Motobecane 70cc Airsal Engine Temp

Hey guys!

After riding for 15 min, my 50V with a 70cc Airsal kit is running 400 deg. Here's the setup:

-2.0 mm BTDC

-Derestricted stock exhaust

-15:15 Dellorto with 82 jet

-40:1 fuel mix

-Stock Head

-B8HS plug

-Stock 50V gearing

Is this a typical running temp for a bike set up like mine? I'd prefer to keep it around 370 max...

Re: Motobecane 70cc Airsal Engine Temp

Set timing at 1.2-1.0mm btdc.

Re: Motobecane 70cc Airsal Engine Temp

Hey thanks for the suggestion! I retarded the timing as much as possible (1.0) and it's still running at 400.

Re: Motobecane 70cc Airsal Engine Temp

It is probably the stock 50cc head is not able to handle and properly dissipate the heat from your 70cc kit.

Get a matching head if you can. Or bore out your stock head to match the dome size for the 70cc kit.

Re: Motobecane 70cc Airsal Engine Temp

Indeed - too much advance on the timing will shoot up the temps, so 1,2-1,5mm BTDC and make sure the contact gap at index is 0,38-0,40mm so you get full saturation on the exciter coil.

Stock head will work like a bad squish band on a overbore kit, best practice is to at least open up the union a little on a lathe (or even a drill press hack) to get a periphery that will angle the edge towards the center of the combustion chamber. Dont go too wild with the rework, but it can stand to drop a few points on the compression ratio anyhow at this stage.

...and Mobys tend to run hot to begin with, so keep that in mind as it will like to creep up to 400 easily. (edited)

Re: Motobecane 70cc Airsal Engine Temp

Jonathan Feldman /

On both my AV7 45mm / 47mm airsal they hit 400 pretty consistently. 45mm was maybe a solid 40 degrees cooler than the 47mm across the board but regardless they both were hot potatoes.

I never was able to get them to run closer to my comfort zone ( mid 300s ). I think it's just the inherit engine design of the moby that makes them always run hot. Sorta sucks but it is what it is. I think temps under 400 are only achievable with the 50cc moby life.

This is just my experience/ opinion doe.

Re: Motobecane 70cc Airsal Engine Temp

Thanks so much for the great advice guys! I'm not surprised the head shares some of the blame for the higher running temp. I'll try to get a matching head since I lack access to proper machine tools. Also, thanks for also confirming my theory that mobys run hot in general.

Re: Motobecane 70cc Airsal Engine Temp

> Ri E Wrote:

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

> Thanks so much for the great advice guys! I'm not surprised the head

> shares some of the blame for the higher running temp. I'll try to get a

> matching head since I lack access to proper machine tools. Also, thanks

> for also confirming my theory that mobys run hot in general.

You are absolutely on the right track with the head, HOWEVER, its a pretty common misconception that mobys have to run hot, its all about tuning

First yes get a new head, the even re-worked the fins on av7 heads are small and work poorly, get an av10 head, the Polini heads work well but have a pretty aggressive squish, if you double up the head gasket it will lower the compression and help you cool properly, the Parma head also works real well

Something that has also not been yet mentioned is a pipe, stock works well for 50cc's but an actual chamber will help get that heat out of the motor

Next jetting, I love me a sha but a multi circuit carb will allow you to tune for each portion of your throttle, if you wanna keep the sha, the UpJet 3d printed sha slide will help as it moves a lot of your top end throttle onto your main jet and away from the slide

Timing seems like your fine, I run 1.1mm btdc on my aggressive bikes, 1.4mm on less speed oriented bikes

Re: Motobecane 70cc Airsal Engine Temp

You can carefully cut your own with a baseball and a drill press the old school way. Modify the baseball by drilling and inserting a piece of 1/4 rod so it will go into a chuck. You need to be well centered on the press to get it accurate enough that it does not wobble. Then use some cloverleaf grinding compound, Grade C will cut quick enough without gouging. Use the headgasket for the cylinder bore to trace the bore to the head with a sharpie. Chuck the ball in the press and center the head, smear on a generous amount of cloverleaf. You are only using the press for accuracy, let the compound to the work dont force it. If it sounds like the compound is not cutting anymore then wipe out the expired grease and add a bit more. Cut until you reach the drawn bore marker line and double check with the gasket. Continue until you have a match.

The compression humps will take a bit of the cutting, but those are in the way of good flow to the spark plug, which is also shrouded quite a bit and could take some rework as well while you are fixing the squish.

Re: Motobecane 70cc Airsal Engine Temp

it is actually very common for 70cc kits to push 400F on mobys. even the high performance 50cc kits get hot, but shed heat a lot faster than the 70cc kits. your best bet is to switch to a more tunable carb than the SHA so that you are in optimal fuel/air mix for the entire throttle range. SHA is usually either too lean at the top or too rich at the bottom - always a trade-off

Re: Motobecane 70cc Airsal Engine Temp

I was running the AV7 on my old 50v with an the 75cc Airsal kit that I derestricted,15 SHA, Treats CDI and a derestricted stock pipe with ceramic variator balls. I had the stock head with the decomp reworked to match the bore and was jetted rich at a 40:1 ratio and Temps would vary from 340-390 and I could run at wot all day. B6HS plug and 1.2mm. I forget the exact size of the squish band but it was mild.

I got greedy and switched to a 19phbg and a Doppler pipe and Temps frequently hit 500* on the blast and heat soaked the engine before I got racked by an Uber and destroyed the bike. Regrets. I could try and dig the head out of storage if your interested. I'd recommend doing a 50cc kit on an AV10 If you want to get aggressive.

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