VM26SS main jet sizing in thousands of an inch?

Does anyone know about what size the bore on main jets are in thousands?

My '77Kz1000ltd has 115 mains and I believe they are lean for the bike. I am running stock exhaust and pods. (wish I had the original airbox setup, think I might have found one!)

I have gone thru the other stuff, valves, timing, vacuum leaks, repowered coils with relay and everything helped but I still have a coughing, sputtering at WOT. I moved the needle clip down one position and that helped ALOT, it moved the bad spot from 1/2 throttle and up to about 3/4 and up.

So at this moment I have the carbs off and am thinking of drilling out the main jets but dont know how far to go. The way I figure it, if it gets better after drilling I know I am too lean, gets worse than I was too rich. Either way the mains are wrong so no loss to me.

I can get a #58 - (.0420) drill thru the #115 main now. Just wondering if I should step it up 2-3 or more like 5-6 thousands. I have no idea how much difference there is between the sizes. Anyone know about what a 120 main would be?

And how much do main jets cost?

Nebr_Rex wrote: And how much do main jets cost?

Like I said, this is to confirm that I have a lean main jet. If I drill out the bore and it runs better than I know that 115 mains are too lean. From there I can order some larger size mains and get it right. Cost has nothing to do with it.

What I dont know is how much of a size difference there is between the different mains. If I knew the bore size of any other main I could figure that out. If the sizes increase evenly that is. I would like to know how much to drill to get to around a 120-125 main. I'm thinking about .005 larger from my .042 (115) size now but not sure.

IIRC the stock jets on that bike were a 105 main, a 15 pilot, a 5DL31 jet needle set to the 3rd slot, an O-5 needle jet and 1.5 cutaway on the slide. So you have already come up 4 full sizes on the main and dropped the needle one slot. Have you changed the pilot jet?

You can run a simple test to see if you are lean rich. When the issue happens put the choke on partway and see if the issue gets better or worse. If it gets better you are lean, if it gets worse you are rich.

AS to sizes of jets, IIRC it is as follows

115 jet = 1.15mm or 0.045275 of an inch
117.5 jet = 1.175mm or 0.046259 of an inch
120 jet = 1.20mm or 0.047244 of an inch

Thanks for the info gd4now! If you're correct the jet sizes are in MM. That makes it easy to know how much to drill out.

Yes, your info on stock carb setup are correct. When I bought the bike in March the engine was all stock except he had put pods on it and changed the jets to a 115 main and a 17.5 pilot. Bike still has the stock exhaust.

At first I thought maybe he had overjetted the bike, didnt think just pods would make that much of a change. But based on what I have ready here and the way it runs I was lean. So I moved the clips down one slot and that made a HUGE difference in how it runs. It used to be really touchy to opening the throttle, had to roll into it very very slow or it would start lurching and caughing. After moving the clip down I can get into it more than before and it pulls much harder but it still has the issue at WOT.

I have tried the choke trick and it seems to help maybe. Hard to tell because any change is very small. Plus running WOT in 5th gear the bike is moving and I'm focused on driving the thing.

What I can do is be going about 40-50 in 5th gear and quickly roll the throttle to WOT and the bike will start lurching and loose speed and never recover. A friend of mine with a 78 KZ650 will not do that. His will lurch and then recover and accelerate.

Main jets are to small. Would you like an airbox? Would only cost you S&H. I have some of the clamps. All the rubber is pretty good except oneis ripped. I use a Henry Abe air filter. It works much better than the pods. Mine is off my '77 KZ 1000

kaw-a-holic wrote: Main jets are to small. Would you like an airbox? Would only cost you S&H. I have some of the clamps. All the rubber is pretty good except oneis ripped. I use a Henry Abe air filter. It works much better than the pods. Mine is off my '77 KZ 1000

Sounds like lean main jets to you? What is a Henry Abe air filter? Have to do some checking on that.

Yes I would like a stock airbox!!! I would rather just go with the stock setup instead of messing around with jetting changes. I'm not trying to set any speed records, just want a bike I can enjoy so the stock setup is fine for me.

The guy I bought the bike from has some of the airbox parts but not everything. He is missing the flange parts that hold the tubes to the box. Looks like three parts, two gaskets and one large flange? Just guessing from seeing the drawing on bikebandit. Also missing the clamps, and the tubes are ripped.

Is the airbox setup the same on a LTD and non-LTD? If so I will take what you have. I'll message you.

I drilled the mains out to what would be about a 127 jet. It was late when I finished so just went for a very short test ride. It is for sure pulling harder than it was. It will now accelerate when rolling quickly to WOT from a cruising 40 mph in 5th gear instead of just lurching along and never accelerating.

Tomorrow I will have to go for a longer ride and see how it does. Then looks like I will be buying some 125-130 mains.

May also help to bear in mind that main jet size influences fuel mixture primarily at wide open throttle.

During mid-range throttle positions, fuel mixture is governed by the jet needle, needle jet, and slide position (unaffected by a larger than stock main jet size).

For example, if the fuel mixture at half throttle is too lean with the stock main jet, a larger main jet would not be expected to enrichen the mixture at half throttle.

For another example, if the fuel mixture at half throttle is too rich, a smaller main jet usually won't produce a leaner mixture at half throttle.

The most common method of adjusting the mixture during mid-throttle range is by changing the clip position on the jet needle. Whereby a relatively lower clip position enriches the mixture by raising the jet needle's position inside the needle jet.

The carb's pilot circuit governs fuel mixture at idle and continues influencing the mixture until about 1/4 throttle position where the mixture then becomes governed solely by the jet needle, needle jet, and slide position.

Good Fortune!

Patton....I have moved the needle clip down one position. That made a huge difference in how it ran. Now after upping the main jets I'm really getting the feel for how different jetting and clip positions make it run.

I took the bike for a longer ride today to see how the new main jet size (127) did. WOW!!! Bike pulls much harder! This is my first bike and when I got it it was not running very well at all. So I had no idea how the bike should have been running or how fast it really is. The sound that I am getting out of it now is also amazing. Might just be crazy but to me it sounds like a 308GTS.

It still has a little more lurch than I like when rolling very quickly to WOT from a steady cruise speed in 5th. The bike will now recover and accelerate from doing that, with 115 mains it would never recover, so I am for sure on the right track. So I will try a bit larger mains. At 127 now thinking about trying 130-132ish. I would rather go too rich and then have to work back to be sure I am giving it all the fuel it needs.

Never would have figured that it would need so much fuel for pods over the stock airbox setup. I am at sea level so that helps too.

I have to recommend for anyone with a stock setup to repower their coils with a relay. I have done it on my bike and a friends '78 kz650. On both bikes it made a HUGE difference in how they run. So much better now, more power, smoother idle, easier starts, barely have to choke 'em on startup, etc. Well worth the effort!

Man dont drill jets. You never get them the same no matter how good you are. New one are cheep. Put your clip back to middle of needle and change to a 112.5 main. If lowering the needle helped, you in affect leaned the mixture out. So go with a smaller main to get back on track. 3/4 throttle is all main circuit.

I drilled the mains just as a test to confirm that I need larger mains. This also gives me an idea of about what size to buy when I do buy them. Drilling them out has made the bike run so much better! So now I know for sure that I need larger mains. Plus I have a good idea of about what size to buy.

I moved the clip down one position to raise the needle not lower it. The bike was running lean in all throttle positions when I got it.