Hi all. Here's a question. I have read many times on this forum that some engines will not shut down with the throttle barrel fully closed until after the engine is thoroughly broken in. I have even experienced it myself with a Super Tigre.

Why is this? What is/are the mechanical change(s) that take place inside the engine during break in that allow it to eventually be shut down using the throttle?

Thanks,
David

I can't think of any changes a run in engine could have. The usual suspects for not shutting down are air leaks from improperly seated O rings on the carb, leaky front bearings etc but none of these change during running in. A carb could still draw in fuel when completely closed because of the then very low pressure inside the crankcase but the engine still needs some air to burn the fuel. The only way it could get some air is back through the exhaust port if there was some oxygen still in the muffler but that would happen after the engine was run in as well. I think I'll wait for some more ideas to get thrown around .

David,the only way a engine can continue running with the throttle shut is if a air/fuel mixture is leaking in from somewhere else. if anything, this is more likely to happen in a engine that has logged some hours and developed leaks from failed gaskets and seals, along with loose tolerances insleeve bearing typeengines.

so what some people have said is; the engine wont shut down unless the engine is broken in [ run in ] what a load of you know what.
this problem seams to happen with worn out engines where the front bearing leaks air into to crankcase of the carby develops some slop in the barrell / needle valve / or carby to case connection. GMS engines are bad for this .

I had a Webra speed  .40, that had the shaft with quite a bit of play in the housing for best power. That play was enough to keep the engine running.
I solved it by adding some teflon oil to my after run treatment. Then bye the bye, as the teflon built up on the shaft, the engine would stop reliably.

This is exactly why I asked the question. I simply could not conceive of how wearing in parts would help the front half (carb, crank, etc) create a better seal. Of course, a piston ring does develop a better seal from break in, but that has no direct link with air entering the carb.

It's most likely that enough castor oil built up around the carb on my ST that some small leak was stopped. But, again, that has nothing to do with break in.

Still, I can go through this forum and find statements that claim it happens. Look, I don't believe everything I read. In fact, the claims struck me as perplexing enough to merit raising the question.

However, one thing is true... an engine will idle and transition better, and a low speed needle can be set more easily, after the engine has been run in. Maybe this is what people are referring to. Maybe they are not able to set the low speed needle lean enough until the engine is broken in. It could be that with a very rich LSN enough fuel continues to enter the engine that it will continue to run for a while. Though, that would still require an inflow of air from somewhere.

David

People mention about "leaky" front bearings. A metal shielded front bearing with 1/2-2 thou clearance between the shield and the inner race will never be air tight. The only air tight ones I have seen are the rubber sealed ones. In the R/C car forums, people blame a leaky front bearing for lean runs. Well I have Two OS CVR .15's, One with a metal shielded front bearing and One with a rubber shielded bearing. Both engines run the same with the same needle settings. I don't believe in blaming front bearings for lean runs.

The car nitro forum people are obsessed with "air leaks". Any inquiry a poster makes about engine problems/tuning is almost always met with "you have an air leak, better seal it up". But then again, most of them break-in ABC type engines by letting them idle away for afew tanks of fuel.