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Old 08-21-2005, 08:50 PM   #1
d0gWateR OP
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2003 640 Top End Power Problem

I have a “funny” top-end power problem when I’m doing 65-70mph at about 4750 rpm and I increase throttle position aggressively. It hesitates and won't produce power. If I'm really aggressive it sound sort of like someone is trying to chainsaw their way out of my engine. If I want to accelerate past 65mph I have to do so very gradually and it helps if I’m heading downhill.

This problem is new, I have had 8000 miles w/o any problems. It started after running for about an hour at 5000 rpm on my way down I-5 last Sunday for the start of the Nor'Wester TSD Rally.

The best way to describe the lack of acceleration above ~5000 rpm is it’s just like when on an old car with a vacuum advance distributor and the vacuum line has fallen off. Acceleration is almost impossible, it has to be done very gradually.

Anyway, normal riding offroad the problem doesn’t affect me, it seems if I’m on the mainjet for for short bursts I have lots of acceleration. It’s only when I’m running 1/2 throttle for a long time and then crack it to ¾+ throttle that it balks. Oh and one more little gem, if I was experiencing this problem while climbing a small hill (typically) then slamming the throttle closed then back open again to almost WFO would make the problem go away (temporarily).

The problem is intermittent, of course… Sometimes during last week (rarely) it worked fine and power-o-plenty rocking up to 80+mph on the MT21 (rear) and D606(front) without any hesitation.

I just spent the weekend dirtriding, and the problem just doesn't present itself under those circumstances.

I suspect I do have the fuel level in my carb too high (engine dies under stutter bumps) so I’m going to remedy that this week. Clean the carb thoroughly and reseal everything back up to check the possibility of fuel delivery issue. I'm suspecting and hoping it's a fuel delivery issue (stuck slide - I ride in the dust a lot and have probably f#@!'d up my canisterectomy long ago). Hmm, just typing all this out helps me get it a little straighter in my mind. I'll bet it's that damn slide, dust coming in through the vent on the carb and getting the slide all gritty... Or perhaps a hose or diaphragm problem?

It's a 2003 640 Adventure: snorkel in, side airbox screen, one size larger mainjet, stock pilot, KTM Comp Silencer c/w drilled baffle, scratches on both sides of the tank :lol


Any other suggestions? Thanks in advance.

Paul
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Old 08-21-2005, 09:33 PM   #2
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I think several of us have had issues with gas tank venting. I inadvertently plugged mine when I rigged up a fancy check valve. The symptom was that on occasion the bike would just die at half a tank whilst riding continuous hwy speed.

Suppose you could have a vacuum leak on the carb.. on either carb spigot, or perhaps the top diaphragm chamber could be sucking air.

Chainsaw, hmm. You know I'm about to repack my silencer (as soon as the packing arrives) because it went downhill really fast. My top end is near non-existant at this point and it sounds rather obnoxious. Can't find any other problems, so I suspect it is all a packing issue with mine.
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Old 08-21-2005, 09:58 PM   #3
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Quote:
Originally Posted by potatoho
Chainsaw, hmm. You know I'm about to repack my silencer (as soon as the packing arrives) because it went downhill really fast. My top end is near non-existant at this point and it sounds rather obnoxious. Can't find any other problems, so I suspect it is all a packing issue with mine.
Hmm, mine sounds pretty much blown out too! Does that cause a loss in top end potentially? <--prolly a noob question, but I don't care.

Also, I was once a victim of the tank vent problem and it definitely isn't that. I'll repack the silencer anyway, it's just too damn loud right now.

Thanks potatoho

Paul
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Old 08-21-2005, 10:44 PM   #4
meat popsicle
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i lost my top end when my main jet came unseated... doesn't sound like what yours is doing - mine was dead at 3/4 throttle and up. But its an easy thing to check and make sure its seted properly and the spacer is ok.
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Old 08-21-2005, 11:54 PM   #5
potatoho
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Quote:
Originally Posted by d0gWateR
Hmm, mine sounds pretty much blown out too! Does that cause a loss in top end potentially? <--prolly a noob question, but I don't care.
To be honest I'm just going by what I've read. My understanding is that as the noise increases from blown packing, the mid-range and top-end go away proportionately. A few weeks ago I began getting annoyed at some cackle, and now it's pretty noisy beyond 3,000 rpm and it just feels all around mushy beyond half throttle.
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Old 08-22-2005, 07:15 AM   #6
Jan from Finland
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Quote:
Originally Posted by d0gWateR
I have a “funny” top-end power problem when I’m doing 65-70mph at about 4750 rpm and I increase throttle position aggressively. It hesitates and won't produce power. If I'm really aggressive it sound sort of like someone is trying to chainsaw their way out of my engine. If I want to accelerate past 65mph I have to do so very gradually and it helps if I’m heading downhill.
Is the chainsaw noise like someone is trying to saw your engine in parts, or do you mean noise of two stroke engine? If it is the former, it could be a cam bearing failure. In that case it should be repaired immediately. If you are lucky, you have to pay only the bearing. If you are not (like me) the chain will break, jam the engine and smash the oil pump, counter balance etc. And if you are really unlucky, the valve train might fail.
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Old 08-22-2005, 12:29 PM   #7
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Power loss above 5000 rpm

Check all vents:
fuel tank
float bowl
slide diaphragm

They must all freely flow to the atmosphere, not the air box or other pressurized area. They should not face into wind or turbulent area either.



