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  Topic Review (Newest First)
06-21-2019 03:57 PM
PhillyBobber717 UPDATE - after a full week of turning wrenches, I think I may have finally found the problem.

I had her running earlier while I sprayed carb cleaner around the vacuum in an attempt to find a possible vacuum leak and I actually saw my one spark plug wire sending sparks to the outside of the engine.

Now I’m off to the local bike shop to pick up an ignition coil, hoping that this whole ordeal will soon be behind me.

Could leaking spark cause it die the way that it did? I found an old post where somebody said it felt like they ran out of gas and they suspected the ignition coil, but they never provided an update, so I don’t know if that solved their problem.. but it did make me wonder because my FIRST initial thought was that I had run out of gas.. any info at all would be appreciated. Thanks guys.
06-20-2019 10:57 PM
flitecontrol With the rear wheel raised, I just turn the engine by putting it in gear and slowly turning the wheel.
06-20-2019 09:54 PM
PhillyBobber717 Also - the brand new plugs had carbon on them..

So, bike starts & idles.. popping sound with throttle and fouling plugs.

New plugs, new CDI, cleaned carburetor, new stator, charged battery & checked compression... what/where could I possibly look next? What is going on??
06-20-2019 08:37 PM
PhillyBobber717 It’s the exact same engine and everything else as a first gen cmx250.

The thing that doesn’t support the sticky valve theory is the fact that it idles fine.

Could it possibly be a vacuum leak? It idles fine and if I ease the throttle open it’s (somewhat) smooth.. but if I crank the throttle it pops immediately and stumbles.. almost like when she’s under a load and sucking in a lot of air, she’s pulling from a crack in the rubber boot (just a theory.) I guess the way to test would be to spray carb cleaner around the boot and if she stumbles, I’ve found my problem..

I feel the need to mention that this bike HAS been modified, by both myself and the PO. Stock air box with straight pipes, main jet is one size larger than original. It ran beautifully until that day when it died. I think it died due to my fried stator and in working on it I have either uncovered (or created) a new problem.

In other news - I did a dry compression test on a cold engine and got 110psi exactly on both sides.. a little low maybe, but the fact that they match makes me feel like compression isn’t the issue.
I also need to rethread my one spark plug hole - when I pulled it out to do the compression test, it wouldn’t seat back in nicely. So another delay as I do not have a thread chaser - has anybody done this tho? I’m worried about little pieces of metal falling into the hole.
06-20-2019 08:12 PM
Guba If it's like a Rebel engine...You can take the valve cover off and then take the plug out on the side of the engine case, use a ratchet to turn the crank by hand to see if the valves are going up and down as they are supposed to be.
06-20-2019 05:53 PM
PhillyBobber717 Still backfiring with throttle..

The ONLY thing I can think of is I have a valve sticking..

Anyone have any other ideas?
06-20-2019 06:53 AM
Guba 16mm x 1.5 thread are hard to find in the U.S., I ended up going to the dealer and just buying the Honda tool. Not cheap!
06-19-2019 11:09 PM
PhillyBobber717 Hopefully they fouled because of my ignition/carb problems and I don’t have a compression issue..
They’re black with carbon, not oil.. which is a good sign.

I’m looking into adjusting the valves - at this point, why not? Might as well do all the preventative maintenance I can while she’s sitting rather than wait for more things to break.
06-19-2019 11:04 PM
PhillyBobber717 No - but I do hear a slight valve tap. Idk if it’s “normal” or not, I know this is a noisy engine..

My backfire is definitely an exhaust explosion rather than the lean popping.

Pulled the plugs and they are BLACK.. I’m surprised it started at all to be completely honest.

Going to autozone to pick up some plugs in the morning. Will report back.

Also, the ONLY bolt I could find in m16x1.5 is a drain plug for $12.. which isn’t a lot of money, but it’s more than I wanna spend on a bolt. So, my flywheel project has been postponed for the time being.
06-19-2019 08:11 PM
flitecontrol I believe the engine is an inline twin. It has a wasted spark ignition system. Each time the pistons come up, whether on compression or exhaust, both plugs fire (e.g., the "wasted spark"). When one cyclinder is compressing, the other is in the exhaust cycle, so they don't fire at the same time.

It sounds improbable, but backfiring is a sign of a too lean condition, not too much fuel. A compression test would reveal if a valve is sticking or not closing properly. Have the valves been adjusted recently?
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