1985 Aero 50 backfire and then...dead

[theuplift]

Okay,

So I installed a 63 big bore cylinder kit from VT Cycles and a 21 mm carb kit from Taz on my 1985 Aero. I got it all tightened up and fired it up. I ran it around 25 mph in short increments for 3 or 4 miles each over a few rides and it was running beautifully. On my last ride, i got to a stop sign and then the motor made a sound like a misfire or slight backfire...I accelerated off the stop and it happened again and the scooter immediately cut off. Now it wont start at all.

The engine still turns over smoothly but it doesn't even sound like there is any spark or attempt to fire up. I pulled the plug and its brown and dry and sparks when i test it. If it is still getting spark when I test it then I'm assuming its not fouled? I am still new to the wrenching and trying to figure all this out as I go. This is stumping me because mechanically everything still seems to be tight. I dont have I way to test compression and couldn't feel any leaks anywhere...Even if compression was bad I would think that the motor would attempt to fire...is that right?

What other things am I missing here? What are some other tests or signs to look for? I havent tried a new spark plug yet so maybe that is the problem?

help me please!! she was running so awesome and now im bumming!

Thanks!

[Mystic]

do a compression test, you may have experienced premature detonation of the piston.....

[motormike]

Yes, do a compression check for the most important fact to know about an engine that has back-fired is compression. head gasket blown out possible. reeds broken, maybe. case gasket, could be. few things to check. good luck duuuudde

[Bear45-70]

Backfires often blow the reeds in 2 strokes.

[theuplift]

Thanks for the replies. I will test the compression today. I pulled the carb and intake manifold already and looked at the reeds and they look to be in working order. I will check again to make sure. If a gasket blew out, what can I do to prevent it from happening again?

[theuplift]

Just tested compression and got 30 psi. guess I blew something ...time to start tearing it down...Hopefully didn't destroy the piston as its a brand new kit...if so, I hope I can figure out why and not do it again!!

[Mystic]

Ouch that is super low, blown gasket perhaps...i still say there is a hole in the piston. Let us know and take some pics hope its just a gasket

[theuplift]

Well I pulled the cylinder and got my answer. My piston and bore are destroyed from what looks to be a combo of losing a circlip and the connecting rod being worn out. I removed the piston and checked the rod for side to side play. It has around 1/16" play which cannot be good. I noticed that it rubs the two outer pieces as it rotates when side pressure is applied. There is also a visible mark on the rod where it has rubbed.

Also, the top of the piston and cylinder head are very black and sooty. It looks like the piston was actually making slight contact with the cylinder head on one side.

So I guess I'm in the market for another big bore cylinder kit and a new crankshaft...lesson learned...make sure circlips are in securely and while the cases are apart, check and replace crankshaft if necessary! Especially if the engine has over 9,300 hard abused miles on it like this one had when I bought it.

Please check the pics and let me know if there are any signs that I'm missing for other potential problems. Thanks!!

PS - anyone with a good crankshaft I'm in the market! (I'm assuming that this much play is not normal!)

[theuplift]

two more pics

[Wheelman-111]

Greetings:

Is that a round hole in the center of the piston crown?
If so, consider an air leak causing extremely lean running. I'm not sure how that would cause the piston to drop its circlip, but stranger things have happened. The clips have to be NEW and be careful not to compress them beyond their elastic limits when you install them. Put the gap in the clip inside the gap in the groove for added "insurance".

[theuplift]

yes it blew a hole in the piston just as Mystic had predicted. I really need to figure out what the ultimate causes were for this before i rebuild and it happens again!

Right now all the possible issues that I know of are:

1. circlip came off and destroyed piston and bore
2. connecting rod worn out
3. Fuel/Air Carb tuning issues...

What would be the cause of blowing the hole in the piston? I'm still learning so bare with me!

Thanks!

[Mystic]

there are any number of reasons why it blew a hole, mainly excessive heat. Whether from too small jets in the carb to a vacuum leak. One thing to do when you get a bigger bore is to consider using a colder spark plug to prevent the piston getting to hot right there in the center. Where you using the stock head or was it machined to match? As far as the circlip goes it happens to the best.

[theuplift]

Mystic-
Thanks for the reply. I was using the stock head that Steve from VT gave me when I purchased the big bore kit. I had been riding the scooter with the stock carb for +/-10 miles before I purchased the 21mm kit from Taz. I did not change any settings or jets on the new kit. The old stock carb was functioning very poorly sputtering at 25mph and very bad response even after two cleanings.

I am using the spark plug that Steve from VT sent with the kit. He mentioned that it was the best fit for the setup.

would the side to side play in the connecting rod have anything to do with the problem? I tried to find the tolerance in the manual but couldnt see how much play is acceptable.

Thanks again

[Mystic]

hmmmmm its really hard to give you a definative reason why it failed like it did. I dont necessarily think a small amount of side to side on the crank is an indication that it caused this problem. Do you still have the stock piston and bore or did you replace it with the BBK because it was bad? if the stock piston and bore are good why not put them back on and see how it runs to eliminate whether the crank is good or not. also did you premix gas and oil or do you still have the oil pump installed and what are the jets sizes in that carb? we need more information to properly diagnose this issue. Maybe it overheated burning the hole because the clip had already came out and was causing extra strain and friction. Also how long did you have the bore kit installed and how many miles had you put on it already? Generally you have to put about 400 miles of easy riding without running wide open to break the bore in

[Bear45-70]

Greetings:

Is that a round hole in the center of the piston crown?
If so, consider an air leak causing extremely lean running. I'm not sure how that would cause the piston to drop its circlip, but stranger things have happened. The clips have to be NEW and be careful not to compress them beyond their elastic limits when you install them. Put the gap in the clip inside the gap in the groove for added "insurance".

Normally a hole like that is caused by not enough octane for the compression ratio.