Fixing a GZ4000 Air Valve

[Chris-PA]

OK, first, it's not really a GZ4000 air valve, it's from one of the early Jenn Feng (Taiwan) McCulloch saws which were clearly licensed from Zenoah. The top of the valve is different:

But the bottom part of the valve is the same, and has the same problem - the housing is molded Bakelite (phenolic resin), and the inner shafts are some sort of nylon. There is a lot of side thrust on shaft due to the carb linkage, and the bakelite housing eggs out. There isn't any seal on that side, so then the shaft gets sloppy and you can't set a consistent idle. It's no different than a worn throttle shaft in a carb.

I decided to try to fix one by printing a drill busing and drilling out the hole, in order to recenter it and put in a sleeve of some sort. But the snout on the housing broke off:



So I got frustrated and set it aside. Today I decided to print a new bushing and drill it out to 3/8". Then I turned a piece of 3/8" brass into a bushing:

I used one of the printed drill bushing and a bit to align it, and epoxied it in with JB Kwik:




Reassembled it, and it fits very nicely and has a good feel. It seems to have less friction too:


I hope to put it on and try it this weekend, although I may have to cut a gasket.

 

[grizz55chev]

OK, first, it's not really a GZ4000 air valve, it's from one of the early Jenn Feng (Taiwan) McCulloch saws which were clearly licensed from Zenoah. The top of the valve is different:
View attachment 554313
But the bottom part of the valve is the same, and has the same problem - the housing is molded Bakelite (phenolic resin), and the inner shafts are some sort of nylon. There is a lot of side thrust on shaft due to the carb linkage, and the bakelite housing eggs out. There isn't any seal on that side, so then the shaft gets sloppy and you can't set a consistent idle. It's no different than a worn throttle shaft in a carb.

I decided to try to fix one by printing a drill busing and drilling out the hole, in order to recenter it and put in a sleeve of some sort. But the snout on the housing broke off:
View attachment 554316
View attachment 554315

So I got frustrated and set it aside. Today I decided to print a new bushing and drill it out to 3/8". Then I turned a piece of 3/8" brass into a bushing:
View attachment 554317
I used one of the printed drill bushing and a bit to align it, and epoxied it in with JB Kwik:
View attachment 554318
View attachment 554319
View attachment 554320

Reassembled it, and it fits very nicely and has a good feel. It seems to have less friction too:
View attachment 554321

I hope to put it on and try it this weekend, although I may have to cut a gasket.

I've not had good luck with the Quick JB Weld, the original is far more resistant to gas.

 

[Chris-PA]

I've not had good luck with the Quick JB Weld, the original is far more resistant to gas.

I'll have to keep an eye on it - I had actually used it inside a clamshell engine and did not see much problem. Here it's on a strato air valve so it won't see all that much fuel.

 

[grizz55chev]

I'll have to keep an eye on it - I had actually used it inside a clamshell engine and did not see much problem. Here it's on a strato air valve so it won't see all that much fuel.

Good luck with it, nothing ventured nothing gained. I've repaired many plastic fuel tanks and had good and bad results, the failure rate is much lower with the original.

 

[Chris-PA]

I got it back together and ran it today, and it worked great. The idle was consistent and the throttle trigger feel is improved a lot. I'm happy with how this turned out.