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49CC 2 Stroke Engine only runs with choke on


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06-17-08, 05:29 PM   #1  
49CC 2 Stroke Engine only runs with choke on

Ok -- I posted a while back about this motor in the other section and noticed its the wrong section anyways

but here goes

Got a small 49cc Motor -- ran fine one night - turned it off all was well -- next time motor wouldnt stay running without the choke on (on as in Choke CLOSED Restricting airflow)

ive tried everything so far
Cleaned all fuel lines, petcock, filters
cleaned carb top to bottom and jets are perfect so is needle
Flushed Gas Tank out cleaned it up good

nothing worked so far

now I did the good old finger pressure test!
and oddly enough I can push my finger into the hole and keep it there - even with someone turning the motor over
I felt a little suction and a little pressure but that was it

so what ive done so far here --took off the Cylinder head, and Cylinder and took the piston head off (Left Rod attached to shaft - I didnt tear apart the crankcase)

what I found was this
Cylinder chamber walls perfect shape - no scratching, scoring, not even dirty!!

Cylinder Head (Combustion chamber) -- u can tell this motor has been used LOL - wasnt like caked on but there is a film of burned out oil and fuel

Cylinder Chamber and Cylinder Head looked normal
now The Piston was another story -- its got a thicker film of burned gases and oils on it and looks a bit nicked and dinged around the edges of the head -- the rings are good and it seemed to seal - but its a bit dinged in a few spots

however it looked right for having a good seal!!!

so im lost!!! -- are pistons supposed to be burned and blackened i mean what can be causing the loss of compression?

ive checked over everything i can think of on this thing to figure out Y it doesnt run without the choke on!

I did find one paper gasket that looks like its been there so long its pressed into the metal - but its the Gasket that seals Crankcase -->Cylinder -- so its Behind the Piston
is it possible that Pressure goes behind the piston too and if its escaping via that old gasket i can loose compression?

let me know
Thanks
SangrelX

Wanted to add this a Link to this image -- this is what the top of the piston looked like
http://www.bikemanperformance.com/im...iston_lean.jpg

not that actual piston but thats what it looked like -- just a film of carbon
and the spark plug is all full of carbon too but it does spark - but not a flat blue -- is it possible the spark plug cant keep up without the choke on?

im slowly learning more about small engine repairs


Last edited by SangrelX; 06-17-08 at 07:38 PM. Reason: added
 
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cheese's Avatar
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06-17-08, 11:25 PM   #2  
Did you take an actual compression reading? Do the sides of the piston also look like the one in the pic? Is this a little chinese bike?


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06-17-08, 11:50 PM   #3  
no

No i didnt use a pressure guage

its a Kinetic TFR Moped its from india

-- decent lil moped -- but like i said it just stopped running one day -- the piston rings are in good shape too - and the piston just has that light carbon build

im thinking that one gasket and the spark plug being replaced should fix it -- but same problem as in first post -- it wouldnt stay running without the choke on


Also side of piston looks same -- has build up on it too
i think the piston needs replaced and the gasket between crankcase and cylinder since its a paper gasket and was basically smashed and molded into the metal from years of use (1000 miles on this thing)


Last edited by SangrelX; 06-17-08 at 11:54 PM. Reason: added info
 
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06-17-08, 11:53 PM   #4  
Does it run well with the choke on, or does it run rich like it's being choked? Sounds a lot like it has an air leak into the crankcase or between the carb and chamber. Does it have a rubber boot between the carb and cylinder? This is a common area for air leaks on many engines.


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06-17-08, 11:58 PM   #5  
hrm

when holding the choke on - and running it as in actually sitting and driving it

it has some power and can move me around decently (215 pounds)

the carb does have a rubber boot around the pipe it mounts onto

i was wondering if this was a restrictor of some sort

 
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06-18-08, 12:02 AM   #6  
Check the rubber boot for cracks or spots where it is seperating from the metal.


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06-18-08, 12:05 AM   #7  
hrm

the lil rubber boot im talking about is actually removable

it slides onto the end of the tube and u slide the carb overtop of it and tighten the carb down onto it

it can be moved on or off the metal pipe itself

 
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06-18-08, 12:07 AM   #8  
Can air leak past it anywhere? What I'm getting at is that it sounds like you have air getting into the cylinder that didn't get there through the carb throat. It came from somewhere else, like through this boot area or another leak.


