CN250 Rebuild

[sargentargent]

Hi All,
Attempted to rebuild my 250CC Engine...(twice)...Each time I did the starter side of the engine would lock up after a time. Engine will move freely by hand from Variator Fan. So far I have trashed 2 crankshafts and 2 flywheels. Keyway on Crankshaft and Flywheel destroyed. Engine ran very well after each rebuild but only ran for 2 days (a few hours of driving each day) before the engine would no longer crank. Never went beyond maybe half throttle. I have the engine manual and am positive I installed all of the parts correctly and had the flywheel nut good and tight. Noticed that there is insufficient clearance between the oil pump chain and Starter clutch gear (found grooves on rear of starter clutch gear after dissassembly) but did not see any indication in the manual for a spacer on the crankshaft between the gear and oil pump chain. Also noticed that the timing chain was rubbing on the chain guide on the right hand side of the cylinder as you face the engine from the front right case cover. The 3 pronged metal starter clutch gear retainer also has resistance when you turn the gear by hand. So...What am I doing wrong? Just ordered a brand new crankshaft, woodruff key, starter clutch gear with clutch, flywheel, gasket set and brand new bolts to secure the starter clutch to the flywheel. Don't want to ruin another crankshaft and flywheel. Any information you may be able to provide will be greatly appreciated. This is a Roketa GK-06. CN250 Vertical 250CC Engine. Approximately 18HP and 10 to 1 compression ratio. Already adjusted valves by moving the valve shaft retainers away from one another one at a time then towards one another until I felt resistance and backed off 1 graduation. Had piston ('IN' mark facing front of engine or intake valve) at Top Dead Center and installed camshaft with hole on top and lines scribed into it parallel to head. Flywheel line (right next to the 'T') in center of clyinder body. Did not forget circlips for piston shaft. Did not forget the black pin and spring that go into the front of the crankshaft which is responsible for oiling the crankshaft. Yes...the rollpin that goes in the hole at the tip of the crankshaft on the starter clutch side (right crankcase) is installed. Did fit front cover by hand turning the water pump impeller and seated the front cover firmly by hand before installing the bolts. Obviously did not forget starter idler gear which meshes with the starter and starter clutch gear. Yes I installed the cam tensioner properly...released the tension by removing the bolt with the spring and releasing the locking mechanism and pushing the tensioner shaft all the way in before installing tensioner and nut with spring. Did not forget to install the cam chain guide on left side of cylinder head and bolt it in along with the washer. Did not forget rear bearing cap on camshaft. I bolted it down after setting the timing and then installed the valve cover. Piston rings and oil rings properly installed along with all new gaskets. Engine went together fairly easy with the exception of the clearance issues indicated. Also...Did not forget to tighten the cone shaped bolts that attach the cylinder head to the crankcase and made sure the copper washers were in place. Also installed the 2 bolts on the right crankcase side that bolt the cylinder into the crankcase. Had no oil or antifreeze leaks. Did not find antifreeze mixed with oil in crankcase. Did everything by the numbers. Don't understand what I have done wrong. After each rebuild engine started right up, idled well and had good acceleration. Then the starter clutch lock-up. Each time this hapenned the engine turned freely from Variator fan.
Thank You,
Steve

[sargentargent]

Seems nobody wants to offer any suggestions. What about the woodruff key and flywheel? Should the woodruff key be installed as far forward as possible in the crankshaft, directly in the middle or towards the rear of the space in the crankshaft? Once the nut is on and torqued to spec's should I expect the flywheel (overrunning clutch already installed) to be back far enough to engage the larger starter clutch or should I install the overrunning clutch on the gear and then install the flywheel, pull the gear toward the flywheel and set it in flush before installing the 3 Allen bolts?

[Masteryota]

I know Tom form SYC will know exactly what is up, but I know he is super busy as well. Send him a PM and I'm sure he will get back to you sooner than waiting here for an answer. His screen name is SYCARMS.

[sargentargent]

Thanks Masteryota. I have already asked him for his advice and so far he hasn't provided it. I don't know how to 'send him a PM'.

[SLESTAK75]

Click on his screen name in any thread and you will get a dropdown menu that gives you the option to PM. some others who are more educated on these 250 engines don't come on as often. So try PM to Syc and be patient. I am not familiar with the 250 so I dare not give advice.

[sargentargent]

Thanks SLESTAK75.

[sargentargent]

A very special thanks to all of you. Tom and I had a nice long talk and I am confident that, with the additional knowledge provided by Tom, my next rebuild will be successful. My thanks to Tom for the information he provided. He is very knowledgeable and a pleasure to speak with.

[Masteryota]

So what are you guys thinking was the culprit?

[SLESTAK75]

Cool. Let us know how it goes.

[sargentargent]

Will do. Will be a few weeks until I get back out to PA.

