RPM's out of gear

turbinedoctor · #1 09-30-2009, 04:05 AM

I think I understand why you should not rev you engine over 1200 to 1500 when running on muffs. It runs lean and can burn a piston, right?

Well I have heard also that you should not rev above these rpm's even in the water and out of gear. Is this correct and if so why?

Turbo

RidgeRunner · #2 09-30-2009, 07:44 AM

Due to the fact that the rod bearings are needle bearings there is more clearance than in babit bearings. The extra clearance is necessary so the needles don't get in a bind. Free revving a 2-stroke above 2000 rpm with no load causes the bearings to get hammered and can ruin the bearing surface on the big end of the rod. I am not sure if it would ever hurt the wrist pin bearings as they are not turning at the same velocity as the rods at the crank. I found some good info on this topic on screamandfly.com.
I have personally seen many more rpm on my engines than 2000 out of gear. I don't think I ever hurt one by doing it but I won't take that risk any more.

turbinedoctor · #3 09-30-2009, 06:01 PM

Thanks Ridge, thats along the lines of what I was thinking but I wanted to hear it from some one else to confirm it. Do you happen to remember the thread title at Scream and Fly. I may look it up to read it.

Turbo

RidgeRunner · #4 10-02-2009, 07:33 AM

9/20/09 - Scream and Fly thread started by bigbluechevy454 Post #8 complete with pictures.
http://forums.screamandfly.com/forum...d.php?t=197402

Karl_Childers · #5 10-13-2009, 10:57 PM

Motorcross guys ruthlessly rev their 2-strokes with no load and drop the clutch and go through this cycle over and over. I wonder what the difference is between a 2 stroke bike and a 2 stroke outboard. They obviously know something I don't

spareparts · #6 10-14-2009, 06:38 AM

just guesing, it might have something to do with the mX motors having the bearings and pins laying in a horizontal plane, and haveing splash oiling to the bearings from teh crank case, whiel OB motors have the bearings laying vertically and having onluy the oil thats induced thru the crank case. Just a thought?

I know the shop I used to work in sold several of the Land & Sea torque shift props. We dialed each one in pretty good(water tested them on the customers boat) before we let the customer have the prop. Every boat(5) we sold one on came back with in 6 months with rod bearing failure, could have been unlucky chance. Our theory was the props were allowing the crank to unload right before it shifts, allowing the rod bearings to chatter. We couldn't prove or disprove without spening a lot of money and breaking some parts, so we figured it would be easier to stop selling the props

turbinedoctor · #7 10-25-2009, 06:56 PM

How about chain saws, they are like outboards, no oil bath there but they do have a chain attached to them? All in all I am impressed with 2 strokes, I like their design and operation.