Wheel locking on startup

I am having a problem where when I try to start my bike the chain will jam in the engine and I have to forcefully roll the bike backwards to get it unstuck. Thisis not all the time, it is totally random. Sometimes it will go a day without doing it and some days it will do it every time I try to start it. 110% unpredictable.

Last summer I had an incident with the stock chain jamming in the engine at high speed and it threw me over the bars with the bike landing on me.

Since then I have installed a new 41 chain (From the Canadian equal of Tractor Supply) as well a a spring loaded chain tensioner.

Since then the chain has not locked up again while riding but back to the current problem of pedaling the bike and letting go of the clutch to start it.

Worst case so far was it took 6 tries of pedal - release clutch to start - tire jams - roll bike backwards to unjam - repeat

So far I have flipped the rear sprocket over and loosened the motor mounsts and reseated the engine but its not helping.

Is there anything else I could try? Is this a common problem on these bikes? (This is my 3rd MB and ONLY one thats acted like this). Has anyone ellse ever had this and if so what did you do to fix it?

OK well I got the spring tensioner AFTER the first lockup but before getting the 41 chain. I am going to ditch the spring tensioner & go back to the regular one. Now that I got the 41 chain that should not be as prone to stretching as the stock chain.

As for my sprocket it seems to be the flat tooth style. If going back to the stock tensioner doesn't work instead of filing mine down I'll probably see if I can just find a replacement on ebay or something.

There is not really much slack in my chain and I got a pretty stiff spring on my tensioner as it is. Not sure if I want to go as far as a case saver though. Never needed on before. The bicycle is nothing special. Just a standard walmart mountain bike.

FIXED. After my problem in my other thread about the getting the sprocket on the hub of my new bike it was still doing it. Stuffed 3 screws in there to make it stationary like the original non moving tensioner (Not my permanent fix, I'll be getting a proper nut/bolt/washer later today) so I can still have spring tensioner if I want it later. It worked.

What these 3 screws are telling me is that the spring that is currently on the tensioner is WAAAAAAAYYYYYY too weak. The tensioner needs to move LESS. I need a spring that is 2-3-4 times stronger I think. Using just the spring the end of the tensioner arm comes down and hits the frame (removing paint) and when the arm hits the frame it cant go any further and everything locks up. Hasn't jammed once since putting those screws in there.

Also on the previous bike this motor was on when I originally started this topic only locked up SOMETIMES (But gradually got worse over time). On the new bike it happened Id say 45/50 attempts to start it so pretty much every time.
Which makes sense because this is a different kind of frame. I had to remove 3 links from the drive chain to get it to even FIT properly so this is definitely a smaller frame.

BOTTOM LINE: Less movement in the tensionser should solve this one. I know I should scrap it but I really want the spring tensioner. It fits great now. The part of the frame where it clamps onto is oval making a perfect fit, where on the last bike it was on, the back part was more roundish than oval. With enough force I could move the tensionser by hand on the old frame. This one is a solid mount.

(After getting the picture for upload I notice the hole to the left <I have 3 holes drilled in there> I see some space in the hole to the left of the middle nut/bolt. Gonna stick 2 screws in there as well until I get to the parts store this afternoon.)

Major success. Got a hardened bolt (I think thats what he called it was "hardened") but its brass colored. Flawless start everytime now. Will start hunting for a better spring soon but this is doing awesome for now. I think even with a better spring I'll still keep the bolt in there though.

Chain is a 41 about 2 months old so not sure how long it takes them to finish stretching but if it stretches anymore then I'll take out the bolt and there will be s SUPER STRONG spring there instead of that weak thing that is current there.

This way I have an option. With bolt = stationary like stock kit tensioner or take it out and its spring loaded. I love having the option available just in case.