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Old 04-02-2013, 09:34 AM   #1
KayElAre250 OP
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HELP! KTM RXC 400 LC4 Starting Problems Cold or warm/hot - Runs great otherwise.

Hi All,


Problem is I have to kick forever (literally 30 min) before it'll start. Even when it's warm/hot. But, once its started, it runs like a dream: idles smooth and lots of power through all RPMS. I rode it 60 miles without any problems (oil level was the same after 60 miles so I don't think it's the top end).


Any input would be greatly appreciated!

Thanks!

KayElAre250 screwed with this post 04-30-2013 at 12:57 AM
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Old 04-03-2013, 10:35 AM   #2
wrk2surf
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well now that you have a thread started.. why not take the black plug out replace it and then double check the carb to see where you are at with jetting.. then clean and oil the airfilter .. fiirst things first .. you may need to do some rebuilding of the carb or perhaps even switch over to a Mikuni BST that is easier to get parts for.
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Old 04-05-2013, 06:32 PM   #3
KayElAre250 OP
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Changed the plug, cleaned air filter, and rejetted to smaller needle and main jets. Now it starts easily, but only when cold. Once it's hot, it won't start. I didn't change my idol jet (pilot), so it still could be carb related. Maybe the idol jet is still too rich for a warmed up engine?

Also, it was very easy to kick after it was hot. Gonna do a compression test tomorrow (if i can find the right fitting). I really hope it's not the top end .

Anyone have any ideas?
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Old 04-05-2013, 08:20 PM   #4
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Quote:
Originally Posted by KayElAre250 View Post
Changed the plug, cleaned air filter, and rejetted to smaller needle and main jets. Now it starts easily, but only when cold. Once it's hot, it won't start. I didn't change my idol jet (pilot), so it still could be carb related. Maybe the idol jet is still too rich for a warmed up engine?

Also, it was very easy to kick after it was hot. Gonna do a compression test tomorrow (if i can find the right fitting). I really hope it's not the top end .

Anyone have any ideas?
Have you checked the valve clearances?
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Old 04-05-2013, 08:28 PM   #5
KayElAre250 OP
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Have you checked the valve clearances?
Yes, valves are set to spec.
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Old 04-05-2013, 09:07 PM   #6
gunnerbuck
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Ok, so it was hard starting hot or cold in the beginning... You reduced the jetting and it starts OK cold but wont start warm:

If the engine is rich it will start better cold than hot.. I have not dealt with a phm 40 carb before, what kind of choke /enrichment setup does it have and is it possible for this to be stuck open?

Sometimes ignition/coil problems don't show themselves until the engine is warm, is there a healthy spark at the plug when the engine is warm?

Do you have a manual decomp lever on the bars? If so is there slack in the cable as to where the lever on the valve cover is against the stop? Sometimes the cable may be adjusted too tight as to where the lever is partial opening the E valves and not allowing full compression....
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Old 04-05-2013, 09:24 PM   #7
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Quote:
Originally Posted by gunnerbuck View Post
Ok, so it was hard starting hot or cold in the beginning... You reduced the jetting and it starts OK cold but wont start warm:

If the engine is rich it will start better cold than hot.. I have not dealt with a phm 40 carb before, what kind of choke /enrichment setup does it have and is it possible for this to be stuck open?

Sometimes ignition/coil problems don't show themselves until the engine is warm, is there a healthy spark at the plug when the engine is warm?

Do you have a manual decomp lever on the bars? If so is there slack in the cable as to where the lever on the valve cover is against the stop? Sometimes the cable may be adjusted too tight as to where the lever is partial opening the E valves and not allowing full compression....
I appreciate the input. The choke is just a cable/spring set up... don't think it's stuck open as there is slack in the cable. Manual decomp lever has plenty of slack, and I even opened the valve cover to make sure it was disengaged, and it was disengaged.

I guess i'll move on to the ignition/coil.

Thanks again for the input.
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Old 04-08-2013, 06:23 PM   #8
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Originally Posted by KayElAre250 View Post
Turns out I have low compression, like 65psi low. For the record, I had the decompression lever disengaged and WOT when performing the test. After putting a tsp of oil into the combustion chamber, I got 75psi. I'm not sure if that's a big enough jump to indicate rings, but I'm pulling the head regardless to check out everything.
If you have an auto decomp and factoring it in, 65 PSI is right on the fringe being too low to allow the motor to run... If your engine is not equipped with an auto decomp then 65 psi is downright terrible... Either way, your on the right track to pull the head and investigate further... Pay attention to the valves, it is possible that one is not fully closing and allowing some compression loss...

The 10lb jump with oil in the chamber does indicate that your ring seal may be part of the problem, look for cylinder scoring...
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Old 04-09-2013, 01:03 AM   #9
Aussie Steve
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Go to the 640 adv thread and check back through the last few pages for a post I think by overlander? about tech tips in sommer ktm brochure. Hot start issues are common on dellortos and there are tips for fixing it in there.
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Old 04-09-2013, 03:16 PM   #10
gunnerbuck
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Originally Posted by KayElAre250 View Post
Yeah, I just realized that the 1996 rxc has auto decomp on the camshaft. I had been looking at a 1995 parts diagram, which does not have auto decomp.

Is 65-70 psi is stil too low, even w/ auto decomp?

Gonna do a leakdown test before I pull the head off. I'll also mess with the carb a bit more.

Thanks for the input!
65-70 is still pretty low, especially when you take into to account that at 60 PSI there is likely not enough compression to fire the engine... Something 80-100 PSI would be a better #...

A leak down test is a much better method to diagnose problems with the top end on an autodecomp equipped engine...
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Old 04-09-2013, 03:27 PM   #11
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Originally Posted by gunnerbuck View Post
65-70 is still pretty low, especially when you take into to account that at 60 PSI there is likely not enough compression to fire the engine... Something 80-100 PSI would be a better #...

A leak down test is a much better method to diagnose problems with the top end on an autodecomp equipped engine..
+1 on this, take a leak down test to figure this out. Your PSI should be around 90..
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