351 XE on LPG
Join Date: Jul 2003
Location: Perth, Western Australia
I come to my Ford brothers for help . . .
The 351 in my XE came with a set of bolt-on YT non adjustable roller rockers. For some reason the previous owner had machined quite a bit of metal off each rocker foot, (between 1 and 2 mm), but I wasn’t aware of this when I performed my rebuild, and torqued each one down to the required 24 ft/lbs thinking that because I was using a hydraulic cam that no adjustment was necessary. Since then I’ve learnt from this forum about lifter preload and methods for measuring it.
Two years after the build I start hearing a knocking sound from the top-end so I perform my first lifter preload check. This is when I discover the shaved feet. BUGGER. Each lifter had WAY too much preload. I ordered a set of new feet from YT and they were a lot higher then the ones I had installed. I also ordered a set of CRANE shims of various thickness, so there was no excuse for me not getting a perfect preload. I also bought a set of new lifters.
I discovered a valve with a damaged tip. Instead of being flat it was concave, as if the roller had hammered a channel into it. I checked all the rollers and they were fine. I know that valves are supposed to rotate, but obviously this one wasn’t. I rotated it manually with a set of vise-grips, to try and loosen it up. I then covered the head with an old towel and made a little hole in it for the valve tip to stick out of, then used a miniature grinder and carefully removed the groove by grinding the tip flat. The towel caught the metal filings. Though a very rough approach - I figured it was worth a try, and I could always shim it to the correct preload.
Once assembled and back on the road it was quiet and ran stronger then it’s ever been since I built it. I was so happy. Having the correct preload really made a noticeable difference. As it turned out, I didn’t need to use the shims (except for the damaged tip), because with the new feet they all came within spec (between 20 and 60 thou).
But wait . . .
My happiness lasted less then a week, when suddenly while on the freeway the motor started making a loud knocking sound, and started missing (along with the occasional LPG carby backfire). BUGGER. No matter what I do I cannot fix this problem. I checked the roller rockers and preload over and over and couldn’t find the cause. I finally thought that maybe the damaged valve was getting stuck open and being tapped home by the piston, so I shoved the XE under a tarp for the last 9 months while I set about building my new garage.
Since then I’ve had the heads redone. The excess preload had taken it’s toll. Some valves needed replacing and all the inlet guides needed new K-lines. The Stellites on the exhaust needed a slight grind to make them flat again. One of the heads required a slight shave as well. Some of the valves were quite loose in their guides and all the stem seals had gone hard.
Then while reading Ford Forums I discovered that I’d stuffed up again and should have run the new lifters in in the same fashion when I ran the new cam and lifters in over 2 years ago when I built the motor. It never crossed my mind. BUGGER. Maybe I’ve damaged a lobe on the cam. How can I tell?
During assembly I noticed that a couple of the lifters didn’t have their pistons hard up against their circlips. A little tap set them right. I used the same new lifters and was going to try and bed them in once the motor started, but the damn same knocking noise came back. I’ll be buggered. The erratic missing and occasional carby backfire is all still there. Changing the timing had no effect.
Any ideas PLEASE?