How to Ruin a Good Moped

Move over Fred's Guide, here's Anthony's guide to ruining your moped in 3 easy steps:

Step 1.) Shear the guide pin off of your throttle slide while reassembling your carburetor.

Step 2.) Rather than buying a new throttle slide, J-B weld a piece of scrap metal in place of the guide pin. A harder metal, like steel, works best (this is important for step 3.)

Step 3.) Enjoy the grinding sounds your moped produces as the jury-rigged guide pin breaks off and goes right into your crankcase. Congratulations! Your moped is ruined.


Given that I followed these steps, and my flywheel is now immobile, what are the chances that my engine will work again after I retrieve that guide pin? I guess my cylinder is OK, but the cranks and bearings and possibly the piston are probably shot. :(

Re: How to Ruin a Good Moped

depends.. unless that thing ran for a while after the evil guide pin got sucked in (which is doubtul) the pin might be relatively whole and is now jammed between the crankcase wall and a counterweight.. and the bearings are untouched.

pull the cylinder, get a nice bright worklight and search for the pin.

Re: How to Ruin a Good Moped

hmm.. that engine has piston ported intake?

That means the pin had to get past the piston skirt in order to enter the crankcase.. so dont be alarmed if the piston skirt has damage.. possibly the walls of the cylinder too.

However, these are relatively inexpensive to replace and minor cylinder wall damage can be fixed or lived with..

i followed the carb thread and the solution you chose looked like a good idea.. mighta tried it myself if there was no better way to provide the slide with an alternative guide slot / guide pin.. Don't get too depressed about it.

Re: How to Ruin a Good Moped

Thanks for the encouragement, joew, I'm not looking forward to opening it up and seeing the damage. I'm not sure what exactly you mean by ported intake... my impression was that the pin would end up below the piston, but are you saying it could have got up above, in the combustion chamber?

Re: How to Ruin a Good Moped

no.. not in the combustion chamber .. well.. hopefully not.

Fuel enters through a hole in the side of the cylinder on a piston-ported intake. The piston, as it moves up and down, opens and closes this intake hole. The renegade slide-pin would need to get past the quickly moving piston. When it does it, it ends up below the piston, inside the crankcase.

If the pin got past the piston and into the crankcase and was chopped into little pieces, the pieces might get blown upwards and into the combustion chamber through the transfer ports.. this is the normal fuel pathway and might carry the small, broken pieces.


Another system is a reed intake. Here, fuel (and the pin) would enter the crankcase directly. The piston's movement does not "time" the intake pulses.. a reed valve does it instead.

dont be a freakin coward! open it up.. :)

Re: How to Ruin a Good Moped


You really hit the jackpot, my friend - sorry to hear it. It really was a clever and worthy repair attempt. My inner Edwardian Englishman raises his pint to you.

Chances are that it's either the top or bottom end that is damaged, but not likely both. If you were lucky, the pin got snagged in the intake or transfer port(s), and wedged up the piston & cylinder. New top ends are not that expensive - you can just go for it, and buy a 70cc Airsal kit for around $120, if you want.

Pulling off the head & cylinder will pretty much tell the tale. If it's all clean and nice in there, you know whee to look next.

I've got a spare Motobecane 40T engine here, which is also locked up, but might contain some useful parts. I'll try getting it apart in the next few days, and see if anything is useful. Moby engines show up regularly on eBay, and our comrade MoPedLar might also have what you need - he's been a great help in the parts department.

Re: How to Ruin a Good Moped

George Smith /

i did something similar to that with a honda's just a sorry way of ruining it and everyone does it at least once

Re: How to Ruin a Good Moped

I dug into the spare motor, and there is nothing useful in it. This is actually an understatement, as it's completely and utterly destroyed. Worst I've seen; it's filed with some rusty, greasy, sticky fibrous stuff, and locked up solid.

It's (The Worst) Motor (In Scotland) .. for you Trainspotting fans.

Re: How to Ruin a Good Moped

i commend you on your creative story telling style.

I cant help but notice that pretty much whenever JB Weld is mentioned on this forum, it is in the negative.

Re: How to Ruin a Good Moped

I have some spare model 40 top ends, and pistons.

email me if you are in need.

Re: How to Ruin a Good Moped

"My inner Edwardian Englishman raises his pint to you."

LOL Good one


Re: How to Ruin a Good Moped


JB Weld has many good uses, and I have used that stuff myself. They REALLY work. Although, I wouldn't have used it for a guide pin...

raises a pint LOL


Re: How to Ruin a Good Moped

Yeah, I have no hard feelings towards J-B Weld.... the surface area of the joined parts was pretty small, maybe 2.5 square millimeters. It's got a tensile strength of something like 3900 lbs/sq. inch, which works out to only about 6lbs per sq mm, so it only took about 15 lbs to break it off. I wouldn't have used it if I'd done the numbers in advance.

Re: How to Ruin a Good Moped

J-B Weld is great stuff, I've used it to fix everything from carb bodies to engine crankcases. Solves a lot more problems than it creates, in my experience.

It has a very long working time, and takes at least 24 hours to completely cure - but this is one of the reasons it's so strong. With epoxy-type products, the longer the curing time, the stronger the bond. This is why 5-minute epoxy has a much lower strength rating than the 24 hour stuff.

Re: How to Ruin a Good Moped

George Smith /

hey i had a spark plug jbwelded into a cracked head for 3 finally was tossed and replaced....probably still be that way if i didnt find a head cheap

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