Low compression.

[toglhot]

Since the two blokes next door test rode my TX I haven't been able to start it again. Carbs, timing points, valve lash, checked everything over and over - no go. Just on the off chance, this afternoon I checked the compression, damn 100 psi. This is on a rebuilt engine- rebore, new pistons, rings, valves regroumd,, New head gasket, now itt's got me scratching my head.
Tomorrow I'll check between the fins for a head gasket leak, retorque the head bolts and squirt some oil down the plug holes. If that all checks out, I'll have to pull the motor.

 

[atom4488]

Since the two blokes next door test rode my TX I haven't been able to start it again. Carbs, timing points, valve lash, checked everything over and over - no go. Just on the off chance, this afternoon I checked the compression, damn 100 psi. This is on a rebuilt engine- rebore, new pistons, rings, valves regroumd,, New head gasket, now itt's got me scratching my head.
Tomorrow I'll check between the fins for a head gasket leak, retorque the head bolts and squirt some oil down the plug holes. If that all checks out, I'll have to pull the motor.

The compression is obviously a concern, but I'd say that an engine with 100 psi compression will run, ie, its not the low compression that is preventing it from starting. The original engine in my XS 650D had 110 psi in one cylinder and less than 80 psi in the other. It was still a runner when pulled.

Plugs have fire(at the right time) and are getting fuel? Should at least fire...

 

[Jim]

Agree. Even at 100psi it'll run. And if the bikes sat for an extended period, a squirt of oil will prolly jump it up 20-30psi.

 

[AUTOMAN]

Usually the wrong compression gage with the check valve near the gage itself.
This will add 8cc or so volume and resultant low compression.

should be a sticky

 

[toglhot]

Yep, but I've got to look at the compression. Should be way more. Something wrong in there that needs looking at.
It pops from both carb and pipes, no leaks in manifolds or carbs, timing is correct, checked absolutely everything.
Funny thing, it went beautifully for both Adam and Stuart, started easily, ran sweet as a lolly, no popping or spitting. So, shut it down, wheeled it around to the wokshop pushed the start button next day, nope, hasn't gone since.
The more I think about it, the more I'm leaning towards the head gasket. I'll explore this afternoon.

 

[atom4488]

Yep, but I've got to look at the compression. Should be way more. Something wrong in there that needs looking at.
It pops from both carb and pipes, no leaks in manifolds or carbs, timing is correct, checked absolutely everything.
Funny thing, it went beautifully for both Adam and Stuart, started easily, ran sweet as a lolly, no popping or spitting. So, shut it down, wheeled it around to the wokshop pushed the start button next day, nope, hasn't gone since.
The more I think about it, the more I'm leaning towards the head gasket. I'll explore this afternoon.

Be very interested in knowing the outcome of your troubleshooting... vicarious learning!

 

[bosco659]

Odd it ran well just before you shut it down and now it won’t go. I’m suspecting and hoping for you that it’s something simple. I’ll be watching this thread with interest.

 

[AUTOMAN]

Sounds like timing chain skipped a tooth.

 

[toglhot]

Sounds like timing chain skipped a tooth.

That's what I thought at first, but I put a degree wheel on and checked the valve timing, no fault there. Inlet opens 90+- degrees BTDC, shuts at 120+- degrees ATDC according to smedspeed. That's at .02 valve lash. Mine is set wider than that, so that would bring the duration down a few degrees.
.

 

[Jan_P]

Yeaa ..hh well we had a spirited discussion regarding kick starter .. So I assume this is a e --start bike
and here is no mention of battery status or age or charging
I would check that ...measure voltage across battery at the usual situations most interesting ignition on and cranking if it drops
It wont start.

What could happen out in the desert also. Should one ride there. .Not saying it is the case here but mentioning as
phenomenon

 

[jpdevol]

That's what I thought at first, but I put a degree wheel on and checked the valve timing, no fault there. Inlet opens 90+- degrees BTDC, shuts at 120+- degrees ATDC according to smedspeed. That's at .02 valve lash. Mine is set wider than that, so that would bring the duration down a few degrees.
.

Valve timing - on a stock motor - Inlet should open @ 36 degrees BTDC, close 68 ABDC.......

Quick check - for stock cam - is advance locating dowel (hole) should be 12 O'clock @ TDC

 

[gggGary]

Easy enough to set at TDC put compressed air line in the plug hole, slowly add air pressure see (if) where it leaks out.

 

[AUTOMAN]

Intake opens WHEN?
Not an adiabatic miller cycle engine.

cliff

 

[Jim]

Intake opens WHEN?
Not an adiabatic miller cycle engine.

cliff

Well sumbich... JP's numbers agree with the factory manual.

 

[toglhot]

I got the specs from smedspeed.
http://www.smedspeed.co.uk/tech.html

 

[jpdevol]

I understand the Smedspeed article you reference to be a discussion of valve lift at various degrees of crank rotation rather than valve timing specs. No matter as - on a stock motor - determining cam timing can be done (without disassembly and various dial guages etc) by putting the motor at TDC and Noting the location of the advance unit's locating dowel. If the cam timing has changed - and not correct - it would effect the compression reading and also cause your no-start condition.

 

[toglhot]

That's my next step. I think the chain skipping over a cog is a long shot, a very long shot: New chain, chain tensioned correctly, new guides. Although, when I brought the bike home the cam chain was so stretched the PO had filed the adjusting slots on the points backing plate out another few mm, but still it couldn't be timed, so that will have worn the crankshaft cog. Should have rebuilt the crank, but I didn't, silly me!

Unfortunately, I don't get to spend much time in my workshop now, the RA has fused all of my cervical spin, as a result I've developed foraminal stenosis, which means I can't bend my neck without causing a lot of pain, so my days in the workshop, for now, are very limited.

 

[mrtwowheel]

Sorry, but, no one asked. Wide open throttle when you're checking compression, right?

 

[toglhot]

I didn't know about that hole on the advance unit, but that appears to be the problem. The hole is at 3:00 o'clock at TDC, so it appears the cam chain cog on the crank is very worn.
I'd be surprised if the valves or pistons aren't damaged.
Now to get the engine out without scratching the crap out of the frame.
I made a start on an engine removal tool, just have to buy some RHS to finish it off.

 

[toglhot]

Well, Doesn't matter anyway, crank sprockets are not available, so I'll probably strip,the bike and sell whatever parts I can and dump the rest. No market for XS650s here. That was an expensive trip to nowhere!