1985 Mikuni BS33 carbs Can you rejet them?

I've been told you can't rejet the 1985 Mikuni BS33 carbs, someone on here should know for sure. I have a 85 XJ700 Maxim X, with a MAC exhaust, it bogs between 3000 and 5000 at wide open throttle.

 

Can we take it that the valves are in spec, the carbs synchronized & the pilot mixture screws set correctly.

 

Stock or mods other than the exhaust?

 

Just the exhaust, a local bike shop tried to shim them, didn't work. The guy was going nuts trying to get it right.

 

Bill,
I'm guessing from your post above that the shop guy only got the exhaust valves set within spec? This is the part that scares me and should be a bright red flag that you can do this kind of stuff yourself..."didn't work"! I'm not sure what that means but the intake valves are done exactly as the exhaust so it "should work".

Chacal can tell you about jetting since he knows more about the ins and outs of each different carbs set issued than maybe anyone else. Bogging at 3000 is mightly low on the RPM range to indicate a main jet issue.

Typically, if it ran well at one time with the intake and exhaust setup that you have, it shouldn't now need to be "rejetted". Float levels and clogged tubes & passages would tend to have a greater effect then all of a sudden needing larger or smaller main jets.

Once the valves are within spec, all of them, then I would clean and regap the plugs, sync and adjust the carbs and see where you are. If you are still having an issue, I'd pull and clean, really clean each carb.....that would get you to a baseline to work from...and eliminate the other things as well.

jeff

 

I agree with Jeff....there's no reason why a valve shim job 'wouldn't work'! You're wasting your money dealing with those guys, and could easily end up with motor damage because of it.

Your approach of going back to stock is a good one....trying to resolve an isolated issue (bogging @ 3K) without covering the other bases is like trying to grab a handful of smoke....almost impossible.

Get the stock pipes back on, adjust your valves (use HAP's shim table spread sheet, it will save you a ton of calculations) clean your carbs (making sure all jetting is OEM) then sync & adjust the carbs.

After all that, I'll be very surprised if you are dis-satisfied with the way she runs....these 5 valve motors are quick when properly tuned.

There's plenty of info available on these processes and lots of helpful, knowledgeable guys here who will be glad to help you over the humps.

 

Bill I told you in your other post that the BS33 CAN be rejetted, it's no good starting another thread because you don't like the answer. The "Shops" that told you that they can't are cowboys as far as the XJ's are concerned, they DON'T know what they are talking about. In fact, you'll be far pushed to even find a YAMAHA DEALER who can help, or is willing to help on these, mainly because it's a bike before their time.

 

Hi this is Roy. I’m having trouble with my BS33 carbs. First I rebuilt them sloppy and quick. Was in a hurry and broke one ear off the #3 carb. I had it repaired but bike still not starting. I didn’t think that repair was possible but the guy did a good job. And he knows these carbs. He said they were at spec and bench set ready to go. It will hit but not start. Abbnd I can’t seem to find another aet

 

Why is this hard, and why do you expect a load of guys on a forum to know more than someone "who knows these carbs"?
If he did the job less than half right the bike would start and run, to a fashion. If he's as good as you say it would start, ride and even idle somewhere near right.
If he knows anything about bikes then take the bike back to him. The likelyhood is you don't have fuel or a spark, but did you check by opening the float drains to see if there is fuel in there? Did you put a plug across the head to see if there is a spark there? Did you check valve clearances, or at least compression?
By all means adk here for help, but meet us all half way?

 

Yes I’ve done all that. There’s fire and fuel. What is the correct I don’t think any one person may be the best to talk to. I talk to everyone I can. Gather all that info. And make an educated decision. Can you tell me what the float height should be on the xj700x? I’m reading something different everywhere I ask. I do value your input more than others

 

I can’t find it in the service manual

 

Mikuni BS33 (all XJ700-X and XJ750-X):

-Fuel level measured via the clear-tube gauge: 3mm +/- 1mm (.12 +/- .04 inches)

-Float height: 17.5 +/- 1.0mm

 

I’m measuring from the gasket surface to the top of the float. I’ve got about 19 mm with floats resting all the way down. When I hold it at an angle to get it off the needle a little then I got 24 mm. I’m not the brightest but I don’t see what I’m doing wrong.

 

this is where the 17.5 mm comes from.


this is where the 3mm +/- comes from

 

You may want to review the article below. Note that we do not recommend setting the fuel levels via a "float height" measurement procedure, as it leads to the exact type of questions/issues that you're experiencing. Your goal is to achieve a correct FUEL LEVEL in the bowl, regardless of whatever "float height" measurement it takes to achieve that level. The "float height" procedure can sometimes be useful as a "shortcut" step to get the levels "close to correct", but it can be an inaccurate (and frustrating) way of dong things.......

https://www.xj4ever.com/setting fuel levels.pdf

 

Well, you’ve been told wrong. You remove the current jets and put in different jets..... you’ve re-jetted it. Theists the simplicity of it. Now, the complicated part is getting the RIGHT sizes— that’s where Len’s write/ups come in.

 

AND.....if the rest of the intake system (airbox and stock-style paper air filter) is original, then my experience (and others) shows that re-jetting the water-cooled engines almost never results in an "improvement" in performance. The 700-X engines are very finicky in regards to jetting changes (or airflow changes) from their stock configurations, as they were already properly tuned to within an inch of their life right out of the crate (for street riding).

BTW, the above is not true on earlier air-cooled engines (especially 650's and 750's) as they were INTENTIONALLY jetted a bit lean from the factory, in order to meet EPA emissions regs/testing procedures of that time. Those models, even in completely stock configurations, can usually benefit from a slight richening of the pilot circuit (either via re-jetting or adjustment of the pilot mixture screw "out") and the main jets by one size higher (bigger).

 

Agreed 100%

 

Plus one to the original jetting not needing any changes - why would they?
Then the float height Vs fuel level with tube, the float heights is a starting point, measured and set with the float just resting against the spring inside the needle valve. Then, once they are all identical, refit the float chamber and supply with fuel, open and close drains a few times to check readings. If you need to adjust the fuel level use the float height measurement to help you.
All the information you want is here, but if your battery is good, have spark and some fuel it will start - did you clean the enrichment jets in the float bowls?