AV7 upgrade/rebuild issue help

I've recently upgraded an av7 from stock to 74cc kit, and i got the matching head with decomp, all installed with treats cdi and Dellorto sha 14.12

- Motor starts, runs, back tire spins, but when i sit on it, it won't go.

i thought it was the clutch slipping so i replaced it in full with a Brand new stock clutch, and it's the same.-

Using a #66 jet right now, but likely down jetting until it runs better, but it starts,runs, bogs a little which i will mess with idle and jetting- my issue is it won't...drive.

I've done a leakdown and it holds 15psi,

This is on a 79 motobecane i've been rebuilding, while i have a 78 that is mostly stock with a cdi/and sha carb. Been through the tutorials up and down, and even swapping parts off the stock bike to figure it out, is there something i'm forgetting?-

appreciate the help,

Re: AV7 upgrade/rebuild issue help

So it will idle all day? But doesn't move? Throttle/throttle slide working? Petcock/is fuel flowing? Filling up the float bowl? I would clean the carb/jet. Check for an air leak?

Re: AV7 upgrade/rebuild issue help

id triple check timing w a gun, also wouldn't downjet, 66 is small for a kit stock exhaust? if it dont move at all, it just might might might be in pedal mode. check your pulley is locked w the small inner gear.

Re: AV7 upgrade/rebuild issue help

Agree on jetting being too small for that set up. Second on fuel flow and throttle slide.

Re: AV7 upgrade/rebuild issue help

So a nice saturday, i swapped motors and went thru everything. The culprit was slightly loose intake and head bolts, and the primary culprit was my fiberglass packing on the stock exhaust in effort to quiet her down. So that doesn't work! It just clogs your exhaust, (i remember rebel mobys posts consistently stating this)

So the kitted motor runs nicely on a 15/15 sha with a 68 right now, i do have jets in the 70s to try, at the same time the throttle slide is finicky and holding open, i guess its

Clean the carb time and the do some plug chops. Do you guys recommend waiting till the head is broken in to do plug chops?

The 14/12 with a 66 on the stock motor 50cc doesn't have much power, now im looking to go 74cc on that one as well, or you guys keep posting about the doppler 50cc kit out performing the 74

But we’re running and rolling here,

Re: AV7 upgrade/rebuild issue help

> Ian Yaple Wrote:

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

> So a nice saturday, i swapped motors and went thru everything. The

> culprit was slightly loose intake and head bolts, and the primary

> culprit was my fiberglass packing on the stock exhaust in effort to

> quiet her down. So that doesn't work! It just clogs your exhaust,

Have you considered using Brass wool ? It doesn't rust and doesn't pack together like fiberglass .

Re: AV7 upgrade/rebuild issue help

never heard of breaking in a head, run a bit rich to help seat rings. on the sha the throttle cable goes outside of the spring, that might be the sticky.

Re: AV7 upgrade/rebuild issue help

PD- Will try the brass wool, as right now im loud, and it's not completely necessary

pat- thank you about the throttle cable outside the spring- i think i have other sticky issues.


So i upjet to 74, and was bogged out, so i coasted back home and put in a 70,

the 70 revs nice and smooth, and a fairly calm idle. But i have a new issue.

Revving at say 20 mph, or 1/4-1/2 throttle, the motor sounds great, but when i go to open her up, and push her higher, there's a Rattling, clanking noise that comes on, only when the revolutions get fast. I would compare it to a harley letting off throttle, but add the sound of a fork getting slung around inside the motor.

Im nervous it may be crank, the crank is stock, i have 74cc airsal, kit with a matching head, from DOS- stock pipe (which has no restriction now)- and a brand new stock clutch, and treats le partie cdi,

did i mis-place a fork? or can the stock crank not handle what i'm trying to do...


Re: AV7 upgrade/rebuild issue help

Jonathan Feldman /

I've never rocked a stock Moby crank, I always go Doppler etc but assuming the crank is in decent shape I don't think the av7 47mm kit is too much. It's been awhile since I've ran one but my understanding is they're pretty detuned from the get go.

