XJ 400 Idle/choke/issue.

So, bought this bike 6oo miles and 2 months ago, it ran then, with a flutter around 3200 rpms. Above or below that it ran fine.

The flutter got worse and never better, and eventually the bike began to backfire and smells like gasoline when it runs (rich I presume).

There is and always has been a small exhaust leak off the manifold.

Now the bike won't start every time. I check the plugs, hit em with a wire brush, then sometimes it starts.

Other points of interest:

1) It will idle with the choke on until warmed up, will idle at reduced rmps when the choke is released, but dies INSTANTLY when the throttle is touched.

2) I pulled the exhaust off the other day to sand and paint it, and when I put it back on it let me ride it around the block, but only had about 1/4 of the power it normally does. had to fully open the throttle to even get it to lurch forward.

3) The next day I let it warm up for 20 minutes and it ran perfectly for a few blocks, where I shut it off and worked on it some more. I pulled the air filter, fuel line, fuel filter, and carbs off. Air and fuel filter were good and clean, the carbs were a little strange.

The left hand carb was clean, the right had significant carbon build up. I cleaned the right carb with spray, check the float, and pulled the diaphragm and needle to spray in there. Reinstalled everything, and now its back to the "starts with choke on, dies with throttle" thing again.

Here is what I haven't done yet, primarily because I am short of the tools to do so at the moment.

-check the compression
-sync the carbs
-adjust the mixture screw

The mixture screw is in between the two carbs and has two springs on it, so one screw controls both carbs. Do these springs wear out and would that cause this?

Lastly, the night I pulled the exhaust (day before it ran again), i cleaned the plugs. The left was wet, right was black. Yesterday, (the following day and the day it ran again), I check the plugs after cleaning the carbs and trying to start it again and the left was same as when I had cleaned it, but the right was white.

I know there's a lot of posts like this out there, but this is my first bike, and I know little about carbs. I am eager to learn, but can't find much in the way of advice. They are mikuni carbs to the stock airbox, some website I found suggested they were mikuni bs 32s. This is all I know about the carb set up - don't know about the jets etc.

I have a picture of the carb before I cleaned it if that makes any difference to anyone.


Thanks, I am dying to get riding again!

-P

 

Sounds like a vacuum leak/plugged carbs. A few questions for you:
Is this the 4 Cyl XJ400 or and XS 400 Paralell Twin?
If it's the XS, do you have the rear monoshock? Is it a 'YICS' engine?
If your carbs are still out, punch out the emulsion tubes... those brass tubes the needles slide into and the main jet screws onto. Clean it SUPER good, there should be 8 little holes in the middle, mine were clogged SOLID.
There are alot of little passages and things that could be clogged, best to tear it COMPLETLY apart and soak in carb cleaner for a day or so. Take a thin wire... I use one ripped off of a brush, and push it through all the holes and passages you can find.
If you still have issues after putting them back together and on the bike, grab a spray can of carb cleaner and lightly spray around the carb boots. If the engine surges, you have a vacuum leak. I had one at the petcock, so check there too! Welcome aboard BTW!

 

Thanks, it is the XS400, thanks for clarifying, I couldn't find a reliable source for that! Its a parallel twin, and a monoshock.

I will pull the carbs again, no big deal there, didn't take long. I have a can of carb dip which I didn't use, but I will this time. Figured I'd give it a quick and dirty clean to see if it was just not a huge issue.

I will double check the boots around the carb, you are suggesting that spraying cleaner on them while it is running will indicate a leak IF the engine surges from burning the additional carb cleaner?

If that is the issue, and there is a vacuum leak, I can readjust the hose fittings, and try to solve it. If it is not, I will pull the carbs, tear them down, and dip them.

Finally, if I do all this, reinstall it, and the engine still doesn't like my touching of the throttle, what would be the follow up suggestion?!

YICS- I have no idea what that means, I just googled it, but I don't know if thats how my engine is, or how to tell. I have not seen anything saying YICS on the bike, but I may not have looked in the right place.


Thanks a lot,

-Phil

 

If it's the XS 400 with a monoshock it'll be a 'YICS' engine, although I actually can't find a YICS port on mine... funny that.
It should say YICS on the engine cases in RED, but may not be there with replacement covers etc. Really nothing to worry about. Only reason I ask is I'm just finishing putting mine back together, and my other bike, an FJ600 uses the same carbs, the Mikuni BS32's. So I'm kinda familiar with them. ^^
These carbs use the engine vacuum, so if you crack the throttle and she dies, it's because of a vacuum issue. Weather it's with the boots and surrounding hoses/connections between the carbs and the engine, or an internal carb issue (air passages/fuel passages clogged) I couldn't tell you.
Look for the Church of Clean or the carb cleaning thread by RickCoMatic, it'll help.
Make sure you take apart the carbs COMPLETLY before soaking in carb cleaner... that stuff DISSOLVES plastics. Make sure it's only metal left!
After a clean and insure no hose/boot leaks there's really nothing else it can be!

 

I got it to work! the vacuum hose from the carb to the petcock wasn't connected well.

So, in other news, since you seem to have some insight on these carbs, I want to switch to K&N filters, and bend some new exhaust (straight pipes).

This will require some new jets, and I will probably take it to the local performance shop to get a dyno read on the exhaust, but I would like to do the work myself. Do you know what the factory jet size is off the top of your head or how I could take a measurement of it?

I will have the shop tell me how much larger I need to go, but don't want to have them pull the carb (I'm cheap and don't want to pay their ridiculous shop rate to do something I can do just as well at home).

Thanks again!

 

the screw between the carbs with 2 springs is the sync screw
each carb will have it's own low speed mixture screw
be careful with that carb dip, there are seals where the butterfly shafts go through the carb body and that dip will melt them

 

Yeah that hose was a problem for me too, told ya! The main jet sizes are 127.5 for the left and 117.5 for the right. Someone on here did the pod filters and told me in a thread the jets he used, can't remember off the top of my head tho, try searching for threads I've posted to, I'm sure it'll turn up!
I myself am sticking to the stock airbox and I'm re-stuffing the mufflers to be nice and quiet. I am by no means an expert, especially compared to some of the folks on here, but I've been tearing apart the BS32's and cleaning 'em for a while now, so it's all pretty damn fresh in my mind!
I've blacked out my engine and will be replacing all the crappy aluminum phillips-head screws with nice shiny polished SS allen-head ones. When I'm all done I'll post to my build thread the list of all the sizes needed for all us XS400 owners.