Help from the vintage two stroke gurus

[motoqueiro]

OK, as the topic says, I need the knowledge of the two stroke vintage guys
I have a 1982 125cc two stroke single cylinder with piston port system intake,
I’ve rebuild the engine two years ago, and drove for a few miles, the bike have been storage since then.
I’ve decided to bring her back to live again, but something is wrong.
She starts and idles but when I try to rev she enters in a type of cut of system, like the 4 stroke? Accelerates/decelerate Accelerates/decelerate Accelerates/decelerate, I’ve open the carb (bing 53) and clean it 5 times, I’ve swapped plugs, I even fitted a new ignition module (electronic motoplat 6v)
So far the only thing I discovered is that if I cover half of the intake with my finger she revs
Se the vid please and share some light in to this as I’m going mad over it
Thanks
https://www.youtube.com/watch?v=Bh46pOmOHek&list=UUNh8QOC1IbMjx8lY_YFFLsw

 

[oh9bolt]

 

[d_adams]

Possibly a leaking crank seal. It probably dried out and shrank a little from sitting. Had one do something similar, but it went WOT on me. Pulling the spark plug wire didn't even stop it once it got going. I'm surprised it idles as well as it does though.

 

[motoqueiro]

thanks oh9bolt for posting it, don't know why it didn't work before?
Thanks d adams, I guess I can try and swap the crank seal for a new one without spliting the cases
that idle is exactly the thing that astonishes me, can the seal be good enough for that engine speed and them let air in in more speedy rotations?
I'm really going mad on this one

 

[d_adams]

One other thing to check is the float height in the bowl to see where fuel actually shuts off at. It may be _just_ high enough to run at idle, but as soon as throttle is applied, it's leaning out.

 

[motoqueiro]

thanks, I did that to, checked the level and increased it, just to see the response, its the same

 

[rchuff]

the fuel filter doesn't have a lot of fuel in it in the video. At least it doesn't look like it does.

 

[motoqueiro]

yes, it appears that way, but does the same thing without it

 

[twobuells]

To test a two stroke for leaky crank seals you need a vacuum tester and a block off for the intake and exaust.

 

[motoqueiro]

hum, that would be a nice thing to have
can it be tested with compressed air? or does it need to be a vacum?

 

[mrlogix]

twobuells correct me if I'm wrong, it's done with a small operated hand vacuum pump. it doesn't take a lot of air to get a pound or two of vacuum. Good ones have a gauge with a tell tale max indicator, then watch the vacuum leak off rate.

 

[Matteson]

Does it have a choke, or a throttle enrichment circuit.

 

[twobuells]

If my remember it was three psi vacuum for 5 min. It would be easy to make using the exaust flange gasket and a rubber cork with a hole to stick a nipple in for the carb flange. Yes a mighty vac would work fine.

 

[motoqueiro]

Does it have a choke, or a throttle enrichment circuit.

yes it's been checked also, seems ok

 

[konarider94]

Did it ever run with the same carb settings its at right now? Seems like its starving for fuel.

There are no mice or blockages in the pipe right? It looks like shit is blowing out of the carb instead of getting sucked in. Are the reed valves bad?

 

[motoqueiro]

Yes, always the same carb, with the same settings, from new in 1982
mine as no reed valves, is a piston port system intake
for what I can tell, there is no blockage in the exhaust

 

[motoqueiro]

Ok
I've just made a vacuum teste, it maintains a constant 12 in HG (is this ok or do I need more vacuum?)
what the hell is going on with her [confused]

 

[Buellysses]

I would agree, it sounds fuel starved. Confirmed by it running correctly when you partially cover the carb.

When you cleaned the carb, did you tear it down?

I'm assuming the idle and low speed jets are good, main jet is probably plugged.

Check this site out:

http://www.bingcarburetor.com/bmw/bmw.html

 

[motoqueiro]

one pic to answer your question

I've cleaned that carb more times than I can remember
I'm really at a loss here

 

[Buellysses]

Personally, I'd order the rebuild kit. I'd bet a beer that it's starved for fuel.