Two Stroke theory/cooling question

[Mark P]

I was thinking about this yesterday running my Stihl leaf blower in thi Texas heat. Years ago, I believe I read in a bike magazine that air-cooled two-stroke motorcycles should never be ran continuously wide-open. That is, the engine required the "cooling effect" of a "dump" of cooler air/fuel mixture from the carb by rollingoff the thrpottle for a second and then reopening it. Is there any truth to this? Would the life of a small air-cooled two-stroke engine be extended by blipping the throttle occasionally when you are running it WFO? Thanks

[tomale]

there are lots of guys who should weigh in on this, but let me say what I know about the subjects. It is not that big a deal to hold the trottle open. The problem comes when you chop the throttle after holding it open for a long time. The reason is because as a two stoke runs a certain amount of oil builds in the crank case and that covers the running motor as it idles, but if you hold it open for a long time, that oil gets used up, leaving the cylinder dry and the possiblity of seizure probable. Frankly is is harder on a motor to idle for a long time than to run it wide open under a load because there in not enough oil getting to the cylinder to keep it well oiled. I am pretty sure that a blower or even chain saw does not run wide open, and more than likely, there is a fan that move air over the fins of the motor to help keep the motor cooler.Air cooled motors should use more oil for this very reason

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)
74' 1/2 440 maico
70' 400 maico (project)
72' cr125 Husky (project)
93' RMx 250 suzuki

[thrownchain]

Thom, you covered it right there, you state that if a motor runs for a long time wide open it uses up the residual oil in the motor, leaving it prone to seizure. The improvements in oils over the years probably helps, but wfo is not good in the long run. Worry less about weedeaters and blowers, but on a bike, not good.

[Bob Bean]

You guys should have seen the motor we ran on a dyno for an hour straight at WFO...   A Husky 125wr.  A guy from ND was developing a alcohol filled head ( no pump) and we used my bike as a test bed for it.
  You would be suprised at how long you can run a 2 stroke wide open.
(if jetted properly)

Bob

1986 ISDE Italy
1987 ISDE Poland
1989 ISDE Germany

1970 125 Six Day
1973 100 Berkshire
1973 Jackpiner 175
1974 Penton 250 Harescrambler
1976 Husky 250CR
1981 Husky 250CR
1982 Husky 250CR
1983 Honda CR480
1985 Husky 400WRX
1985 Husky 400WR

[Paul Danik]

"Back in the day", the then older and more experienced Penton enduro riders would explain to us youngsters the virtues of applying the choke every once in awhile if holding the throttle wide open while making up time on a road section. This was during the Sachs powered Penton era where the spring loaded choke handle was mounted on the handlebar, the trick was to have it in a position where it could be thumb activated.

Mark, maybe this doesn't exactly fit in with your question, but your question sure reminded me of those great times

Paul

PS...  Those same older and wiser enduro riders also recommended keeping your fingers at the ready on the clutch lever, just in case....[8D]

[brian kirby]

If jetted rich enough a two stroke can run wide open a long, long time. Remember the scene from On Any Sunday with Malcolm Smith coming across the long stretch of lake bed wide open? When they usually seize is when you shut the throttle off after a long wide open section. Into the 80s Malcolm Smith sold a kit for Mikunis on Huskys that had a lever that opened the enricher valve just enough to dump some more fuel but not enough to reduce rpm. If you dont have a handlebar mounted choke lever, when you get to the top of a long hill, or at the end of a long stretch of wide open throttle, instead of chopping the throttle, leave it wide open and hit the kill switch. Doing this dumps lots of cool fuel into the cylinder and prevents seizing.

Brian

[454MRW]

Yeah, I remember riding my then geared to run around 85 mph '74 Hare Scrambler WOT on the black top road that ran past my parents house, only to have the engine seize and slide almost sideways before feathering the clutch and getting it straight enough to pull it all in, and then to my amazement pop the clutch when the bike slowed to around 25 mph and break the piston loose and riding it timidly the remaining 1/2 mile home. LOL. Another thing that still amazes me is that the engine is still on the honed original bore, although on a New coated piston. The rings got stuck in the old piston and was missing a chunk above the top ring. Gotta love that Golden Spectro! Mike

Michael R. Winter
I enjoy rebuilding and appreciating Pentons!
1974 250 HS Pentons-1980 KTM 175-400'S
1975 Can Am 175 TNT & 77 250 Black Widow
1979 Husqvarna OR390
1976-78 RM & 77-79 PE Suzuki's
1974 CR250M 07 CR125R 79 CR250R

[Bob Bean]

I almost hate to tell this story, as I dont want to start a oil use debate, as that has been done many times over and over, but this one stands out some.

