Hey y’all,

I have 200 miles on a 351 bb build and it decided to detonate on me.

The spark plug is shredded, the piston is peppered with debri, and the head and valves are trashed.

I am leaving early august to finish the tat and need a working bike before then, if anyone can help me find a new head it would be greatly appreciated.

Thanks!

 

You could do some serious research on Loncin 300 head kit that is essentially a clone of the KLX. Question if anyone here has tried one. Also hit the ADVrider forum and put a thread in the Thumpers sub forum asking if anyone has tried one. It may be the ticket for half the price of OEM. Click the link below.

Loncin 300 head

Been a lot of speculation that the new engine is very similar to the Loncin. Can't hurt to do some digging.

 

The big question, what caused the engine to detonate?

 

As far as I can tell, the chain must have skipped a link or otherwise lost timing suddenly causing the piston to collide with primarily one of the intake valves cracking it from the stem and throwing debris around causing large pitting on the surface of the piston and the head surface, completely shearing off the spark plug.

I was riding at about 50-60mph when it happened, after trailering it home I opened up the engine. There was plenty of oil and coolant, after removing the valve cover I inspected the cams they were fine and there was plenty of oil up there so it wasn't burning or not delivering oil.

The engine hand cranked fine but couldn't reach top dead center, after removing the head I realized the reason why, the valve was protruding into the combustion chamber making the piston unable to complete its cycle.

I was about to fill up my tank for the second time on this new 351 bb kit, it rode completely fine for nearly 200 miles. I checked my timing marks meticulously before reassembling it so I doubt timing was off, I think it suddenly went bad.

I am running the cv carb: 40 pilot, N1TC needle 2nd notch from the top, 160 main jet, k&n filter, airbox lid removed, stock exhaust, stock cam chain tensioners

I am wondering what difference the krieger manual cam chain tensioner could have made, possibly saving this whole problem completely.

 

Sorry to hear about it. Are you able to salvage the BB cylinder?

 

The cylinder is actually fine thankfully, after talking with Bill we may be able to salvage the head and piston, if not I will need to find a replacement.

 

If your OEM tensioner was faulty and bad enough it could allow massive slack in the cam drive allowing the chain to skip at the crank. I will bet if you look at the tensioner the bottom of the plunger will be polished over 3/8-1/2 inch and there will be a pattern worn over four to six teeth on the toothed part of the plunger. That shows how far the tensioner is pushing in when the back side of the cam drive goes tight, like under any deceleration for sure, then ratcheting back out when the front of the drive pulls tight again.

It is the momentum of the cams aided by the valve springs that will cause them to overrun slightly when the crank is decelerating, pulling the back of the drive tight and pushing the plunger in. It is supposed to lock in, but doesn't. The spring is to push it forward, the ratchet is to lock it in place. The spring isn't all that stiff. If it was stiff enough to hold the plunger it would be putting too much load on the drive pulling the cams into contact with the head bearing surface, there's only a few thousandths of oil floating the cam in the head bearing surface. You don't want too much tension and the manual tensioner can easily be adjusted to have little to no slack with no tension. That's a good thing.

With a manual tensioner you fix the adjustment where it is at. If the chain wears or seats in it will start ticking, but the slack will only be a few thousandths, not enough to skip by any means. Plus I just had a previous customer contact me to get another tensioner for another Concours. He ran the first one over 20,000 miles without adjustment, sold the bike, and now has bought another. Got with me for another manual tensioner.

Manual tensioners require very little attention and the ticking at the tensioner tells you when it needs maybe 1/4 turn in. Very little movement, the problem is with the OEM tensioner design relative to the way the chain wears. I have around 5000-6000 miles without adjustment and still no ticking on the 250, went over 10,000 miles on the 650 without adjustment, Zephyr has gone 12,000.

 

That piston is a paper weight now.
The head is going to need seats, valves, and probably guides as well as some welding/grinding to repair the chamber.

 

I pretty certain that the manual tensioner would have prevented this. i had a similar problem. my bike was stock with only a slip on muffler, all of a sudden bad noises happened. took it apart and found a failed timing chain tensioner caused a whole lot of broken stuff. got all new parts and switched to Kriegers manual tensioner, it was golden. KLX678 is correct, thats probably what happened to you as well. sometimes we gotta learn the hard way

 

More critical question, how many miles on the cam chain?
If you find a head, I would probably get a new piston, that thing could be dressed up and run probably, but why?