Compression release stuck?

[EVOL Tweety Bird]

Predator 212 non hemi. Has started becoming a bit of a pain to start. Before you barely had to pull on the start cord and have the throttle at idle to start. Now I have to hold the throttle about half open, and really yank on the cord to get it to fire with the choke on. Which then has the bike want to launch at the T/C tries to engage.

You have to keep it running by blipping the throttle choke off, till it warms up, then it will idle, but the idle speed screw is almost fully in. Anything less and it just dies. Once warmed up she runs normally, except for the higher then normal idle.

This is a stock cam, and yes I have checked the valve adjustment to make sure the exhaust wasn't too tight.
In making sure it wasn't a fuel problem, I dribbled a little gas in the carb, left the throttle at idle and choke on. Pulled the cord normally several times, and no start. Opened up the throttle half way, and again yanged like heck on the cord and she fired up.

Something I realized is the start cord pulls really easy, not once has it yanked back on me, like it would if I would try pulling the cord when not up on the compression stroke. I'm beginning to wonder if the compression release is sticking on, which is making it so you have to spin the engine over really faster than normal to get it to fire.


Before I tear off the side cover to check I figured I'd post this to see if anyone might have a different idea of the problem,or conformation of what I'm thinking is probably what it is.


Add on, has the 140 e tube, 37 main, slightly larger idle jet, billet rod /flywheel, pod filter, header,22 pound springs, ported/polished, governor delete, oil sensor delete.
All were in place from day one, long before this started.

 

[OND]

Sounds like you've covered the basics , Never seen a compression release on one of these go bad yet ….but who knows ! Thought about a flywheel key ? :shrug:

 

[TheDullCarbide]

I would say flywheel key or clogged idle jet.

 

[EVOL Tweety Bird]

Yeah,I checked that too, even double checking the coil gap while I was there. And even swapped out the idle jet with another, and shot carb cleaner through the port in the carb.

Was going to check compression, until I realized if the compression release is stuck I won't get much of a reading.
Oh well, guess I'll check the cam tomorrow.

 

[TheDullCarbide]

Yeah,I checked that too, even double checking the coil gap while I was there.

Was going to check compression, until I realized if the compression release is stuck I won't get much of a reading.
Oh well, guess I'll check the cam tomorrow.

If the compression release is stuck you'll get full compression pulling the cord. I would pull the valve cover and see if you can spin the exhaust pushrod on the compression stroke. If it spins, you have a stuck/broken compression release.

 

[OND]

Evol , I know you've tried a new spark plug ? Right ?

 

[Raskin]

Maybe weak spark plug..:doah:

 

[EVOL Tweety Bird]

If the compression release is stuck you'll get full compression pulling the cord. I would pull the valve cover and see if you can spin the exhaust pushrod on the compression stroke. If it spins, you have a stuck/broken compression release.

Going up in compression stroke or at TDC? At TDC I can spin the the push rod, and no play in the rocker.

Yep new NGK and even an Autolite racing plug, and yep got good spark.

 

[TheDullCarbide]

Going up in compression stroke or at TDC? At TDC I can spin the the push rod, and no play in the rocker.

Yep new NGK and even an Autolite racing plug, and yep got good spark.

As the piston comes up through the compression stroke, the exhaust valve is supposed to open about 0.015" if the compression release is working. If the exhaust valve doesn't move at all during the compression stroke, the compression release is broken.

Spinning the pushrod just tells you if the valve is trying to open or not.

 

[EVOL Tweety Bird]

Got it. I'll double check that. Thanks.

 

[EVOL Tweety Bird]

Okay an update of sorts. As DC had suggested, I checked how much the exhaust valve was opening on the compression stroke. To my Mark 5 eyeball it appeared that the valve was moving at half throw and staying open almost to TDC. Thumb over the plug hole and barely a whisper of compression, verified with compression gage, just nudged 10 psi.

Well I have a possible direction to look, but something told me to go back over everything just to make sure I didn't miss something else.

Pulled the flywheel, key in place and ditto the flywheel, so good there.
Coil gap at 35 thos. and coil centered over the magnet.
Yanked the carb and tore it down completely blew out all the passages, verifying they were all open and flowing. Reinstalled the 140 tube/37 main and larger idle jets. Verified fuel flow from the needle, and yeah the tank was full of fresh fuel.

Loosened up the exhaust valve to eliminate the release, and compression jumped to 80 psi on the first pull. Got up to around 120 psi, and held it, so no compression loss. Readjusted both valves, spun the engine over several times and rechecked the gap. Good to go.

Wicked blue spark from the plug, which highlighted something. Before this thing ran slightly rich, now the plugs, two, used different brands for testing, showed nothing, like they weren't getting much fuel, odd as I was running the heck out of it just to keep it running.
Well I'll check back on that.