[sargentargent]

I had purchased a 250CC scooter for $700 with shipping from an auction site that had 1 mile on it and removed the engine because Roketa wanted $1800 for a replacement engine. Scooter appeared to have the exact and I mean exact same engine. Tom suggested I check my original center crankcase for cracks since the original engine was leaking and couldn't stop the leak + it was over 7 years old so I thought it was time to replace it. Left crankcase of scooter motor was too long for my buggy so I used ALL of the new parts from the scooter engine except for the left crankcase (had no choice). On my buggy the left crankcase is attached to the differential from the inside of the differential. You know what those buggy manuals are like. Worthless. So I made the decision to rebuild the engine while it was still in the buggy. Plenty of room once you remove the CV joint and swing arm from the passenger side and remove the 2 motor mount bolts allowing the motor to drop a few inches in the front. Tom believes the center crankcase from the new motor may be a bit too wide so he suggested I clean up and reuse the original center crankcase with the brand new crank, flywheel and starter gear and clutch. He said that if I see anything (during the rebuild) that I think doesn't look or fit right to take pictures and post them. So we'll see. I think he is right. I think the crankshaft was moving from side to side in the engine and it destroyed the crank and flywheel slot in the process. If that was the case it could have allowed the starter clutch to disengage and eventually lock up shearing the woodruff key and ruining the crank in the process. I will let you know after I'm sure everything is ok and back to normal. I will check to make sure nothing else broke too like a valve or maybe the piston but I don't think so. The problem is on the starter clutch side and if that crank was moving inside the case it is definitely the cause of the failure. After I get the center and left engine halves assembled I am going to try to move the crankshaft back and forth inside the case. If it doesn't move I should be okay.

[sargentargent]

I hope this post helps out others who are considering rebuilding their engines. Don't try to mix and match to save a few bucks. Everything has to be perfect for the engine to run reliably. Check and recheck your work before starting the engine. Listen for any unusual sounds and turn off the engine if you hear anything strange and recheck your work again. Make sure the engine turns freely by hand (which is what I did...too bad I didn't check to see if the crank was secured properly-not moving back and forth inside the case) and get the timing set and valves set properly. It'll save you money and time in the long run.

[sargentargent]

Hope this worked. Tried to upload a photo of the destroyed crankshaft.

[Masteryota]

I definitely agree with double and triple checking anything and everything important, especially engine internals. I would have thought almost any of the scooter style 125-250cc engines based on the GY6 would have been identical, too, but as with almost any small engines, including dirt bikes and atv's, I know parts can be scarce and if it can be repaired, you are better off repairing it. If it is cracked, I'm sure you can find a local welder to fix it up.

[sargentargent]

Yes Masteryota...There is a place nearby up in PA that can weld anything...I can do some Mig welding but welding Aluminum is not for the inexperienced.

[sargentargent]

Just one more quick question. When you guys install new gaskets do you use high temp automotive silicon, liquid solder or just put the gaskets in and tighten the bolts?

[SLESTAK75]

Thus far I have not replaced many gaskets. But for me it would depend on where the gasket was. If its somewhere I thought I may want to get back into; like the CVT, I would not. But for the bottom of the bore, or case halves, or someplace that holds back oil I would and it would be the hi temp rtv. Head gasket might be a good candidate for the spray copper sealant. This is just what I would do.

[sargentargent]

Thanks SLESTAK75. I did mean the crankcase halves, front cover and cylinder body. Don't use a CVT cover. It was not machined properly from the factory and eats up the belt. First time I pulled the CVT cover off after I had the buggy for 1 month there was lots of black powder inside and the belt was shot. Slipped like mad. Besides, I think it keeps the Variator and Clutch cooler without the cover. My Clutches, Variators and belts last me about a year now. Sure it's not safe so I don't recommend it.

[sargentargent]

http://www.youtube.com/watch?v=bE-KCAOOO8A

By the way. I did watch this video about a dozen or more times before I attempted my rebuild. If nothing else it will show all the parts and what they look like after they are installed.

[sargentargent]

Never use air guns or impact wrenches on the crankcase bolts or the head nuts or bolts. These must be torqued to specs.

[sargentargent]

Will be attempting the final rebuild tomorrow. If all goes well I'll be driving by Friday. Wish me luck!

[sargentargent]

Well...my original center crankcase has a structural crack in a semi-circle pattern. Cleaned up left and center crankcase halves. Had to use the one from the scooter engine. New crankshaft fits like a glove. There is very little front to back movement. Just enough for proper clearance. Starter clutch is still giving me a problem. It locks up as soon as I tighten the flywheel nut. Not sure what I'm going to do but I'm certainly not going to finish the built till I get this sorted out. The starter gear, clutch and flywheel are brand new. Really can't put my finger on what is happening. There are no more clearance issues with the timing chain and oil pump chain. Going to try again tomorrow to see if I can figure it out.

[x-bird]

can u put up some pics of the starter clutch assembly?

[sargentargent]

These 2 parts make up the assembly. The starter clutch itself is bolted to the flywheel with 3 6mm Allen head bolts. Had to get a special socket at the auto parts store. Don't have a digital camera so I am showing the photo's of the parts I purchased. They are the same as the original ones.

