1977 GS-750 re-jet. Here is where I am at.

[TBC]

Okay, so here is an update. I put in 135's and went for a ride. When I did WOT input off the line in first gear ( I have been doing this by the way at around 2500 to 3000 RPM ) it still bogged but did not necessarily die. Before with the 127.5's it always died. Always. When it didn't die out but bogged down, I held WOT. It would clear itself and rocket off. I think I am really close. I have not touched the jet needle and it is still in #4. Based on what it was like with the 127.5's and what it is like now with the 135's, I am off to pick up 140's. I might be right or I might be wrong. I'll find out and report back guys.

Tom

[coltrain]

130's or maybe even 132.5's is where you will probably end up. 140's is crazy talk maing.
CafeKid needs to get that thing running too. Damn that G.

[TheCafeKid]

Okay, so here is an update. I put in 135's and went for a ride. When I did WOT input off the line in first gear ( I have been doing this by the way at around 2500 to 3000 RPM ) it still bogged but did not necessarily die. Before with the 127.5's it always died. Always. When it didn't die out but bogged down, I held WOT. It would clear itself and rocket off. I think I am really close. I have not touched the jet needle and it is still in #4. Based on what it was like with the 127.5's and what it is like now with the 135's, I am off to pick up 140's. I might be right or I might be wrong. I'll find out and report back guys.

Tom

K, just a note here... You may find that you are going to have this problem no matter how big a jet you're going to have...the VMs will NOT deal with wacking the throttle open like that well to begin with. You're basiclly aleviating all vaccum from the motor when you do that. This is WHY they switched to CV carbs (well, one of the big reasons anyway) Because that slide, when you wack the throttle open, is suddenly WIDE OPEN, the motor cant deal with all the air at once. It cant pull it. Suddenly there is no vaccum to pull gas thru the jets. This is why they also used VM28 "Pumpers" on some of the other bikes, and race bikes. When you wack the throttle open, there's an accelerator pump (hence the nickname Pumpers) that shoots a shot of gas down the throat of the carb, so it can get fuel to bring the vaccum back up. The 26's dont have this, So it takes a second for it to catch up. Put your 127.5s back in, move the needle, if THAT doesnt work, try 130s. 140s is, IMO on the way high side of the realm of possibility...

[hampshirehog]

As TheCafeKid says, you can't whack open slide carbs that aren't fitted with a pump, though you can roll the throttle on pretty quick if you're jetted correctly.

I think I'd be tempted to do a 2 or 3 cylinder plug chop - pull 1 or 2 plug leads and go for a ride with the throttle wide open - running on 2 or 3 cylinders means you won't be going 100 mph! Hit the kill switch at WOT and read your plugs on the cylinders that have been firing. At WOT your mains totally overwhelm every other part of the carb function so your reading will tell you which way your mains want to move. 140s are on the high side of average for a setup on a 750 but as I said in my earlier post well within reason.

CV carbs without a pump can bog too. Massive difference between a CB900F (no pump) and CB1100F (pump) where throttle response is instant.

The CVs were the Jap answer to meeting emissions levels. All the racers were of non CV design and if you know what the Japs copied - the British SU carb of the 1970s and earlier - you'd be amazed that the Jap bikes out perform a duckbilled platypus - ask any Morris Marina owner!

[TheCafeKid]

As TheCafeKid says, you can't whack open slide carbs that aren't fitted with a pump, though you can roll the throttle on pretty quick if you're jetted correctly.

I think I'd be tempted to do a 2 or 3 cylinder plug chop - pull 1 or 2 plug leads and go for a ride with the throttle wide open - running on 2 or 3 cylinders means you won't be going 100 mph! Hit the kill switch at WOT and read your plugs on the cylinders that have been firing. At WOT your mains totally overwhelm every other part of the carb function so your reading will tell you which way your mains want to move. 140s are on the high side of average for a setup on a 750 but as I said in my earlier post well within reason.

CV carbs without a pump can bog too. Massive difference between a CB900F (no pump) and CB1100F (pump) where throttle response is instant.

The CVs were the Jap answer to meeting emissions levels. All the racers were of non CV design and if you know what the Japs copied - the British SU carb of the 1970s and earlier - you'd be amazed that the Jap bikes out perform a duckbilled platypus - ask any Morris Marina owner!

True HH, however, the CVs are LESS likely to bog simply due to their design. As Ive explained before, and was explained to me very understandably, the reason CVs are considered a more STREETABLE carb is simply because of the way they work. They use more or less a dual valve set up, with a real set of butterflies as primaries, and the slides act as, what they are, vaccum controlled secondaries. when you wack the throttle open on CVs, the butterflies fly open, but there is still some blockage of air flow due to the slides, so the motor still retains a decent amount of vaccum. As the vaccum increases further, the slides start moving UP, allowing more air, more draw, more power, LESS bog... The VMs do not have the primary butterfly valves, but use the slides AS the primaries. SO...whacky whacky...boggy boggy... Class dismissed

[tkent02]

SO...whacky whacky...boggy boggy... Class dismissed

Except they don't all do it.
Are you talking whacky whacky from idle or from some realistic RPM?

[TheCafeKid]

Except they don't all do it.
Are you talking whacky whacky from idle or from some realistic RPM?

Sounds from his post, hes going WOT from a standstill...Not going to work, no matter what you have on there. From realistic RPM, you should be able to whack the throttle open, or more precisely, roll it on QUICKLY and feel little or no effect, if jetted right. But WoT from low rpms or dead is not going to do anything but bog.

