Yet another jetting question!

[KW]

Can some of you more tecnically aware enlighten me as to the function of main jets in a carb.

From what I understand the main jets come into play from about 3/4 to full throttle. Do they do anything at all below this throttle opening?

The reason I ask is this - I've fitted individual pod (Emgo) filters to my GS750, and to start with I've increased the main jets from 100's to 110's. I set the idling mixture using a Colourtune plug and she ticks over perfect. I went out for a run today and she runs great until you hit around 7500rpm and then runs out of steam. I only gunned her a couple of times during the ride to try this out, the rest of the time I was riding normally to a max of about 6000rpm. When I got home I took out a couple of the plugs and they are the perfect tan colour.

So if the mains do nothing below 3/4 throttle then should it simply be a matter of putting in bigger mains to solve the reluctance to rev to the redline? Or if they do have an effect at less than 3/4 throttle why would she be reluctant to rev but still give the perfect tan coloured plugs?

I wish I was cleverer!!!

[KEITH KRAUSE]

Any jetting chart will show you the main jet does'nt have much effect below 3/4 throttle position. It does sound like the bike is lean and starving for fuel at high rpm's. You said nothing about raising the jet needles.
The pod filters WILL require raising the needles at least 1 position. The needles regulate the fuel from about 1/4 to 3/4 throttle but there is an overlap effect with the pilot and main circuits. With the VM carbs, any time you adjust the needles you MUST synch the carbs with a vacuum tool. I don't know how well the Emgo's flow, especially with a stock exhaust, but it sounds like you may need to increase the main jet size too. Is your 7,500 rpm problem at full throttle?
Also, for improved float bowl venting and correct fuel mixture at all rpm's, remove the two float bowl vent tubes. When installing pod filters, this is necessary. The increased intake requires more fuel for a correct mixture. The stock venting can't keep up with the increased demand and the jets can't draw fuel easily. This results in fuel starvation. Removing the tubes stops a "vortex" forming in them and also minimizes resistance to venting. Removing the tubes is also required by jet kit makers.
In short, if it was my bike, I would raise the needles a position, and remove the vent tubes. You also must synch the carbs and adjust the side air screws for the highest rpm's to do an accurate re-jet. In your case, I don't know if I would increase the main yet. I'd probably wait and do the above adjustments and test. If the 7,500 rpm problem still exists, I would go up a full size (5) on the mains.
I also suggest new manifold o-rings.

[KW]

Thanks for the reply Keith,

I didn't raise the needle as I had such good coloured plugs during normal riding, so I thought if I raised the needles it would result in running rich. But if you think I should then I'll have a go. But I have fitted new manifold 'O' rings.

I've remove the vent tubes as you suggested and attempted to synch the carbs tonight. This is where I hit another problem!

I fired her up and checked the reading on the gauges:

Carbs 1 & 4 reading 10
Carbs 2 & 3 reading 6-7

So I decided (as it would be easier) to adjust carbs 1 & 4 down to the same as 2 & 3. I took the top of carbs 1 & 4 then fired her up again. Now I get the following:

Carb 1&4 reading 10
Carb 2 reading 6-7
Carb 3 reading 3-4

Strange, but ok I'll set 2 & 3 up to 10 like the other 2 carbs instead. Took the tops of carbs 2 & 3 and fired her up again. I adjusted carb 2 to read 10 then tweaked the tickover so it was back to 1500rpm. Then I went to adjust carb 3. Everytime I moved the adjusting screw the revs either shot up or the bike stalled, with a corresponding change in the vacuum readings, but all I could get on the gauge for carb 3 was a downward movement of the needle towards zero. It would not increase beyond 4 on the gauge. I tried it a few times but with the same result, then it was too dark to continue so another job for tomorrow night.

But what the hell is going on with carb 3? Do you think maybe the adjuster screw is stuck/seized and turning it is moving all the carb slides via the actuating rod?

[Sandy]

A problem I had on my 1979 GS1000 (VM Carbs) when syncing the carbs was what seemed to be that the slides were over adjusted up at some point in time and caused me a lot of grief trying to get them to sync properly. I discovered the problem by turning the idle screw all the way out. The bike should have quit but didn't because the slides, or one of the slides, seemed to be hanging or adjusted partially open. I then dropped all the slides down using the slide adjusting screws and tried to keep the slides as reasonably synced as possible. And no it wasn't hanging up on the throttle cable. I then started syncing the carbs all over again from stratch as recommended. If you have the carbs off the bike, or you are running pods (you are), I understand you can do a preset on the slides using something like a drill bit as a gauge. If someone else has done this maybe they know a good size to use, I've never used this method. If other people have messed with the setting in the past this might be the problem.

[KEITH KRAUSE]

Hi. First of all, if your "normal" riding was in 4th/5th gear and you were cruising around, say 40-65 mph, then you're using both the pilot and needle ciruits. Your plug reads would not give a good indication of the needle circuit mixture. For a good needle read, you have to go at least 1/3, or 1/2 throttle is better. I go several miles at this position before reading the plugs for the needle circuit. You need a safe road to do this obviously. If your plugs are tan colored after this test and your needles ARE in their original positions, then those emgo's don't flow any better than the stock airbox. Don't do any plug reads until the carbs are vacuum synched.
As for your synch problem, I've seen carbs do what you're describing and you can get to the point of the carbs idling high with the main adjuster screw completely backed off. You can turn one slotted adjuster to the right so much that you lift the other slides. This happened to me many years ago when I first was learning. At the point you are, it's always time to start over with a manual synch. I do wonder if you created a vacuum leak when #3's level dropped further. Attempting to lower the "high" levels to match the "lower" ones is the correct method though.
I would do a good manual synch per the factory manual. That's always worked for me. On your 750, I believe #3 carb is the "master" carb. So you're supposed to set it to its fully closed position first and then do the others. I'm not sure on that. But if there is no master carb, then it's fine to start with #3 anyways.
You first have to introduce a clearance between the main idle screw and the throttle pulley. I would then finger loosen all four slotted screws adjusters and open and close the throttle valves a few times to seat them. Then, starting with #3, adjust the screw so #3's slide is at the fully closed position and tighten the holder nut to 3 ft/lb. Then carefully do the other ones. You'll be turning the screws to the right to achieve this. You look at the engine side of the carbs while doing this. You get the "nicks" as uniform as possible. When you see each slide stop dropping, stop turning. Then you adjust the slides fully open position. Looking up into the filter side, when the slides are raised fully by pushing up on the pulley, the bottom of the slides should be between .5 and 1mm above the top of the main bore. To adjust this, there's a screw under spring tension that contacts the pulley.
Now turn the main idle adjuster knob up several turns after contact, enough to allow the bike to start.
Now you should be able to vacuum synch again. As long as the bike is in good operating condition, the levels should be in a good range at start up and only need basic leveling. Lower the high level(s) to match the lower ones. It should'nt take much. I like to adjust mine at 3,000 rpm. Be sure the timing is correct and valve clearances are good before a vacuum synch. Also, make sure you adjust the side air screws for the highest rpm's and re-set to 1,100 rpm with the main idle screw.