XJ750 carbs

[Sycoholic]

I currently have an 83 Maxim 750 with the Hitachi cv carbs. The dealer can't get anything for it, but did 'donate' some old jets it had laying around. I've built 2 into 1 drag pipes for it with J&P cycles baffles, which add some restriction. I've also added the K&N pod filters. I've been filing the jets out with welding tip cleaners for lack of bigger jet availability. I have the main at about a 140 (compared to a 134) and the pilots I went back to stock on. I filed them out and the bike wouldn't run and fouled the plugs. When I'm revving it once it's warmed up, it's like a rev limiter. But it's belching black smoke. It has about the same power as a 50cc scooter and about the same top speed. This bike should scream! It also backfires out the exhaust. If I go down the road a mile or two and pull over, it won't even idle. It just stalls out. I have to work the throttle a bit and after sitting it smoothes back out. The diaphragms seem good and the slides are all operating the same. I'm stumped and have a poker run in another state I was going to this weekend. Help!! Any and all help is appreciated. Thank you very much.

[Geezer]

Since you're getting black smoke odds are you've gone way past the 140 mark and are in the area of insanely rich.

If you can get stock jets? Try using carb drills next time to open up the jets. At least that way you will have a point of reference.

BTW, a small change in the size of the hole in the jet can make a big difference in how the engine runs.

Geezer

[pggg]

The pilot jet hole is very tiny and drilling the pilot jet will ruin it, but for the main jet(mikuni), a 1.5mm drill bit lightly twirled in and out by hand gives around a 140 - 142.5 which isn't too drastic, but a 2mm drill gives around a 190!!

[Sycoholic]

The plot thickens. I tore the carbs down again, just to make sure they're clean. That makes 1,569,546 times.... or at least it seems that much. I compared them to a Haynes manual I have for the bike. Hmmm. Never noticed this before, but up top where the diaphragm is, I'm missing that little plate that covers those air jets. No plate, no screws for the plate. I know the diaphragms move the slide up and down with the throttle, but will that lead to the problems I'm having???

[GNEPIG]

.. i think those passages are for the choke. i dont think the plates you are talking about seal off anything, just covers. but i have been wrong before.

[Sycoholic]

Ok... think I'm on the right track now. I installed some Mikuni jets in my Hitachi carbs. The Mikuni's are equivalent to the 120 stock mains, and I put the stock pilots back in. The bike pulls pretty good until w.o.t. It's ok, but not great. Now that I'm using Mikuni jets I can experiment a lot more, and for cheaper too!

[HankD]

xj750 carbs

You have mentioned main jets and pilot jets but have excluded the circuit that is used the most. The jet needle controls the fuel between the pilot jet (about 2,000rpm) and the main jet (7000+rpm). Of course there is some overlap. The hitachi carbs don't have adjustable needles from the factory so if you don't have a jet kit, you will need to shim the needles using small washers. You can add washers without removing the carbs as the jet needle is accessible from the top of the carbs, unlike the main and pilot jet.

If you still have the original air box, you may want to consider reinstalling it because from my experience, and experience on the XJ news group, air pods are very difficult to tune and if you do get it right, you can get more power at the top end but you will loose power in the lower rpms. If you leave the pods, you may need 2 washers for each jet needle but I suggest you start with 1.

Hank

[audijunky]

Not to try to step on toes but wouldnt raising the needle jet make it run richer. Well maybe this can help you
www.factorypro.com/tech/carbtun.html

Good luck

[HankD]

You are correct Steve that raising the needles will make it run richer. Installing pod filters in place of the original air box will make the bike run far too lean. Previous posts show attempts to change the mixture with the main and pilot jets but nothing in the middle.
Hank

[TomB]

Jet sizes

The pilot jet hole is very tiny and drilling the pilot jet will ruin it, but for the main jet(mikuni), a 1.5mm drill bit lightly twirled in and out by hand gives around a 140 - 142.5 which isn't too drastic, but a 2mm drill gives around a 190!!

Mikuni jet sizes are metric, e.g. a 130 jet is a 1.30mm hole, a 132 jet is a 1.32mm hole.
Jets are made of brass which will drill tight but never tighter than the drill going through the hole so if you use a 1.5mm drill you will get a 150 jet.
I have spent a long time tuning specials/ competition sprint bikes by drilling jets with great success but you must be accurate, filing jets to an accurate size IMHO is just about impossible.

Tom (Time served Toolmaker, 26 years experience)