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06-18-08, 12:12 AM   #9  
hrm

hrmm perhaps this will help u see what im dealing with

Link --> http://www.kinetictfr.com/blog/?p=227


that is an entire step by step of the motor for this thing

u can see the area in the 9th image down -- thats where this one gasket is and its made from paper -- its soo wore out that its almost become one with the metal of the cylinder itself -- is it possible air can come in from Behind the piston? -- i mean wouldnt that mean the piston is leaking air past itself?

Also note the piston in his pictures is exactly same as mine of course - but mine is totally blacked on top and the sides are exactly same shape as in his pic


Last edited by SangrelX; 06-18-08 at 12:14 AM. Reason: Added info
 
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06-18-08, 12:34 AM   #10  
Yes, if any gasket or seal leading into the crankcase leaks air, it will lean the fuel ratio on 2-stroke engines.


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06-18-08, 12:38 AM   #11  
ok

ok wanted to add - that i have cleaned the entire cylinder and head very well inside and out -- took some 2-cycle oil and have oiled the inside of the chamber well

and I slid the piston into the chamber to see how smooth and how well it it seals -- its got a good seal
i think the piston itself is ok -- after cleaning it i can see the lines on the piston head and even read the top of it

so im starting to think it is indeed that seal between the crankcase and cylinder

- is this perhaps what your thinking of too?
also on the top of the piston head is a letter E and an Arrow pointing over

is this the part of the piston u line up witht he exhaust port right? - line the arrow so its pointed to the exhaust port ?

I think i will cut a new gasket out of some gasket paper and put this thing back togather tomorrow see what i can get from it

 
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06-18-08, 12:45 AM   #12  
I think air is getting in. Where I don't know...you'll have to find that. I would suspect the rubber boot I already mentioned. If not, the gasket you're thinking of could be it.

I don't know anything about that engine, but I would assume the E and arrow to mean exhaust side, like you suspect. I can't think of anything else it would mean.


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06-18-08, 12:50 AM   #13  
ok

ok im going to add one more peice of info i didnt add before I forgot

i think this will verify your idea of it being an air issue

when I run it with the Choke on (restricting the air) the motor stays running

when I take the choke off (letting all the air in from the filter)

the motor revs to a real high RPM then sputters and dies

is this the makings of an air leak? (i dont know why on earth i didnt say that part earlier)


if its an air leak I will make sure to check ALL Seals and everything I Can think of - then im definetly replacing that spark plug cuz its fouled and no blue spark during daylight - just the old dull little sparks


let me know
and Thanks for all your help cheese

 
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06-18-08, 12:56 AM   #14  
Your description strengthens my belief that it is an air leak. Check that rubber boot.


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06-18-08, 01:00 AM   #15  
done and done

ok -- i have the carb and rubber lil boot here in the house

I took the rubber boot that goes onto the stem that the carb mounts onto

and I inserted the rubber boot into the carb and then i will be inserting the entire thing onto the main stem -- so instead of rubber boot then carb - imma do them both at once to ensure a proper and tight seal between them

-- that should take care of the boot and carb issue

-- the only other issue is the Gasket for the Crankcase/cylinder - I know its bad - i mean its basically just an outline of a paper gasket -- so imma cut out a new one from gasket paper and make sure its really snug and tight

those are only 2 spots i can think of for an air leak - i will also be leaving the decomp lever off for a bit until i get the motor straight -- its possible the decomp valve is loose - i will make sure to adjust it properly too

-- if you can think of anywhere else air can come into the chamber let me know

Thanks Cheese I appreciate the help

 
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09-09-08, 09:51 AM   #16  
Same Motor, Different Problem

I have a Kinetic TFR and I was having what many people complain of-the motor runs and seems to run well on the stand, but when I try to ride it it goes for a little while and then bogs down and dies. This problem can also be caused by and air leak according to some sources.

I think I finally got it running today but I'm not sure why. I changed the gap on the spark plug (wider) and tightened the carburetor mounting bolt more than I usually do. I don't know what I set the gap to. It's supposed to be .20 from what I've read but all of my feeler gauges are lost right now.

Also I was leaving the decomp cable unhooked before. I reconnected it and operated it several times. It felt like it might have been sticking a little. I never use it so maybe the mechanism was a little stuck and causing an air leak.

Also I closed the idle adjust screw and opened it only 3 turns. Before it was open a lot more than that.

So one of these things might have been what it needed, but I don't know which (if any) it could have been. It still seems to be a little slow to accelerate but it does run now to where I can ride it. I just got it working this morning and rode it only briefly. I'm going to go out and try it again and see if it still runs and stays running. Then I'm going to make small adjustments to to the idle screw to see if that helps any with the acceleration.

 
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