XR Starting/Carb Adjustment problems

hey yall... sooo im stuck. I have kinda a Frankenstein Xr. my bike is a 1991 XR600r bottom end that is, with replacement cases from 1992. its got a 650 top end but not sure what year. got golds racing rear shock and cr500 front end forks triples wheel all that jazz. I got the bike in boxes and have been building it up for the last half a year. but when I go to start it I am having probs. if I try to start it without the air filter in (its a brand new clean and oiled uni filter) every time it will take about 10-15 kicks before it will back fire once then about another 5 and it will run. after about a minute of warming up it will idle perfect and not want to die at all. if I have the air filter on, all it will do is back fire over and over without and attempt to run. but once its idleing without the filter on and I go to put it on it doesn't seem to affect the way it runs at all. also I cant start it using the decomp, it doesn't seem to even try. kinda stuck at this point. not sure where to start looking besides maybe carb adjustment. not sure at this point what jets are in it, but im in Arizona so if anyone knows what I should be running jets wise, or if anyone has any idea whats up with my starting probs itd help a ton. thanks

phoenix Arizona that is

Have you pulled and cleaned the carb? It maybe the air cutoff valve. The acv can give trouble in the starting department. My two cents is to tear off the carb check the acv and clean the entire carb thoroughly with compressed air and use a fine wire to clean all the hole and jets out maybe use a wire from a wire brush and a good overnight soaking in a carb cleaning solution just make sure to remove any and all plastic and rubber parts from the carb before soaking

Yah I got the carb off eBay with a brand new xrs only rebuild kit in it and went through to make sure it was all clean but haven't taken it apart after running a little gas through it

Check the rocker gaps..Backfiring is a bit of a sign the timing is slightly out. You do not want the gaps crazy tight ..bit loose is better than a bit tight.


You also haven't got crap loads of oil all over the new filter have you..They aren't meant to be dripping with the stuff..If it's over oiled or wet with a say petrol oil mix from it being cleaned it'll interfere with the starting..fumes etc.

Like I said its a brand new filter so there's no cleaning chemicals on it, but I did oil it but not that much. I've seen my friends oil there's and use a lot more than I did, so I don't think its over oiled. But I haven't checked the rockers. And do you think maybe it could be a tooth out of time in one way or the other, Im not sure. Cause when it is running, it runs strong

I don't know whether it may or may not be slightly out in the TDC timing department,,Only way to know is line things up and have a look...Cam lobes down..Cam sprocket marks level with the top of the head,,T on mark,,Piston top of the bore..How you make sure the lobes are down without the head cover off I don't know..May be able to tell by looking through the inspection caps..pretty hard to see in there though.,,That's useless anyway as you can't see the Cam sprocket marks,,Head cover really needs to come off for a proper look..If you did the timing originally and it all lined up then I guess their's no reason it would have changed..Tappet/Rocker gaps need a check is all I can say as they could move around a bit after a bit of a run..

Edited by Horri

I know I'm kicking a dead horse here but did you check the intake boot for air leaks? Just a thought

Yah its also brand new. The one that was on it was ripped so I got a new one

When you say 650 top end, does that include a 650L piston?  I think a stock 650L  piston on an XR600 lower end will give an extremely low compression, maybe somewhere around 8:1.

When you say 650 top end, does that include a 650L piston?  I think a stock 650L  piston on an XR600 lower end will give an extremely low compression, maybe somewhere around 8:1.

Most correct,,How far did the Piston come up the bore when you had the crank/rod/piston fully extended??If it was lower than the top by 5mm or so that could be the issue.

Edited by Horri

Its 600 crank trans cases all bottom end. Jug piston and head is 650. But I haven't done a compression test, but from kicking it over without the decomp I assume its got plenty of compression cause i damn near need to jump on the kickstart to get it to turn over, and I thought maybe something wasn't in right so I pulled top end and inspected the just but no scratches or wear so I think its just got a ton of compression

Just tryed to adjust the valves and may have found the prob. There was no valve clearance what so ever. Won't be able to tell if that fixes it for a while cause I got get some oil before I try to run it. I'll let y'all know if that fixes it

No valve clearance whatsoever..What on all the rockers???Hope you got it on the right stroke when you checked them. I kinda doubt it would run at all with absolutely no clearance on all the rockers

Yah nothing on the rockers what so ever. And I'm positive I have it on the right stroke. And it did run but was a bitch to start

Oh well,,we can but trust your judgement on the settings..Report back on whether it's running better.

Spec on it is. .12mm exhaust and .10mm intake and that's what I have it at now. I just need to get oil so I can run it and test the new adjustments

Hey guys just got the bike to run again and after the valve adjustment it is so much easier to start and doesn't backfire or anything. Now just got some carb adjustment to do. Thank you everyone for the help I really appreciate it

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