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Discussion Starter · #1 · (Edited)
Hi guys, long time stalker but first posting. I'm making this out desperation. I've got a 1980 CM400 with pod filters. Recently replaced the exhaust with a Mac 2-1 and replaced the head gasket with an Athena brand gasket because it was leaking oil externally. Afterwards, it started backfiring through the carbs so bad that it would push them out of the intake boots. Triple checked the timing and it seems to be correct, so I figured it was the carb jetting because of the new exhaust. I ordered a jet kit from 6Sigma that is supposed to be made for this setup (127.5 and 80 jets with needle shims). This seemed to make it run worse. It idles fine if the choke is halfway out, no more no less, and dies if given throttle. I just did a compression test and found that both cylinders are only making about 80 psi, but the cylinder leakdown test only shows about a 10% loss out of each cylinder with no leakage through the valves. If anyone has any ideas what the problem might be, please let me know.
 

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Discussion Starter · #2 ·
A buddy of mine said that the cam might be 180 off, so I tried flipping it around and it doesn't seem to make a difference.
 

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Sounds like your cam could be out of time, maybe off by a link.
 

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A buddy of mine said that the cam might be 180 off, so I tried flipping it around and it doesn't seem to make a difference.
When it's 180° out you don't flip it around, you rotate it half a turn, then re-attach the chain.
 

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Discussion Starter · #6 ·
I'm sorry, Mike in Idaho, that's what I meant to say. I unbolted the cam sprocket, rotated it 180 and rebolted it.
 

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Was the throttle wide open during the compression test?......It NEEDS to be......
 

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On these engines, rotating the camshaft 180 just swaps which cylinder is on the compression stroke and which is on the exhaust stroke. Since it's a wasted spark ignition (both plugs fire every revolution) it still runs the same.
 

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Discussion Starter · #9 ·
Thank you, J-T, I didnt know that, but it does make a lot of sense now. The timing looked right, but I'll mess around with it some today. I'll take some pictures later.
 

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Discussion Starter · #10 ·
Yes, 66Sprint, throttle was wide open. I tried spraying some wd-40 in too and it brought it up from 80 to 90 psi.
 

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I'd try a different compression gauge first. 80-90 psi and 10% leakage really don't add up together.
Since the carbs blow out of the insulators on a backfire I'd replace those. The carbs seat fully into the insulators and when good they are hard to remove. Intake backfire is an indication the the mixture is very lean, read as vacuum leak. Having to add choke confirms that unless the idle circuit is plugged up.
 

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Discussion Starter · #12 ·
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I tried moving the chain one tooth in each direction and it didn't seem to help. Ended up setting the timing back to where I had it. It looked straighter than either of those.
 

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Can you see the piston through the plug hole. Should be at the top of it's stroke.
If not, turn 360 and peek
I don't have anything else to contribute, unfortunately.
 

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Discussion Starter · #16 ·
I'd try a different compression gauge first. 80-90 psi and 10% leakage really don't add up together.
Since the carbs blow out of the insulators on a backfire I'd replace those. The carbs seat fully into the insulators and when good they are hard to remove. Intake backfire is an indication the the mixture is very lean, read as vacuum leak. Having to add choke confirms that unless the idle circuit is plugged up.
They're brand new intake boots. The ones that were on it originally disintegrated while going down the road. It ran fine before I changed the head gasket and the exhaust, but the exhaust that was on there was basically a straight pipe.
 

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Well, if the flywheel key is sound ( if broken, the flywheel can "turn" relative to the crank, so "T" is not actually TDC, piston crown can be seen thru the plug hole as suggested ) and the front cam chain run is taught ( not slack ) then all looks good timing wise. How much cam chain lift is there on the cam sprocket and how much "play" can be seen in the cam chain links and pins??

Compressions are low but equal more or less.

I have come across a 250 unit that badly back fired ( but not enough to blow the carbs off ! ) and instantly stopped in quite a violent way due to excess "lean back" of the cam timing marks due to cam chain wear.

I have been asking for camshaft cast on info which is out there to try and identify different cams via part number.
 

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Did you have the clamps on tight when the carbs blew off? I onde tried to help a guy with a similar problem and it turned out he had been trying to run the engine without the carbs clamped down.
TOOLS
 

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Re-check the compression with the carbs off the engine. The vacuum pistons in the carbs may not be opening at cranking speed.
 
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