1976 Motobecane Cady moped won't start

I hope the members here can solve this problem.

I have 2- 1976 Motobecane Cady mopeds. Both actual 378 miles since new. Got them and cleaned out the carbs, gas tanks, sparks etc. All the normal things. One started and purrs beautifully. This is good as we can diagnose for one to the other. The other had no spark.

1. On the no spark moped we found the timing was severely out of timing.

2. I replaced the condenser and points. With the timing set properly we now have great spark.

3. If I put gas directly into the cylinder we great some spark and the engine runs only on that gas. Thinking it was the carburetor, I placed the carb in question onto the moped which runs and it fires up easily. So I ruled out the carb.

4. Next thinking it was gas flow I hooked an external gas tank and line directly to the carb. Gas flows but again the engine won't fire.

5. I have made sure the seals on the exhaust area are tight thinking a air leakage problem. Didn't help.

6. I must be missing something simple as we have spark, gas and air. Should I disconnect the choke cable? When I move the throttle you can feel the movement at the carb.

I would really like to solve this mystery. Any and all help is appreciated.

Re: 1976 Motobecane Cady moped won't start

It would appear that the engine is not “sucking” fuel from the carb into the engine. I made sure the tube from the carb to the engine was clear and the gaskets that attaches the tube to the engine are tight.

Re: 1976 Motobecane Cady moped won't start

Blown crank seals, maybe?

Re: 1976 Motobecane Cady moped won't start

Pushrod Fifty /

The decomp cable isnt too tight, resulting in compression loss? Muffler clogged? Easy enough to take the end cap off and check.

Re: 1976 Motobecane Cady moped won't start

New decomp cable, muffler taken apart fully cleaned all pipes to motor clean no carbon build up. Thought of a loss of suction from poor exhaust fittings, checked for proper gaskets and re torqued.

Re: 1976 Motobecane Cady moped won't start

Any way to test crank seal integrity? Would there be a loss of compression if we tested it?

Re: 1976 Motobecane Cady moped won't start

Pushrod Fifty /

How are you setting the timing? If it was severely off before, how did it get that way? Its tricky because there are no lines to time to, you need 3 tools or methods- you need to know TDC and from there have a way to know when you are 1.5mm BTDC when the crank is turned in the normal direction, and a way to know if the points are opening at that exact time. Then you need to be sure the cam didnt move when the fly bolt was torqued down.

Re: 1976 Motobecane Cady moped won't start

We are 1.5mm BTDC. it was measured and we have excellent spark. Once we set the timing we checked for spark and we were bright blue. When it didn't start we pulled the clutch off again and checked the timing and we are still 1.5 and the points open at the proper time.

The problem appears to be that the engine is not pulling gas into the cylinder.

Re: 1976 Motobecane Cady moped won't start

Pushrod Fifty /

Dirty carb jets I would say.

Re: 1976 Motobecane Cady moped won't start

carb was cleaned and it didn't start. took the carb in question and placed it on our second moped. The second moped started with ease (with the carb in question) and runs. knowing both carbs work, we swapped out the carbs and we still have a no go.

Re: 1976 Motobecane Cady moped won't start

To check suction I was told to remove the spark plug and have someone place their finger on the hole. If good vacuum then it willl create a good seal. If it doesn't then what do it mean?

Re: 1976 Motobecane Cady moped won't start

Pushrod Fifty /

You need to use a compression tester to check but if you feel compression resistance it should be enough to start. Compare cranking force between both bikes. When you crank it over do you smell gas or do you see that the plug gets wet? Are you releasing the decomp as soon as tbe engine starts to spin?

Re: 1976 Motobecane Cady moped won't start

Crank seals bad ?

Air leak ?

Re: 1976 Motobecane Cady moped won't start

That is what I'm trying to find out tonight. Any easy ways to figure out leaks or seal troubles?

Re: 1976 Motobecane Cady moped won't start

Not easy , but , a leak down test should show were any leakage is .

Re: 1976 Motobecane Cady moped won't start

Pushrod Fifty /

How did you time it just curious.

Re: 1976 Motobecane Cady moped won't start

I would suggest to give it a leakdown test to see if the spi seal is good or not. No crank seal on those isodyne engines. Sealing on the left side is via a 6303-2RS (17x47x14) sealed bearing. Might be the seal on the right side carter cover plate, lots of folks have removed that plate and not considered that it needs a very good seal so it does not suck air.