So pulled the T/C and side cover off, verified timing marks were lined up. Found the old cams I had from my other engine so I could check the releases on them and the current one. When I checked the old cams the action was smooth and snapped right back. The cam out of this engine had a spot where you could feel it hang up slightly and it would drag when releasing pressure on it.

I took a little valve grinding compound to where it pivots, worked the thing back and forth and got whatever was causing the bind. Poor machining or maybe something metal from break in got caught up?

All back together,verify all gaskets in place and right orientation, gas on, switches on, choke on, no throttle, yank the cord and BRAPPP! then it died. Choke off, pull cord, and a couple of sputters. Give it half throttle and BRAPPPPPPPPPPPPP! Well, as long as I held the throttle at half. Soon as I'd start to leave off it would die.
Even with the idle screw almost completely screwed in the whole way. Ran it till it warmed up and could finally get it to run on it's own with the idle screw in that far. Sprayed brake clean around the carb and intake to check for any vacuum leaks, all good there. Pulled the plug out and it was dry. I'm beginning to think I had/have two problems,one with the release and possibly a sour carb. Well we'll soon to find out on the latter, new one coming from ARC. More to follow.

 

[OND]

Evol did you pull the low side jet and check all the passages to the back of the carb ?

 

[TheDullCarbide]

Okay an update of sorts. As DC had suggested, I checked how much the exhaust valve was opening on the compression stroke. To my Mark 5 eyeball it appeared that the valve was moving at half throw and staying open almost to TDC. Thumb over the plug hole and barely a whisper of compression, verified with compression gage, just nudged 10 psi.

Well I have a possible direction to look, but something told me to go back over everything just to make sure I didn't miss something else.

Pulled the flywheel, key in place and ditto the flywheel, so good there.
Coil gap at 35 thos. and coil centered over the magnet.
Yanked the carb and tore it down completely blew out all the passages, verifying they were all open and flowing. Reinstalled the 140 tube/37 main and larger idle jets. Verified fuel flow from the needle, and yeah the tank was full of fresh fuel.

Loosened up the exhaust valve to eliminate the release, and compression jumped to 80 psi on the first pull. Got up to around 120 psi, and held it, so no compression loss. Readjusted both valves, spun the engine over several times and rechecked the gap. Good to go.

Wicked blue spark from the plug, which highlighted something. Before this thing ran slightly rich, now the plugs, two, used different brands for testing, showed nothing, like they weren't getting much fuel, odd as I was running the heck out of it just to keep it running.
Well I'll check back on that.

So pulled the T/C and side cover off, verified timing marks were lined up. Found the old cams I had from my other engine so I could check the releases on them and the current one. When I checked the old cams the action was smooth and snapped right back. The cam out of this engine had a spot where you could feel it hang up slightly and it would drag when releasing pressure on it.

I took a little valve grinding compound to where it pivots, worked the thing back and forth and got whatever was causing the bind. Poor machining or maybe something metal from break in got caught up?

All back together,verify all gaskets in place and right orientation, gas on, switches on, choke on, no throttle, yank the cord and BRAPPP! then it died. Choke off, pull cord, and a couple of sputters. Give it half throttle and BRAPPPPPPPPPPPPP! Well, as long as I held the throttle at half. Soon as I'd start to leave off it would die.
Even with the idle screw almost completely screwed in the whole way. Ran it till it warmed up and could finally get it to run on it's own with the idle screw in that far. Sprayed brake clean around the carb and intake to check for any vacuum leaks, all good there. Pulled the plug out and it was dry. I'm beginning to think I had/have two problems,one with the release and possibly a sour carb. Well we'll soon to find out on the latter, new one coming from ARC. More to follow.

Kinda freaky but my neighbor's ATV had the same exact problem last night. It has a very old version of a VM22-ish carburetor and it wouldn't idle. First we looked for vacuum leaks by spraying carb cleaner near the outlet flange on the carb and it revved when we shot it. Took the carb off, put a very thin coat of silicone on it, still didn't idle.

Then I was going to play with the idle mixture screw and but I couldn't find it. Looked all over the carb. I ended up finding it laying in the driveway.:doah: So it backed it's way out and the carb was sucking air through its hole. When we sprayed carb cleaner on the flange, some of it got in the mixture screw hole and made the engine rev.

Not saying this is a common failure to look for but wouldn't hurt to look. Actually, look for anywhere out-of-place that the idle circuit could suck air and make sure it can't. Think like a carburetor.ut:

 

[Mariah]

This really sounds to me like the idle circuit or idle jet is clogged somewhere. Did you remove the idle jet and make sure it is clear? Are all the idle circuit passages clear? Some idle jets have o rings above and below the cross drilled holes, a sliver of o ring may have been pinched off during assembly and it could be trapped in the idle circuit holes, that would explain the lean idle mixture problem you are experiencing.

