Engine max rpm question

[MichaelKanRS]

I noticed that when I rev my engine with clutch pulled out, it can rev pretty damn high. However, riding down hill at full throttle, the engine seems to slow the bike even though it's not close to max rpm. Shouldn't the engine be able to spin faster since I'm going downhill?

 

[Taffy13]

This is what you would call engine braking, basically the compression of the engine is stopping you because your wheels are out turning your motor even at full throttle. This happens because you only have that one big gear your engine is driving.

The gear multiplies torque in one direction ( engine->gear->tire) but multiplies rpms in the other direction ( tire->gear->engine ) so when going downhill your wheel is spinning at X speed then gets multiplied by first the tires size, then by the size of the gear, and finally by the size of your engine output gear and usually that's more then your engines rpm.

Super technical but given more gears that wouldn't happen.

 

[indian22]

Mike engine braking is a good thing if your motor is a four stroke but on 2 cycle engines it's not. It's not as an effective braking technique as it is with the 4 stroke motor. Using engine breaking won't harm a 4 stroke, but if used habitually on 2 strokes it will eventually destroy the engine. Coasting long hills should be done with clutch lever held in and occasionally blipping the throttle to flow fuel to the engine. The two cycle engines only lubrication comes from the fuel mix and with out adequate fuel flow the engine isn't lubricated and if that's not bad enough at least half the engines cooling comes from the fresh, and cold, fuel flowing into the crankcase...cut the fuel flow often and friction and resulting heat will ruin the engine.

Rick C.

 

[Taffy13]

I never thought it could damage a 2 stroke, good to know.

 

[kelly dean]

i know a bit about damaging 2strokes LOL

 

[Sidewinder Jerry]

Engines either 2 or 4 stroke have a maximum safe rpm. If pushed beyond that you risk damage to the engine. Even with multi gears the there's a maximum speed at the safe rpm using the highest gear. When going down steep hills or very long grades you can cut back on the throttle and/or hold the clutch in when you've reached the max speed. Gravity has taken over and the engine is no longer contributing to acceleration.

I strongly recommend getting a tachometer. It'll be a major help to keep you from over reving going down hill. I like the ones with an hour counter on them. The hour counter is useful for oil changes on 4 strokes, fuel filter changes on both 2 and 4 strokes and cleaning the air filter on both 2 and 4 stroke engines.

The hour counter/tachometer can be found on eBay reasonably cheap. They are also very easy to install.

 

[MichaelKanRS]

I've been noticing this phenomenon on flat grounds as well now. When I am cruising at 24/25 mph, the engine occasionally engine brakes down to 21-23 mph and accelerates back to 24/25 mph. Frequency is probably 2-4 times per minute. I've adjusted choke up and down to see if anything changes but the problem still happens.

 

[Sidewinder Jerry]

I've been noticing this phenomenon on flat grounds as well now. When I am cruising at 24/25 mph, the engine occasionally engine brakes down to 21-23 mph and accelerates back to 24/25 mph. Frequency is probably 2-4 times per minute. I've adjusted choke up and down to see if anything changes but the problem still happens.

This can be caused for several reasons; slight inclines, air/fuel mixture, oil/fuel mixture, filter issues, gas quailty, etc. I once got some gas that looked like oil and vinegar salad dressings. Came out of the pump that way. Still a tachometer can let you know if the problem consistently occurs at a certain rpm.