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Discussion Starter #1
Hello, and HELP!!

I have an '85 Cagiva Alazzurra 650 that started running on one cylinder and forcing gas
back through the carb on the rear cylinder.

What I found so far is the rear cam belt has stripped all of it's teeth off.
Question is why?

I checked the tension on the idler & tension pulleys and they
seem to be fine. I turn the cam over slowly and the valves don't seem
stuck or bent, and look to be working normally. A mechanic friend said
if the valves seem to be fine, replace the belt and check the compression.
That scares me!

I was asked if the Pantah engine is an "interferrence or Non-interrence engine? I
don't know what the answer is to that, does anyone know?

Anyone have any suggestions on how to proceed?

I would like to not remove the head, but I realize I may have to, to be sure
what is wrong.

Thanks for any help you can provide.
 

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Some comments:

"I have an '85 Cagiva Alazzurra 650 that started running on one cylinder and forcing gas back through the carb on the rear cylinder."
I don't know how gas can be forced back through a carb. Your bike should have simple Dellorto PHF32 carbs.

"What I found so far is the rear cam belt has stripped all of it's teeth off. Question is why?"
Belt failure due to age is most likely. It is imperative that you get all the broken pieces of belt out of the motor.

"I checked the tension on the idler & tension pulleys and they seem to be fine. I turn the cam over slowly and the valves don't seem stuck or bent, and look to be working normally. A mechanic friend said if the valves seem to be fine, replace the belt and check the compression. That scares me!"
Why? This is very sound advice.

"I was asked if the Pantah engine is an "interference or Non-interference engine? I don't know what the answer is to that, does anyone know?"
With an interference engine design, the path of a valve extends into the zone where the piston moves; obviously the crank/camshaft timing prevents a valve from coming into contact with a piston during normal operation but if a cam belt was to break, the timing would be out and a valve could then hit a piston doing damage. It is my understanding that the Pantah motor is an interference design so I would expect valve and piston damage if a belt failed but you indicate that the valves seem OK.

"Anyone have any suggestions on how to proceed?"
You have already checked the rotation of the camshaft and the valve movement; follow your friend's advice making sure that when you fit the new belts, all the timing marks are aligned. The compression test is done by attaching a pressure gauge to the spark plug hole and turning the engine over by hand. If one of the valves is slightly bent and not seating fully then you will lose compression. You should see around 120psi; this motor has a 10:1 compression ratio. This is a very safe procedure. You need a new belt anyway and you must buy two; if one has failed due to age, the other will follow. Also peek inside the bore through the plug hole to see if the piston head looks OK. Borrow/rent a borescope if you can; make sure the cam is turned to TDC so the valves are closed then peek inside to get a better look at the piston head

"I would like to not remove the head, but I realize I may have to, to be sure what is wrong."
If you have lost compression then you have no choice. These bikes are relatively easy to work on; dropping the motor and pulling a head is not difficult to do. If you are not mechanically inclined then perhaps your friend could help.
 

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How can you tell that the rear belt tension is OK if the teeth are missing? What's "OK"? Do you have an owners' manual?
Whoa! Teeth on the rubber belt are missing, right? Not the gear? (hmm, bows out to listen to my elders, not sure if they made a toothless gear or not back then,...)

If its just the belt missing teeth, you need to replace belts before finding the tension. Teeth gone = compromised rubber, no way to actually measure usable tension
 

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Discussion Starter #6
Thanks for the info. Here's answers to your questions.

Yes it's the belts teeth that are missing.

The belts were replaced in spring '15, and this happened in June '17, about 3,000 miles on them.
I wouldn't think failure from age, but also wouldn't think a defective belt, that would of failed sooner
don't you think?

I understand that checking the tension with the damaged belt will not be accurate. I should have
said that the belt tension & Idler pulleys where not loose or seized in any way and the belt was
not loose on the pulleys. I didn't actually check for proper tension, just that there was tension on
the belt.


I do have a manual.

I have checked in the cylinder with an inexpensive scope, will borrow a good scope and check again.

Again, thanks for the help.
 

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1) Put new belts on, 2) get the valve timing correct,3) get the belt tension correct 4) check the compression on both cylinders
That will tell you everything you need to know. You will have compression if you haven’t bent valves. You will not have compression if you have bent valves. It’s as simple as that.
 

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Seems like there's consensus here! If I recall correctly, an over-tightened tensioner is more likely to cause belt wear. With the engine cold there is supposed to be 5mm of slack on the pulling side of the belt between the smaller belt pulley wheel and the camshaft. Belts are to be changed out at 20k km so this one failed way too early.

Good luck yellowrat01.
 

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There is a date code on the belts, normally. Belts were will age and become brittle regardless of how well they are stored.
 

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Thanks for the info. Here's answers to your questions.

Yes it's the belts teeth that are missing.

The belts were replaced in spring '15, and this happened in June '17, about 3,000 miles on them.
I wouldn't think failure from age, but also wouldn't think a defective belt, that would of failed sooner
don't you think?

I understand that checking the tension with the damaged belt will not be accurate. I should have
said that the belt tension & Idler pulleys where not loose or seized in any way and the belt was
not loose on the pulleys. I didn't actually check for proper tension, just that there was tension on
the belt.


I do have a manual.

I have checked in the cylinder with an inexpensive scope, will borrow a good scope and check again.

Again, thanks for the help.

What BRAND was the belt in question??
I have seen brand new "CA-Cycleworks" belts brake in the first month -

Also seen plenty of belts bought at the dealership fail prematurely too!

I ONLY use the DAYCO belts on all my "Rubber-Band Ducati" and for my friends when they require help as well - never had one fail.

and as goofy as it sounds, I have been setting the tension on the last 5yrs or so (about 15 or 20 belt changes on various Ducati) with the "Guitar tuning software" method on my smartphone - works like a champ! they end up not too tight and not too loose, there's a bunch of videos showing how to do this (the app I use personally is called Pano Tuner - it was free on App store)
 

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generally belts lose teeth when the cam stops while the belt is still trying to drive it.

remove the belt and try to turn the cam. won't turn means bent valves.

sounds like shit's gone south to me.

i set the tension on the old ones just by pulling the adjuster and idler together with gentle force. works well. visual here -
 
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