Help! Rebuilt 600 backfiring

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vmax600

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Jan 21, 2005
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16
Age
56
Location
Webster NY
Hi guys,
I just got finished rebuilding my 97 XTC engine(600 twin). I took it apart to do a rering, maybe piston replacement. After getting into it, I found the nicasil pitting and flaking off the cylendars. So I got a whole new top end. I split the case to replace the rest of the seals since I had a good chunk of money invested in the top end. While inspecting the crank I found the clutch side beearings were loose and noisey. So now I have a completely new engine. She started right up, but now at an idle she sputters every ten seconds or so, and I can hear it backfiring. I have been through the carbs several times. I have replaced plug boots. What am I missing? I am getting reallly frustrated with all the money I spent and the time invested, (especially since I got her running) :o|
I see the book calls out a timing spec, but I can't find a procedure for it anywhere in the book. Should I have timed the engine somehow?
The other thing I notice, is that when I kill the engine, the plugs are WET when I pull them. I can see fuel vapors coming up from the plug hole when I pull them.
 
your carbs set to stock spec?, you sure pilot jets are clean?how many turns
out are your fuel screws??? how does it run otherwise???

did it run fine before you took it down??
 
Thanks for the replies guys.
The fuel screw are turned out 1.5 turns, and I have cleaned the pilots with carb cleaner and air. The carbs have stock jetting (45 pilots, 151.3 mains) I double checked the float height last night. I have about 3 miles on her from tooling around the yard. This sled has always had a lousy mid range. I tried dropping the needle one setting on the carbs, but it never seemed to help.
I haven't been hamering on it as everything is new and with the back firing, I don't want to ruin anyhting. I almost want to say that it sounds like it's dropping a cylinder when I do get into it a little. I can hear the pitch of the engine change.
I am going to try a spare coil tonight to see if there is any change.
I know i haven't put a lot of time on the engine, but I am pretty sure and assembly oil has burnt off by now.
I am open to suggestions.
 
Interesting. Sound exacly like my problem. Someone suggested chipped reed to me. How are yours?
 
how many km's any thing above 5000 and you may want to change the emulsion tubes .Over time the metering holes get worn bigger ie RICHER.Adjusting the needles won't properly address the problem.
 
Yes it was a new crank from a yamaha dealer.
My reeds are fine.
I don't understand what emulsion tubes are either. Are they the jet needle tubes? I sort of wondered wheather they could be worn and allowing fuel to be sucked into the engine at an idle?

Last night.
I tried a coil and opened up the fuel pump to check it for dirt or damage. These didn't help at all.
When I start it up, I use full choke to start it but if I don't flip it to half choke right away, it loads right up. So I let her run at half choke for a couple of minutes, but then it wants to load up agian, so I have to turn the choke off. I know it's not warmed up yet, but even with no choke, it sounds loaded (too much fuel). Then when I rev it enough to turn the track she smokes like crazy and still sounds loaded up. Then when it idles down, it starts to miss or drop a cylinder completely. Then the backfiring.
It has always (since I owned it) felt loaded up in the low and mid range, but droping a cylinder and backfiring is new.
I took the carbs off again last night, and I am very confident that nothing is plugged and the floats are right. I syncronized the carbs with a pin guage set and the pilot screws are set to 1.5 turns out. I pulled the choke plungers and double checked that circuit but everything was clean, so the carbs are set right, but it still seems like I am getting too much fuel.
I actually have a spare CDI box that i haven't tried yet, I really dont understand what it does.
Sorry for the novel, I am just trying to include as many details as possible.
 
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I think timtires had the answer, if by emulsion tubes he means the brass tubes that the main needles slide through. The dealership said that they had been selling/replacing a bunch of them on the 600 twins. I put them in, with a clean dry set of just about new plugs, started her up. It took a little bit to clean her out as the crankcase was a little flodded, but even before it was completely cleaned out I could hear the difference in the motor and it ran smoother. Now the she throttles right up, no low end bog. I haven't had a chance to ride yet, but were gettin some fresh snow now so hopefully tomorrow night I can give her a test drive.
Thanks for the help guys!
 
The other thing that may be happening that is related to the emulsion tubes are the rubber o-rings that seal them to the carb body. I have had them degrade to the point that they allowed air and/or fuel to pass by the O.D. of the tube body and cause issues. If you replace the tubes you want to also replace the rubber o-rings that seal them to the carb body.
Mills
 
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