Also make sure there is not restriction in the exhause or ar the exhause port window.

Re: 1976 Motobecane Cady moped won't start

I think the leak down test is our next diagnostic procedure. Can you clarify "spi seal" location. what does spi stand for? Does anyone know a good source to purchase these seals for this motor?

Re: 1976 Motobecane Cady moped won't start

Pushrod Fifty /

Hard to believe that the seals would be so far deteriorated that the engine wont even start. Any evidence of case leakage? Only use seals that have specific markings that they are made in Europe, Germany, Japan, like SKF. No Chinese or unmarked, or you will be replacing them again. Real soon. Waste of time.

Re: 1976 Motobecane Cady moped won't start

should I release the decompression lever at the beginning? I usually get the moped spinning and then release when my legs are strong enough to pedal

Re: 1976 Motobecane Cady moped won't start

Pushrod Fifty /

Release as soon as the engine starts spinning.

Re: 1976 Motobecane Cady moped won't start

Seal issue is not explained well by me so I will try again.

The isodyne and super isodine engine do not have a crank seal, they rely on a seal bearing on the left engine side and the casing seal on the right side. Sealing on the left side is via a 6303-2RS (17x47x14) sealed bearing, C3 specification is the Motobecane specified tolerance. Right side is the gasket for the right side casing to seal the carter. If you have a super isodyne engine then there is a right side bearing as well, it is 6202 (15x35x11) open bearing, C3 is again the tolerance you need.

Re: 1976 Motobecane Cady moped won't start

Thank you for the explanation. In talking to the original owners this is what happened the day it stopped running. Having these mopeds new in 1976, the moped was used by the kids for some 300 miles. The father wanting to try it took it for a spin and when turning he had a crash. He and the moped went down and in the process snapped the right pedal off the moped crank arm.

Since that time the moped has not started. The kids attempted to use it without the pedal by pushing each other to start it but that didn't work. We must be missing some simple thing that is being overlooked. Why would it just stop running. I don't think the seals would be damaged from father's fall off the moped.

When I got it I had the pedal fixed and when we attempted to start it there was no spark. That is how this whole process started. Points, condenser, spark plug, timing all done and now have bright blue spark. Carb is set properly and swapped out on other one and still no go. Checked the wires of the engine for continuity as they may have broken within the plastic coating. All tested so far are testing normal.

To me it appears the gas is not getting into the engine. I will attempt the leak down test this weekend. If I am missing anything , please feel free to respond after having this additional info.

Re: 1976 Motobecane Cady moped won't start

Disconnect the fuel line from the carb and let some drain into a clear glass bottle or jar. Check for good flow as well as "bubbles" of water in the fuel.

If the flow seems to be anemic then use a bicycle tire pump and give it a gentle stroke of air to back flow some air up the fuel line. The gentle push of air up the line could dislodge some crappo at the saran screen on the petcock and allow good fuel flow. Would want to properly drain the tank and at least give it a goodly flush out if it does have crap or water. Very likely the dropping of the bike could have dislodged crap in the tank and it is obstructing fuel flow.

If you have water bubbles in the fuel again you would want to drain the tank well, give it a flush with some methyl hydrate (get that at Homehardware or CanadianTire in the paint section) the 3,78 litre bottle will be perfect for a flush. Drain the methyl hydrate into a catch container for reuse, you can strain the particulates out with some cheese cloth or fine steel wool in a funnel and decant it back to original container that way.

The AR1 Gurtner carby likes to be surgically clean, nothing in there that would be ruined by carb cleaner so you can go to town with that to clear the little orifices in the carb. There is a second saran screen in the carb, that needs a goodly clean. On reassembly the pointy end of the float needle points up (very common ooops on the float).

DO NOT CLEAN THE PETCOCK WITH CARB CLEANER. Or you will need to purchase a replacement. The seal inside will swell and it will be immediately problematic or disfunctional. It does not mind methyl hydrate so you can try to back flush with that if you need to try to get proper flow. This is the replacement if it is kaput - https://www.treatland.tv/motobecane-av88-cock-8mm-thread-p/motobecane-av88-cock-n-line.htm

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