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Discussion Starter #1
I've been having trouble with my '78 XS400 for quite some time now, no matter what I do, nothing makes it run any different.

At an idle, and just off of an idle, I can't get it to run on 2 cylinders. BUT, if you are lucky enough to get it above 3k rpm, it runs great!

-Both cylinders get good spark when tested against the block.
-Both carburators are perfectly clean, and have been fully rebuilt.
-Both cylinders have good compression.
-I replaced both coils with brand new Yamaha coils.
-Points are new and properly gapped (K&S engineering, all Dennis Kirk offered)
-I put new points wires on it.
-Put a new Yamaha condenser on it.
-I installed a MAC 2-1 megaphone (with baffle still in place) to replace the rusty swiss cheesed stock pipes.
-I've tried 3 different sets of new spark plugs, right now it has splitfires in it.

I can turn the ingition timing all the way one way, and then all the way the other, and it only makes a minimal difference.

Messing with the carburators does pretty much nothing, I've tried many different sizes of jets, messed with the needles, nothing.

Right now I've even got it at a shop, and they're doing nothing but scratching their heads the same as I was. I'm thinking of getting it out there this week.
 

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Discussion Starter #4
quote:Originally posted by Ratty-550

Always the same hole that's dead?
ya the right cylinder, sorry.
 

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Discussion Starter #5
quote:Originally posted by Ratty-550

Always the same hole that's dead?
ya the right cylinder, sorry.
 

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I would say it is still a clogged idle passage in the right carb. Those little passages can be almost impossible to get clean sometimes. If you stick the nozzle of the carb cleaner into the idle jet passage with the jet out, does carb claener come squirting into the venturi?

Ken
 

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+1 what Ken said. Use a torch tip cleaner or fine wire and gently, GENTLY poke around and see if you can knock whatever's in there loose.

Today's carb cleaners sorta suck. I've got a buddy that uses muratic acid on super gooey carbs, but that scares me.
 

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Try swapping the carbs (if possible) and see if the same effect happens to the left side. I agree, it's looking like a clogged carb in the idle section. Now if you could get your hands on a sonic cleaner - that might clean out the carb.
 

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Discussion Starter #9
quote:Originally posted by kenessex

I would say it is still a clogged idle passage in the right carb. Those little passages can be almost impossible to get clean sometimes. If you stick the nozzle of the carb cleaner into the idle jet passage with the jet out, does carb claener come squirting into the venturi?

Ken
I got the bike home today and went through the carbs once again. I ended up putting the original pilot jets back in because one of the new ones didn't have holes drilled in it and I missed it when I put it in! WTF? Unfortunately that changed nothing.

Yes all the passages are clean, carb cleaner squirts nicely out of everywhere I spray it.

I uploaded a video to youtube so that you could all see exactly what its doing..it seems its running on both cylinders, they were both heating up evenly, so the main problem is that it just doesn't want to rev.
 

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Check your spark plug caps.I had a friend who had a SRX600,it wouldnt rev out,i popped the cap off the plug and inside on some of those older Yamahas the piece that makes contact with the plug was unscrewed..just get a little screwdriver and tighten it up or replace the cap.Weirdo stuff.
 

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Discussion Starter #12
quote:Originally posted by Ratty-550

What did you use for carb kits? You didn't use the needles out of a Keyster kit did you?
Yes, but it ran exactly the same BEFORE putting the carb kits in.

quote:Originally posted by joesmotos

Check your spark plug caps.I had a friend who had a SRX600,it wouldnt rev out,i popped the cap off the plug and inside on some of those older Yamahas the piece that makes contact with the plug was unscrewed..just get a little screwdriver and tighten it up or replace the cap.Weirdo stuff.
I've had 3 different sets of coils/wires on it, all the same.




I'm thinking maybe the points I bought are wrong in some way? Like maybe they're not able to be timed correctly? It was only points and exhaust that I touched before I had problems with it, never really thought they were the problem before, but now I'm really starting to think. Unfortunately I threw the old ones out because they were cracked.
 

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Is the advance mechanism working?If you put a timing light on it,can you see the marks move from the *F* mark to the *advance* marks when you rev the engine?
 

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look at the diaphragms carefully under a strong light

do you know how to test them before unscrewing the carb top?
 

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and are you sure the emulsion tubes' holes which come thru the side are all clear?

and just because the idle jets are clear does not mean the idle and transition discharge holes are
 

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Discussion Starter #16
quote:Originally posted by joesmotos

Is the advance mechanism working?If you put a timing light on it,can you see the marks move from the *F* mark to the *advance* marks when you rev the engine?
I can't run the engine with the side cover off because it loses oil. So I'm not sure. All I do know is that moving the timing plates changes NOTHING. And no matter what I do the timing marks DO NOT line up with the spark. This is why I'm thinking its the replacement points that is causing this.
 

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Discussion Starter #17
quote:Originally posted by HackAsaw

and are you sure the emulsion tubes' holes which come thru the side are all clear?

and just because the idle jets are clear does not mean the idle and transition discharge holes are
Well there really is no way to be totally sure I guess. I can't see through the tubes because they're so small and curvy. All I know is that Carb cleaner sprays nicely out of every hole I can find.
 

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Sounds to me like you need to do a couple things.One,either drain thei oil out,pull the cover off and look at the advancer and see if you can move it manually,or lean the bike way over on its side(like lay it against a wall so the oil doesnt come out) and check it like that.The advancer is the little cam the points rub against..and if the marks dont line up,well...it might be because the advancer is stuck in a certain position.If it doesnt rotate freely then the timing cant advance.And like what Hackasaw says..the emulsion tubes..if you pull the main jet GENTLY AND VERY SOFTLY push/nudge those tubes out with somethings soft so it doesnt damage them..they usually install from the TOP of the carb...as in,to install them you would insert them through the TOP of the carburetor..inside the bore...so you pull the main jet,and push them UP..thru the body of the carb.You wont be able to see shit unless you get them out,they have fine holes drilled in them to EMULSIFY the fuel...if those holes are clogged,then it runs RICH..also...if theres a washer under one main jet,and not the other,that will cause it to run rich too,on the one WITHOUT the washer..usually a small copper washer or aluminum(i think,its a soft light colored washer if its not copper)...
 

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I was having the same problem. Found one of my fuel needle valves that fills the bowl on one carb wasn't working and no gas was getting into the carb.
If you by one though, get it from JCWhitney ($15)if they have the right size. I bought one on EBAY beforehand ($30). Before that I ordered one from the stealership for $70! Just checked my bank account yesterday and they credited my money back. Good!
Hope I helped.:D
 
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