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jstatham

6 hours on Top End Rebuild and Smoking

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Hello everyone and thanks for all the help in the past.  I went ahead with the Top End rebuild on my 1996 Forman 400 and it started smoking at about the 6 hour mark.  I broke it down tonight and it looks like I have a ton of carbon for only 6 hours of run time.  How can I tell if the problem is the Chinese Cylinder, Rings or valve seals?  Here is a video of it running and pictures of valves and piston.  I am planning on taking the jugs to a machine shop this week and have them looked at the Original OE and the Chinese Junk.  I think I am also going to bite the bullet and try a complete rebuild with a new rod, timing chain and whatever else I might need.  I have never rebuilt an ATV engine but have done boat and car engines. Any help or advice will be greatly appreciated.

 

 

 

 

 

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Watch this. While it is a 420 Rancher, these Honda ATV engines are all similar.

 

 

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from the looks of the top of the piston pic you posted ^^^, that is a ton of oil burnt on the valve area. when you rebuilt the top end, what kit , as in..did you use the complete kit, piston, rings, cylinder, all new ?, if you just used a new piston with new rings, and just used your old cylinder ?, then that maybe your problem. a worn connecting rod wont be good either. any time i rebuild a top end, i make sure the rings fit the cylinder, by pushing them each down into the cylinder to check for fit, if the end gap is too much ?, it will smoke !. its also very posb you bent a ring while installing the cylinder down on the piston. one thing i have learned in all my years of working on atv's and rebuilding engines ?..DO NOT RUSH THE JOB !.

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It's impossible to say what happened but I do know that the sounds coming from the valve train etc. are alarming. A quick method to check piston to cylinder clearance is to insert a feeler gauge between the piston skirt and the middle of the stroke on the cylinder start with .002 and move up. shouldn't be more that .003 in my estimation. Shadetree, Fish....what's your

 opinion?

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I took everything to a machine shop and the cylinder is fine but the rings had a giant gap when placed in the cylinder. The piston was .005 out of specs as well. It was a cheap Chinese crap kit. I bought shindy rings and going to use the good piston and the cylinder. I also put in a new OE timing chain and waiting on new clutches. I checked the specs on the rod and it all measured good. I think it should be good after I get it built back this time. Thanks for all your help. Rookie mistakes

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Edited by jstatham
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13 hours ago, Misterclean said:

It's impossible to say what happened but I do know that the sounds coming from the valve train etc. are alarming. A quick method to check piston to cylinder clearance is to insert a feeler gauge between the piston skirt and the middle of the stroke on the cylinder start with .002 and move up. shouldn't be more that .003 in my estimation. Shadetree, Fish....what's your

 opinion?

MrClean , interesting  , I never heard about the sticking a feeler gauge between  the piston skirt and the cylinder wall 1/2 half  down the bore method before >>>> I send my cylinders to G+H and let them send me a piston, rings and bore job that match  , can go wrong with the measurements or the price and you won't be wasting 6 hours of labor and part money 

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I think the connecting rod is the most overlooked issue on a rebuild , just a little wobble and it egg shapes the bore quick ----   you need to have a good hard connection with your  rod , lol 

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3 hours ago, Fishfiles said:

MrClean , interesting  , I never heard about the sticking a feeler gauge between  the piston skirt and the cylinder wall 1/2 half  down the bore method before >>>> I send my cylinders to G+H and let them send me a piston, rings and bore job that match  , can go wrong with the measurements or the price and you won't be wasting 6 hours of labor and part money 

It's a quick method to get a "Ballpark"  measurement.

 

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On 1/14/2022 at 4:05 PM, Misterclean said:

It's impossible to say what happened but I do know that the sounds coming from the valve train etc. are alarming. A quick method to check piston to cylinder clearance is to insert a feeler gauge between the piston skirt and the middle of the stroke on the cylinder start with .002 and move up. shouldn't be more that .003 in my estimation. Shadetree, Fish....what's your

 opinion?

i never go by ring clearance on cylinder walls..why ?, because i always have my cylinders freshly bored..lol. what i DO check: is end gap on the new rings, even brand new rings from the factory has been known to be off.

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On 1/15/2022 at 8:52 AM, Misterclean said:

It's a quick method to get a "Ballpark"  measurement.

 

most times ( in all my years of engine building ) cylinder to ring gap is pretty common...YOU MUST HAVE SOME GAP !. but !!!, its not critical either..what is critical is the end gap. the oil rings is where you must focus. i'm not saying the other two rings should not be checked ?!..but you start with the oil rings. if the oil rings are loose, large end gaps ?, what will happen is oil gets past the top two rings..and ..well..here you have a smoking engine. I ALWAYS CHECK MY RING END GAP !. if they are wide ?, then i will not waste my time installing them. and no..i do not use feeler gauges !..lol. i go by sight..you just learn what will work ?..and what wont work in 30 yrs of doing this..lol.

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