couple of porting questions =P

first off, im not gonna go running off and destroy my engine juuust yet.

i did read the wiki bit on porting a motobecane, and it is far from a complete guide. it is totally just specifications for people who already know exactly how to do everything involved. so my questions are....

when it says "Raise" as in the exhaust port.... what exactly does that mean? =P do they want me to shave 3 mm off the top of it and then weld 3mm of steel under that so that its exactly the same size, just physically "raised"?

what does it mean by remove all material directly below the window on the piston?

does anyone know of a guide that will give a bit more detail and maybe sort of a step by step on how to do porting in general? or at least specifically to small engines or 2-strokes or whathave you?

and finally, if i were to do the intake, exhaust, and piston modding, what kind of performance gain is usually expected on a 50v engine? (and ever since i put that dellorto sha 14:12 on there my top speed has gone down from 27 to 24 or so :(. )

thanks for any advice you can give me!

Re: couple of porting questions =P

By "raising" the exhaust port, the author means that you should take a handvile, dremel or whatever grinding tool you feel comfortable with, and increase the height of the port by 3mm. You don't need to wel any material on the bottom.

Removing any material directly below the window of the piston is a bit vague for me, I'd need to see the piston first. But my guess is that you need to cut a piece out of your piston, to increase the intake timing.

And there aren't any real step by step guides. Mostly because this isn't really something you learn from a guide, but rather by trial and error. Try it on some shitty cylinder you don't mind throwing away if it goes wrong.

Performance gain is hard to tell, as I have no clue how these engines run.

I don't know if the article you mentioned tells you to widen the exhaustport, but that's always a good thing to do. You want the absolute width at about 67% of the bore. So you would need to make a portmap to calculate how wide it whould be ;)

Re: couple of porting questions =P

thanks for that reply, definately cleared that bit up.

there be the brief guide i read. should have linked to it on first post

it also says widen the transfer ports up a bit. so lower, and widen the intake, raise exhaust (the same amount that you raised intake), and widen transfer ports. ill tell you now i have no idea what a transfer port is =P

how could one lower the intake on a moby anyway? the intake is simply a thin tube that comes out and snugs up to the carb.

unless these ports arent external? if so im totally looking at it wrong. in fact i must be? cause you wouldnt be able to get the exhaust on if you messed with its mounts......... unlesss

heheh its referring to the end of that "tunnel" where it meets the piston. but if thats so i THINK the carb intake is pretty direct...

and i understand what it meant by removing the material in the piston now. it means to make the "window" more like a door, as in it no longer has a "bottom" in that area.

haha sigh can anyone clarify a little more?

24 mph just isnt cutting it for me. and the acceleration is nothing to brag about... thankkssss for putting up with my noobishness =P

Re: couple of porting questions =P

Hmm, you're wayyyyy off base trooper.

You have no friggin clue what this is all about, do you? :P

Ok, what is meant by intake is not the intake-manifold (The thing that bolts to the carb) but the intake that is IN the cylinder, in the bore as to speak. (I hope you know what the bore is). Never raise an intake, it will make you very very sad if you do. You can lower it (make it go more to the base of the cylinder)

By exhaust they don't mean your exhaustpipe, but the exhaustport. Again, it is in your cylinder, in the bore. You need to modify the window in your cylinder wall to the new specifications.

The transferports are ports that are in your engine-casing and in the base of the cylinder. They move the fuel from the crank-case to the combustion chamber.

But please, do me and yourself a favor... If you don't know EXACTLY what you are doing, don't do it. You can very easily turn a perfectly good cylinder into an ashtray.

Have you tried other tuning options yet? Like an expansion exhaust, bigger carb or high compression head. Try those options first, in this case it will gain you more MPH than messing with the ports.

Re: couple of porting questions =P

Re: couple of porting questions =P

Nice work with the 2 stroke tuners handbook.

There are a lot of things that you can do with porting. I'll post some pictures up here soon of my latest attempts.

what I posted on the wiki was on persons idea of a system for changing the windows on a motobecane av7-style cylinder.

It was scanned from an old french magazine.

That is not a guide of the definitive way to port a motobecane. It's just one persons porting that worked out for them.

Re: couple of porting questions =P

i do need a good ashtray... but yes, i know a liiitle bit more than you give me credit for... not much, but some. by raise i meant lower =P i did read the diagram =P

i didnt realise the port connected to the carb was actually bolted on till i looked at it a half hour ago =P

and i was having a blonde moment when i was talking about the exhaust. i know exactly where it means to expand it. the small hole where you can actually see the piston itself, behind where the pipe screws on =P

so hopefully thats just a little relief for you there. i realize these other things are easier and give better results (the expansion exhaust and high comp head and all,) but i happen to be quite broke. i do however have a dremel tool, and dont worry, like i said up top, im not about to rush into anything.

i imagine i need to take the engine apart to do this as to not get little metal bits stuck in the cylinder and kill my engine in the blink of an eye =P

and that prolly means timing adjustments, which i hate, and new gaskets, which i dont have.

rest assured gents, just colleting information. probably gonna get a rebuilt kit at some point if i can find one for a dandy price, and at that point is when i'll take the porting into consideration.

money is the obstacle here. thanks for your reply!

Re: couple of porting questions =P

Just buy some gasket material and raise the cylinder with base gaskets until it runs like you want it. then measure everything out to where it needs to go to have the port in the same spot, cut it out and fly. I've never worked on a motobecane engine, but thats what I do with my puchs and I gained about 5mph. I could've gone more, but I wanted to be on the safe side. Widening the intake and exhaust are harder to screw up than port timing.

Good luck,


Re: couple of porting questions =P

Oh god. If you don't even know what a transfer port is, you should read up on how a 2stroke engine works before you start grinding away at the ports.

"Read Up!":

Re: couple of porting questions =P

haha thats exactly why im on here asking =P i said i wasnt gonna do anything hastily. im on here to collect some info =P

but thanks for the link!

Re: couple of porting questions =P

MIke applewhite /

IDK but to me raise means to increase the size of the port.So as to get more volume of fuel without increasing the fuel /air ratio.IDK just trying to get it too.Did I say that right:)Got to increase (grind) in the right place

Re: couple of porting questions =P

Really just try your best to get the general concept- follow the guide in the wiki, expect that it won't be perfect, learn from that first experience and keep trying. I don't think understanding every element of the 2-stroke handbook is possible until you have run both good and bad cylinders of your own to compare. So get out there and make some good mistakes to learn from. Good luck.

Re: couple of porting questions =P

I think the thing is to get it like glass. Some say ruff the intake port back up after opening the hole and area inside the port. If you do mess with any port windows make sure you chamfers it right in the cylinder 2 port window... my 2cents. :)

Re: couple of porting questions =P

how about somebody with a rough intake ride with it, takes notes, then smooth out the intake, ride it, take notes and then see if there is an actual difference at all. Both arguments sound convincing -but what actually happens from experience???? A lot of logical conflicting 2-stroke theory gets tossed around without real-life application.

Re: couple of porting questions =P

that isn't directed at anybody in particular, but an ongoing observation on the performance forum- the theory camp/the experience camp.

Re: couple of porting questions =P

MIke applewhite /

This is only the second time I've seen this handbook.You nice folks must have had it read to you in the womb.I am an idiot .Idiots should be forgiven for they don't know any better.The part that said the price to of this is paid in reliability was a downer.I do understand (better) how rings seal against all that pressure without leaking.I realize now where I am compared to others on this forum.Oh the shame on me.Thanks for enlightening.

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