'88 XT600U bog and backfiring
 

I recently bought an 88 XT600 that wouldn't start easily or stay running, but sounded good when it did. It sat for 2 years so I figured clogged carbs. Well I cleaned them(twice actually) and made sure no plugged holes. I got it running and it ran great, full throttle and all.
I filled with gas(it was old gas in it), 93 super unleaded here in the states, went 2 miles toward home and it suddenly bogged like it was working real hard and kept popping/ backfiring. till it died, wouldn't restart. after a bit with full choke it did start but same thing and died. Recleaned carbs, made sure no air leaks(even put light silicone sealer on some things).

It then started up and ran perfect, idle smooth, let idle for a couple minutes then bog and popping again and it died. I belive it would be the cdi or coil( it didn't seem to have great spark, but the bike is new to me so I don't know how good it should be) the plug color looked a bit lean and even after kicking on full choke it was dry. That says fuel issues, but outside of some internal air leak or out of spec part it isn't a clogged jet or port. BUT the popping and bog when it gets warm says cdi. Any of you that have been running these bikes have some possible solutions? Thanks JJ

No I didn't , the pin wouldn't come out easily and until I had a new needle valve and pin I didn't take it out, which I have ordered because I wanted to replace it anyways. I also didn't know it had a screen like that and that would make sense. The one question on that would be, when it did quit, I opened the bowl drain screw to check for fuel and it did come out, but whether there was enough,I don't know. It wouldn't start this morning after 8 kicks on full choke. I'm going to squirt some gas and a little oil in the cyclinder tonight. If it fires up, I know it is mostly fuel related.

Thanks for the picture, it shows what is in there.

Try a simple one first - a new spark plug. I had an XTZ that did something similar thing once and it turned out, after multiple carburettor cleanings, to be that. The plug gave a fine spark when out of the cylinder but was breaking down when under load.

A quick update today, I put in a different plug, still nothing but backfiring( had to stop and sleep with icepack around my ankle, afraid of it being broke from one nasty pop). Now decided to kick with my motocross boot on right foot.
I decided to rollback to when it quit running, about 1.5miles AFTER filling up with 93octane GAS. Drained gas out of tank and float bowl and got 87octane from a different company. About 6 good kicks it fired up and is running, let it idle for 3-4minutes, went around the block a couple times. It does still have a skip in the firing, runs perfect under acceleration(easily hit 85mph and still going), with a steady throttle it has a stumble.

Any ideas what the steady misfire would be? Has no backfiring, plug now looks good color. Over tightened valves? Do these motors get tight or loose with wear? No knocking or ticking at all, entire motor sounds smooth and has great power. I'm going to really like this bike once it is straightened out.

Dirty fuel filter
Carb failure
Faulty ignition coil
Faulty pickup coil
Bad CDI unit

Leaking inlet rubbers causing a weak mixture.

They are rubber bonded to steel and they deteriorate over time. Unless they have been replaced in the bikes life you almost certainly have this problem.

The boots are in perfect condition, the previous owner apparently recently replaced them. This morning it took squirting gas down the spark plug hole to get it fired up without a bunch of kicking, then it ran as normal. Dies when I pull the choke out, so it is working. Spark plug now is all sooty black. I'm looking into getting a CDI and coil, even if they turn out to be backups. More carb work is in the works also.

Yes, it has now been replaced along with the needle(or plunger) since the parts came in yesterday morning. It wasn't plugged, but it wasn't completely clean either. ran it to work today and bombed around, takes about 8 kicks on full choke or a squirt of gas down the plug hole to get started after it sits for a couple hours and has that steady stumble but stays running and runs perfect accelerating. It's definitely something in the pilot circuit(low speed) or an electrical thing.
Time to put in my pair of Mikuni's from a Raptor 660:funmeteryes:

This bike is awfully skittish, it wants to shift back and forth like on glare ice. Picture hanging the bike by the frame and swinging it side to side, that how it drives, even on asphalt, worse on gravel. Needs some better tires.

There was no inline filter other than the petcock tank one. I have one that is going in. Right now it will hardly start unless I squirt fuel directly into the cylinder through the spark plug hole or kick with all I've got 8 to 10 times(it messed up my ankle again this morning when it backfired trying to start it normally:censored:). That says it isn't getting enough gas to start, BUT, once it starts, it'll run till the tank is empty, so I know it is getting enough fuel into the carb, plus I've made sure the bowl is full repeatedly.
It simply is the carb start circuit or a spark issue, I've checked spark multiple times now and it really snaps, but whether it is doing it on-time(aka bad cdi or coil) or under compression is the question. If it was an issue with fuel delivery to the carb, it would run out of gas after it is started, but it doesn't and plenty of fuel comes out of the bowl when I open the drain screw.
My ankle is telling me to walk away from this bike right now.Or should I say limp!! My Honda XL600r starts up in one kick!! But the Yamaha is in way better shape.

your problem with having to kick it 8 times is either technique or decompression cable is not well adjusted.

