Noob vs. Mikuni VM carbs: Questions

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vdubplate

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I thought today I would try try to see if I could get my bike running. This bike has always ran like shit ever since I got my hands on it. I might need to rebuild the engine or get the timing, valves, and can chained adjusted. I wanted to see if the carbs might be jacked up before I start messing around with all this other stuff. Originally I tried to just messing with the throttle and the idle screw. Ive been scared of the idle adjust until today. Today I took off the filters and figured I would start from idle adjust unscrewed and see where my slide is at and work my way up. I was reading everything I could find online as well as the tuning manual for mikuni vm. The popular approach was to use different sized drill bits to determine slide height and work your way up through the bits. What I found on my bike is I can not get the slide to close all the way. Is the slide supposed to close all the way or no?, I loosened the throttle and the idle adjust all the way to see if that made a difference. I'm attaching pics. The difference in the idle adjust all the way open isnt much different from all the way closed.

open
img0301v.jpg


closed
 
The slide has a cutaway that is higher on the intake side; when the slide is fully-closed, you'll still still see an opening as in your first pic.
 
vdubplate said:
Thats good news. I was thinking the cable might be to tight
Click to expand...

It's always good to double-check that too. Back off any cable adjustment you have at the handlebar throttle housing and make sure you have slack there before setting the idle adjustment screws. Remember the cable adjusters on top of the carbs are to synchronize slide lift-off, not to take up slack or adjust length.

Once the idle screws are set, take the slack out of the cable with the handlebar housing adjustment. I like to leave just the slightest free play at the handlebar; lets you know that the slides have dropped fully to the screws instead of hanging on the cables. It's good to be able to feel at the bars when the slides have closed.

If you want to be absolutely sure, unscrew the carb top caps and pull out the slides, then re-install without the cable It's very self-explanatory once you get in there - just be careful with the needle. The spring will fully depress the slide, and you'll be able to better see how the slide moves and adjusts.
 
That's a good idea. After trying different methods I wasn't any closer to getting the bike to run correctly. I was feeling like there was still some tension on the slide. If I unscrew the caps them ill know for sure.
 
Listen, guy, you're doing things assbackward. The only way to tune a carburetor is by engine function. That's why you make sure everything else is in adjustment first. It's also a good idea to do a simple compression check at the git-go so you know whether or not you'd be wasting your time with adjustments.
 
What grizld 1 just said was, set your cam chain and valves FIRST damn it. Then compression test, make sure you you have a strong spark at the proper times. NOW it's off to the carbies.
 
WHAT??
Individual interpretation takes a statement different ways.
At the git go must have a different meaning up north.
In my woods it means 1st, first, numero uno.
But there are many paths to the well.
And if we argue about what he said, he may stop talking.
Grizld1, Thanks for education over the years, looking forward to more wherever you post.
 
I'm going to order a compression tester today and order what I need to do the valves from Mike XS and see where we're at.
 
I am not arguing with grzld1 far from it, point is the carbs are down the list. The OP while well meaning has HIS order wrong. Probably the easiest and most ignored or botched maintenance task on an XS650 is setting the cam chain. Once that's done why not set the valves. Then a compression test will reveal if there are other issues.
We don't know if this is a points bike, if it is there can be lots of "make it run bads" in the ignition.
 
Compression tester and feeler gauges can be had locally, both are rather cheap. Feeler gauges with the .0025 gauge normally less than $10.00 All you need to do the valves are the feeler gauges and a couple of wrenches. You can do a good job of setting the cam chain with the electric starter and the spark plugs out.
You want a compression tester that threads in to the plug hole, IMHO the push on ones are junk. Harbor fright has one that works fine.
 
I completely agree. Its as if the bike is almost there. I thought I would take a go at the Idle Adjustment before I try anything else even though I have been told that thats the last thing to mess with. I'm thinking of rebuilding the engine. Its a 75, Its leaking oil and when I changed the sump filter parts of the Cam chain guide were coming through. No metal shavings though. Do you think before I rip into my engine I should get my hands on another engine and see where I get on that one? How hard is it to do a rebuild?

I've checked out the cam chain adjustment video on the site. Its a matter of tightening a bolt on the engine or no?
 
Not tighten adjust LOL, adjust til the head is more or less flush with the adjuster collar, while cranking with the starter it needs to be moving in and out a bit, if it stops moving it is too tight. If you hear an erratic "tinging" at idle it is too loose. but yup just a couple of wrenches 22mm is correct for the cap 7/8" will work "fine"
 
Since your bike isn't idling?? A buddy to kick it while you watch would be nice.

On your engine read the recent thread about re torquing head bolts the leaks often can be stopped. Your 75 would have points unless someone has upgraded to electronic. Points need some TLC to work correctly. Recent thread on that too. I would go through all the maintenance on what you have before deciding on an overhaul or replacement. The maintenance skills are needed after an overhaul anyways so might as well learn them now and possibly avoid the over haul or put it off till after riding season.
 
I can get my bike to idle but I have to give it constant gas. I am not sure but I think the ignition has been upgraded by the bike builder but to be honest I'm not sure how to tell if it has been switched over. There is a little black box attached under the battery that could be some sort of ignition. All the electronics are stored in a fake oil can. Ill attach a pic in a minute. I'm thinking about buying a bike from a guy later on to get my hands dirty rebuilding the engine. I'm a little scared to take my bike apart so that might be a good precursor. Also like you said I should do the maintenance first before I jump to conclusions.
 
I'd listen to every word these guys say. Set your cam chain tension, then check the valve clearances (both way easier than anything you're gonna run into with carb tuning), and then start dealing with the carbs.

As a side note, you may have an issue with your carb jetting with those pods on there... Especially if it has free flowing exhaust as well. I'm not sure on the sizing with the VM's though.

Good luck... you've come to the right place for help. This site is full of XS badasses.

Deft
 

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