decent.
4 full sizes up on the mains..120 Mikuni.
I would raise the jet needles to their highest position (e-clip in the 5th/bottom groove).
I would try to make the stock 15 pilot jet work, along with richer pilot fuel screw (underneath) adjustment. An additional 1/2 turn out from where they were (if it was running well before) is a good starting point. Probably will be about 1 1/4 turns out?
Then adjust the side air screws using the highest rpm method. Usually about 1 3/4 turns out.
Remove the two floatbowl vent lines and leave their ports open to breath.
Bench and then VACUUM synch the carbs. Don't skip the vacuum synch.
Be sure the spark quality/ignition timing and valve clearances are good before the synch.
Test at full throttle for the mains, 1/3 throttle for the needles and minimal throttle opening for the pilot circuit. Start with fresh plugs. Chop off and read. Do what the plugs/performance say.
The stock jet needles sometimes work OK with the cheaper pods, but never with the K&N's and a quality pipe.

[wolfie]

Thanks Keith. That gives me a starting point. I tried to convince him to get the K&Ns, but couldn't sell him on the price($179 from local cycle shop). Looks like he'd have to shell out another $115 for a jet kit, so it's just as well. This is more about getting him some filtration as his air box is in a bad way. Bad enough that I found sand in his #4 carb when I cleaned them.

[KEITH KRAUSE]

OK. Good luck with it.
You and Lynn were on the right track before I posted. Maybe I added a few things.
Don't expect to buy the jets/adjust the needles and be done the first time though. My suggestions are just an semi-educated guess and I'm just making an attempt to get it right the first time.
Just be sure the other things are also done, such as good compression, clean carbs, float levels, timing, vacuum synch, etc, or you'll have trouble.
SOME shops will swap you main jets if you bring them back in new condition.
Adjusting the jet needles is the most work with the VM carbs. Also, you MUST re-synch whenever the jet needles are disturbed. Hopefully you'll be lucky and get the stock needles to work for you.

[wolfie]

Just an update. The main jets finally came in. So I put them all together and fired her up tonight. I even took it for a little ride. I think your suggestions definitely put me in the right ball park. I'm going to need adjust the pilot fuel screws out some more. The mid range feels good, so I think I'm close on the needles. I am out of poop for the night, so plug tests will have to wait till later. Thanks guys!

[wiredgeorge]

Your baseline for the jetting is #100 main jets, #15 pilots and 5F21-3 (clipped 3rd slot) jet needles. The OEM jetting for the 750 runs to the lean side, even with the stock airbox. My experiences with the Dynajet kits have been less than thrilling. The jet needles they use for most VM applications are the same jet needles so while they might work OK in one application, it seems to me that they won't work well in all. It is similar to buying a suit. If I owned a clothing store and sold every customer a 42R suit, it would surely work OK for that fella who was 5'10" and 175 lbs with normal length arms but for everyone else? Also, they don't provide for rejetting the pilot circuit and given the Suz version of the VM26 is fussy about the pilot circuit, I would want to start there anyway...

First, your pods and 4 into 2 aftermarket system will require large pilot jets... I suggest #17.5s since you didn't mention any other mods. Next, you will likely have to remove the jet needles and reclip them BUT there isn't any point in doing this, if perhaps you can get away without doing it. Your carbs will need resync'd if you remove the jet needles. So first, just pop in your new main jets. #110 is proabably what you need. Take it out and ride the bike. It should start easier with the larger pilots and you may need to adjust both sets of mixture screws but I would leave those alone till I found out how it started and idled before adjusting. Next, take the bike to a speed where you can get a sense of whether it is gas starving or bogging... The #110s I recommend are pretty close and if anything, #112.5s might be needed (depending on pod type / pipe type) but in any case, if it pulls cleanly up towards redline, then the mains will be correct. If it seems like it wants to run out of steam after 7K rpm, then the larger jets would be the way to go. Don't over jet as this will also cause the bike to bog... Starting slightly lean when doing this experimentation is easier to figure out than going rich and moving backwards in jet size.

Last, with the "free flowing" pipes, there isn't a great way of guessing what to do about reclipping the jet needles. Most aftermarket pipes with no baffles only create flat spots that are really hard to mask. If the free flowing pipes have decent baffles and provide some flow restriction, you MAY get by with no reclipping of the jet needle. Ride the bike and see if there are any flat spots. If there are, reclip DOWN to the 4th slot. The slots are numbred 1 to 5 from top to bottom. You will have to check for flat spots again. If you have one, try the 5th slot. At this point, you will at least have an idea where the least problems arise in mid-range. RESYNC your carbs as pulling the slides out affects sync. Good luck!

[wolfie]

I read some jetting mods a guy did to his 750. said he out 120 mains and put the needle to the lowest position. im gonna try it when i get home next week.
Reply on what you used. thanks

I went with Keith's recomendations. 120 mains, stock needle in the highest position. I used the stock pilot jets, but had to tweak the mixture screws individually a few times before it ran right.

Thanks Keith. You are, as always, brilliant.