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I got a light bore motor (72cc kit) that didn't run for free. Took the cam cover off and noticed that the cam bolts sheered right off the cam -

273270


273271


Got a new cam, sprocket and bolts. Tossed the cam in, set the flywheel to T, put the sprocket back on so that the O matched the notch and bolted it up.

It has briefly started twice, both times at high RPM's and then cut out moments later. A few times I got a very loud backfire from the exhaust but still wouldn't start.

Some other notes -
I ultrasonic cleaned the cab/jets. So that's good.
No air filter.
Exhaust has silencer/baffle removed.
Spark plug doesn't look to be the correct one, but I do have spark.

My concern is that when I put the cam back into the head,it was 180 from where it should have been. Is this possible? I've never had so much trouble getting a motor running.
 

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Typically, the cam bolts are off center so they can only be installed one way. Yours look parallel which is confusing. Are you sure you don’t have any air leaks on the intake manifold?

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The newer ball bearing two-bolt Honda cam bolt holes are 180° opposite each other.

Shane, when the "O" mark on the cam sprocket is lined up with the notch on the head and the "T" mark on the flywheel is lined up with the notch on the case, you should be able to jiggle the valve tappets ever so slightly, that is indicative of the cam lobes pointing downward.
 

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Is the centering dowel for the cam scprocket missing from the center hole of the cam?
 

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Discussion Starter #5
Typically, the cam bolts are off center so they can only be installed one way. Yours look parallel which is confusing. Are you sure you don’t have any air leaks on the intake manifold?

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I pulled the intake to check it out and the port is larger than normal.. I'm starting to think I have a XR70 motor and not a light bored 49cc. To answer your question, it has an oring that is gooey but no paper gasket and I dont have one. Maybe I can pick up some gasket paper tomorrow. Everything is tight tho.

Is the centering dowel for the cam scprocket missing from the center hole of the cam?
Yes, in this photo. In the new one I purchased and put in, it has it the centering dowel.
 

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Discussion Starter #8
did you adjust the valves after installing the new cam?
Sure didn't! It was another OEM cam and I don't have my feelers, they are still making their way across the ocean with everything else I own.


Post up the engine VIN to help identify it, Shane..
DE02E-5007097.. whats confusing me is the jug has an YX stamp on the underside..
 

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While you have to intake off, always good to check it quick to see if it’s flat. Good idea to keep of glass handy in the shop for things like this. Hold the mounting surface on the glass and using (when they arrive off the boat I guess. Ha) feeler gauges, see if you can slide any under the middle. Years of over tightening can warp the mounting flanges.


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DE02E-5007097.. whats confusing me is the jug has an YX stamp on the underside..
That VIN number comes back as a 1998 XR70R engine, Shane, post up some pics of it when you get a chance, it may be a mix-n-match hybrid. The smaller Chinese engines (50-70cc) generally use the same engine stud spacing as the Honda engines, I've assembled a few in the past with no issues at all.
 

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I got a light bore motor (72cc kit) that didn't run for free. Took the cam cover off and noticed that the cam bolts sheered right off the cam -

View attachment 273270

View attachment 273271

Got a new cam, sprocket and bolts. Tossed the cam in, set the flywheel to T, put the sprocket back on so that the O matched the notch and bolted it up.

It has briefly started twice, both times at high RPM's and then cut out moments later. A few times I got a very loud backfire from the exhaust but still wouldn't start.

Some other notes -
I ultrasonic cleaned the cab/jets. So that's good.
No air filter.
Exhaust has silencer/baffle removed.
Spark plug doesn't look to be the correct one, but I do have spark.

My concern is that when I put the cam back into the head,it was 180 from where it should have been. Is this possible? I've never had so much trouble getting a motor running.
Some things that your pictures show: 1.there is a lot of wear on both sheared screws, indicating the sprocket was loose for a while, which is the most likely cause of their failure.
2. The screws are incorrect for your head/cam. Those are 9mm hex screws that are for the early style cam, and they WILL contact the center post of the cam cover (possibly how they got loosened in the first place.) Inspect the cam cover post for signs of wear. The correct screws have either a socket head or 8mm head. You must also have the correct cam cover with the little external arm that keeps it from rotating.
3. If it doesn't end up being air leaks as others have suggested, the revving you describe sounds a little bit like fuel starvation (if you have ever ran an idling carb till it was out of gas, it will rev at the very end and sometimes pop out the exhaust.) Even though you cleaned the carb, there can still be stubborn deposits, especially in the pilot jet and circuit. Inspect all that again as well as the float valve assembly. Check for the oddball stuff like a carb slide in backwards, a missing slide spring clip allowing the main jet needle to slide up and down on its own, and if sediment is coming from your tank and re-clogging your carb, and a clogged petcock. Lastly, it needs an air filter, as the lack of filter will make it run lean.
Finally, these engines are 360 degree engines, meaning they fire every time they hit top dead center. A cam sprocket rotated 180 degrees will run just fine. On a 180 degree engine, the valve timing and ignition timing would be wrong, and it would not start.
 

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Discussion Starter #13
Thanks everyone for the information, I really appreciate it all.

Tim - I'll get you photos tomorrow!

Davmo - Whoever previously owned the motor did some whacky stuff. The gaskets were just cardboard and silicone, not even copper RTV. The cam bolts were obviously incorrect and yes, it did damage the side cover, which IS missing the tab. I have clear gas lines so I can tell the flow is good. I will go through the carb again, I've cleaned it multiple times. Gone through the jets and even cleaned the float needle seat because it kept dumping gas, which fixed that issue. I have an air filter due in the mail tomorrow. It sounds lean, I have the clip up one on the needle (it has the retainer spring) so I'll move it back down to center and get the air filter on it.

Again, thanks for all the input guys. I could easily just shelf the motor and put in another but I don't like having busted sh!t laying around.
 

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Discussion Starter #14
Follow up -

New spark plug in, air filter on. Clip one down on the needle and after about 10 minutes of carb tuning, it's running great.

Thanks everyone for your in put. It looks like it was just running VERY lean and I'm sure the random spark plug that was stuffed in there didn't help either.
 
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