 

[EVOL Tweety Bird]

In answer to the last three, yes. Shoot carb cleaner through the eye in the front of the carb with the jet removed and get a geyser of spray up through the hole. Plug the idle jet hole and spray again, yep 4 holes in the side of the carb throat shoot fluid out. Sprayed into the idle jet itself and it's open too. And yes both o rings are on the jet. I'll pull it and double check o ring condition.

On a side note, do not buy the RuiXing idle jet OldMiniBikes sells if you have a Predator 212, it does not fit. It has a larger diameter section at the top that won't allow it to seat fully. I found that out.

I even swapped jets with one of my other engines to see i it made a difference, this one is a larger diameter hole then the stock one. Made no difference.

Lastly, this carb has a sealed idle mixture, no adjustment other then upping the jet size.

All in all, I agree something is going on in the idle circuit. I'm going to pull the carb again and go back through it,while I wait on the new one.

Stuff like this drives me silly, I have to know why something is not working. It "works" under pressure of carb cleaner, yet not under vacuum of the engine. I'll keep digging...

 

[EVOL Tweety Bird]

GOT IT!!!

Yanked the carb and went back through it all over. And finally saw/found something.

There are some tiny ports in front of the throttle plate and a larger one behind it, in the side of the bore.

A quick spray of carb cleaner shot through the front hole air intake on the carb, and I'd get a shot of fluid from all the ports in the bore.

Gave it a second longer blast and the front holes would peter out while the rear port kept spraying. Well that's odd.

Rigged up a setup to back flush first the larger port and then the smaller ones, pulled the idle jet and put my finger over the front intake hole. Sprayed the heck out of it letting the cleaner blow out through the idle jet hole.

Took my finger away from the front of the carb and what's that? There on my finger tip was something black about the size of a pinhead.
It was rubbery. Piece of an O ring? Not from either of mine, I used a magnifying glass to check them both and not any damage on them.

Reinstalled the idle jet and blew carb cleaner back through the circuit and had full flow out of all the ports.

Reinstalled the carb, set the choke, throttle at idle, and gave the thing a yank. BRAPPPPPPP!!!! Turn the choke off and Brap Brap Brap she sat there idling like a champ. Even managed to back off the idle speed screw a few turns and she kept on idling.

Let it sit for a few hours, and tried it again. Fired right up and and idled like a champ. :thumbsup:

Wherever that debris came from I guess it would get sucked up against the tiny front port circuit and block it off, dropping the fuel flow enough to make it a bear to start. Holding the throttle half open would allow fuel to pull from the main jet to get it to start, but not as soon as the throttle was released the idle circuit starved for fuel and the engine died.

Man that was a bugger. ut:

 

[EVOL Tweety Bird]

Oh and before I forget, two things.

#1 Thanks to all who pondered this with me, and fired out all the things to look at. Love this place, always someone willing to help.

#2 I'd show some pictures, to make it a little easier to understand, but whatever is going on with the site recently won't allow me to...

 

[Davis]

I’ve had that ethanol fuel eat a rubber fuel line from the inside out on a harley and kept gumming the carb up.

 

[TheDullCarbide]

GOT IT!!!

Yanked the carb and went back through it all over. And finally saw/found something.

There are some tiny ports in front of the throttle plate and a larger one behind it, in the side of the bore.

A quick spray of carb cleaner shot through the front hole air intake on the carb, and I'd get a shot of fluid from all the ports in the bore.

Gave it a second longer blast and the front holes would peter out while the rear port kept spraying. Well that's odd.

Rigged up a setup to back flush first the larger port and then the smaller ones, pulled the idle jet and put my finger over the front intake hole. Sprayed the heck out of it letting the cleaner blow out through the idle jet hole.

Took my finger away from the front of the carb and what's that? There on my finger tip was something black about the size of a pinhead.
It was rubbery. Piece of an O ring? Not from either of mine, I used a magnifying glass to check them both and not any damage on them.

Reinstalled the idle jet and blew carb cleaner back through the circuit and had full flow out of all the ports.

Reinstalled the carb, set the choke, throttle at idle, and gave the thing a yank. BRAPPPPPPP!!!! Turn the choke off and Brap Brap Brap she sat there idling like a champ. Even managed to back off the idle speed screw a few turns and she kept on idling.

Let it sit for a few hours, and tried it again. Fired right up and and idled like a champ. :thumbsup:

Wherever that debris came from I guess it would get sucked up against the tiny front port circuit and block it off, dropping the fuel flow enough to make it a bear to start. Holding the throttle half open would allow fuel to pull from the main jet to get it to start, but not as soon as the throttle was released the idle circuit starved for fuel and the engine died.

Man that was a bugger. ut:

:doah: I assumed this was a Mikuni for some reason. That rubber could be from installing the new idle jet. A piece of an o-ring could have caught and ripped and those four tiny holes near the butterfly lead directly back to the idle jet.

 

[OND]

Yea most people don't realize where the low side or idle circuit runs to , It has to be on the backside of the butterfly for it to idle right . I'm glad you found it ! :thumbsup: