Compression release rpms

David Maner

What rpms does the compression release on the High Performance VC 460 engine shift at. I'd like to lower my rpms some from the 960 I have it at now but don't know if I still need to tweak my carb adjustment to get there of if it's the compression release killing it. One thing about it at that low rpms and with the high compression that bugger stops within a couple revolutions when I shut the key off. Almost like it had a brake on the engine to stop it like a chain saw chain brake does. Which is awesome in itself. Starts just as fast as it shuts down too.

Anyways what is the lowest RPMs I can expect to be able to get this engine to idle at?

BGW

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raydhd

The S/G is going to kick in somewhere around 800 rpms.

Shepard

I just did some reading on B&S's compression release. Anything above starting rpm and it supposed to kick out. They do not give an outright number it turns off at. I would try taking your idle down 50 rpm at a time till your happy. You'll know if the compression release hits as it will emit an additional noise in the exhaust as it's burping the compression and trying to keep running.
Also B&S says when adjusting your valves to turn the engine over TDC by a 1/4" to eliminate any chance of release lever interference.

David Maner

If I don't forget, tomorrow I'm going to turn the gas off and run the carb out of gas and then see what my tinytach says the rpms is when I'm turning the engine over with the S/G. For shites and grins only. You have me wondering what that'll be. I suspect it'll be as you mentioned earlier in another thread to be in the 800 rpm range. But knowing me I want to know for sure. This engine is a tad bigger than a stock engine and thats what you were talking about I do believe, so it may be slower, not sure though. I know that when the S/G relay/solenoid isn't picked up/energized the s/g is neither a starter motor nor a generator rather just a parasitic load

I'd like to get this engine to idle in the 750 to 800 rpm range. if I do I'll go back to the red springs in the 780 clutch. For what I use the cart mostly for, mushroom foraging, puttering along at a snails pace is ideal. With the low end torque of this 460 I don't think it will hurt me any on the hill climbing any either. Right now in the comet 780 with the 106 gram weights and the green springs I can tear up the hills spinning the tires all the way. Some of these hills are 20 degrees plus too.

I'm fully aware that I may not be able to get this engine to idle that slow and if I can't no big deal. Cammed up as it is and all I think it'll sound awesome at that low idle and that is part of my motivation too :-)

ETA: I've got my intake adjusted to .002 where a .003 won't go and my exhaust set to .003 where a .004 won't go and a .002 is way loose. I go ahead and put the intake fully open when I adjust the exhaust and the exhaust fully open when I adjust the intake. Back in my air cooled VW days I use to set my valves on top dead center on compression stroke. They didn't have a compression release to worry with like these engines do. As the engine warms up the gap gets bigger. I've checked that just to make sure. The last thing I want to do is burn an exhaust valve

raydhd

Mine was in the 750-780 range when I did a similar test. Of course that is somewhat dependent on the state of your battery. Some of us also noted way back when we went to the higher ratio rocker arms with the original camshaft our compression releases worked a lot better.

sho305

I've had the comp release stop working when lash got too big, have it work better with high ratio rockers. Its all about the spring thing on the cam and when it wants to move up, likely you could adjust it but have to take the cam out to do it. My 420 with the Vegas cam will idle right down until it starts missing from the comp release, it does have 1.3 rockers, it does have start/gen that likely helps it idle slow better. I'm sure its below 1000rpm but no tach on it at the moment. I always set my lash to almost zero on clones, just so the rocker slides a little on the keeper. Its a little harder to set with the roller rockers but they still have a little play in them.

Cavemandrew

I was trying to figure out what was in my stock cam. Is that what it is? Its only on one cyl.

Attached Images

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raydhd

Those balls are for the compression release. Not one cylinder, just the exhaust valves on each cylinder. Sort of looks briggs like