125-200cc Can't get my 165 running right after meltdown

I melted the top end on this bike last Sept and have been chasing my tail ever since. After meltdown, I had the cylinder reconditioned, new top end, new mag side crank seal (original one blew out when head oring failed), new-to-me 165 head (1.1mm-new vs 1.3mm-old squish) and now I cannot get rid of heat/detonation problem after riding it for a while. Especially at constant throttle cruising speeds (like road transfers). The motor seems fine when doing more corner to corner blasting with short bursts of full throttle, but anytime the throttle is constant at more than half throttle it'll start rattling bad.

When I replaced the crank seal I noticed my case does not have a timing mark, so I had to use pictures to set the timing. I believe I had set spot on, but was experiencing detonation which I could not get rid of with fatter jetting. The fatter jetting would temporarily help and then back to running like garbage again after a short while.

Here's what I've tried to remedy it:
-richen jetting
-change Lectron rods between 3-1xl and Ms3
-gaskets on both sides of reed cage
-adjust timing plate full-CCW
-sprayed for leaks and found none
-checked impeller and it looked fine minus what looked like silver RTV caught on the leading edge of each blade, which cleaned up easily.

Either this new 165 head is setup too hot and I need race gas, my timing is messed up or the dry side crank seal I used (a double lipped seal that matched numbers, but wasn't OEM on my 2014 cr125) isn't sealing properly once the motor gets hot. I'm pretty sure the double seal isn't the issue, so its either not enough octane or my timing is off.

The head I got was from Canada. Do they have better fuel and are therefor setup a little hotter?
How do I get my timing set without marks or the tool?

One last thing....when I melted it last year it got hot enough to crack the water passages in the head. Could that much heat in the motor cause other issues (such as electrical issues)?

Any help is appreciated!

Here my two cents:

I chased a hidden air leak on 2006 Wr 125. It was gasket between center cases. So, do a leak down test first.
It was higher up on cases and wasn't drawing the gear oil.

Second: My 09 blew a base gasket and got real hot.

Third: If all else passes. See what your squish is? You can always take an old base gasket. Use 3M spray adhesive. Glue it to a piece of xyz thickness metal, to cut out a spacer plate, and use two base gaskets on top and bottom of new spacer, to get a xyz overall thickness, to drop compression. Cheep test. I've not had good luck, stacking 2-3 regular, various thickness gaskets, without a blow out.

Fourth: had a crank bearing going bad, on ignition side, and it caused just enough crank movement on the seal, to make it leak when running.

Mix tank of race fuel using vp110 or sunoco 110 and see if things get better. You can also rotate the stator with the direction of the crankshaft rotation which will retard the timing. The bolts that hold down the stator are in slots and allow for a few degrees of timing change, a very slight amount makes a difference so I would not go to far at a time and begin with the stator in the same position as it was before by observing the marks the bolts made.

My 165 would diesel on pump 93. Just a little too much compression/tight squish (didn't check to see what it was though).

It was recommended to do a compression test and see how much psi it would make. I had read here that about 200 psi was the cut off point before needing race gas. Mine made ~220 and I had to run 98 to make it happy.

Another is the petcock screen. It's plastic! Clogged petcock = insufficient fuel supply = fuel starvation.

Hey PC,

If it is an octane issue. Send me your head and I will reduce your compression as much as possible with this head.

Thanks all!
A compression test is a great idea and would answer a lot of questions. I was under the impression that this new head was less compression than the old one, but maybe I'm wrong. It does make a lot of power.

Retard timing same direction as crank rotation.....good info, thanks!

I'll report back after I acquire a new compression reader and maybe try a little 110 octane in the mix.

Hmm, very interesting. I was having some strange stalling issues prior to my 165 kit being installed, seemed to be a fuel starvation issue, I had thought it was a plugged gas tank vent issue. Bike would stall, I'd try to start it, no go. Kick, kick, kick, no go. Would pop open the fuel cap, fiddle around a bit, put cap back on, bike would start right up.

I'll have to check my petcock, thanks for the tip! -Eddie

I agree with this, if the compression is at or near 200psi you need race gas or seriously de-tuned for 93 pump gas. By de-tuned I mean jetted rich and maybe even retard the timing a bit but be warned if you do this you won't get near the performance and you'll be spooging up your silencer packing and fouling plugs more often.

That sounds more like the fuel tank vent check valve is stuck and not venting.

I agree, but every time I have the issue, the vent seems to be venting fine. No restriction at all if I blow through the line. I've even had the cap off and examined it, same deal, it seems fine. And checking the petcock won't cost me anything but some time so I'm going to check it out. If it checks out fine, no loss, just another thing I can cross off the list then.

I stopped and got a $30 tester from Sears last night and she blew 225-230# cold. At least I'm not chasing my tail anymore!
Gonna try some race fuel this weekend and see how she goes.

That bike should be a ripper when you give it the race fuel it needs. VP110 or Sunoco110 should work. AV100 will not be good enough for that kind of compression. One of my friends had his 2001 Honda CR250 modded for race fuel and his compression is the same as yours and his bike will detonate on AV100. Someone on this forum or maybe another shared a write up that explains how AV100 is actually only about 96 octane when rated the same way as automotive fuel.... Just thought I'd point that out before you ran into another problem.