Motobecane SHA 15.15 Dellorto..? or others

julian schroder /

I was wondering what is the best carburetor for a 78 Motobecane Mobylette I was gonna go with a SHA 15.15 but i heard there are better carbs for it. also was wondering weather, the arisal 70cc kit or the 70cc parmakit and is it compatable just bolt on or are there some issues. P.S. i have a av7 crankcase if that helps. Also is it alright for the head from a 50cc good for the body of a 70cc kit?

Re: Motobecane SHA 15.15 Dellorto..? or others

Michael Sheeler /

Depends.

Re: Motobecane SHA 15.15 Dellorto..? or others

julian schroder /

Depend on what .?

Re: Motobecane SHA 15.15 Dellorto..? or others

Brent Bublitz /

It sounds like you are pretty new to this. The 15.15SHA is perfectly fine and can get you some real speed. Stick with that until you learn a little more.

Re: Motobecane SHA 15.15 Dellorto..? or others

Replace the crank. You can then use the cheap LePartie CDI with a woodruff key and stop fucking with timing settings. The Airsal is cheaper casting than the Parmakit. With a little prep work they are close to the same.

15mm or 16mm will be fine. The stock head is fine with a gasket or polished up to run without a gasket, the stock pipe will blast but no power gain like a chamber pipe. The stock crank is like a 3musketeers candy bar. Soft as fuck on the inside.

Re: Motobecane SHA 15.15 Dellorto..? or others

stock head will work ok depending on cylinder deck hight. if the piston crown goes up to cylinder deck, you may get too much compression, causing overheating and failure to wind out to potential.

Re: Motobecane SHA 15.15 Dellorto..? or others

julian schroder /

Thanks man and any exhaust favorites.

Re: Motobecane SHA 15.15 Dellorto..? or others

My moby had a stock pipe. I liked it a lot had plenty of acceleration where I was able to keep up with cars or even faster on take off. top speed of 43mph. I didn't even consider a aftermaket pipe cause the stock one did everything I needed it to. (edited)

Re: Motobecane SHA 15.15 Dellorto..? or others

Brent Bublitz /

moby mcfly Wrote:

-------------------------------------------------------

> My moby had a stock pipe. I liked it a lot had

> plenty of acceleration where I was able to keep up

> with cars or even faster on take off. top speed of

> 43mph. I didn't even consider a aftermaket pipe

> cause the stock one did everything I needed it to.

That's about where I'm at on my moby. Does 43 with the stock pipe and an Airsail. I'm looking to upgrade with a pipe and carb though. I'm running hot and I think the 14.12 can't move enough air to keep me from leaning out. Need to get my head temps down and looking for that magic 50.

Re: Motobecane SHA 15.15 Dellorto..? or others

I'm now running the Gianelli black gun, but have yet to retune for the pipe. still going low 40's like it did with stock deresricted blunderbus, but is running rich with the new pipe, we'll see after I rejet and/or retime.my set up- 74 airsal, lowered deck, with Mars head, 19 phbg, black gun, 11x48 gears.

Re: Motobecane SHA 15.15 Dellorto..? or others

how hot brent? with the 15.15 my temps rarely peaked 410 or so after i fixed a exhaust leak. I was using a lot of choke to keep temps down. I was recommended a 19phbg or 16.16 for more tuning options. but the 15.15 was definitely efficient (edited)

Re: Motobecane SHA 15.15 Dellorto..? or others

well, sha's do go lean on top but that's why you gotta tune the main at WOT and use modifications to lean out the low end, although usually that is more a function of a pipe.

if you're running that hot i'd say pull timing, pull compression

the 10 cent solution might be just going to a thicker base gasket, giving you a little free porting and dropping compression a smidge. otherwise some additional de-restricting of the stock pipe might get you there, you can cut the tube or cut it shorter for the outlet nozzle snooter.

Re: Motobecane SHA 15.15 Dellorto..? or others

Hey Graham, what do you mean by "free porting? when adding a thicker base gasket?

Also, if I wanted to reduce compression but I have indexed the exhaust port to BDC I could just toss a metal gasket between the head and cyl correct? even though its an O-Ring head.

Re: Motobecane SHA 15.15 Dellorto..? or others

I wouldn't mess with the head gasket on an o-ring head. Head gasket is a higher pressure seal and will be more likely to leak if you do some janky shit.

base gasket is a lower pressure seal so you can get away with more jank.

going to a thicker gasket (the cardboard from the back of a high-quality legal pad is really good stuff, like 2.5mm thick- 2mm compressed) will raise your exhaust and transfer ports, which has a similar effect to porting them.

some people call this a 'rev plate'... its not ideal, much better to do acutal porting, but it is 100% reverseable and lets you see how raising the ports 1mm or so will affect your powerband.

using thick cardboard is more likely to compress unevenly if you don't use a torque wrench, which can bind your piston and smoke ya. You can also cut a plate out of aluminum and sandwich it between thin gaskets, or anneal it and use motoseal, or any number of things to raise the cylinder a little bit.

it will also drop the compression on top.

