cam grinding -

Did you ever do a cranking compression test on the engine before and after cam phase changes?

Stan

Something like that thing on Winky's Workshop?

- now at the

CVT to an intermediate jack shaft really helps with that.

Generally you advance timing with an offset key (if it has a keyed balancer) giving you a degree or two. Much better than a sudden shift of 14.

Generally they go the other way with those engines at the cost of engine life. MORE rpm, with more gas, more air, and a more open exhaust, elimination of the governor, and stronger valve springs. That is basically stage 1. Sometimes before upgrading engine internals they will mill the head for more compression. I've seen YouTube hotrodders do it on a sanding belt.

Generally, instead of trying to build low end torque with a tiny engine they built horsepower, and increased the gear ratio and they launched at higher RPM.

Some of the older early videos from guys like Red Beard's Garage and Cars and Cameras from when they were still doing it mostly for fun really do some interesting cheap mods on those small engines. Sure a top fuel mini dragster built around an alcohol fuel turbo charged predator 670 is entertaining, but I more enjoyed their older shorter videos where they took an EX21 with basic mods and stretched another 5-7 mph out of it. If you must play with the 670 dropping one bonestock on an old Paughco Springer frame was more approachable and made it more interesting.

For reference I've got a couple Manco Intruder II carts I bought for the kids years ago. Even with the Subaru Robin EX21 (6.5hp) they run a CVT to an intermediate jack shaft and then a tiny gear on the jack shaft to a giant gear on the drive axle and run only 16 inch tires. They do run turf/sand knobbies, so take off torque is important, but I think they are locking up at a much higher rpm than you are thinking.

When I was a kid those who were lucky enough to have an old classic 5HP cart with a simple belt/lever friction clutch were running tiny tires.

I'm not sure an RV cam (long dureation) translates significantly to a

I'm just talking. I didn't really analyze your post thoroughly.

Can you grind some off the base circle and get the lost lift (and more) back? You will have to add length to the follower region (pushrod, bucket follower, whatever you have) to keep the geometry correct at the valve tip, I don't know that valve train so can't help there.

There's a engine good discussion website called speedtalk.com (unfortunately it's been fighting off bots for a week so response time is very slow at the moment). One of the long time posters goes by SchmidtMotorWorks, he has developed a browser-based set of software for cam design you might take a look at if you want to evaluate your proposed shapes. See his announcement at

with cam advanced it was OBVIOSLY lower cranking compression. My50 year old screw in tester does not work too well right now.

Looks like hell though

Generally that's how "they" do it. Not how "I" do it. The original had a 1.5HP engine with 8:1 cam drive.The engine ran an atmospheric intake valve and a very rudimentary carb. I am going to duplicate it as closely as I can

I am moving JUST the intake valve closing event - the most critical for low speed cyl filling to improve dynamic compression under 1800 rpm. Whet this project needs is thr EXACT OPPOSITE of what you want for a minibike or dragster

Why would I want to do that? the intake valve lift is designed for

The cam is ground and installed in the test engine and broken in. runs good and obvious increase in compression when pulling the rope.The engine has a distinct sound at low to mid rpm - a more "authorative" exhaust note, but not the cackly one from having the whole cam advanced (where the exhaust valve was opening with the piston only past way through the expnsion phase of the power stroke)

Actually the reverse should be true. You should see higher cranking compression / PSI because of the earlier IVC.

Stan

it SHOULD have, but since all the other events were also advanced 14 degrees all the advantage if the advanced ivc went out the window.

Is this a moderately stressed part that could be assembled from the gear and separate lobes and bearing sleeves on keyed shafting? The lobes wouldn't need to be hardened during testing, they and the bearing sleeves could be slid on, Loctited and pressed together with an end bolt while you experiment, by melting the Loctite to change timing. The lobes could be split and overlapped to allow individual control of rise and fall timing.

If RPMs will be limited the ramp rise could be steeper and valves fully open longer than for 3600 max RPM.

See what's acceptable on this one:

Once the lobe positions are satisfactory they could be made permanent by removing the gear and passing a keyway broach through the shaft slot. Then they could be case hardened, ground or sanded smooth and polished. Keys with the hub and shaft sections offset for fine adjustment can be bought or easily milled.

The end bolts could be cap screws with heads reduced to a narrow flange that occupies some end bearing length. I do this sort of mod enough that I made threaded bushings that fit 5C collets and run the bolt threads centered; the heads and shanks rarely are. The 3/8-16 bushing has end grooves for O rings that hold it together and was hacksawed into 3 pieces to clamp tightly on short sections of thread, the others need a locking nut.