Motobecane AV7/AV10 cylinder struggle

Peter Booth /

Jan 07, 2023 02:11PM

My son and I got two complete Motobecane Mobys from a guy and are trying to make on runner out of them.

We took both motors apart and the motors are pretty shot. We bought a new stock crank, crank bearings, seals, piston, cylinder and gaskets from Treatland (pretty much everything but the head and the cases).

We put it all together and all was well until we got to putting the head on. With the head NOT in place, the piston goes up and down in the cylinder fine. However, when you put the head on and begin to tighten the 10mm nuts on top, the piston gets bound up in the cylinder before TDC. So we're not hitting the head or anything and the sparkplug is not in. If you push the cylinder forward with force, you can get the piston to move.

My conclusion is that something is making the cylinder and piston "out of true" when the head is bolted on.

Things I've looked at/considered and my conclusions...

(a) Crank (which is new) isn't centered in the case. (This wouldn't create the current behavior as the piston can move side to side on the connecting rod)

(b) Connecting rod (which is new) is bent. I checked this fairly carefully (by eye, just watching the connecting rod go up and down with no piston/cylinder and using the seam of the cases as my "micrometer" to look for movement)

(c) One or more cylinder studs (which are old) are bent and torquing the cylinder out of position. Nope, I pulled them all out and they're dead straight.

I took the motor to my friend who is the head mechanic at the local Yamaha/Kawasaki dealership ("What a cute little motor," he said when I brought it over to his house. 😂) He's the one who found that pushing on the cylinder can "free up" the piston. Pulling on the studs can also free up the piston which is what made him think a stud might be bent. He did say that he was shocked there weren't locating pins at the bottom of the cylinder/top of the case and said it was weird to him that there was so much free movement in the cylinder before the head is installed.

So I come to all you hoping someone has had this experience or can offer a troubleshooting idea I haven't thought of. To my mind, the only thing I haven't checked are the sides of the cases themselves.

I'm just completely stumped....

Re: Motobecane AV7/AV10 cylinder struggle

Fred Braue /

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fredb

Jan 07, 2023 02:22PM

pictures would help some. lets see the assembly without the head on. some quick odd thoughts. cylinder stud bent or crooked in the crankcase. something wrong with the stud centers in the crankcase, a bad cylinder, i would also want to know did you buy a stock cylinder or performance overbore/big bore type. when putting the head on use a little silly putty on the cylinder head to see if you are just ticking the head or the chamber is a bit out of round . doubt it is the crank but for stock crank designs on the AV7 the bearings are 6307 with an odd taper that goes towards the crank side only

Re: Motobecane AV7/AV10 cylinder struggle

Peter Booth /

peterbooth OP

Jan 07, 2023 02:34PM

Here's a link to a Google Drive with a photo of the assembly and then videos of....

piston moving with no head

piston moving with head resting on cylinder

piston moving with difficulty as head nuts are put on

piston locking up when head nuts are tightened up.

https://drive.google.com/drive/folders/193kHtT5HwfTx1mfhgfzIDXc3gTwdny8-?usp=sharing

Re: Motobecane AV7/AV10 cylinder struggle

Totally normal JBOT /

capturedbyrobots

Jan 07, 2023 03:48PM

have you tried a thicker base gasket to see if maybe your squish band is getting hit by the piston at tdc?

In order to see if it’s indeed hitting the head, put spacers on the studs between the cyl and nut and tighten down with no head.

If it doesn’t stick then you are hitting the head and need to add head gasket or add base spacer.

My bet is that you’re hitting the head or the new seals and bearings are making it hard to spin over. Did you lube the seals during installation?

