Motobecane av88 - only runs at high rpm

Need some help please! This is about a ~1967 AV88 motobecane mobylette (the blue ones).

New to me a few months ago. Started/ran fine every time (30mph) until the carb (dellorto sha 14.12) rattled off during a short flat run. Visual inspection fine, popped it back on and it ran as good as before at WOT. But, wouldn't idle and wouldn't run below half throttle, as soon as you slow down it dies. Takes a lot of peddling to get it going fast enough to start if it's off the stand!

Popped carb off, inspect/disassemble/clean/reassemble, but it was very clean with clear passages already. Still no improvement.

Checked spark, it's strong and the plug has less than an hour on it, still in like new condition.

It's puzzling to me since it worked fine before it rattled loose.

Any thoughts appreciated. Thanks!

Re: Motobecane av88 - only runs at high rpm

It could be an air-leak "issue" with either where it bolts to the manifold, or maybe the top cap gasket was compromised during your cleaning/reassembly. Make sure you have an o-ring to seal where the carb meets the manifold (since you said it started happening) after the carb "fell off" to seal that junction. You have to physically push the carb against the o-ring and manifold to get a good seal.

Another possibility is that there was an air leak in the first place and the previous owner compensated by up-jetting, now that you have it sealed properly, you're having a too-rich condition?

Re: Motobecane av88 - only runs at high rpm

The idle passage is below the slide and easy to miss.

Re: Motobecane av88 - only runs at high rpm

Throw in a oring at the union between the carb and the intake, 5/8 x 13/16 x 3/32 oring is a great fit into the 14mm carb sizes.

Might have smeared the piston a little when it spooled down with no fuel if it happened when the engine was already fully heat soaked. May need to clean up the piston and throw in new rings.

Re: Motobecane av88 - only runs at high rpm

Thanks everyone. There's def an o-ring in there and I held it on firmly when tightening, but in case its leaking I ordered a new one. Will try that and post back.

Re: Motobecane av88 - only runs at high rpm

OK, time for plan B I think.

Swapped it out for a new Dellorto 15.15 with a 60 jet. New muffler has basically no carbon build up. All carb seals good and tight. And same thing happening, only starts if I pedal the heck out of it and dies below half throttle. I have other jets but I think 60 should be close enough to tell if the carb was the problem which it doesn’t seem to be.

Could it be the timing is off a bit? The points are clean and gap is correct. Any other ideas?

Help appreciated!

Re: Motobecane av88 - only runs at high rpm

What is the compression reading ?

Re: Motobecane av88 - only runs at high rpm

Seriously, check the Piston and cylinder before you run it anymore...

Undo the head bolts and tilt the engine down. Pull the Carby an slippy the cyl off...I'm with rebel here, your gonna find something inside.

Re: Motobecane av88 - only runs at high rpm

If you upgraded to a SHA15:15 I hope you also changed out the intake from the stock 13,8mm ID intake or you may just be compounding the issue by adding a restriction after the carb.

Thinking that you might have smeared the rings when the union between intake and carb failed. Needs a bit more TLC and freshening up the top end is a goodly thing to do for a old old bike.

Get a upper gasket set (head, base, exhause, intake) and a set of rings (FG type) pull the jug and check that the poor old old rings are not stuck in the grooves. Chances the rings are tired anyhow over the years so goodly time to fresh them up. Gap the rings before installing them to the piston (0,15-0,16mm), they do not come pre-gapped (close but they will be a wee bit too tight)

If there is any slight smear of aluminium on the chrome cylinder wall you can use a Qtip to carefully apply a little Drano to the smear ONLY (it WILL eat aluminium quickly so careful not to get it anywhere else) then buff out the now flaky smear with a clean rag.

Decoke the head and the exhause tract while you have the jug off. Use some oven cleaner inside the exhause (dont get any on the aluminium, some are very harsh on aluminium). Follow the instruction on the can and flush out the old burned on coke with a hose. Maybe you have to do this twice if it is very coked up. Take the end cap off the silencer, remove the body sleeve and clean out the baffle holes and the holes in the piccolo tube exit on the cap. Use a old drill bit in your hand as a reamer for that, also clean out the exit pipe with the drill bit.

Reassemble after cleaning everything, use some two stroke oil as assembly lube so there is no scratching.

IMPORTANT you MUST use a new crush gasket on the exhause big nut when reassembling AND you must check it is right tight AND recheck it when the engine is hot hot so that it does not shake loose from normal engine vibrations. If the nut shakes loose it will wear out the threads in the cylinder rather quickly.

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