So I have an OHV motor on my pocket bike. Every car I’ve ever owned has been OHC, so it kinda new tech to me. There are absolutely ZERO performance parts for this little engine. It is not a predator or 2 smoke. It is Chinese clone of a Honda gx31.
So, I want to try and get some more lift out of the cam. Not duration, lift. It has a cheapo plastic cam. And, no, I’m not going to learn all the crazy camshaft building tips for a cheap plastic can on a pocket bike. Rather, I’d like to know the little secrets about getting more lift by shimming the valves so they open further. Or any other push rod tricks.
My thoght was either a metal disc going under the rocker arm between the arm and sprint seat. Or taking the little push rods out, welding some material on the end and then grinding it down until I achieve desired length.
http://www.me.sc.edu/research/AARG/images/AARG/Camless%20Page3.htm
this is the engine.
Adding lift involves changing the rocker arm ratio, not shimming things. If you make the pushrods longer, the valves won't close anymore, and it will lose power, until it burns the valves and stops running completely.
I was thinking shim buckets like Fiats have, but StreetWise is right. How about a port & polish, nice valve job, perhaps some dry Moly on the piston skirt and sleep well knowing you've done everything you can?
Dan
The best fix for underpowered Honda engines is bigger Honda engines. 
It sounds like he wants to reduce the lash to increase the lift. While in theory those motors require almost no lash and you should pick up some lift (and duration at the same time) it's not likely enough to notice. Increase the compression and make an offset key to advance the timing. Those Honda motors are built to last a long time and are very conservatively built.
Interesting. So the rocker arm will only go as far as at it can stock. Even with a small shim between the valve spring and rocker arm pushing the valve in further?
Ok, well... maybe it’s time to brush up on the cam lobe specs and get a plastic welder guy to help me do a one off regrind.
The rocker arm may be able to move further, but you can only get a certain amount of travel based on cam and rocker design. Since one end of the travel has to correspond to the valve shut, that means you can only go just >so< far open. Add more height to the cam lobe or reduce the base circle and you will get more travel, or change the rocker ratio and you get more travel and either way you can get the valve open further.
Depending on rocker geometry, you CAN get more lift. If you use longer pushrods and then move the rocker pivot up to correct the lash, you will get a small amount more lift, because the rocker geometry changes. The rocker ends up rotating more for the same amount of pushrod motion, and the pushrod end gets affected more than the valve end. It also mucks around a lot with the wipe pattern on the valve tip, so going too far is ill advised. You don't get very much, either, maybe 5% or so.
Nissan guys used to do this on the L-series engines to get more lift because of the idiosyncracies of the follower setup that those engines had. They would shim the cam towers up .100 or so. I forget if the math required that they shim up the adjusters or add lash caps on the valves, my brain was too stuck boggling at the idea of shimming up the cam towers.
This is what we are dealing with
Hal
UltraDork
2/3/18 3:21 p.m.
About the only way to get more lift in that is to change the cam profile. Looks like a job for some of our 3-D printing guys if they can get the right material.
Th cam profile should probably be the last place to look for power in this application anyway. It looks like the cylinder and head are one piece so increasing compression is probably not going to happen so... like I said, increase the timing advance, and also see if you can fit more carb and bigger exh. in there.
I'd reduce the base circle with a file or something... More lift and duration.
In reply to Suprf1y :
Agree. More timing. Lap valves. Do some porting work. More carb
