MonsterAir

Hey guys. This will be my first post on the site and could really use someone's help. Just getting back into the hobby and this time I will be doing all work myself rather then paying an arm and a leg to the rip off artists (they actually just closed down) down the road to do all my work. I pulled my Cen GST 7.7 outta the basement for the first time in about 4 or 5 yrs! Everything is in almost new shape except the motor which just needed a new sleeve and piston and crank (crank has rust pitting). So I ordered a new crank housing, crank, piston/sleeve, and clutch (broke a spring removing one of the shoes)...... Everything arrived today so I figure I'd slap the parts on break her in and have it ready for bashing in the snow this weekend. Well didn't work out like that, which brings me to my problem. The new crankshaft I got isn't exactly the same, on the backside of the crank where the piston would be fixed to the piston there is a pin and spring instead of a fixed pin on the original. I put everything together all snug and when I went to fire up the motor and right as the motor would start to get going it would die and I'd loose grab of my one way bearing that turns the piston/crank. So I pull it all apart to find this pin fell out. Replace it and the same thing happens again and again and again, altogether I did this 11x with the last time ending with that pin tearing up my piston. Metal filings throughout the motor. I should mention I was using a drill as a crank for my roto start. Which I think I caused some damage by cranking to fast and long but it seemed the pin only fell out when the engine would just get going....... If anyone could help me out here I'd be so grateful!


I can post pics of this new crank compared to old crank if that would be helpful.

earlwb

The little tiny pin and spring is for the pull starter on the back of the engine. if you do not have the pull starter, you leave that little spring and pin off and do not put it inside the engine.
The little spring loaded pin was used to engage the rear mounted pull starter unit.

earlwb

Oh yeah. it is possible the pull starter unit has a problem and the tapered slot is buggered up or something too. That would allow the pin to come out. That spring loaded pin serves as a one way rotation device. It engages so the pull starter can be used to start the engine, but after the engine starts, the engine runs fast enough so that the pin cannot engage the rear slot to hook the pull starter.

MonsterAir

Thanks for the reply earl, it's much appreciated! Leaves me with a few more questions though. Is that back plate for a GST? Because mine doesn't look like that (pic below). Also if I leave the pin out, the back plate has no way to grab the crank (pic below)..... I'm so lost as what to do next. As of right now I'm out $100 in parts and I'm afraid to get more and have the same thing happen and be out another $100 when I could have just bought a new engine for $200........ On another topic, I'm using the original carb, If I had an air leak somewhere (I took it apart sprayed and cleaned with contact cleaner) would/could that keep the motor from starting?



For some reason I can't upload my pics. I hope my description is clear enough.

earlwb

I would not be surprised if the engine manufacturer has changed things. They do it all the time, especially the Chinese ones. Unfortunately, it sounds like you somehow wound up with the wrong parts for your particular engine. They do change things quite often, even more than once in a year. The engines have evolved over time.

The engine is made in basically two major types, one is with a pull starter (like using a weed trimmer or lawn mower starter). The other engine does not have the pull starter. They use some sort of a electric starter box or device to start the engine. There is some drag inside with the pull starter, so one can get more power and speed by not having a pull starter on the engine.

MonsterAir

Earl your completely right! They changed the starter shaft! I didn't realize until last night. It's really messed up because the part numbers are all the same ya know. I'm trying to reach out to cen maybe they can do something for me. I never thought they would change parts around in a engine like this. I should have noticed somthing was funky when the crank was different and the crank case is a much softer metal. Ya live and learn I guess. Thank you for your help.

One last question. I'm gonna go with a different engine, thinking about the LRP ZX .32 spec 2. Do you think this motor is powerful enough for the GST? Really just going to be my basher.

1QwkSport2.5r

Unless it's a 20lb truck, a .32 will move a 12-15lb truck around pretty good. The .32s are a little stronger than decent .28s and my 13lb LST2's wheelie and scoot around on .26 and .28 power easily. A .32 will have a bit more torque at a little lower rpm than typical .26-.28 engines.

MonsterAir

I just weighed it with my boga scale. 18# with half a tank of fuel.

Mr Cox

What was wrong with the stock crank?
A little rust on the surface doesn't really do any harm, and it is easy to remove anyway. Just change the ballbearings as they might have rust too.

MonsterAir

Well, it sat for 4-5 yrs so when I pulled it apart the bearings were shot crank was rusty so I figured it was shot being it gave me problems last time I ran it. Come to find out this motor don't hold a tune well.