some info about compression release

[gone_fishin]

doh! i knew that!

i see how it would be a mechanical help for starting, but i guess i'd have to ride something with one to see how it would be used for decent control at speed.

 

[smitty]

The Jacob's brake, (or j-brake, which is a compression release) on diesel trucks, provides "engine braking" without having to gear down.( Wouldn't it be nice if we could gear down?)

 

[Sianelle]

I'm hunting for the model of Villiers cylinder barrel that's fitted with a compression release for my own retro motor-bicycle replica.

sianelle...
keep us posted?

The Villiers engine with this feature has a boss specially cast into the cylinder barrel and the cooling fins are arranged so everything clears the decompressor. The standard barrel lacks for this feature and I don't think it's possible to bore a 14mm hole anywhere as the casting isn't thick enough. Some barrels have a small boss cast into the head to take a 1/4 inch mounting bolt for a fan cooling shroud and it may be possible to do something with one of these if I hold my mouth right.

 

[Quenton Guenther]

Back in the day when I raced my 450 CC Ducati I had a 10 MM compression valve assembly screwed into the head [stock]. In the beginning I needed the release to start the big 4 stroke single, but in time I learned to start it by just going past the compression stroke and using my entire [at the time 155 lbs] body to force the kick starter downward. The compression assembly was then removed and replaced with a 10 MM spark plug attached to a second coil for more fire in the head from the magneto. During the same period of time I witnessed many, many, 2 strokes locking up at the end of the straight away when the riders used the compression release as a brake [no courage] to slow down for the corner. It always amazed me how many racers didn't realize when the release was open [and the throttle closed] and the motor was at maximum RPMs and NO OIL in the cylinder that the motors locked up. It didn't take long to watch the compression release vanish from the flat track circuit, because it made an expensive brake [broke].
Quenton

 

[ibdennyak]

Back in the day when I raced my 450 CC Ducati I had a 10 MM compression valve assembly screwed into the head [stock]. In the beginning I needed the release to start the big 4 stroke single, but in time I learned to start it by just going past the compression stroke and using my entire [at the time 155 lbs] body to force the kick starter downward. The compression assembly was then removed and replaced with a 10 MM spark plug attached to a second coil for more fire in the head from the magneto. During the same period of time I witnessed many, many, 2 strokes locking up at the end of the straight away when the riders used the compression release as a brake [no courage] to slow down for the corner. It always amazed me how many racers didn't realize when the release was open [and the throttle closed] and the motor was at maximum RPMs and NO OIL in the cylinder that the motors locked up. It didn't take long to watch the compression release vanish from the flat track circuit, because it made an expensive brake [broke].
Quenton

Saaaay, maybe you can answer a question for me. Years ago I took a YZ 125 and supposedly a 500cc Ducati thumper in on a bill. I didn't realize the thumper had a compression release the first time I tried to start it. Yup, just like a cartoon...backfired and over the tank I went. Still have a scar. (Geez, another candidate for silly pics thread) Was it actually a 500, or maybe the same as yours? Thing was phenominal though idle is 2nd, crack it open, and an endless wheelie.

Denny

 

[Sianelle]

I found that really interesting Quenton Guenther, but for my 1920s designed low reving, soft tuned, heavy flywheeled, two stroke engine a decompressor isn't going to cause it to lock up or do anything else that a well bred elderly English engine shouldn't.

 

[Quenton Guenther]

Hi Denny & Sianelle,
As far as I know the largest Ducati single was in fact called 450 CC [actually less, I think it was around 438 CC]. The Ducatis I raced were the Desmo version.
The average 2 stroke motor with a compression release wouldn't be in danger of locking up. The problem only occured when the compression assembly was opened with the motor at maximum RPMs with the throttle closed [supply of gas with oil mixed in was stopped], stopping the oil needed to lube the cylinder & piston.
Quenton

 

[Herrmanator8]

my dirtbike has a decompression lever but it releases too much compression and doesnt help in starting it at all...the 03 model has auto decompression for starting.