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Discussion Starter #1
Hi All,

I bought a CX650C from JB in SC back in late March and rode it a good thousand miles up to Boston over a weekend.

It's been running pretty great until it blew both head gaskets back in June, which were an easy fix. The radiator temperature sensor had stopped working, so I replaced it with a switch. Unfortunately, I couldn't track down any OEM gaskets so I used aftermarkets (The Disaster Motors set). Things were still going pretty well, but I could hear that the starter clutch was starting to fail and it finally went in August.

After getting the motor out of the bike and opening it up, the water pump seal went and it was pretty obvious the timing chain needed replacement so I ordered replacements and started on a "double bypass" (the stator checked out on a quick resistance test and looked to be in good order). I threw the new parts in, replaced the starter clutch springs (which had more or less dissolved) and got the bike back together and running again. Halfway through a post-build carb sync, the engine sputtered and died. A quick check showed that one of the head gaskets had gone, so I replaced that the next weekend. Still, the bike refused to start. A spark test showed a weak intermittent spark on both sides, something I'm pretty confident is me failing to properly torque the mechanical ATU upon putting the engine back together.

When I was diagnosing the spark, I did a quick compression test on both cylinders. Both read 235psi and 240psi, numbers I checked on two separate gauges, twice. That high compression combined with an aftermarket head gasket indicates to me why the new gasket failed less than a thousand miles after swapping it, but I have no idea what could cause compression that high. Could it be that the aftermarket gasket is much narrower than the OEM one, leading to increased compression of 70psi?

To sum it all up, I have a few questions

1. What could be causing that high of compression?

2. Are there any good sources for head gaskets, aftermarket or otherwise? I'm very unimpressed with the quality of the disaster motors ones.

3. Have I correctly diagnosed the weak/intermittent spark on both cylinders as the ATU?

4. Are there any good sources for a stator re-winder? Custom Rewind has been recommended, but I'm having difficulty reaching him on the phone.

Thanks!
Fred
 

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Your exhaust valves aren't opening.....
 

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First rule: never buy parts from anyone who's name has "disaster" in it. The factory 650 gaskets are available if you search hard enough. I can't imagine the gaskets being your issue. You can send one to me or Murray, or a number of other members to compare with a stock gasket, but I doubt that is the issue. A used factory head gasket should measure around .045" or 1.35 mm on the metal ring that sits between the cylinder sleeve and the head. Having done a bit of machine work on the heads, I doubt that even if you ran the motor without gaskets that it would have too much compression to damage the engine. One of our member runs 14:1 plus compression and his motor holds together (sort of) enough to run 160 mph. Many of us have taken 30/1000's off of the heads without any problems. I am wondering if you possibly have the cam off one tooth to affect this. I have never tried this, but I am trying to think of what you have done, or what could have caused this. How did you check the compression, with the gauge screwed into the plug hole and the throttle wide open?
 
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Discussion Starter #9
Hi all,

Willys, I did think that it might be a valve issue but I tested clearance and it checked out fine. I'm inclined to not believe that it's that or timing as the motor did start and run for a good half hour after finishing the bypass

Murray, I'm in Dorchester (Massachusetts) which I swear is a nicer place than what most people hear about it.

edinlr, I checked with the gauge screwed into the spark plug holes, throttle wide open.

CXPhreak, I'm definitely going to check the timing when I get the engine back out. Unfortunately, I'll be traveling for work starting next week and Boston winter is starting to roll around.

Next engine out I'll be sure to give the head bolts a nice wipe-down, I didn't do that last time. Head gasket I used measures 1.00 mm thick at the metal ring. I'm going to re-re-check these measurements Friday morning to make absolutely sure that they're sane.
 

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Make sure you blow all the bolt holes out
Permatex copper head gasket spray on both sides of the gasket

Make sure you get the oring around the oil orifice
Follow the static timing procedure on the gsm to set timing
Check the locating pin on the atu is not burred and the notch on the rotor is also clean so it fits down properly

I would do the resistance test on the pulse generator coils before re installing them it only takes a minute
 

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"Next engine out I'll be sure to give the head bolts a nice wipe-down, I didn't do that last time. Head gasket I used measures 1.00 mm thick at the metal ring. I'm going to re-re-check these measurements Friday morning to make absolutely sure that they're sane."

Use the ARP bolt (available at any speed shop in very small tubes) lube on clean head bolts and torque to factory specs. Your gasket appears to be thinner than factory, but that would be very little extra compression and certainly not 200+ psi. Does the head appear to be recently machined? I am wondering if a previous owner may have machined the heads to take out a warped surface. If they took too much out, that could have lead to the high compression. There is no simple solution to check this. The best way would be to pull the heads off and cc them to see if they match factory specs. You might be to measure the surface height against some casting point for reference, but cc'ing would be the most accurate. When I had my turbo heads ported we did the opposite and took a lot of material out of the combustion chamber and had to mill the head to reduce the combustion chamber measurements. What I would look for is if the stock heads had a 70cc spec and yours measured 50 cc, (I just threw these #'s out, have no idea of the stock specs) then we would have an idea of the problem and you could open up some material in the combustion chamber. I really doubt this is the case, but you can't rule out any possibility on a 30 year old motor.

I am wondering about your comment that both head gaskets went at the same time, this is very strange unless there was a big mismatch in the engine.
 
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