Piston Contacting Exhaust Valve
Posted: Mon May 31, 2021 11:05 pm
Hi All,
I completely rebuilt my Mach 1 engine and started it up recently. It was a little hard starting, but ran reasonably well on the handful of times I started it in the driveway. Still working out some other bugs so wasn't able to take it for a spin just yet.
On the last time I ran it (to show it to a friend) it started immediately but was bogging and had no pep. After a few minutes of running it just quit rapidly - like something internal got jammed, or it backfired and quit. Immediately I tried kicking it over, and it did so nothing was locked up, however I couldn't feel the stiff compression at TDC like before. I did a compression test which got no reading at all. I could however still feel the puff of air from the piston out the tail pipe when kicking it over.
I put it in top gear and rotated the tire with the spark plug out to see if everything seemed good. On the top of the exhaust stroke I thought I could feel the piston hitting something, but I could still rotate through. At TDC I could easily spin both valve shims so I knew the rocker wasn't pressing on either valve. I left the engine at TDC and pulled the head, and I could see a piston mark on the valve where it was making contact and removed the carbon:
The measured gap on the exhaust valve shim was 0.35mm so that was in spec and not causing the interference.
At this point I'm thinking the problem is a bent valve leaking out the exhaust, but reading zero on the compression test seems like too much to leak out from a bent valve, no? I released the springs/catch/collets, and tried to slide the valve through the guide. The valve would move smoothly at first, but got stuck about halfway out. I had to twist and pull to get it out and the barbed catch at the top of the valve stem scraped the inner diameter of the guide taking bronze chips with it. I think this confirms the bent valve theory?
When I reassembled the engine during rebuild everything went exactly as the manual said, and it ran well the couple times before (5-10 minutes each time) so I don't think the bevel gears are off by a tooth or anything like that.
I don't mind replacing the valve and guide, but I don't want this to happen again.
Anyone have any ideas how this might have happened?
I completely rebuilt my Mach 1 engine and started it up recently. It was a little hard starting, but ran reasonably well on the handful of times I started it in the driveway. Still working out some other bugs so wasn't able to take it for a spin just yet.
On the last time I ran it (to show it to a friend) it started immediately but was bogging and had no pep. After a few minutes of running it just quit rapidly - like something internal got jammed, or it backfired and quit. Immediately I tried kicking it over, and it did so nothing was locked up, however I couldn't feel the stiff compression at TDC like before. I did a compression test which got no reading at all. I could however still feel the puff of air from the piston out the tail pipe when kicking it over.
I put it in top gear and rotated the tire with the spark plug out to see if everything seemed good. On the top of the exhaust stroke I thought I could feel the piston hitting something, but I could still rotate through. At TDC I could easily spin both valve shims so I knew the rocker wasn't pressing on either valve. I left the engine at TDC and pulled the head, and I could see a piston mark on the valve where it was making contact and removed the carbon:
The measured gap on the exhaust valve shim was 0.35mm so that was in spec and not causing the interference.
At this point I'm thinking the problem is a bent valve leaking out the exhaust, but reading zero on the compression test seems like too much to leak out from a bent valve, no? I released the springs/catch/collets, and tried to slide the valve through the guide. The valve would move smoothly at first, but got stuck about halfway out. I had to twist and pull to get it out and the barbed catch at the top of the valve stem scraped the inner diameter of the guide taking bronze chips with it. I think this confirms the bent valve theory?
When I reassembled the engine during rebuild everything went exactly as the manual said, and it ran well the couple times before (5-10 minutes each time) so I don't think the bevel gears are off by a tooth or anything like that.
I don't mind replacing the valve and guide, but I don't want this to happen again.
Anyone have any ideas how this might have happened?