Moby fine tuning

Finally got my Moby on the road, woo hoo! Everything seems to be basically in order, starts right up and idles fine, clutch is smooth, etc. It pulls great, but seems to top out at about 23, so I'm guessing I need to do a little fine-tuning.

Two specific questions:

1. I'd like to clean out the exhaust port, but I'm concerned about the copper crush gasket that seals it. Are replacement gaskets available somewhere, or is there an alternative that's commonly used?

2. Is there anything specific to these bikes that I should look for other than the basics? I've cleaned the carb and gas tank and done the usual stuff, but this is my first Motobecane and it hadn't run in about 20 years before today. Is there anything less obvious that I need to check? It's a 1979, variated, 12v system, Gurtner carb, about 900 miles.

What a great ride this thing has, I'm really excited to get it running its best. Any help is appreciated, thanks!

Re: Moby fine tuning

no experience with motobecanes but cant you just remove the baffle and clean it? that should help.

Re: Moby fine tuning

🇮🇹💦 Of the Loin /

the exhausts dont tend to get too gunked up, but yes you can just unscrew it and the crush gasket should be fine.

while you are at it pick up a dellorto 14.12 or a 15.15 and take off that old nasty gurtner

Re: Moby fine tuning

Thanks. I'm also wondering about the belt--I've heard that belts can make a big performance difference on these, and don't know what's on mine as it was there when I bought it.

It's a v-belt, a gray/whitish color and it looks to be covered with nylon or some other kind of fabric. I'm guessing that it's the original belt as the thing only has 900 miles on it, but I have no way of knowing. Anybody have any idea whether this is the right belt, and if not, what should I look for?

Re: Moby fine tuning

Mike Scouty McScoutington /

you can go to any auto parts store or hardware store and you should be able to get a replacement. i suggest the 17320 from a hardware store. they are toothed and are slightly fatter then the other possiblity the 4L320. i have run both and they work just fine. you can get one for between $3.50 and $6 most places. you may find that a 320 size belt is a little big or small. if that happens to be the case (due to either really good or really bad engine mounts ect.) you may need a 4L315 or 4L325. those belt sizes are a little tougher to find. you may want to go through and check/lube/change your wheel bearings and grease your clutch (careful not to get the clutch shoes). in addition clean all of your electrical connection points. you would not believe how much power and responce you can gain from having a super nice and clean electrical system. i suggest a product called "lectromotive" to clean the electrical parts (again avail. at an auto parts store) and some fice grit sandpaper or steel wool.

Re: Moby fine tuning

Leon Swarmer /

With only 900 miles I'd leave the exhaust port alone.

Re: Moby fine tuning

Mike Scouty McScoutington /

to be honest, with a little mechanical know how you are not going to mess anything up. i would even suggest you grab a copy of the repair manual and rip the whole engine apart this winter and make sure you have good rings and such. thats how i learned everything i know about these bikes. if you do feel comfertable doing that it gives you a great chance to shine the bike and engine up real nice.

Re: Moby fine tuning

I think I'm going to take the head off and see if there's a bunch of carbon in there (assuming I can get a wrench on those back two nuts) and then decide whether to take the exhaust off based on that. Probably won't take the whole engine apart, it seems to run very well, no smoke or bearing noise or other bad stuff.

There seems to be a lot of rolling resistance, as in it's hard to push forward and hard to pedal, even though the wheels, drive pulleys etc seem to turn freely. So I'm going to go through that stuff again and see if I can grease the variator bearing a little better. And I'm looking for a toothed belt, haven't found one yet.

One other question, I bought the clutch/ignition cam puller from Moped Warehouse, but it seems to be too big to thread into the variator. Are there maybe separate pullers for variated and non-variated clutches, or am I doing something wrong?

Re: Moby fine tuning

Mike Scouty McScoutington /

in order to take the head off of a moby you need to drop the whole engine. as far as the resistance goes, you should have that. the way that the clutch is set up you are always turning the engine unless you disengage the pulley. you will notice that you have almost no resistance when you pull the decompression lever and push the bike forward. as for your clutch puller, the large side is for the timing cam. both the dimoby and the variator use the same size puller. if it is a double sided puller all you have to do is take the long screw out of the puller and flip it around.

Re: Moby fine tuning

Bummer, I just had the whole engine off when I cleaned the gas tank.