[sargentargent]

I kept readjusting the position of the woodruff key in the crank and eventually got everything back together. It cranks well from the battery. Only cranked her for a few seconds. Tomorrow I'll put in the oil and antifreeze and start the engine and let it idle for a bit. Then turn it off and try starting her again and so on. Then I'll button her up and take her out for a bit to see how well she accelerates and runs overall. If she runs for more than a few days I should be good. So far I've only gotten 2 days out of each rebuild.

[SLESTAK75]

Just make sure to give that engine a proper break in period. Dont go full throttle right away. Keep it half throttle and below for at least one tank of gas. Some others may have some suggestions on how they break them in.

[sargentargent]

Will do SLESTAK75. Going to baby her for awhile. Did that with the 2 previous rebuilds too. Sure hope it worked this time. Want some seat time.

[sargentargent]

I also threw in a Dr. Pulley Variator with the sliders to go with the Dr. Pulley hit clutch. Transmission is fine. Checked it for binding. Couldn't find any. Moves very smoothly. Replaced the belt too. Should be real nice when I finish buttoning her up. Still need to reinstall the cv axle on the passenger side, replace the fuel line (leaks) and connect up a hose that goes to the exhaust. Well, the car shows are over. Time to head back into the garage. Peace.

[sargentargent]

She's all back together. Takes me a lot longer now, with the arthritis, to get things done. I put in fresh oil and antifreeze and started her up. She fired right up. Just let her idle and gave her a little gas every so often. Let the engine run for about 2 hours to see if I heard any unusual noises. She runs quiet. Heard nothing strange. Now I have to go through the wiring again because she won't turn off from the key. If I can't find the problem I'll just install a kill switch off the CDI or the coil. Time will tell whether or not this build is successful. If the starter clutch locks up again I will be forced to invest in a new motor. Going to try and find a motor with a different set up because I don't like the fact that there is a woodruff key in the crankshaft. It would have been better if the shaft were splined and the flywheel was machined to fit the spline with a different set up for starting the engine. Nothing I can do about it now. Time will tell.

[sargentargent]

So my new Dr. Pulley Variator nearly destroyed my brand new belt. Put in 2 of the spacers. Does anyone know anything about these Variators. Came with no rear cover like a stock variator has. Are you supposed to put a stock cover on or use it the way it is? Drove for about 4 hours today. Engine is running very well. Starts right up and has good acceleration. Going to change the oil tomorrow. Used about 3/4 of a tank today. Hope she keeps running this way. Discovered that proper placement of the woodruff key is how to get everything to work properly. Had the starter clutch on the starter gear and kept repositioning the woodruff key until the flywheel slid all the way back against the starter clutch by hand. Put on the nut loosely and lined up the holes for the 6mm Allen head bolts, installed them and torqued them to 24ft/lbs. Tightened the flywheel nut to about 80ft/lbs. Hoping to get several years out of this motor. Has a brand new crank (CFMOTO just like the original crank), brand new cylinder body, brand new piston and rings along with a new flywheel, starter gear, starter clutch, idler gear, oil pump and oil chain.

[sargentargent]

Forgot to mention that thread lock is required on the 6mm Allen head bolts that attach the starter clutch to the Flywheel. I also used thread lock on the 3 pronged retainer for the starter clutch gear. Got a lot of seat time in today. Everything seems fine with the motor. Just hope it stays that way.

[sargentargent]

One more thing I should mention. For anyone who plans to rebuild a cn250...if you buy a new crankshaft it will ship without the woodruff key and without the small pin that fits in the hole in the front of the crank on the starter clutch side. That pin must be installed and the little black pin with the spring on it must be put in so that the little pin goes into the slot on the black pin with the spring. A little confusing but once you see it you'll understand. Also...the oil pipe uses 3 bolts. 1 is large with two copper washers that go on both sides of the oil pipe. This bolt goes into the front bottom of the engine. There are 2 other bolts. 1 is silver and it must go into the top part of the front cover into the crankcase with 1 unusually shaped copper washer and 1 round copper washer. The last oil bolt is black and goes into the valve cover with 2 round copper washers. If you put the black bolt where the silver one is supposed to go you will damage the engine. Don't mix them up.

[sargentargent]

Day 3. Got a lot of seat time again today. Great combo with the Dr. Pulley Variator and hit clutch. Lot of fun to drive. Had to use one spacer against the crank and the other 2 in front of the Variator fan. Works fine.

[sargentargent]

Still running fine. No sense in posting over and over. If anything goes wrong (sure hope it doesn't) I'll repost. Thanks again to everyone.

[sargentargent]

Purchased a socket on line. Has 41mm on one side and 39mm on the other. Works with a half inch air impact wrench. Makes changing clutches a breeze. Got new sliders for the Dr. Pulley Variator. Buggy is really fast. Am very happy and hope things keep going well.

[sargentargent]

Just installed a vacuum operated fuel pump, an E3 36 Diamond Fire spark plug and a high performance orange coil. Engine is idling much more smoothly, has better acceleration and is running really sweet.