Help - identifying Mikuni carbs 85 xj750x

Hey guys, sorry if this has been answered but I've spent all morning searching forums for an answer and I'm having a hard time. I need to identify these Mikuni carbs so I'm able to order new main jets as I have converted to pod filters and my mixture is way off. I believe they are BS-33's but the pictures I've seen do not match up, and since they're in-line 4 I have a sneaking suspicion that they're actually an RS model? Idk, me brain hurt now. I've attached a couple pictures, hopefully that helps.

Also will I also need to replace/upgrade the pilot jets and/or needles?

Thanks!

 

Hi, they could possibly be bs32s they look very similar to the ones I have fitted to my Fj600 but not sure. I'll post some pics I have of mine tomorrow which am in the middle of cleaning and see what you think.

 

I have an '86 XJ700X that I'm currently working on cleaning/rebuilding the carbs now. Your carbs look very similar to mine (which my literature says are BS-33s). I'll try looking for numbers (your Mikuni Corp' stamp looks like exact match)

 

The Maxim-X has unique carbs made for it.
The body is much shorter so #4 clears the water pump.
You can see in your first pic the little angle iron holding them together is cut out for #4.

 

Have you measured the inner diameter on the throttle blade side. I believe the "33" in BS-33 means the size in millimeters. Mine measured 32.99mm

Also my throttle blade itself has a '130' stamped on the lower portion, if that helps any.

 

Contact info@xj4ever.com
Len has the correct parts for those quite special carbs.

 

Cleaning your Mikuni carbs max x carbs featured

 

I did measure the inner diameter and it is 33mm. And mine also has the '130' stamped onto the blade so that helps a bunch thanks!

 

Hey K-moe, does he have bigger main jets than stock size? Mine are 105, I think I need to go up to at least 110

 

If they are available he will have them.
Have you ever ran pods on CV carbs before?
Getting it to run as good as stock is going to be a royal pain.

 

Glad to help!

 

One reason I'm glad I have a spare set carbs for my X. Can't imagine how difficult or expensive it might be to buy another set. Although I wonder if BS32s or 34s could work (never seen a swapped set of Mikunis on an XJ700X)

 

They will not physically fit. The BS33's on the X have a shorter body.
Here's a pic showing typical Mikunis, BS35's from an XJ900 next to the BS33's from the X.
You can see the bodies are about 1/4" shorter and this is to provide clearance for #4 carb and the water pump.
I planned to put the 35's on my X since mine displaces 911 cc's.
I didn't see this until I had them ready to go on.

 

Good comparison Simmy, so I guess I better hang on to these carbs then huh?

Questions: With 911cc displacement, did the BS33s still work well for you? What mods did you do to dial them in for the extra displacement?

 

The problem with eliminating the air box on CV carbs is you alter the air pressure entering the carbs.
The air box does present some restriction to air flow so most guys assume, hey lets eliminate that restriction, more air, and now if I just compensate with more fuel I can make some big hp gains.
The problem is that the air box and CV carbs are engineered together as a package.
Eliminating the air box increases the pressure at the inlet to the carb. Now you've also altered the air pressure in the diaphragm chamber on top of the carb which is responsible for raising your slides. Everything is now askew.
Most CV carbed bikes without the air box experience a big dip in power where the restriction to air flow of the air box becomes most evident.
It is almost always at 5,000 rpm. You'll find your slides just not cooperating with your right wrist.
The bike is stuttering, sputtering, jerking because your throttle slides do not have the correct pressure to smoothly rise as required.
Below 5,000 there isn't this restriction to air flow and it runs ok.
Usually if you can get well above the 5,000 mark your slides will open but they will never feel connected to your wrist like the OEM set up.

One way to compensate for this is to begin by chopping 1/2 a coil out of your throttle slide spring so it allows the throttle to open with the decreased vacuum above it.
The Stage 3 kits tell you to do this 1/2 a coil at a time until you achieve smooth operation. The problem with this is that you're trying to do this while still guessing if you have the jetting correct, too many variables to deal with all at once.

Now to my experience, I first put pods on the XJ750-X and it was actually rideable with stock jetting except that I'm running a sidecar and the 5,000 dead spot was amplified. It was not good.
I have a local bike shop where all the road racers go to, he has a dyno with gas sampling so I took it to him.
This was more than 20 years ago. To be honest I don't know what jets he actually installed but he also put softer springs in to eliminate the dreaded 5,000 dead spot.
Since then I put the 75mm pistons in it from the FZR1000. As a result I think these carbs are now a bit undersized which now simulates the effect of an air box as the carb mouth itself becomes a restriction. The carburation now is excellent, I'll go so far as stating it is as good as an OEM set up. I can cruise through a town at 50 kph in 4th or 5th and just lay on the throttle and it picks up speed without stuttering, no gear change required.

If it's not too late I would strongly recommend you keep the air box, if not I hope you have a little more understanding of what is going on with your bike when you hit the 5,000 rpm dead spot. Correct jetting alone will not fix this.