As for ¨chainsaw noise¨
I cannot guess without hearing it in person.
Try the ¨stethoscope method¨ with a screw driver handle on your ear touch blade to engine while running. Touch various places on engine and listen for odd sounds. Works excellent as long as you know how it should sound. Get a known healthy bike for comparison.

Steve
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Old 08-22-2005, 12:49 PM   #8
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esteban

what to ride tonight? 2 or 3 hours at rampart?
see post in rockies,can leave south side at 5:00pm

firstworks
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Old 08-22-2005, 11:01 PM   #9
d0gWateR OP
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Quote:
Originally Posted by Jan from Finland
Is the chainsaw noise like someone is trying to saw your engine in parts, or do you mean noise of two stroke engine? If it is the former, it could be a cam bearing failure. In that case it should be repaired immediately. If you are lucky, you have to pay only the bearing. If you are not (like me) the chain will break, jam the engine and smash the oil pump, counter balance etc. And if you are really unlucky, the valve train might fail.
Saw in parts, but I only heard that once. I never twisted the throttle that hard again after I heard that noise!
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Old 08-22-2005, 11:02 PM   #10
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Quote:
Originally Posted by Esteban
Power loss above 5000 rpm

Check all vents:
fuel tank
float bowl
slide diaphragm

They must all freely flow to the atmosphere, not the air box or other pressurized area. They should not face into wind or turbulent area either.



As for ¨chainsaw noise¨
I cannot guess without hearing it in person.
Try the ¨stethoscope method¨ with a screw driver handle on your ear touch blade to engine while running. Touch various places on engine and listen for odd sounds. Works excellent as long as you know how it should sound. Get a known healthy bike for comparison.

Steve
Thanks Steve, I'll check all of those things. I'm probably going to rip apart top end for the peace of mind it will bring to know all is OK .

Paul
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Old 08-23-2005, 05:26 AM   #11
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The chainsaw noise might be detonation. Are you running a cheap assed low grade gas?

Also check for airleaks leaning out the mixture.
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Old 08-23-2005, 08:15 AM   #12
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It's possible that I had bad gas, but I had just tanked up with 91 octane(R+M/2 method). It could have been some detonation, the engine temperature gauge read correctly at 4 bars, but the case of the engine seemed very hot.

OK, you guys are going to kick my ass for this but:
I had just added the side airbox screen before this trip w/o making any additional jetting changes. I had never done any long runs on the mainjet before this one. Could it be that with the blown out packing, drilled baffle, side airbox screen, that I was just too lean on top?
The stock mainjet is 154, and I had only jetted up 2 sizes (whoops thought I was running a 156.5) to a 160 w/o changing the pilot air jet.

I appreciate all the feedback guys, and I'll be able to get out to the garage and start working on the bike tonight.

Thanks again!
Paul


Quote:
Originally Posted by UngaWunga
The chainsaw noise might be detonation. Are you running a cheap assed low grade gas?

Also check for airleaks leaning out the mixture.
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Old 08-23-2005, 09:41 AM   #13
meat popsicle
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if its too lean it will run hotter and the bluing on your headers will start creeping aft.
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Old 08-23-2005, 10:09 AM   #14
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Quote:
Originally Posted by d0gWateR
Could it be that with the blown out packing, drilled baffle, side airbox screen, that I was just too lean on top?
The stock mainjet is 154, and I had only jetted up 2 sizes (whoops thought I was running a 156.5) to a 160 w/o changing the pilot air jet.
What is the procedure for drilling the baffle on an SXC competition silencer? Is it a hole saw deal, or just little holes or something.

Pinging sucks. If it is too lean, you've got several ways to find out. You can put the stock airbox side back on, or tape up the airbox screen. The snorkel, you could put it back in if you want, but I don't recall it drastically changing the jetting requirements the last time I experimented. You could go the carb route and increase your main jet, or move your jet needle up, whichever works better for you.
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Old 08-23-2005, 10:41 AM   #15
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Quote:
Originally Posted by potatoho
What is the procedure for drilling the baffle on an SXC competition silencer? Is it a hole saw deal, or just little holes or something.
I just drilled a 1/4 pilot hole then used a uni-bit (step drill) to open it up to as big as I could fit. I then drilled one or two other smaller holes. I'll take a pic when I repack the silencer.

Quote:
Originally Posted by potatoho
increase your main jet, or move your jet needle up, whichever works better for you.
The needle is up one position (clip down one) and I'm jetted a little richer on the main [160]. The problem appears when I'm on the mainjet, so I'll try recovering my airbox and repacking the pipe and see if the problem disappears.

Quote:
Originally Posted by meat popsicle
if its too lean it will run hotter and the bluing on your headers will start creeping aft.
I have some creepage

Thanks all!!
Paul
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