Ct90 good spark won’t start

dostrem

New Member
I have a 1968 ct90 that looks great, mechanically sounds but after new piston and rings I can’t get it to start. Chain timing correct. (Not 180 off), spark advance assembled and installed correctly(not 180 off), compression stroke identified by exhaust valve position, points timing set statically. Points gap set, spark plug gap set, great spark, but absolutely no pop! Compression 100, tried starter fluid, no pop. Replaced spark plug/ no pop. Tried wiggling spark plug to see if positional- still good spark. Air filter connected but not exhaust. Argh! Next option new engine. Any ideas??
 

dirtbkr188

Active Member
With only 100 PSI compression, you might want to make sure the valves are adjusted to spec, and then consider that it might be a fuel issue.
 

dostrem

New Member
I know what you mean - I’ve checked and rechecked. Even tho I have spark outside the combustion chamber, I wonder if there’s really spark inside? Maybe a funny position related short in the plug cable? Unless someone comes up with a really good idea, I’m probably just going to switch motors. I’ll still have to check out the plug cable or use a different coil
Thanks!
 

kungfutoast

New Member
I have a 1968 ct90 that looks great, mechanically sounds but after new piston and rings I can’t get it to start. Chain timing correct. (Not 180 off), spark advance assembled and installed correctly(not 180 off), compression stroke identified by exhaust valve position, points timing set statically. Points gap set, spark plug gap set, great spark, but absolutely no pop! Compression 100, tried starter fluid, no pop. Replaced spark plug/ no pop. Tried wiggling spark plug to see if positional- still good spark. Air filter connected but not exhaust. Argh! Next option new engine. Any ideas??

You do not mention the carborater in your check off list. I would look into that. Pull your plug and check for fuel. Might be flooded or oil fouled or non of the above? Your float level might be too high restricting proper flow. A jet could also be blocked, not set correctly or not the correct size.

Best of luck
 

dostrem

New Member
You do not mention the carborater in your check off list. I would look into that. Pull your plug and check for fuel. Might be flooded or oil fouled or non of the above? Your float level might be too high restricting proper flow. A jet could also be blocked, not set correctly or not the correct size.

Best of luck
 

dostrem

New Member
I think I have the answer- turns out I was using the wrong spark plug- a D8EA rather than the correct D8HA. The longer plug was being crimped by the intake valve rendering it useless. The contact was also interfering with the function of the intake valve itself. This was a week of work to figure out a simple but highly significant, some might say stupid error! I don’t even know why I had this H around! Thx for all the suggestions
 

-Nate

Active Member
So it now starts & runs well ? .

Don't beat yourself up, you learned and that's priceless .

I always have a bunch of new spark plugs that fouled, once I get the engine running the plan is to install them and ride them clean again, I rarely take the time and so buy another plug...
 

dostrem

New Member
Lots of ideas- thought I had it solved by changing to correct spark plug. No joy. Took the head off, removed new valves put old ones back in, thinking that they might match better. Did some valve grinding, put it all back together valves looking like should seal fine. No start(still good spark). Compression test repeat: 75. Argh. Leak down test at 70 psi- air roars out of carb and leaks out around the rocker arm cap. Loosened the valves way up, still major leaks. Nothing out of the exhaust and can’t really tell on the main body. Conclusion: 1) intake valve not sealing at all and 2) bad top gasket. To be continued…
 

dirtbkr188

Active Member
Stating above that you installed a new piston and rings, the compression range with those new parts should be 160 to 185 PSI, optimally. If you installed a new piston and rings in a worn cylinder, that could possibly explain why you only have 75PSI.
These little engines will run with 100PSI...just barely, usually after having to push-start it in gear to warm things up.
 

-Nate

Active Member
Each time you surface the valves they should be checked by holding the cylinder head with the port facing up as you dribble some gasoline in the port whilst watching the valve in the combustion chamber ~ if you see a droplet of fuel right away the valve face isn't matching the seat as it should .
 

dostrem

New Member
Each time you surface the valves they should be checked by holding the cylinder head with the port facing up as you dribble some gasoline in the port whilst watching the valve in the combustion chamber ~ if you see a droplet of fuel right away the valve face isn't matching the seat as it should .
Fascinating! I tried the dribble test and it’s clear the intake valve is leaking!! I’ll relap it and try again!
 

-Nate

Active Member
Use the finer lapping compound and when you lift the valve off the seat look for a thin equal line 360 degrees around the valve seat .

Don't just spin the valve, twist it back and forth, moving a bit forward every few twists .

Don't use water to test, it has too much surface tension .

A little dampness 'round the valve is okay but any droplets is bad, needs more lapping .
 
Top