Personally, I believe the decompression cable/adjustment is The weak spot on the XT

I do not have the decompressor assembly fitted on my 1990 XT600E (retrofitted the kickstart) and I find it very easy to kickstart her in one, sometimes two kicks if she's been sitting in the garage for weeks..

Have you gotten your ankle checked out before?
heal fast!

When it pops is imediately after the decompression lever snaps back shut. With it set right now, as soon as the kick starter starts its cycle the cable begins pulling on the lever. It opens it further all the way till about the halfway point(kick start lever is approx level with the ground), then the lever on the motor snaps closed, so from that point on there is no decompression. Its at the point the the motor fires, if it starts, or pops and kicks back on the lever.
As for my ankle , right now it is swollen and a big black and blue bulge , hurts like h**l to walk on. If it turns greenish-yellow, time to get an exray!

With some time to look and tinker, I hate the decompression system on this bike, and have an extra handlebar perch and lever for my Honda's manual system, I am thinking of running that on the Yamaha so I have control over it. The whole setup is similar on both and all it would take is a long enough cable to loop around the motor so it comes from the same side as the kicker cable. As long as the bike can fire without full compression, it should fix a big part of the issue.

JJrider,
I feel the pain of the kicking. :funmeterno:

It might help, or even amuse, you to know that I have been down that road back in 2007 with my ttr600.
I suggest you take a quick read of this thread about kicking over that engine:-
http://www.horizonsunlimited.com/hub...arting-26306-2

By the sounds of it the TT600 has a handlebar mounted decompression lever? The TT model must have been a Europe version of the XT or something like that, none of my OEM parts sources lists a TT600. I found a picture of a TT head and it has the same decomp. lever and bracket as mine so the handlebar perch and cable from a TT should be a direct bolt on.

I did rotate the kick start lever by hand slowly till it hit tdc and just barely went passed it, my brother gave it a good kick and on the second try it fired up and ran smooth, no squirting gas down the sparkplug hole first. It idled smooth with no miss or stumble for about a minute then slowly started to do its normal uneven firing.

We went for a 15mile ride and it ran fine with just the stumbling at 2600 to 3200rpm range, above that it didn't miss. Once warmed up, at idle it sounds like it fires only every other compression stroke instead of each one. Doesn't die or seem to even want to die but just has that slow and steady pattern of firing. Motor is dead quite as far as ticks or knocking.
I did notice that my headlight dims each time the turn signal blinks( engine at or just above idle rpm), I think it's normal for that age of electronics. I'm going to switch all bulbs out with LED's if possible.

I'm hoping to get a spare cdi and coil to swap out as a possible cure to the missing(unless this is the normal way these engines run, as I said, this line of bikes is new to me). I doubt it though.
Thanks for all the help and suggestions , it saves lots of hours of guessing and parts changing(when they are not needed).bier

An update, after a week or so, I had been running the bike, it started good in about 2 kicks on full choke, it always ran perfect for about 1 minute, didn't matter if choke was on or off(high idling). then it would start to do its stumbling, never acting like it wanted to die but just nothing will stop the miss. Going down the road, under acceleration, it ran perfect, steady throttle=miss. Filled up with premium(it had 87octane in it), ran it 2 miles home, missed the same. Next morning started up first kick, ran for 5seconds, quit and won't start, even dumped some gas down the sparkplug hole, it just backfired at messed up my ankle again and actually broke a buckle on my motocross boot plus broke the kickstarter stop bolts inside the motor.

This is the last straw for me and this setup. If I can't make it electric start( I can't find "E" model parts here in the US, only kickstart years, except brand new$$$$, I wonder why?? Since I will not ever attempt to kick it again, I won't be able to even try to fix it unless there is some mechanical way to turn it over soooo

I'm ripping it apart to
1) put in a Honda motor I have that Will not kill me
2)recoup my money by ebaying as much of the parts and say good riddens.

The sad part is the bike is in mint shape, never tipped over, never run offroad, adult owned, it looks like it is maybe a couple months old!! It just does no good if it can break bones at any moment or be impossible to keep running. Ya for another $500-600 I can buy every electrical component and "hope" it works, nobody can say what is failing, so all that can be done is to buy, buy, buy and hope! I bought this bike to save money on gas, :thumbdown: