Hammerhead 150, I swapped top end because I was having an issue with low compression and still having the same issue??

Have good spark
Good fuel
Compression is 90 after lapping new valves
New gaskets and copper spray
Verified timing and valve lash .004
New rings clocked at 120 degrees
Ring end gaps were .008
Head torqued to 13.5 ft pounds

What else could be causing low compression?
The original issue before new top end was it wouldn't idle or run at low rpm. (Low compression and low vacuum)
Any ideas welcome.

Are you measuring compression with the throttle wide open?

If you add a few drops of oil to the cylinder does the compression change?

I didn't have the carb on to check but, I did add some oil and it didn't help at all.
I'm going to run a smoke test and see if I can spot where I'm loosing compression?
I would think a new jug and piston would show at least 110psi until after break in then it would go up.

Did you clock the piston rings so the gaps do not overlap? A quick test for bottom end leakage is to hold your finger over the valve cover vent while cranking and feeling for pressure.

Yes clocked rings at 120 degrees offset. The cylinder looked to be lightly honed with decent crosshatch so I didn't bother doing it again.
Valve cover vent seems to be a check valve, only intake.
Pressurized cylinder with smoke and valves look to seal good.

Correction on the cover vent, I am getting a little pulse in and out while cranking

Added a little oil and got to 120psi.
Would rehoneing the cylinder help or should I get another new set of rings?

If the rings and cylinder are new, it is possible that surfaces are not mated well. The pulse you feel on the vent means there is some blow by past the rings. Perhaps the engine does need to be run in before you will see anything higher. Funnyman, Jersey Devil or Sycarms on this forum have done more top end rebuilds than me and may have additional insight on what to check. Maybe send them a private message to get their attention.

Update, the re-hone helped and I got it running. While waiting for it to warm up and set idle mix, the timing chain jumped off!
Pulled head and everything looks good, no strike on piston and valves not bent 👍
Chain tensioner seems to be functioning correctly when I installed.
What does a bad top guide look like?
If my chain is stretched, how do I know how many links to order?

Update, cam chain still has slack on cam with tensioner fully extended. Best source for parts and what is the easiest repair option?
I counted 45 links on chain.

Make sure the chain is actually on the crank sprocket and not sitting next to it. The ca. Tensioner is just a bolt, spring and plastic guide. The guide is hinged on one end and tensioned on the other. These items should be easy to see when the top end is disassembled.

On this forum is a list of trusted dealers that know the parts they sell and will support you after sale. Look for the trusted dealers in the dealer review section.

The chain I have is 45 links like you said, mean you have the right chain. The chain should not be loose. Check the tensioner, install everything, release the tensioner to extend and after that, you can put a piece of metal to lock it in place. Remove the tensioner, from the cylinder and remove the piece of metal to keep it in place. You will see if the tensioner still extend or not.

Here's a picture of what I mean.

As you can see an new one come with the little piece of metal to hold the tension when you install it. Soon you release the metal piece, it will extend and look like the picture under.


If the tensioner extend to the end, PGH might be right on the chain position or the links extended over the year and you need to change it.

I do have all parts to fix it but before, make sure you check everything and only buy the parts you need otherwise you will spend money on parts that are good.

If you need to confirm parts, post pictures of them and I can check with the one I have on my shelve.

Here are some pics,
Chain on crank gear
Tensioner fully extended only comes up 3/32 when loosened
Chain on can with everything on adjusted and tight.

Looks like a dozer track when the pins are stretched, not meshing with the gear.

If this is stock the chain stretched.

Just my experience with top ends!
I would get new guides….cam chain….cam tensioner.
Please don’t use a harbor freight tester. I recently did my hh250 and I was reading 125 psi with the Harbor freight tester and 210 psi with a tester rented from auto-zone.
If your valves are sealing, i would recheck it with the new parts mentioned above, it shouldn’t be a problem after that.

Update, finally got around to installing new cam chain and guides. Chain was stretched almost a whole link. It fired right up and I got the new top broke in. Still a bit of a dog but I have to keep reminding myself it's only a 150cc not, the 700+ Rotax's I normally work on.

There are ways to make the 150s pretty peppy. 😁