1976 Z750 twin

What needle are you using? is the y-22 generic needle?

No, the needle is 4JN19

Since you can't find the z-4 needle jets, you could switch over to Z-2 needle jet and 4HL12-3 needle which was what came on the US import. both are available on ebay.

another thing to look at would be the o-ring on the needle jet. yours looks pretty worn.
what state is the float bowl gasket in? if it leaks at all it can mess with the jetting as the jets are in the float bowl itself.

Hmm that's a good idea. But first I will make new jets on lathe. Since the internal diameter is 3.15 I will ream the pre-drilled hole with reamer 3.1 mm (I can't find 3.15 mm reamer and I believe it doesn't even exist) and if needed raise the needles. Then I will report back to you

I bought new O-rings. The gaskets are new.

Hi guys. At last the new jets are made. So let’s give it a try...

I realize this is a different bike and that this info might be a bit late, but on the early Kz550's, the needle to needle jet clearance is very close.

The OD of the stock needle is 0.099" and the needle jet ID is .102". That leaves roughly 0.003" clearance.
The OD of an aftermarket needle is 0.100". That leaves roughly 0.002" clearance.

0.002" is about 0.05mm.
0.003" is about 0.07mm.

A defective/unusable needle from a "counterfeit" kit had a needle with shaft measuring 0.098". No matter the amount of shimming etc, the needle was way too rich.

These measurements are on the parallel portion of the needle shaft.
The needle jet bore was measured using a round shaft that fit with a light interference-fit, then shaft was measured using the same micrometer used to measure the needles. (Measuring multiple positions to confirm roundness.)

Very nice looking jets, BTW.

Hmm here the old jet (the good one) to needle clearance was 0,16 mm (0.006").
I reamed the new jets with reamer 3,1H7 (so if we assume that the reamer is actually in the middle of tolerance field, the actual diameter is 3,105 mm), so it leaves a clearance of 0,115 mm (0.0045"). In both cases the clearance is larger than in your case.

Nevertheless, I will give it a try. Ordered new diaphragms though, just to be sure. The old ones are a bit tired.

Thank you

Hi guys,

it's been a while since my last post, so here's update:

I've sorted out the problem with carbs. I've changed the diaphragms and installed new jets and they are now working well. I've ordered some custom made diaphragms from the US just because they were cheaper. Here in Europe you can only get whole assembly (diaphragm with piston) and they are a lot pricier. Since then, I had no further problems with the carbs.

But there are some new problems (of course they are...). Last winter the bike lost its compression. It gave around 30-60 psi of pressure per cylinder. I've decided to take everything apart and see what's going on. I've ended up with replacing the piston rings (although the old ones were not really bad) and installing new valve seals made of blue viton rubber. I've also installed new gaskets. One of the valves didn't seal right, so I've also lapped the valve seals and adjusted them (damn 32mm shims!). The bike started in the first try and I was very satisfied...

...until I went for my first few rides. I found out that it burns oil pretty badly. I did run the new piston ring in gently (cca 1000 km). It didn't get any better. I can't really tell you how much it consumes, but yesterday it burned about half of inspection window in 50 km. I've topped it up to about 3/4. After another 120 km the level was about 1/2.

I'm a bit confused, since the consumption is not consistent. I can see no oil leaks on the floor and a friend who drove behind me said that the bike is not smoking (it did smoke a bit going downhill with throttle fully closed on high revs). The spark plugs are nice brown/white-is, but I can see some oil on plugs' washers.

I am not a bike mechanic but as a mechanical engineer I kind of know how engines work in theory. So my guess would be that they are three possible leak points: valve seals, piston rings and head gasket. And all of them are new.
Did any of you had a similar problem? And how can I check where the oil gets into the chamber?

Did you have the bores measured to be certain they are within specification for diameter, roundness and taper? Were the bores properly horned for for the new rings? Did you check the valve stems, particularly the intakes, for wear?

No, unfortunately I didn't do any checks on the bores and valves. But in my opinion there shouldn't be much wear, since the bike has only about 30k km (~18500 miles). 

Do you know what was done to the bike during those kilometers? Was the oil ever changed? How many times did the tachometer needle fly past the redline? How many times was it overheated? It does not take long to damage an engine that is neglected and abused. For the engine to suddenly lose all of its compression, something pretty serious must have happened. What caused that valve to suddenly not seat? Was it bent? What caused compression loss in he other cylinder?

When you had the engine apart, did you notice if both cylinders had clear cross-hatching from top to bottom, or were they shiny and smooth? How did you determine that the old rings were not bad? Did you check for proper ring end gap on the new rings?

I apologize if this seems like an interrogation. You are correct that there is a pretty short list of suspects when it comes to excess oil consumption. I bet the loss of compression and the increased oil consumption have the same cause.

Have you checked the air filter box to see if there is oil in it from excess oily blow by coming out of the crankcase?

P.S. What is the compression reading now?