Hey guys, its been a while since I been on the forum, but I have a problem with my 2009 KLX 351s (3,370 miles). The bike would not start, so I took the motorcycle into the a dealership service center and was informed that my 351 big bore kit cylinder walls had scorn marks, looked like a lot of heat was in the cylinder and was not making any compression. So, this is what I did to try to fix the bike.

1). Bought new 351 Big Bore Kit from Bill Blue
2). Purchased brand new OEM battery
3). Bought a new/used starter off Ebay with 1,600 miles on it
4). Had the carb rebuilt
5). Had Decompression Mod done

The mechanic installed all the new parts and adjusted all the valves and what not while in the engine. It was hard to start when I got the bike back and took almost an hour to get going. Had to put engine jumper unit to get the bike enough power to start because it eats up battery charge because it is so hard to start.

Finally, got the bike going and rode it around for about 4 miles and the bike has totally loss of power. It will get up to speed, but it takes a long time and the power delivery is nothing like my first 351 kit installed on it. Its like night and day.

The mechanic seems to think that if the bike starts and it can get up to 85mph in 2 miles the bike is fine. Also, he wants to compare my bike performance with a stock KLX 250s and if it runs just as good as a stock bike, he feels his job is done and nothing more he can do. That's BS, I can not even throttle up the front wheel in any gear. Sad to say this is not the beast I had before by far.

My question for everyone is why is the bike so hard to start or won't start and why do I have loss of power when it runs? Do you think the carb was not adjusted right and not have the correct jets installed? Should I take it to a place that does dyno tuning and adjusts the carb? Not sure what to do next and this has cost me a ton of cash already....

Thanks

 

I wonder if the mechanic checked the valves and valve seats. Aside from carb jetting (which you should be learning/doing), that's the only other thing I can think of that would make for hard starting and soft power.

Which carb are you using?

if it's the CVK when the mechanic rebuilt it he may have put it back to the stock settings which would certainly be lean.

 

I told the mechanic to check the valves, cam chain tensioner and any other part that needed adjustment, since I did not want the engine open up for awhile.

I am running a stock carb on the bike and have no idea what jets they installed in the carb. I was messing with the Kouba T-Screw on the carb and it seemed to want to start easier, but did not. My thoughts, it might be the carb was setup wrong. Question, could having carb set up wrong make it loss that much power and torque?

 

When mine went out, A bunch of metal ended up on the ignition pickup due to the strong magnet in there. Maybe check that to see if it's messing up your spark / timing.

 

haha, wow, that's definitely one of those little things most people would never think to check. If there was excessive wear on the piston rings and cylinder, there may be metal down there. That's a good point.

How hard would it be for OP to check that?

If I were OP I'd take that carb out and at least check the jets and float level. It's pretty easy to do and even a monkey can do it without breaking anything. Especially considering the slide won't need to be removed - it's the most delicate part I'd say. Check the numbers on the main jet. There's a small chance the mechanic is one of those "by the book, no matter what" kind of guys and put the stock jetting back in.

Based on your signature, that dj132 jet would seem awe-fully lean at sea level for a 351, you may want to consider going to a dj136 at a minimum. My stock bore 250 was running the 136 nicely on the top end (my cvk had mid-range fueling issues that caused overfueling in the mid-range though). Several other riders here are running jets larger than the 136 on their 250's at sea level as well.... anyway, check your current jetting, put in a bigger main jet if you can and test it out.

You might consider having a leakdown or compression test done before trying to dyno tune.

On a side note, you should really consider going with a mikuni when you can, OP

 

Just pull the left side cover. On mine it didn't even tear the gasket. When mine was going out due to failed big end bearing, it was nearly impossible to start towards the end.

In a fit of optimism, I thought that perhaps the rattling was something to do with the starter idler bushing, so I pulled the side cover to inspect it. There was metal all over in the stator / rotor and on the ignition pick-up. I'm not sure how it was running at all since there was actually a bridge of shavings between the two solder points on the pickup.

I can't tell you for sure if this caused the hard-to-start condition because during the same inspection I found the failed bearing and decided to cut my losses, sell out, and buy an FZ-07.

Regardless of whether that is the cause or not, I'd recommend OP pull that cover and clean all that stuff up just to get the metal out of the engine. It's a pain in the **** to get all the filings off of those magnets, but I'd sure do it if I were you.

Anyway, not to be a total pessimist, but when my bearing went out it was in the process of taking out my bore. It's been speculated that the debris was slinging up into the bore and grinding. Before you spend any more money you might want to check your big end bearing - at least one other member here has had that failure.

To check mine, I set the engine to about 90 degrees - Put it at top dead center by the timing mark then turn it 90 degrees either way. I then dropped a small wooden dowel through the spark plug hole until it rested on the piston. Apply light finger pressure to the dowel and gently turn the crankshaft back and forth a couple of degrees. The piston should move up and down directly with crankshaft rotation. If you have play, which I sincerely hope you do not, you've got bigger problems.

Good luck!

Edit - Decompression mod? Re-timing the ACR you mean? I'd look real hard at that also. Lean jetting on the CVK could certainly cost a good bit of power, but I wouldn't think it would make it particularly hard to start - not an hour of cranking hard to start.

 

Yes, what is the decompression mod?

 

I'm going to assume it was the ACR, but I could be wrong.

Automatic Compression relief - it's the extra bit on the exhaust cam that's centrifugally deactivated. It basically opens the exhaust port up a tad early to decrease the workload on the electric starter... it also makes the engine less efficient at starting.

Typically people doing the MCM mod do this, but I don't see it in his sig... It's possible he just disabled the ACR, but who knows.

 

I gave up on trying to jet the thing myself. Even after trying everything mentioned on this forum (from larger/different jets, dynojet kit, drilling out things) it NEVER started right, and performance was good but not great with the 351 kit. I also had the occasional "kickback when cranking, clock resets to zero" thing that everyone says to replace the battery about.

Although it had been back to the dealer several times and they said the same thing that your dealer said, I tried another shop and they solved ALL of these problems.

It can happen, you just gotta find the right guy.

 

How was the bike running before the 1st big bore kit? After the 1st 351 cylinder/piston was wrecked, was the engine torn down completely to determine the cause? Was it the same mechanic who installed both 351 pistons?

Was the piston installed in the correct orientation? Were the valves inspected before the 1st installation? (inspected is not the same as afjusted) there could be a problem with lubrication.

Carb, battery and starter don't have anything to do with premature wear of the piston /cylinder.