Originally Posted By: boraticus
So, in reality, the "oil build up" issue is of no significance if the machine is to be run a high rpms most of the time.
I've yet to work on a two cycle engine that had bushing type bearings. I know they exist, I just haven't seen nor worked on one. Even the old Lawn Boy mowers I own have a rather crude needle bearing arrangement in the big end. They come in a strip that must be laid into the bearing seat on the conrod then the into the seat of the conrod cap then the two are bolted together. I used to use grease to hold them in position until I could get the cap on.
Despite the crudeness, it makes bearing replacement much easier than disassembling the crank to change big end bearings as is necessary on most two cycle engines I've worked on.
I've got plenty of two cycle OPE and ALL of them smoke on startup regardless of the mix ratio. Doesn't matter if it's 50:1 or even 100:1 as with my outboard engines. They all smoke on start up, primarily from being choked resulting in lots of fuel&oil in the system to get things rolling. Once warmed up and running, even at slow speeds, smoke is virtually invisible.
By the way, just for clarification, from anything I've read, oil injection was invented and used simply for the convenience of not having to mix oil with the fuel. I've yet to see any literature stating that it was invented to eliminate oil build up in the crank case.
As far as the ratios it pumps oil at, it varies from virtually nothing at idle speed to very rich at full throttle. Can't recall the exact numbers but I'm fairly certain that at full throttle it can be more than 25:1 on the old Yamahas. The first thing people who race RDs (any any other racing two cycles) do is disconnect the oil injection and go straight to premix usually 32:1.
Being that you're a trained two cycle mechanic, I'm certain you've heard of the legendary A. Graham Bell. If not, you should read his book "Two Stroke Performance Tuning". Serious reading for the two cycle aficionado.
OIl build up is usually not a problem at high rpm since the fuel (and some of the oil) move through the engine quickly, however with enough oil in the premix or bad injector pump it can become a problem.
Bushings were used in some two strokes many years ago, but are no longer seen, The Lawn Boy big end needle bearing you mentioned is easy to replace and almost never gives problems.
I feel I am doing a lot or repeating. When a working oil pump delivers a small amount of oil at idle and more as rpm increases as it was designed to do. oil build up in the crankcase is kept to a minimum without the owner knowing what the ratios are. Many of these pumps proved troublesome and had a tendency to go out of adjustment, hence they were disconnected and the more reliable premix was used, coming full circle and bringing back the problem of oil build up with incorrect premix ratios.
I never claimed any expertise, nor am I a trained two cycle mechanic. I was employed as a motorcycle mechanic and learned on and off the job.
