Hello and welcome Daniel. No mention of the year and model of your sick bike but I always propose to anyone attempting jetting or tuning complaints to consider the age of the bike and consider that the bike has probably started and ran many times with its original or inherited factory jetting and tuning is likely NOT the problem. I always advise basic troubleshooting should be performed before swapping brass. Spark, fuel, compression testing are first. Cylinder leakdown percentage and valve clearances would be next. You also mention decomp removal. Can't imagine that's ever a good idea for a big 640 motor. Reckon that would need to be corrected with replacement. Big performance motor like that would require decompressor to start reliably.

 

Thanks for the reply. Interesting what you have said about the cam based decompressor. We ran a compression test and only came up with 50PSI. do you know what number we should be looking at?

 

50 psi compression is not likely to start at all. I advise compression testing and cylinder leakage testing in this sequence deliberately. My experience shows when compression is low it can be caused by mechanical failure (rings-valves) or cam chain timing or tight valves. Your cylinder leakdown testing will help isolate mechanical failures causing low compression. Reckon compression would test 125psi minimum for it to run decent. I'd expect at least 150-175 psi. When auto decompressor is operational you would observe very low(50 psi) compression. Common practice to disable decomp pin for compression testing. If decomp is disabled and you're getting 50 psi you may have a skipped tooth on cam chain or burned valve or perhaps valve clearance is too tight and not allowing valve to seat.

 

Ah meant to say its a 2005 model.

Interesting chatting on here, I've thought a lot about this today and I need to do the compression test again.

When I am kick starting the bike there is enough compression that I cannot kick it over TDC without using the manual decompression lever. If it only had 50PSI of compression I think I would be able to kick it over without any decompression.

 

Ha. Reckon she's got some compression if ye can't kick her. You may find you can kick the bastard with a compression gauge installed. There is a bit more "give" when kicking with gauge. Testing with decomp would be unhelpful. I guess she don't have Estart? That would be a real bitch to troubleshoot starting problems on that big bastard while only using a kicker. I'd tow the bastard to a local hill and coast er to 30mph and bump start her. Don't tow start it. Seen that end badly too many times

 

Yeah, kick only. We tow start bikes often - sketchy as it is, sometimes its the only way to get a kick bike running so we can fault find. We towed this one and even with towing it was still a bitch to get going. I notice that the generator cover is adjustable so maybe the timing is out also.

When running - she runs lovely and rides well too. Do you have stock jet sizes you could let me know so I can check that.

Also, from what I read from you - there is no good reason whatsoever to remove the cam decompression device?

 

Sorry. No I don't have jetting specs for you. You may find jetting in bikes owners or service manuals. You may be able to find downloadable manual with online search. Most are free, others charge small fee. I would agree with you regarding removal of auto decomp. Kick only motor on that big old dinosaur is a hard way to go. Perhaps it was broken or worn out. You may find a paddock starter is worthwhile investment for that old monster. Gas and electric are available and will start that big nasty bitch. Put er in 4th gear and pull clutch lever and spin er up and pop clutch and she spins over with no fuss. I had to start using electric paddock starter for the bigger bikes. Old bones and body parts can't kick the big motors anymore. Big old monsters just hurt me and laugh now

 

Found a manual online and it refers to Technical Information publication.

Installing The Ignition (Kokusan 4K-3); Adjusting The Ignition Point (Kokusan 4K-3) - KTM 400-660 LC4 1998-2005 Repair Manual [Page 106]

KTM 400-660 LC4 1998-2005 Manual Online: installing the ignition (kokusan 4k-3), Adjusting The Ignition Point (Kokusan 4K-3). – Block The Crankshaft With The Crankshaft Locking Bolt. – Insert The Woodruff Key Into The Crankshaft. – Clean The Cone Of The Flywheel And The Crankshaft And...

www.manualslib.com

Has anyone seen this and is the information useful that it is referring to?