If you're really getting the fear about the noise id inspect the top end real quick to make sure you're not detonating and confirm the crank is turning smoothly. If it's neither of those id go ahead and assume it's something with the variator. Everyone says stock variator makes all sorts of god damn noises. I'm more of a kickstart er3 guy cause it doesn't have all of the hard to access circlips and can be tuned easier/ better.

Keep us posted on your adventure.

Re: AV7 upgrade/rebuild issue help

the stock variator has metal balls and have a clanking sound especially at idle. if u havnt changed motor mounts thats probably it.

Re: AV7 upgrade/rebuild issue help

Heres the sound, sorry its deadened in my pocket

The 74 kit certainly revs up and throws me pretty good, and im over 200 lbs

I tightened the motor main mount bolt, greased the clutch (new clutch) before this ride

Process of elimination, swap the clutch and see if its that first i guess

When i had the motor open i feel like i asked everyone how to tell if the crank was good or not, and it didnt have any play, but it is original,

Thanks for the replies

Will let you know how i make out

Re: AV7 upgrade/rebuild issue help

I found that putting a overbore kit on a engine will not necessarily increase the rpm but it will increase the torque of the engine. AV7 stock crank is good to 10k before it wants to come apart, just the added weight of the piston and torque will give more vibrations.

If you have upped to 15mm carb then use a 15mm intake (or larger) and check the intake is 15mm right through, if it is cast intake then there is a restriction where it transitions between round and rectangle, you want to chase a 15mm drill through the bore to make sure it is open, or carefully with a dremel.

I usually run rich so I can ride all day long and into the night, mid 70s jet works well on a 70cc kit. Best practice would be to have jets in the 70s range, suf in a fatty and get the engine up to temperature and run it hard at wide open throttle. If it 4 strokes then come down 2 sizes and take her out again for a fast hot run. When you find the fattest jet size that does not cause 4 stroking at WOT then you are good. Some folks argue to go leaner but this is good safety until you feel you know the bike better and you maybe want to experiment. Lean makes more heat so the engine will pump out more and more power until it is so lean it seizes.

Check the outlet of the exhause, if the ID is less than 11mm it is undersize and causes a restriction. Exit pipe at 11mm is a goodly size for a 70cc motor if you are streeting the bike. If you are tires of packing getting all gummed up in the silencer you can replace it with course steel wool or make a new baffle with a piccolo tube inside a piccolo tube to diffuse the sound.

Check your timing, you want to be in the 1,2-1,5mm BTDC range, this will keep theheat down as well as give you acceptable starting, idling and power. If you have a CDI ignition set it at 1,2mm for thebestbang for the buck.

If you are still on points, do the external condenser mod if you have not done it yet. Also make a piston stop tool to help with the timing setting, do not try to adjust timig by messing with points gap, it needs to be 0,4mm to ensure exciter coil saturation.

Always replace the bonding strap between the motor and the frame. Best suggestion is to use a wire to go from one of the intake studs to connect to the lower external coil stud.

If you are on WOT all the time then gap the plug at 0,38. If you are doing stop and go city/suburb open them up to 0,46 so they do not foul easy at idle and low rpm. Lots of factors in selecting plugs, go for B8HS thru B5HS range, select the coldest plug (higher number is colder) that does not foul at low rpm. Hot plugs can get on a glow at WOT and cause detonation and kill your motor.

For urban-suburb riding, the top engine mount on a Moby likes to be just snug and use a nyloc nut. If it is too tight then the radial torsion on the rubber will finish them in short order. Tight just enough so there is no side to side play but loose enough so the engine will swivel. If you are at WOT al the time then swing the engine in to where it sits at full variation and tight the long bolt goodly, that will allow more vibration dampening at WOT.

Hope some of that helps.

Re: AV7 upgrade/rebuild issue help

Also read your plug for more than just rich-lean, it holds a lot of informations to help tune.

« Go to Topic — end of thread

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

Login or Create Account