Stock 420cc Cranking Compression?

geoffw

I need a small favor if anyone has a bone stock 420cc clone engine(no milling, cams, etc). Ive got 4 of them sitting in front of me but none of them are stock :)

I need to know what the cranking compression of an out of the box 420cc is.

Thanks in advance!
-Geoff

BGW

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geoffw

I need actual cranking pressure. The compression release on the cam reduces some pressure so its easier to start.

Geoff

sho305

The stock clone cart is in storage and a long way away from me, or I'd test it for you. but it uses gen to start it may not turn it over without comp release.

geoffw

sho,

Does the stock gen turn it over faster than the clone starter?

-Geoff

sho305

I would guess around the same, but the clone starter is much louder. Stock gen sounds same as it did with stock engine. I know it is under my idle rpm which is 900 on the tach and even then it wants to miss from the comp release trying to work at times. I plan to bump up the lash a little so it stays off at 900 where the stock clutch stays un-engaged. My best guess is the start rpm is same maybe hair faster, we only used the clone starter a few times to make sure it worked on the other cart then it went haywire and locked on. I had to pull the solenoid wiring to turn it off. It was/is hooked to the clone key switch. My clone is a recoil. I'm pretty sure the gen would not run it without the comp release, it slips the belt if not tight now. If the comp release kicks off imagine the gen would slow down. From my experience you can usually (but not done this yet) increase the lash a couple/few thou to make the release stop at lower rpm so you can idle it lower. I did it on the other clone but don't recall what I set it at thinking 6 thou on valve that does it.

geoffw

Gotcha!

I just went to Harbor Freight and tested the 420 on the shelf and it make right around 50psi. Im guessing the Duromax is right around 60 because its higher compression.

Ive been setting my cams to crank at 80 psi but the blocks are cracking right at the starter ear. Im going to file down the release less so Its in the 60psi range.

I personally like to idle the engine at 1100-1200 because it seems to run soother with the large cam and 33 degrees of timing. 900 and it dies too easily.

Hopefully I get time to do a stock gen setup soon. I like the idea of having a cart that starts like stock.

-Geoff

sho305

I don't think the stock g9 had a comp release, they are 285cc iirc. But they do have zero ignition timing at idle. So it may turn faster with the release for all I know.

Yes starting is an issue with the cams, few things will start them without the release.

I don't doubt it needs more idle rpm with a cam lol, my 900rpm is mostly a function of the stock clutch I might raise it up when I get a different clutch. I do idle it at times, but still pedal start. I have about 1" free play in the pedal from turn-on to when the carb opens. I could put a bypass switch on it to let it idle but not bothered.

I'm really temped to try the G9 ignition on it somehow, since I can get ~30 degrees advance above 2500rpm and zero at idle. But not sure how the pickup works or what it needs. Will have to pull a stock flywheel and see.

geoffw

Between the cam and 10.5:1 compression, i dont see a way to start it other than with a comp. release. Its just hard to find the happy medium. I just set it at 60psi and Im hoping it works without cracking more blocks/blowing the starter.

The kart guys get away with it because they use those big hand-held electric starters which turn the engines over at 2000rpm even with 14:1 compression.

On another note, This will be my 7th cam that I've tested and these big engines really like big cams. So far the best is the 260 Duration / .410 lift. Requires a built valvetrain and a rod but its seriously dangerous fast.

Ill post vids this week. I think we have a winner.

sho305

That is great you been able to test the cams, sounds like fun to me lol. Not sure I want a huge cam in this cart (maybe another cart) as it needs to be reliable. Another 1K rpm and more than stock power would be ideal, and to live for a while. Would rather put a rod in with any cam for same reasons. Unless you have no problems running a cam that big. I plan to run trails for hours on end, though WOT would be pretty limited spurts here and there unlike racing is.

I don't know what compression I am at, just told him to cut it .050 as they said I could get away with regular gas up to .060.....but I think the kart guys also engage the clutch over 2K rpm so they don't get the knock I do down low. Only happens once in a while, its the stock clutch. When I floor it the rpm go up and its fine after the moment it takes to shift down.