2 stroke explosion

R

Razors Edge

New Member
Had my first engine problem yesterday with my second hand motorbicycle.
The engine is a 49cc Grubee 2 stroke. (Has the slant head, but not sure specifically what it is beyond that) and it is mounted in frame on a Macargi “The General”. It’s really a pretty sweet looking ride and the performance was great until yesterday.

This bike is meant to be my inexpensive/reliable commuter. Maybe if I describe what happened, I can get some tips on how these engines are meant to be treated. As I understood it the 2 strokes were better for commuters because they can handle higher rpms and longer running…?

Anyway, my commute is 11 miles each way with a short stretch of highway in the middle. There are mile long steep hills at the end of the commute each way, so the extra torque is nice. I have been riding the bike for 60-80 miles getting the feel for the bike and making sure it wouldn’t let me down before commuting to work yesterday. I got to work with no drama and was quite early.
Going back home, the bike was difficult to start and a couple times when I was going 6-7 mph and popped the clutch, the rear tire would slide and the engine wouldn’t even turn. (I do know that my fuel/oil mix is closer to 20:1 right now, maybe that is a contributor to my problem?)

I got the engine going eventually and headed out of town. I was pushing the bike pretty hard to make up time and was averaging around 26 mph for a little over 6 miles. When I was about 4 miles from home the engine had a catastrophic failure with the spark plug blowing right out of the engine. I stopped and grabbed the spark plug and peddled my angry butt the rest of the way home. I was too angry and tired to stop and look at what all was damaged, but I’m guessing that I will be ordering a new head for the engine soon. Once I am home from work again, I will take a close look and update with what I find.

Any glaring mistakes or genius suggestions from the most knowledgeable motorbicycling community on the web? :)
 
Hard starting...did you turn the fuel OFF while at work? If not you might loaded the crankcase with excessive fuel mixture. Could have too much compression, if so, a added head gasket will help. Or you could just have bad spark plug threads.
Fuel mix at 20:1 ok for break in, then go to 50:1, if not at least 32:1
My first engine (not a forum member at that time) I broke it in at 16:1 (by book)... it ran fine but later found out way too much oil.
 
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Yeah, I have been pretty OCD about turning the fuel line off. Does it make it start any easier if you turn the fuel line off a half mile before your destination and let the engine sputter out?

I am planning on doing 32:1 ratio from now on, but will I need to do 20:1 for a tank after I put on a new cylinder head?

Is the 50:1 ratio okay to use with the standard air cooled 2 stroke oil, or should you switch to semi-synthetic with a ratio that high?

Thanks in advance!
 
Razors Edge said:
Yeah, I have been pretty OCD about turning the fuel line off. Does it make it start any easier if you turn the fuel line off a half mile before your destination and let the engine sputter out?

I am planning on doing 32:1 ratio from now on, but will I need to do 20:1 for a tank after I put on a new cylinder head?

Is the 50:1 ratio okay to use with the standard air cooled 2 stroke oil, or should you switch to semi-synthetic with a ratio that high?

Thanks in advance!
Click to expand...

I turn my fuel off about 500 feet before I park it.

When wanting to ride after the fuel is turned off, open the fuel flow and press on the tickler button (lets air out of the carburetor while filling the bowl up), holds float down. This is not a prime button.

Personally I'd do a 20:1 for the first two tanks (replacing head has nothing to do with it), then onto a 50:1 mixture. There are some doing a 100:1. I use Amsoil, but any GOOD 2 cycle air cooled oil works.
Due to the fact, I have a heart condition, I always start my bike with a short spray of either starting fluid or throttle body cleaner. Never have to move the bike more then the length of my truck before it fires off. I drilled a small hole in the filter cover.

That was my first, now I drill from the side, in my case on the left, as that is how I mount the bike, and it it easy to get too.
 
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How tight did you install the spark plug? Are you sure you need to replace the cylinder head? If the plug just vibrated loose and fell out the threads might be just fine.

Tom
 
2door said:
How tight did you install the spark plug? Are you sure you need to replace the cylinder head? If the plug just vibrated loose and fell out the threads might be just fine.

Tom
Click to expand...

Wow! Good call. The threads were fine and the plug went straight back in. Guess I will need to be more careful about checking the nuts and bolts every so often.

Is starting fluid okay to use with 2 stroke engines? This may help me out until I can work out a little more of the high oil mix I have in my tank...

Riding to work tomorrow, yeah! .duh.
 
Starting fluid will make starting easy but you shouldn't have to use it. Once you find the 'sweet spot' where your engine fires quickly you won't need the extra kick. The Chinese 2 strokes are all different and all like a little different fuel/choke/tickler on first starts. It takes some practice but you'll eventually find the right combination that will let the engine fire in a few feet of pedaling. Just keep playing with different combinations until you hit on the right one and then you can leave the starting fluid on the shelf.
Good luck, ride safe.
Tom
 
I never used to use the primer button or tickler as some refer to it, but have discovered the engine warms up a lot faster if I prime it. I had a spark plug come loose once. You have to tighten them up pretty good or they will loosen up, but dont keep wrenching on them or you will strip your plug threads and then you will need a new head.
 
I had a plug blow out of a lawnmower once. The neighbors freaked out, fearing it was a gunshot. Nope, just didn't tighten the new plug enough. When it shot out, it yanked off the plug boot, but that's ok. The threads were still good.

Did your head crack, or are the threads stripped? If not, just make sure your plug has the little metal washer if it's supposed to have one, else it will have a cupped edge that seats against the head.
 
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