AV7 porting question

i think i have an AV7 SLOW cylinder (with square transfer ports)

my question is: is this cylinder worth porting? i have a Sim circuit exhaust and a Dellorto 14.12 carb and a single speed clutch and i wanna be able to atleast keep up in traffic

also ive never ported before... thoughts? advice? flaming critisism?

thans for the help!

Re: AV7 porting question

Well it's always worth it to match your intake port to your larger intake, no matter what you're running. If the air and fuel hit a wall when going to the cylinder it's going to fuck your shit up.

Lacking a variator is your biggest performance limitation. You'll never go fast on 50cc without one. I know, i had a Peugeot 103LS, but luckily that bike still had a spring so I just got a shorter belt to pull the gearing back and lightened my clutch to regain some of the low end.

You have a 40t? Does the engine swivel at all like it did on my non-variated 103?

Re: AV7 porting question

i really have no innformation on this bike whatso ever, the attached pic is my motor

ive been looking at getting a stock MBK variator from 1977mopeds and ive read the wiki on whats needed, but the only t hing im having touble finding are the engine springs, because the ones 1977 sell look exactly like the ones i have...so...i dunno thats later on

but the engine does swing on the frame with tension springs

and i think its a 40t


Re: AV7 porting question

Well, is there still a vin tag attached to the front fork?

50v = variator (V). 40t is no variator.

I can't tell, the clutch is on the left side of the engine. When you rev the engine on the stand, does the engine pull back, pivoting at the head where it's bolted to the frame?

Do you have a spring behind your engine attached to the frame?

When you take the belt off the clutch, is there a cheeck that moves back and forth?

If so... congratulations, you have a variator.

If not... you can't simply buy a variator from 1977 because the crank shaft on the left side isn't long enough. You need to do an entire bottom end re-build if this is the case.

Re: AV7 porting question

i DEFINATELY do not have a variator...the bike doesnt even run atm, i bought it with a trashed carb and is been in storage for years, i guess im gonna do what i can

but were getting off topic here, is the slow cylinder worth porting?

Re: AV7 porting question

40T is a different bike, the non variated 50V counterpart is the 50L, also some Travelers.

you can rig up a spring on your own without too much trouble, something the mounts to the stock exhaust bracket on the bottom of the case and to the frame where the stock spring retaining clip mounts up.

there are pictures somewhere, but basically what you will need is a long bolt which rests against the frame (at the point where the retaining clip mounts above the kick stand), with a hardware store coiled spring along the length of the bolt, and something to stop the spring at the case (on the stock exhaust bracket), but allow the motor to move along the bolt.

alternately, you can use a gas strut which is much better than a spring, i should have a picture of that.

Re: AV7 porting question

gas strut

doppler spring is pretty easy to copy for less than $10 hardware

Re: AV7 porting question

The slows are definatly worth porting, the tranfers although not as tall are far wider than a fast cylinder. Now here is the part many find difficult to understand.

Remove the exhaust and look at the piston thru the exhaust port. Looking at it? Good. Now turn the crankshaft to Bottom Dead Center. See how much piston is still in the window? Now what you want to do is raise the cylinder how ever much it takes to open the exhaust port fully, then subtract that same amount of materials from the top of the cylinder.

AV7 cranks are Av7 cranks yes you can stick a variator on it

the springs are exactly the same you just have to notch out a non-variated springholder farther back to allow for the travel of the engine. That hook looking thing that holds the spring to the frame.

Re: AV7 porting question

Yeah, the porting is pretty easy to do. One of the infamous PMFMs featured a ported "B" cylinder. The springs look pretty much identical. I thought that the cranks were the same between the Dimoby and Variator clutches. I'll go and measure. Make sure that your intake is efficient and air tight. If it isn't, go and snag yourself a shorty intake. I think it's also possible to go fast on the Dimoby, but your acceleration will suffer. The variator is a huge advantage, but once you've reached your top end all things should be equal so long as you can get the bike there.

Re: AV7 porting question

listen to crackstar. slow cylinders are where its at. im currently running a ported 20mph cylinder and doing just over 40 all day long, with only a 15 delltrolo

Re: AV7 porting question

ok, crackstar i think i get what your saying, if im looking at the piston theough the exhaust port and its say... 1.5-2mm of the piston still covered, take 1.5-2mm off the top of the exhaust port so that its opened sooner? should i also round the sides of the transfer ports so there more ovals rather than rectangles?

Re: AV7 porting question


Raise the entier jug with a spacer or lots of base gaskets so that when the piston is at BDC the crown of the piston and the floor of the exhaust are at the same level. As my experiences with slow jugs goes, at BDC you will still see 1.5 to 2.5mm of piston blocking the exhaust. (NOTE: Thick base gaskets will blow out easier)

So if you needed to raise the jug 2mm to get the floor of the exhaust "up" to the same level as the piston crown. Then you will need to "remove" 2mm off of the top of the cylinder.

Look thru the exhaust at the piston while the piston is at BDC, you should probably see the top ring in full.. Literally Half of the exhaust is blocked by the piston at BDC.

You don't have to grind anything. Put the Dremel down.

You are raising all the ports timing by raising the entire cylinder.


Re: AV7 porting question

crackstar, what your saying makes alot of sence, but wont that drop my compression WAYYY down? would i have to shave the head to fix this?

Re: AV7 porting question

NO! don't touch the head

if you raise the jug you need to remove the distance raised from the top of the jug, or what can also be referred to as the "deck".

The head sits on top of the deck.

You need to bring "the deck" back down to the top of the piston.

Best way to do this would be taking to a machine shop and paying for it. If you are cheap and can pay attention to what you are doing you can "lap" away the excess deck height. Fine grit sand paper, oil, and a very flat surface and lots of time. Stoping often to verify that you are still flat and square to the bore. Yeah, you are gonna have to measure all four corners often. It took me about 8 hours, not in a row, to remove .093 inches from the top of on kit I was assembling.

Later, you may think about trying to remove material from the head, in order to further increase compression. However, because of its very hemispherical shape this will not yield as dramatic a reduction in combustion chamber volume as you would expect

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