But, may do it anyway, 23 mph seems awfully slow for this bike, so it seems like there may be a few things holding it back.

So far, I'm going to try:


fitting a toothed belt

re-adjusting the throttle cable and decompression cable

re-gapping the plug

loosening the chain, I think it may be just a little tight and I like to err on the loose side to protect the bearings

re-checking the carb for air leaks

cleaning the electrical contacts

Am I missing anything? Wheel and engine bearings seem good, carb is clean and it seems to be variating properly.

Re: Moby fine tuning

🇮🇹💦 Of the Loin /

do you have the straight intake? its only 10mm and the carb that comes with it is smaller, therefore restricting the bike to either 20 or 25 mph

Re: Moby fine tuning

AH HA! My carb looks like the one on the right in the attached diagram, and it's a 1979 bike. Does this mean it's a 25 MPH? If so that would clear up a lot of things--if I'm going 23 on a 25 MPH bike, I'm at least in the ballpark of where I should be, speed wise.

If so, can I change out the carb and intake to gain some speed?

I was thinking of putting a Dellorto replica on it anyway...


Re: Moby fine tuning

Find the numbers on the carb labled

tell us what they are

Look at the fork plate

tell us what the "miles" character is

If the gurtner is a, and it is not leaking, then tell all these delorto users to suck it.

you can put on a different carb later, but you have to get the bike running as well as possible stock before you can solve any of your potential problems.

I suggest checking the timing. I do not recommend pulling apart the engine just for the heck of it. check compression, if it is good, then let it be.

by all means go ahead and recrush the copper washer... no biggie

Re: Moby fine tuning

OK, here's the skinny on my Moby's carb:

The number is AR 2.10-737

The "miles" character is "B"

The intake is about an inch-long tube that comes straight off the carb, its inside diameter is 10mm.

So, it seems to me that I have a 25MPH carb. Am I right? If so, what are my options?

Re: Moby fine tuning

you have a 10mm intake and carb. possibly smaller porting within the cylinder.

find a larger "bendy" intake (it is about 13mm)

a gurtner AR2.12 or other suitable carb (search the performance tuning section)

Everybody is going to recomend getting a 15mm carb, since the largest intake you can get easily is around 13-13.5mm anything larger then that is not going to be helpfull.

I say do not buy this stuff yet. the 25mph version should be able to go about 27mph on flats. get it running better, or find the reason it is currently slow before jacking up the carb

Re: Moby fine tuning

Great, this has all been really helpful. I'm going to go through all the stuff I mentioned above anyway, make sure everything is good. I had thought this bike was supposed to go 30, and was thinking something must be seriously wrong. But like I said, at least now I know this bike is somewhat close to where it should be.

It actually seems to run very well, I can probably squeeze a few more MPH out of it with some fiddling this weekend, then I'll worry about the carb.

One more question: when I was taking the carb off, I noticed that the drive belt is still readable and says "Made in France." So I'm thinking it must be original, though it's not toothed. I'm confused--are these things supposed to have toothed belts or not?

Re: Moby fine tuning

🇮🇹💦 Of the Loin /

silly gurtner lovers

dellorto is the wave of the future, and also there isnt much of a performance increase from a 14.12 dellorto to a 15.15 unless you want to do porting, so you could get a high flow and a 14.12 and be zooming along at 35 easy once you get a bendy intake

Re: Moby fine tuning

Make sure your varriator is is the little brass fitting on the lefthand side of engine inside the spinning part of clutch. I used a needle greas fitting with the needle removed. the straight intake is for a slower machine...some states require these....My gurtner works fine...bad engine bushings can contribute to slowness by allowing the engine to move to far babck changing final drive ratio. origional belts do not have teeth but some new ones sem to and many have reported good results with them! Good luck and listen to every one on here many smart people! good luck!

Re: Moby fine tuning

Yup, that's on my list for this weekend. I've tried all sorts of things to get that stupid fitting to take grease, but no luck so far. Guess I'll try again.

The carb seems to work well too, it's just not big enough :). My engine bushings should be good too, this bike is is in almost new condition with the exception of a bent fork plate and some minor paint wear from the cables.

Again, thanks to everyone, you saved me from a weekend of racking my brain trying to get 30 mph out of a 25 mph bike.

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