Does this problem sound like a blown head gasket??

O

Ocean7

New Member
Hi guys, I'm kinda new to motorized bicycles. I installed an 80 cc kit on my MB which went smooth.

The bike has been running well for around 8 weeks, and ride it everyday! It is awesome! I have been taking it slow for engine break in purposes and have been following the instructions that came with the kit regarding engine break in, and all was going well until today. I took it out for a spin and after about 20 min it lost power and started spitting fuel/oil mix out the air intake on the carb with loud "popping". It seemed as if the engine was back firing out the carb or something?

I'm curious if this has ever happened to anyone and what exactly the problem might be?

Any advice much appreciated :-)
 
Hmm sounds like a ring problem. When you are getting spitting out of the carb it's from blow by from the rings pushing the air fuel mix back out.

More people will be around to help you out with their expert advice too.

Good luck
 
that sounds bad...if thats the issue do you know if it is "fixable"? Will I need to strip the engine down to get a look inside?
 
Take off the cover on the left (mag) side and look at the spinning magnet. Make sure it hasn't come loose and rotated to throw off the timing. It should have a key in it. It should not touch the mag frame when it spins. If your head gasket is blown or leaking you'll have oily goo around the head. It will give you a loss of power but shouldn't make it backfire thru the intake. If you have a broken ring you should be able to feel the compression loss when you try to start it. If not pull the plug and roll it over with your thumb blocking the hole. You should not be able to keep it plugged. A broken ring will scar it up the cylinder pretty good usually. You can just pull the head off and check the cylinder wall for damage. If you have to pull the cylinder off Chris Hill's site has a great walkthru for putting back together. GL
 
A leaking head gasket will have oil residue around it. sounds like the carb bowl is overflowing. It got thrash at the seat of the needle valve or it simply need adjusting On my head gaskets I tork them to 20 foot pounds. I also use a very little amount of permatex high heat gasket maker type RTV silicone. Very thin application you dont wanna get it in the cylinder. The orange color RTV in that brand works the best . I have used the black color and it works great two. Its my chinas best friend LOL. http://motorbicycling.com/f4/carburetor-pictures-how-install-rebuild-195.html
 
one more thing...how "responsive" is that fuel/air adjustment screw on the side of the carb supposed to be? I mean, if the engine is allowed to idle at a constant RPM and you start adjusting it (clockwise or counter clockwise), should you be able to hear the engine revving up or revving down or something?

I had mine always set at the default (3 and a 1/2 turns counterclockwise from fully in) but whenever I tried to play around adjusting it, it did not sound like it was having any effect on anything? Fully in or fully out... did not seem to make any difference?
Just wondering incase this might have been a catalyst for the current problem? Thanks to all you guys for your great advise :-)
 
Ocean7 said:
one more thing...how "responsive" is that fuel/air adjustment screw on the side of the carb supposed to be? I mean, if the engine is allowed to idle at a constant RPM and you start adjusting it (clockwise or counter clockwise), should you be able to hear the engine revving up or revving down or something?

I had mine always set at the default (3 and a 1/2 turns counterclockwise from fully in) but whenever I tried to play around adjusting it, it did not sound like it was having any effect on anything? Fully in or fully out... did not seem to make any difference?
Just wondering incase this might have been a catalyst for the current problem? Thanks to all you guys for your great advise :-)
Click to expand...

That screw is idle speed only. Not fuel/air.
 
Ocean,
The carb adjustment you're talking about is only an idle speed screw. It simply blocks the throttle barrel from moving too low and when screwed in it increases the idle speed. Screw it outward to decrease the idle. It works best if you crack the throttle open a little before moving that screw. It engages a slot in the throttle barrel.
You original problem could be one of several. The magneto rotor, magnet, is indexed to the crankshaft with a keyway and woodruff key. It is rare but there have been reports of that key shearing allowing the magnet to shift its position on the crankshaft which would alter the ignition timing. That will certainly cause a backfiring that might appear as popping and fuel being pushed out of the intake. The alignment of the key and keyway are hard to confirm unless you remove the magnet from the shaft to inspect it. Be careful here and don't damage the crankshaft or the seal behind the magnet. Have a look and get back to us with your findings.
Tom
 
If turning that screw in or out has no effect then either there is a kink in the cable at the carb or you don't have enough slack in the cable. Grab the black cable housing at the top of the carb and pull up on it. You should be able to feel the slide go up and down. When it hits the bottom you should be able to move the housing up and down about 1/8" before you feel the slide start to lift.
 
Thanks guys, I now see exactly what you are meaning with regards to the engine idle screw. I was totally mistaken. I carefully took the carb apart today and examined it, and now I have a much better understanding of how it is working. I inspected the float and chamber, and that thrash at the seat of the needle valve; everything was clean as a whistle. If it is the carb bowl giving the prob I'm not too sure what it could be because everything in the carb seemed to be in place and the float was not stuck or anything?

Thanks for your input 2Door: I carefully took the magnet off to get a look at the keyway and woodruff key. To me it looked un-sheared, which is good thing I guess, but also bad because the breakdown is still a mystery? I'll try post I pic to get that 2nd'ed though?

I have been searching about this forum and read a thread about how bad water can be on the magneto? Thinking back, my bike did get rained on that morning it broke down; I did not think it would have been too drastic tho.
 

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:-||Hi Junster, I followed up with your suggestion regarding loss of compression. I didn’t notice a loss in compression when trying to start it up, but I just now tried rolling the engine over while my thumb was covering the spark plug hole after taking the plug out. I could feel the pressure slightly building up under my thumb, but it was not enough to force it off the hole nor did any air escape past my thumb. Sounds like maybe a ring is bust? I could also hear and feel air being pushed back out the air/fuel inlet… hmmm
 
Ocean7 said:
Hi guys, I'm kinda new to motorized bicycles. I installed an 80 cc kit on my MB which went smooth.

The bike has been running well for around 8 weeks, and ride it everyday! It is awesome! I have been taking it slow for engine break in purposes and have been following the instructions that came with the kit regarding engine break in, and all was going well until today. I took it out for a spin and after about 20 min it lost power and started spitting fuel/oil mix out the air intake on the carb with loud "popping". It seemed as if the engine was back firing out the carb or something?

I'm curious if this has ever happened to anyone and what exactly the problem might be?

Any advice much appreciated :-)
Click to expand...

Your exhaust pipe is clogged.

Jim
 
Hey ya Jim,

after reading your post I straight away checked the exhaust. There is a bit of black carbon build-up, but it doesn't seem to be clogged up. I can blow in it freely and feel air coming out other end. Are you meaning totally clogged, or just restricted?
I'll run some de-greaser through it. Unfortunately, I can’t take the end off the exhaust because there is no screw or end cap, this bike kit came with the end of the exhaust totally welded closed.
 
I'm thinking it is restricted. Here's the test!

It is not a good idea to run your engine without the pipe for prolonged periods...but for a short test nothing detrimental will happen! Just don't get the engine hot.

Remove the muffler and go ride it for a couple of minutes...does it still pop through the carb?

Does it have more power?

Jim
 
That’s a great idea … but only one problem; I had already whipped the head off much earlier on today, crap!

I was checking the rings and inside of cylinder for damage. The rings looked ok, the inside of the cylinder looked ok too except for a small scratch. I’m no professional when it comes to engines so I’m not sure what to make of it. Here is a pic if it helps, it might be difficult to see though…
 

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I took it into the shop and apparently it is a bigger problem than I thought. I'm probably gonna scrap it and get a new Kit or alternatively a new engine...

Any advice on good, robust engines on the market?
 
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