No Start - Shims / Valve Adjustment

[pulmenti]

Hi all, I'm still frantically trying to sort everything out on my '82 Maxim 400 before the winter sets in so I can get at least a taste of riding this season.
So far, I've rebuilt and cleaned the carbs, adjusted them (mostly) correctly, sorted out some sticky slides, and cleaned the tank.
All this was done to fix these issues:

• Running rich
• Won't rev above 4500 rpm
• Revhang / sudden revving with no throttle input

I've sorted out the carbs well enough so that it doesn't seem like it's running rich anymore (although there were some pops and bangs the last time I ran it - didn't smell gassy though) but I began thinking I was also having issues with timing / valve adjustment. It also would sometimes only rev to 4500, but once revved to 6000 before dying off. When it was able to get to 4500, it would idle fine, but when it could rev to 6000, it wouldn't idle.
So I took off the top cover to take a look, took some measurements to find the spacing between the cam and the shims, and ended up with the following:

Left exhaust: .06 mm
Left intake: .06 mm
Right intake: .05 mm
Right exhaust: .05 mm

(These were taken with the lobes facing 180 degrees away from the shims, so fully disengaged)
Then I took out the shims to see what they were numbered, and strangely only the right intake was different. They were:

Left intake: Y285
Right intake: Y280
Left exhaust: Y280
Right exhaust: Y280

I don't really know what the shim numbers mean, or what I should do next. If anybody could tell me what I'm looking at / where to go from here, that'd be great! It'll kill me if I don't get at least one good ride in before the PNW winter coats the world in black ice for the next 7 months.
If I'm looking in the wrong place for my issues, tell me that too!!

 

[tstidham]

I wouldn't worry about the shim numbers, just the final valve lash they provide. Variations in machining and normal wear will dictate what thickness of shims to use but if .06 and .05 mm is within spec, you're good to go. They don't have to be identical, just in range.

 

[pulmenti]

Right, so after looking through some threads, it seems like the proper spec is .1 to .15 mm - mine are at half to a third of that. It's weird, I assumed the gap would widen over time. How can I go about adjusting it to spec?
Do you think this extra - tight clearance causes some of the issues I'm having?

 

[tstidham]

You'll need to get extra shims of the correct thickness to increase valve lash, so measure the ones you have and get thinner ones that will bring you into spec.

 

[pulmenti]

You'll need to get extra shims of the correct thickness to increase valve lash, so measure the ones you have and get thinner ones that will bring you into spec.

Gotcha, how do I know which shims are right? I've measured my shims, and know that I need 3 that are 2.65-2.7 and one that is around 2.75 mm thick, but how do I know which ones to order? Are they specific to the xs400 or are there general bike shims? Sorry, this particular thing is insanely confusing to me for no reason

 

[tstidham]

Any that are the correct diameter should work, but I just googled xs400 dohc valve shims and a few hits popped up. One being https://www.mikesxs.net/motorcycles/Yamaha/XS400/engine-products/top-end/valves/valve-shims.html

 

[pulmenti]

Thanks for the replies - ordered some new shims, put them in, all but one valve is at .15 mm, with that left intake being at .1 because I suppose I did my math wrong - but they're all in spec. Thing is, it doesn't start now. I just replaced my spark plug boots, checked float bowls, tightened and then adjusted out the idle air mixture screw, but no luck. Timing?

 

[tstidham]

You've changed a lot of things so you'll have to start at the basics, spark, fuel and air. Pull a plug, plug the wire back on it and hold it to the cylinder head to ground it and crank the engine. If you have spark, try spraying fuel or starter fluid in the carbs and see if she coughs. Air is usually not a problem unless you have a severely clogged filter or blockage.

 

[pulmenti]

alright, I just went and checked on it. Both cylinders are getting spark, there's fuel in the float bowls (and vapor comes out of the spark plug holes when cranked without plugs), and the filter isn't an issue- it's off at the moment. The vapor coming out of the spark plug holes is what's making me think compression / timing, but it's electronic ignition on the dohc.. and again, all the valve clearances are in spec, and it ran before.. what's your opinion?

 

[tstidham]

Certainly, if you can do a compression test, do that. Make sure you have the throttle wide open when doing the compression check. What did you reset the idle mixture screws to? Seeing fuel mist shooting out the plug holes may mean it's really rich so you may have to turn them back in some or make sure the choke (though technically it's not a choke but an enrichener circuit) isn't stuck on.

 

[pulmenti]

Certainly, if you can do a compression test, do that. Make sure you have the throttle wide open when doing the compression check. What did you reset the idle mixture screws to? Seeing fuel mist shooting out the plug holes may mean it's really rich so you may have to turn them back in some or make sure the choke (though technically it's not a choke but an enrichener circuit) isn't stuck on.

Think I'm adjusting the wrong screw, again.. I only adjusted one screw, the one that's in the middle of the two carbs and closest to the engine. I turned it in until it was difficult to turn and then out one and a quarter turns. I don't have a compression tester at the moment.
The other thing that I forgot until yesterday was that when I was taking out the old shims to inspect them, I misunderstood the instructions and accidentally took out the screws that hold in the intake cam completely, and the chain tensioner did its job and squeezed the cams really close together.. I took out the tensioner and pulled the cam back to where it was, and didn't see it jump a tooth, but it could have easily changed something I think. I'm not trying to act like an idiot I swear!

 

[tstidham]

Oh, yea. That middle screw is the idle speed adjuster, not a mixture screw. Best to try and set that back to where it was as if it did start, it'd probably be screaming.