Re: Motobecane SHA 15.15 Dellorto..? or others

Graham so what temps should I be looking for? I read somewhere that 400 was a good number. I have been confused on this for a while now and want to set this straight in my head so I no longer have to worry about. Im about to re-read the tuners handbook to straighten my confusion out.

Re: Motobecane SHA 15.15 Dellorto..? or others

Gotcha.

I have a library of gasket thicknesses as I had a puch TCCD 70 cc kit and you have to deck the hell out of that cyl.

last year 3 av10s setups with Doppler heads and cyl. about 1.0mm squish.

over the winter (since I fried a couple the last year) I placed a thicker gasket on the base. .7mm and 1.0mm. the squish on one was 1.2 the other 1.43mm.

really didn't see a huge drop in performance between the 1.0 and 1.43mm as far as top end speed. and I was willing to sacrifice 5 mph but I don't think I lost anything.

On a build I am currently working on, I was going to deck the cyl to match up the exhaust port and BDC and then use a manufactured moby 50cc metal head gasket to add to squish. Reason is I really am in the dark about over decking the base gasket and having the exhaust port and piston not synced together.

what are (if any)the negative impacts on exhaust timing if you "over" deck the base gasket?

I also switched the 3rd av10 to a bidalot head. this vs' the doppler head with the same base gasket added a significant amount of squish to the setup.

Re: Motobecane SHA 15.15 Dellorto..? or others

hmm, well... unless i'm getting radical with crazy stuff, i line up the piston to the bottom of the transfers, there are some theories about minor misalignment in the transfer-to-piston-crown interface leading to badness... there are also some theories about anti-reversion stuff that says its good to have a misalignment there... woah! too much thinking for me, i just set my cylinder base spacing to get that perfecto, then i port the exhaust to match. Usually i let the piston crown dip a cunthair below the exhaust port floor, just so i don't get a hot spot on the piston. maybe i made that up, maybe it works, i dunno, my pistons run pretty cool.

if you've got fancy schmansy head machining stuff, you can cut your head to compensate for whatever the deck height comes out to, or machine your cylinder. If i don't have that equipment, i get it close or would pay someone to machine my jug if it was super far off. You can fix your squish a little bit with the head gasket, but you gotta use good ones, aluminum or copper or whatever. Beer can is ok, but copper is better.

i haven't really seen too many of the commercially produced heads that i like, i think they are set up for high rpm AV10's with crazy exhaust timing. I prefer something with a smaller squish and a bigger taper. 1.0-1.3 squish clearance is usually what i'm shooting for.

As for the temp thing, i'm assuming you mean CHT... this is tricky, not an easy answer. First of all stupid swingy french motors run hot head temp because they don't get good airflow. Second the fins are small and third they are variated so they are ripping high RPM when not moving very fast and getting good cool air.

The piston temp is really what you care about, not the cylinder head temp. You can get your CHT up to 700 degrees where aluminum begins to loose strength, without causing any harm to your engine.

if your piston face gets hot, however, you will sieze... bummer.

If you're careful to keep your piston cool, 400, 450, all day long on the CHT. If your piston is hotter than your head for some reason you can sieze at 380.

I watch the piston face and underside for black carbon buildup. That means your oil is burning and you're piston is 400-450 degrees. If you're paying attention and notice your temps are consistent, say, 410, and your piston isn't building up black carbon, you are safe at that temp and can assume short blasts up to 420, 430 are safe. If you are running 390 and getting a lot of gunk on the piston, you need to do something to cool the piston down and you will know that a head temp of 390= danger zone.

the CHT thing is really only useful as a comparison tool, to tell you if something has gone wrong or to compare from one similar setup to another.

Re: Motobecane SHA 15.15 Dellorto..? or others

great info Graham! cheers....

Re: Motobecane SHA 15.15 Dellorto..? or others

Brent Bublitz /

Jumping back in, motor has an airsail on it and I was seeing temps pushing around 430 and up. I started to feel what might have been really light soft seizes and started getting really nervous about it.

I'm getting the tank brazed right now anyhow as no one has been able to come up with a clean replacement frame, so the point is moot for a few days anyhow until I can get it all back together. I'm going to pull the head and check the cylinder in the mean time. I like the idea of doubling up the base gasket as well, might try that and see what my compression numbers look like.

« Go to Topics — end of thread

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account