My big problem is back again!!!

[ianders]

1997 Yamaha G16. About a month a go, the engine blew up (broke the connecting rod) I think it was due to low oil. This was a great learning experience for me, I now know the the red light is the low oil light and not the "on" light. Anyway, I found a friend to help me rebuild the engine as I am not the "shade tree" mechanic that he is. Finally got it back together and rode it for about an hour. It ran great for the first 45 minutes, then it felt like something was hanging up, like the clutch or brakes. The engine was running fine with lots of power, but something was slowing it down. Frank (the shade tree mechanic) came by today to see what the problem was. While we were rechecking the valve settings, turning the clutch by hand to move the piston, everything went quiet. Frank says he thinks the rod has broken or come unbolted. So out went the engine and is on the way to his house to be torn down again. Any advice? Why would it break the rod again in less than an hour of use?

[Tonka329]

I say time for a BIG BLOCK Kit!

[Freezeman]

There is lots of possible reasons. The clearances of the rods big end to the cranks journal is critical. Too tight or too loose of clearance could be the problem, or it was torqued too tight or too loose, or the crank is out of round by a small amount or large amount. Or the bearings for the crack are worn out, or the bearing bosses in the case is worn, too much end play or too little end play for the crank, or the case is cracked so it can't hold the crank in place, to thin of oil used, too thick of oil used, too many beers in the shade.
But most shade tree mechanics will blame it on the rod being a bad rod which they are right, as you need a new good rod now.

[ianders]

Looks like this project may turn me into a true shadetree mechanic before its over. Guess I better go ahead get my tree planted!

[SwampFox]

I wouldn't brag about being a shade tree mechanic. Do it right and do it right the first time. Make sure you have the proper tools and service manual. I would also recommend one of these easy up tents instead of a tree, a lot quicker.
Be careful about reusing things like rod bolts as well.

[chompy]

Did you replace the crankshaft when you did the first rebuild? If not that may be a source your problem. There might be some residual aluminun buildup on the crank from the con rod. Check that and check your clearances.

[ianders]

I didn't replace the crankshaft during the first rebuild. The guy helping me was able to "buff" the scratches out, or so he said. I wonder if I am overreving the engine. This cart would go about 28 mph when I got it. How many rpms should this engnie turn at full speed?

[Mike Mac]

Ianders, I only run my car at top speed for about 10 to 15 seconds, then lift. Your car probably does not have the govener hooked up, so if you stay in it, that may be your problem.
Also my throttle only opens about 80%, kinda my built in rev. limiter!
Good luck Mike

[sho305]

I've run my stock cart wot with no governor for many years. I let off after a little bit because it goes nearly as fast at half throttle once you get up to max speed. My throttle goes open all the way, but the last 1/4 makes little difference anyway. However when you rebuild you don't go out and run it hard or high rpm, unless it is a race engine built to run out of precision parts. And in a car engine you have an oil pump, these don't. If you do it in your back yard and don't have a machined crank and rod and check the clearance, you take it easy and let it break in before beating it. The clearance if tight will work its way out larger, the parts will fit to each other, then its broke in and should run as well as any other. We now have the capability to machine things near perfect so you don't have to break in things....if that is what you did. We built 500hp 454s and put them in a boat and ran them wide open at 6K rpm for 20 minutes straight the same day, a decade ago, but if you don't have precision its a whole different ballgame. The one guy eventually worked for a NASAR team for a while he was quick with the engines. Sure you can hone/sand a crank and use it, you just have to be careful with it for a while and then there is always the small chance it will come back on you, but that is typical practice for small engines.