Seems like the 351 makes more noise, knocking, when the engine is cold. Like a tapping that is in rhythm with the engine. Not a cam chain noise, or rattle. Not noticeable when the engine is warm. I tried higher octane fuel(premium) in case it was early detonation from the higher compression of the 351. That didn't seem to make a difference. Anyone else notice a similar noise after the 351 install? Ideas?
Dan

 

Piston slap maybe? Just like the Subaru cars-sounds like a diesel when cold. Goes away when warmed up. Just a thought.

 

I thought I read the 351 was lower compression than stock>??

 

Stock is 11:1. BB told me via e-mail that his kit was 12:1 but would still run on regular unleaded, jetted properly.

 

Piston slap would be my guess, it is not uncommon for many engines to have it. A few years ago new Corvettes had it and many owners were complaining about it.

I have not installed my 351 yet but I have looked over all the hardware and I do remember thinking that the 351 piston did not have much of a skirt which would be part of the cause. Usually this is done to keep rotating mass low. In general piston slap is not bad it just is noisy when cold. Have you checked in with Bill about it?

 

dan, you strike me as being pretty capable of recognizing piston slap vs. the cam chain issue, but I gotta ask. Have you heard a loose cam chain on one of these KLX's? Has yours been quiet while you've owned your KLX? I've been around some loose chains, but the one in my KLX sounded way more like piston slap or a knock than the rattle I'd heard from loose cam chains on other engines. In your 351 kit installation, is there a chance that the tensioner is off? Obviously a good chain wouldn't have stretched this quickly after you BB install, but the KLX tensioner seems to be touchy device.

Lots of these kits have been installed at this point, and knocking or piston slap doesn't seem to be a common complaint after the install...but...it might be a possibility. On the piston skirt design and height of the 351, it doesn't look that short to me. If you want to see short skirts, you check out one of those newer generation 4-stroke competition pistons like a CR250F. Many mechanics jokingly call them ring holders. Granted, the piston diameter has increased a good bit in relationship to the skirt height, but slap hasn't been mentioned up 'til now.

dan, I think you can pretty much eliminate the cam chain idea by checking the cam alignment marks at TDC. If they're not dead straight to the plane of the head, you might look harder at the tensioner. I'd hate to hear that piston slap might be an element of this kit. I'm pretty sure Bill does some pretty careful fitment of each piston to the given cylinder that it's shipped with.

 

One thing to check, unless you removed it, is the smog pump (air injection pump). Mine ticks when cold but goes away when the engine warms up. Cam Chain knocking could also be the issue. It too is also louder when at a cold start-up, but it also rattles around the 5~6k rpms range. And piston slap could the problem. Thou uncommon and not heard of yet. The 351 piston skirt does look shorter when compared to the OEM 250 piston skirt.

 

Learjet, the smog stuf is long gone.
Miaugi, I have 2001 chevy pickup that slaps quite a bit when its cold, I hate it. The dealer says nothing to worry about. I have not bothered Bill Blue with it yet. I keep going back and forth to whether it sounds bad or not.
TNC, I'm not sure how capable I am , but I do have the above pick-up to compare piston slap too. If it's piston slap, it sounds different. Maybe because it's one cylinder vs eight?I'm kind of thinking it's not piston slap.
When I installed the 351 I installed the tensioner before the valve cover and I remember feeling the chain for tension afterward. I took a pic too.
I'm sure something could have changed since then. Question: the bike wouldn't even run correctly if one of the cams was off even one tooth right? I just went out and started it and made a video. It didn't seem that bad, and I have gone back and forth since the install as to whether it sounds bad or not. Paranoia? Here's the vid, not sure if you can pic up the knocking. It seems to be in rhythm with the engine, not constant.
I just watched the vid and it sounds a lot worse in the vid than in person. Maybe you can get a better idea from the movie. Never mind me mumbling in there
Click the vid to play. ((EDIT it figure the sound won't play! I'm working on it))


Another thing that's not exactly right is the idle. I don't think it would have any thing to do with the knock, but it will not adjust to the rpms I want. It wants to go either ~1800, or ~700. I think I need to play with the A/F screw, but I'm waiting on a Kouba screw that should be here next week.
Thanks for all the ideas.
Dan

 

Dan888....nice video....you can really hear that sucker!

The good news is though, I think it might be an exhaust leak. If so, it would not be impossible that once up to operating temperature it goes away (seals itself up).

I would suggest you double check for a leak and/or check to see everything on the exhaust is tight right from the head back, including the spark plug (don't over tighten though!).

 

I'm not usually impressed by sound vids, but I can hear what you're hearing there. Yeah, that sounds pretty loud in the vid. If it's slightly less extreme in person, that sounds a little like my cam chain sounded...worn out beyond spec at 11,000 miles because of the tensioner. It sounded like a freakin' knock and not just cam chain which on other bikes sounded to me more like a rattle and whirr...if that makes sense.

However, your cam alignment marks look great. I'm thinking that your marks would be slightly out of plane with the chain within spec but the tensioner not properly pressing the slider...but...correct me if I'm wrong here. The chain is pulled on the crank gear at the front of the chain...or the part of the chain run toward the front of the engine...right? So even a good chain in spec could rattle if the tensioner is malfunctioning. Learjet has played with this more than me for sure, so he may chime in here. Then again, it's not impossible for it to be piston noise unfortunately. Your past work on different projects posted here show a good amount of meticulous attention, so I'd doubt your install was buggered up. Your pics on the 351 install looked good.

I hate this tensioner on our KLX's. There's something not up to snuff on many of them IMO. Mark Krieger will hopefully be getting back to me soon with the tensioner he's making to replace my OEM version. I've sent him all the specs and a hand-rubbing of the cylinder head mounting pattern.