As well, is this a 70kit or a 50cc? Orig stock head with a 70cc kit will hit for sure without modding the head or spacing it out a bunch (edited)

Re: Motobecane AV7/AV10 cylinder struggle

Peter Booth /

peterbooth OP

Jan 07, 2023 04:20PM

Thanks for your thoughts. It is binding up well before TDC. I had the same thought originally: If the head makes it catch, it must be hitting the head. Nope, something is making the cylinder/piston not match perfectly as the piston is catching about 70% up from bottom dead center. (So 30% below TDC)

Re: Motobecane AV7/AV10 cylinder struggle

Gen Xer /

neflfixitdude

Jan 07, 2023 04:49PM

You make no mention of how you set up the rings. Did you gap them properly and how are you inserting the cylinder onto the piston? How are you lubing the piston and cylinder for assembly? Any issues with the chamfer (presuming there is one) on the skirt of the cylinder? Have you read Rebel Moby's collection of posts in the wiki?

Re: Motobecane AV7/AV10 cylinder struggle

Totally normal JBOT /

capturedbyrobots

Jan 07, 2023 04:55PM

Then you need to try the piston drop test and disassemble to see where you’re seeing wear on the cyl or piston. If it’s tight it’ll have rub marks.

And yeah if your ring gap is too tight it can be scraping the shit out of your bore, possibly your ring is catching on a port idk

Or your cases aren’t flat at the base gasket area possibly

What I would do is pull the piston and cyl and put them together with rings and push it thru the bore by hand. If it slides well and feels ok it’s not the kit. Gotta be systematic. Eliminate the kit as the problem first (edited)

Re: Motobecane AV7/AV10 cylinder struggle

Peter Booth /

peterbooth OP

Jan 07, 2023 05:38PM

LOVE all these ideas! I'll start with piston/cylinder alone to see what that does.

The ring gaps are oriented to the right and left of the front of the cylinder so they're not catching on any ports.

I may also reassemble the whole thing with no rings to see what that does. I'm off to sleuth tonight with all the suggestions. Thanks all!

Stand by for updates!

Re: Motobecane AV7/AV10 cylinder struggle

Beach Club Nick /

khool1499

Jan 07, 2023 06:51PM

> Peter Booth wrote:

> -------------------------------------------------------

> LOVE all these ideas! I'll start with piston/cylinder alone to see what

> that does.

>

> The ring gaps are oriented to the right and left of the front of the

> cylinder so they're not catching on any ports.

>

> I may also reassemble the whole thing with no rings to see what that

> does. I'm off to sleuth tonight with all the suggestions. Thanks all!

>

> Stand by for updates!

If you didn't gap the rings and the ring gap is too tight then they can bulge into the ports. It sounds like you didn't measure or correct the ring gap.

Re: Motobecane AV7/AV10 cylinder struggle

Peter Booth /

peterbooth OP

Jan 07, 2023 07:20PM

Well I think I've got the answer! It looks like the connecting rod is scraping against the piston skirt because the cut-out on one side of the cylinder skirt (to allow the connecting rod to pass the skirt) was not cut out properly. (So as the big end is moving from 6 oclock to 9 clock and then from 9 clock up to 12 oclock, the rod is getting locked up by the edge of the skirt.) I'm going to take a dremel to that point on the cylinder skirt tomorrow and hopefully will have good news to report. Thanks for all the ideas folks.

Re: Motobecane AV7/AV10 cylinder struggle

Totally normal JBOT /

capturedbyrobots

Jan 07, 2023 09:07PM

That’ll do it. Just wash it well

Re: Motobecane AV7/AV10 cylinder struggle

Garelli Master✨ /

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stephens

Jan 08, 2023 06:18AM

Your connecting rod is a tad thicker making it slightly hit the inner bottom of the cylinder skirt on the cylinder when the rod swings around. If you pull the cylinder back off and closely inspect you'll see slight markings on the connecting rod and the inner bottom of the skirt. Take a Dremel or a file and clearance that spot and you'll be good to go. I've had this happen on a few AV7's but never a AV10 yet.

Re: Motobecane AV7/AV10 cylinder struggle

Peter Booth /

peterbooth OP

Jan 08, 2023 01:08PM

This was exactly the problem. I just hit it with a dremel and it is smooth like buttah! Thanks for all the ideas and problem solving. Much appreciated!! Now the project can move forward!

MOPED ARMY

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