[TBC]

Sounds from his post, hes going WOT from a standstill...Not going to work, no matter what you have on there. From realistic RPM, you should be able to whack the throttle open, or more precisely, roll it on QUICKLY and feel little or no effect, if jetted right. But WoT from low rpms or dead is not going to do anything but bog.

Couldn't get back at it this afternoon due to rain. Hopefully tomorrow or Sunday but weather is not suppose to cooperate. If you go back through my posts I have been going to WOT from around 2500 to 3000 RPM, not idle and not from a standstill. It has been as I pull away from being stopped. I believe my "off the line" comment leaves everyone with the impression that I have been at a standstill and at idle, and understandably so. My error in wording.

Cafe Kid. I am ready to go with your most recent advice ( as a result of my 135 main's adventures ) but the rain beat me to testing it out. Hopefully this weekend, but as I mention above, weather isn't suppose to be anyone's friend this weekend.

Tom

[TheCafeKid]

Couldn't get back at it this afternoon due to rain. Hopefully tomorrow or Sunday but weather is not suppose to cooperate. If you go back through my posts I have been going to WOT from around 2500 to 3000 RPM, not idle and not from a standstill. It has been as I pull away from being stopped. I believe my "off the line" comment leaves everyone with the impression that I have been at a standstill and at idle, and understandably so. My error in wording.

Cafe Kid. I am ready to go with your most recent advice ( as a result of my 135 main's adventures ) but the rain beat me to testing it out. Hopefully this weekend, but as I mention above, weather isn't suppose to be anyone's friend this weekend.

Tom

Ok tom, but thats STILL pretty low RPM to be going straight to WOT. Try to roll it on rather quickly, but not jerk the throttle all the way open instantly. Again, I still say 135s are a bit big, and im going off of info ive gathered from those MORE knowlegeable than I on this particular subject. As I said, based on YOUR findings, i went ahead and ordered 130s and 132.5s just in case.

[TBC]

Ok tom, but thats STILL pretty low RPM to be going straight to WOT. Try to roll it on rather quickly, but not jerk the throttle all the way open instantly. Again, I still say 135s are a bit big, and im going off of info ive gathered from those MORE knowlegeable than I on this particular subject. As I said, based on YOUR findings, i went ahead and ordered 130s and 132.5s just in case.

That's cool. I am going to take your advice, back up a bit, and try again. I thought it was a little lean with WOT with 127.5's and clip in 4th position. Maybe going to 5th position will work for my bike. Hey, it's all grist for the mill, isn't it? I'm a newbie at re-jetting, so a big learning curve for moi.

Thanks,
Tom

[TBC]

Cafe Kid, I have a question for you. When you go to fine tune your pilot circuit ( small throttle openings ), are you going to preset your fuel screws and then adjust air screws or vice versa?

Thanks,
Tom

[TheCafeKid]

Cafe Kid, I have a question for you. When you go to fine tune your pilot circuit ( small throttle openings ), are you going to preset your fuel screws and then adjust air screws or vice versa?

Thanks,
Tom

You really *shouldnt* have to mess with the fuel screws nearly as much as the air. Use the air for FINE tuning, the fuel, once you decide you are getting enough fuel to the pilot circuit and what not, you shouldnt have to move them. I would start at 1.5 to two turns out on those, and leave it. If you're rich, back off a bit, but i doubt that you'll be lean on those at that large an opening. Stock i believe is only ONE turn out. Air screws play with to fine tune highest idle...

[KEITH KRAUSE]

If your problem is at full throttle, then it's the mains that need changing, not the jet needles. Simple 1/3 to 1/2 throttle tests/plug reads would verify if the jet needle is right to begin with.
Your bench synch could be off significantly, adding to the problem. A vacuum tool synch is a basic part of any re-jet.
Are you sure the jet needle assembly was correct? Thicker factory jet needle plastic spacer on top the e-clip and thinner under the clip?
Did you verify the ignition advancer is operating smoothly and the advance timing is spot on?

[TBC]

If your problem is at full throttle, then it's the mains that need changing, not the jet needles. Simple 1/3 to 1/2 throttle tests/plug reads would verify if the jet needle is right to begin with.
Your bench synch could be off significantly, adding to the problem. A vacuum tool synch is a basic part of any re-jet.
Are you sure the jet needle assembly was correct? Thicker factory jet needle plastic spacer on top the e-clip and thinner under the clip?
Did you verify the ignition advancer is operating smoothly and the advance timing is spot on?

Just a little update, but not much because I have not been able to spend much time at it.

Hi Keith,

To answer your questions. Bike bench and vac synced. There are no spacers on my jet needles. As I purchased this bike stock from original owner with stock jetting, I can only assume there never were. Verified timing is correct and spot on with timing light. BTW compression with hot motor is 110, 110, 115, 115 for cylinders 1 to 4 respectively. Bike has 76K on it. It was re-ringed by me at 50K and also valve job which was done by shop. I am hoping to sort myself out jetting wise later this week.

Tom

[TheCafeKid]

Just a little update, but not much because I have not been able to spend much time at it.

Hi Keith,

To answer your questions. Bike bench and vac synced. There are no spacers on my jet needles. As I purchased this bike stock from original owner with stock jetting, I can only assume there never were. Verified timing is correct and spot on with timing light. BTW compression with hot motor is 110, 110, 115, 115 for cylinders 1 to 4 respectively. Bike has 76K on it. It was re-ringed by me at 50K and also valve job which was done by shop. I am hoping to sort myself out jetting wise later this week.

Tom

Correct he is Keith. The 77 VM26SS set up had NO spacers on the needle. Only the clip. I wonder if one were to add some spacers, if any advantage, or ease in jetting would be gained.