250r knock at idle

[Ryah]

My bike has just started to develop a knock at idle. It goes away at higher rpm. The bike does have a few seasons on the top end. I'm hoping that it may just be piston slap. Anyone that would like to chime in on possible senarios would be appreciated. It keeps me from missing the simple things when I tear it down for a top end rebuild. I was planning on checking the crank for side to side play, but how do you tell if it is the bottom end bearings. It's not using any trans oil, so I'm led to believe that the bottom end bearings are still good since both seals are still good. Again, I've rebuilt many 2strokes over the years, I've just been fortunate enough to not have to do one for over 5 years now. Gotta love Honda quality.

[69HemiGTX]

Without hearing it myself, I would say that the piston would be the first thing I would think of due to its age. The way that I always check crank bearings is by removing the stator cover, grabbing the flywheel, and trying to move it up and down. If it moves, the bearings are done.

[Jason Hall]

Make sure you check the counter balancer bearings also. They will make noise & when they come apart they trash the cases. Also make sure the outer bore of the right side counter balancer bearing is'nt beat out. I have a right side case half that has that problem. You can pull the balancer out without splitting the cases, I would change the bearings while it's torn down.

[TeamGeek6]

how do you tell if it is the bottom end bearings. .

If the noise is in the top end, then its not crank bearings, they are down below.

1.) Air leak. I can set my 86R lean on idle and the piston starts knocking at any temperature. Richen the premix, it stops knocking. Leaking manifold, carb cap loose, bad crank seal on the left side, sucked in case gasket.

2.) Piston slap at about 50 degrees BTDC, thats when the rod kicks over. No way to tell unless its torn down.

3.) Detonation when hot. If the engine is oveheated, detonation will knock like crazy.

Crank seals looking good means nothing as far as bearings being good. Any up and down play in the flywheel end of the crank = bad bearings. Each crank bearing is about 0.007" axial play, but that tightens up when both are installed, so end play shouldnt be much more than 0.005 on the end Im guessing.

For a good general test of all the bearings, including rod, yank the spark plug and indicate the piston at TDC, then see how many degrees the flywheel will turn
before the piston moves. If you rock the rotor side to side, might hear a knock if a rod bearing is bad. There is a little knock even if its rebuilt.

"honda quality" yes for the most part, until you tear that R engine down and look at all the screw ups they made in bearing mounting. The left c-balance and both crank bearings are a double press (which is a major violation of bearing design rules) and the right c-balance bearing floats side to side and ruins the races.

Heres the photo series I did on a complete tear down and rebuild:

http://gl1200harness.tripod.com/2stroke.html

[MN250R]

Ya mine does it to at Idle 85 ATC 250R
I had it apart down to the crank and everything seemed ok, After a fresh rebuild it sounds like a little piston slap at idle then goes away at higher revs. maybe i missed something when it was apart , not sure. It did it when i bought it and and does it since it was rebuilt.

[Jason Hall]

I have built 250-R's that have had new crank, rod & counter balancer, even trans bearings & still sound like something is gunna disinigrate. I found that the gears under the right side cover will rattle or knock at an idle. Especially when the clutch basket is worn. I feel that all the slack from all the turning gears, will travel down to the crank & counter balancer gears, & make the crank & piston sound like it's knocking. I have rebuilt race quads that have been rode hard & jumped there entire life & they are the ones that knock the worst. The last bad one I did blew the crap out of the counter balancer bearings. The counter balancers job is to eliminate vibration. That is where all the slack will travel naturally. I would suggest a new piston / bore, crank bearings & counter balancer bearings. Make sure the clutch basket is'nt real sloppy.

[jason85atc250r]

I have built 250-R's that have had new crank, rod & counter balancer, even trans bearings & still sound like something is gunna disinigrate. I found that the gears under the right side cover will rattle or knock at an idle. Especially when the clutch basket is worn. I feel that all the slack from all the turning gears, will travel down to the crank & counter balancer gears, & make the crank & piston sound like it's knocking. I have rebuilt race quads that have been rode hard & jumped there entire life & they are the ones that knock the worst. The last bad one I did blew the crap out of the counter balancer bearings. The counter balancers job is to eliminate vibration. That is where all the slack will travel naturally. I would suggest a new piston / bore, crank bearings & counter balancer bearings. Make sure the clutch basket is'nt real sloppy.

get the hinson billit counter balancer holder and ditch the stock cast one. it's $100 of good insurance. this one guy where i live just got a year ago a 86 trx250r that was mint. like just of the showroom. the balancer holder broke and destroyed the cases

[Ryah]

Thanks for all the feedback. I'll be tearing into it over the holiday break.

[TeamGeek6]

. I feel that all the slack from all the turning gears, will travel down to the crank & counter balancer gears, & make the crank & piston sound like it's knocking. .

Yep, I suppose it could, especially on a single cylinder, they arent balanced that great. Probably at BDC.

[Fox250R]

I though all 85/86 motors ping at idle..Mine does,My buddys 89 fourtrax does...I dont think its a big deal.....

[Ryah]

Well I went to start the teardown, and first thing I did was pull the plug. The prong was bent almost to the electrode. The plug did have a bit of carbon build up on the electrode. My dad says he had a bike in the late 70's that would get enough carbon built up on the top of the piston to close the gap. Has anyone else ever heard of this? I was thinking of just replacing the plug and see if my issues go away. If it has a streched rod or bad bearing then it should bend the plug over again, correct? Anyone want to weigh in with a guess. I'll post answer when and if I have to do the teardown, so it may be a while with Christmas here.

[Mr. Sandman]

Enough carbon built up on the piston to contact the spark plug and close the gap? That would be about 3/4" of carbon build up so I doubt that's what caused it. I'd guess the plug was dropped at some point before it was put in, or the prong is burned to a crisp and is so thin and pliable that the compression made it move, but that would bee a long shot as well.

[dogger]

make sure you check teh piston close. if you lose teh pistn your looking at a crank and main bearing's for sure. look for unusal wear patterns and crack's.also check conterbalancer while your there as well.and I don't think it's a normal pinging sound because he said he just started to notice it and most people with bike's notice a change in sound when somethings a miss.