 The same Husky (73 125wr) I allowed the guy from Notre Dame test his new alcohol cooled head design on, had a little bit of history behind it.

The original owner bought it new. Had about 2 hours on it and the rod bearing went out.  He took it back to Burroughs cycle shop and they replaced the rod and bearing.
   I thought maybe Ira had pressed the crank back together with too little side clearance's, so I had the guy take it to a mechanic that did cranks full time.  He made sure the clearances were right.
 He got it back and rode it about 3-4 hours and the rod/bearing went out again.  At this point he was not happy with the bike and sold it to me for a cheap price.
   I replaced the rod/bearing and even machined a few small slots on the sides of the rod to allow more cooling/oil to reach the bearing.  I made sure the side clearances were good too.
  The only other switch I made was, I ran Bel Ray oil at 40/1 ratio.
He had been running Golden Spectro.
  We then proceeded to run the heck out of this little Husky 125. I rode it in Enduros, HS, Took it up North Trail Riding. My wife raced it in HS and MX both, Ran it hard for 2 years, Then we did a months worth of testing the ND guys head on a mx track, up and down a 1/2 mile field, and the dyno testing. Wide open for long stretches...
  We tore it down once and put rings in it after 2 years.
And of course all the regular maintanence. Filters and gear oil....
All this time on Bel Ray at 40/1.

Then we were going up north to Michigan for some trail riding.

We loaned the little Husky out to a friend to ride.
I never thought about it, when he gassed the bike up, but about an hour into the ride, the rod/bearing went out.

The only difference?  His gas was mixed Golden Spectro 40/1

Now I have had hundreds of customers using Golden Spectro for over 40 years with no problems (that I know of)  I also use Spectro Platinum (synthetic) in all my bikes nowdays and have had no problem.
Was it just a matter of time before the rod went again ?
Or did the Bel Ray oil offer just a tiny bit more protection ?
I sure dont know, but it had me leary for many years after that.

Bob

1986 ISDE Italy
1987 ISDE Poland
1989 ISDE Germany

1970 125 Six Day
1973 100 Berkshire
1973 Jackpiner 175
1974 Penton 250 Harescrambler
1976 Husky 250CR
1981 Husky 250CR
1982 Husky 250CR
1983 Honda CR480
1985 Husky 400WRX
1985 Husky 400WR

[454MRW]

All I know is that I like it at 32:1 and haven't had another seize. I think we ran pump regular gas back then, so that may have contributed to the problem. I did have one slightly scuff the exhaust side of the piston when the extra oil of the 32:1 mixture started to plug up the exhaust bridge oiling hole in the piston. I like the anti corrosion feature throughout the winter of the blend of synthetic and non-synthetic oil that the Spectro uses. Of course fuel mixture could have also been a little lean back then too. My local Suzuki racing Guru back then strongly urged everyone to stay away from BelRay in Suzuki's, because the RM Suzukis bottom ends were notorious for failure with BelRay. Mike

Michael R. Winter
I enjoy rebuilding and appreciating Pentons!
1974 250 HS Pentons-1980 KTM 175-400'S
1975 Can Am 175 TNT & 77 250 Black Widow
1979 Husqvarna OR390
1976-78 RM & 77-79 PE Suzuki's
1974 CR250M 07 CR125R 79 CR250R

[SouthRider]

A lot of people forget that the more oil that you add to the fuel the leaner your jetting becomes.

Sometimes a "perfect" setup can come crashing down just by switching to a different oil that has a different recommend mixture ratio.

Neither oil would be "bad" or wrong - just not right at that ratio for the given jetting of a specific bike......

_____________________________________________________________________________________

"We the willing, led by the unknowing, are doing the impossible, for the ungrateful. We have done so much, for so long, with so little, that we are now qualified to do almost anything, with nothing."

1972 Penton Berkshire 100
1983 Husqvarna 250 XC
2011 Jayco 31.5 RLDS
2009 Chevy 2500 HD Duramax

[SouthRider]

And when in doubt - just forget about dropping that needle just one more click. It will save you at the end of that long straightaway when you drop off the main jet....

_____________________________________________________________________________________

"We the willing, led by the unknowing, are doing the impossible, for the ungrateful. We have done so much, for so long, with so little, that we are now qualified to do almost anything, with nothing."

1972 Penton Berkshire 100
1983 Husqvarna 250 XC
2011 Jayco 31.5 RLDS
2009 Chevy 2500 HD Duramax