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Discussion Starter · #1 · (Edited)
Backstory:
I got my first CX500 a couple weeks ago. 15k miles. Previous owner hadn’t ridden it for a year due to electrical problems. I found and fixed the broken wire (main 20A fuse wire was chafed). Motor idled really well after a car rebuild.
Took it out for a ride, motor seized at 0.9 miles. Luckily I didn’t crash. Only left a nice 10 foot skid mark. No idea what caused it to seize. Took it apart, mechanically looks fine, but looked like it jumped timing.

I found a motor locally that was included with a running CX500. The motor had been sitting for years, but was claimed to have been a runner beforehand. It was seized from sitting.

I ohmed out the stator coils, they all checked good. Poured some MMO into the spark plug holes, let it sit for a day. It broke free easily. I verified that valve timing looked good and installed it into the bike.

Considering that the bike ran before all this, I assume that the CDI unit & ignition coils are good. When I crank the engine it has spark.

I can’t get it to start however. It will very rarely make any sign of combustion. Sometimes I’ll get a blue flame out the exhaust port rarely (no exhaust installed at the moment).

I verified ignition timing with a timing light. I swapped over the CDI from my running bike. I tried starting it with the yellow alternator wires disconnected as well.

I’m at a loss here. All I can think of is that either the engine has 0 compression (my compression tester won’t fit, I’ve ordered one), or that the spark from the CDI has suddenly become too weak.

The speed at which it cranks is similar to that of my running bike. It definitely feels like there’s compression via the thumb test.


To sum it up, previously time-seized engine has spark, starting fluid/good carbs but won’t run.
 

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Should be adequate to start.

Have you tried spraying a little fuel in through the balance ports to see if it'll fire?
 

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Discussion Starter · #6 ·
I got a compression tester on the motor. 75 PSI on each cylinder. I know this is less than half of the spec of 171. I suspect that it’s so low since the engine is at room temperature and hasn’t run yet.
 

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On the plus side both are equal so I don’t think you have an engine problem. Maybe a gauge problem. Both cylinders being equal is more important than the actual number.
 

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What about the vale clearances? Are they set correctly? If too tight, hard starting. If you are sure they are set correctly, and a compression test with the throttles wide open doesn't make close to the spec compression, well, then.........wrench time.
 
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McReviver may have it.

The engine will struggle to start at less than 100 PSI compression and valve clearances are a possible reason for the unusually low compression.
 

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Discussion Starter · #12 ·
McReviver may have it.

The engine will struggle to start at less than 100 PSI compression and valve clearances are a possible reason for the unusually low compression.
I’ll definitely try resetting the valve clearance today and report back.

I’m just curious what the thinking behind this is? The engine sat, so the valve clearance grew too close? That’s why the engine will randomly become very hard to turn over?

All I can think of is that the engine sat, the contact points corroded, the corrosion broke off, and now the valve clearance is too large. This might cause a delay in valve timing which would cause a loss of compression - since the valves are opening and closing later.
 

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Perhaps just the assumption that the valve clearances had been readjusted. If not it's likely not the issue. But that compression is very low. Likely too low to sustain starting.

Have you checked that the spark plugs themselves are actually sparking?
 

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Discussion Starter · #14 ·
Perhaps just the assumption that the valve clearances had been readjusted. If not it's likely not the issue. But that compression is very low. Likely too low to sustain starting.

Have you checked that the spark plugs themselves are actually sparking?
Yes, I checked that both spark plugs could arc a 10mm gap to the valve cover bolts. I also put a timing light on both spark plug wires to make sure that each cylinder was getting spark at the right time.

Did spark plug wires change between the years? I’m not sure what year this motor came out of, but the frame is from a 78. I’m wondering if maybe the spark plug boots aren’t seating on the spark plugs, if there were any significant changes from 78 on.
 

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It is logical that at 75 compression an engine that hasn’t run in years won’t start. The question then is why low compression? Engine was stuck, which means the RINGS were stuck. Although the pistons now move still leaves the likelihood that the rings are not free in their grooves yet. Yes, check tappet gaps, but the tappet corrosion idea is extremely unlikely. I think more soaking of the rings with periodic spins of the engine is the best approach. When it eventually starts, running it is the well established path to continued improvement and success. In the meantime, Do No Harm. IMHO
 
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Discussion Starter · #19 ·
It is logical that at 75 compression an engine that hasn’t run in years won’t start. The question then is why low compression? Engine was stuck, which means the RINGS were stuck. Although the pistons now move still leaves the likelihood that the rings are not free in their grooves yet. Yes, check tappet gaps, but the tappet corrosion idea is extremely unlikely. I think more soaking of the rings with periodic spins of the engine is the best approach. When it eventually starts, running it is the well established path to continued improvement and success. In the meantime, Do No Harm. IMHO
I’ve been leaving MMO in the cylinders every night to help free things up.
I had an 82 KZ305CSR that also had extremely low compression after sitting for 7 years.
Soaked the cylinders in MMO for a few days, and once it started once, it started reliably thereafter.
 
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