Moby porting

The engine on my 1980 Le Moped is in need of a new seal, so I figure if I'm going to take it apart, I might as well do some performance mods while I'm there, Right?

I'm quite interested in doing some porting on the all stock 50v engine. According to my vin tag it's the B in the wiki says it applies to the fast model only. would it be a waste to try and do some porting on this model?

Paulvolio's poor man's fast moby project is a real motivator, but it fails to mention any specifics.

If anyone out there has some good tips, pics, info, what-have-you, I'd love to hear it.

Also, I figure I'd just ask here; what's the best way to seal the cylinder to the case. Would cork gasket material be too thick?



Re: Moby porting

The pmfm actually started out as a 25 mph ''b'' model and I was able to break 50 with it.

I generally followed the guide in the wiki but payed a lot of attention to the case halves.

The 25 mph cylinder has bigger transfer port windows which saves a lot of tough porting work on them. You want to raise the exhaust port by quite a bit and lower the intake by the same amount.

You can widen the intake window by a lot because the piston rings never travel over it but the exhaust can be much less forgiving so dont go too far to the left and right about 2mm either side.

The ''devil horns'' on the crankcase halves needs to be matched up and you can open them up to the pont where they will still maintain a seal.

I also milled the head by about 3mm with wetsand paper and a perfectly flat patio block and I use high quality Hondabond grey silicone for a base gasket and no head gasket.

Contrary to what has been said you can mod the exhaust with a larger exit pipe. I used an old small bottle jack pump handle and welded into the endcap and removed the first baffle then trimmed the exit pipe to fit in the upper of the three holes.

You might have better luck with an after market pipe though just nothing huge or you will have no low end.

Re: Moby porting

awesome. That's just the info I needed!

Thanks paul.

Also, just a few quick ?s:

Do you run high octane fuel with the head milled down that much?

what carb are you running on the pmfm currently?

I was thinking about sticking with the stock Gurtner, only because I'm really cheap, lol. Unless I can find a cheap enough dellorto.

Re: Moby porting

I run high test gas in it from a station that has a seperate hose for each grade and I have advanced the timing I have yet to hear any preignition.

I am currently running a 14/14 sha dellorto and an opened up to 14mm stock shorty intake.

If you have the 25 mph engine it should come with the 10mm gurtner I hit 50 on a modified one but I had no low end power with it.

Re: Moby porting

The thing about the gurtner is you'll never be able to re-jet unless you drill your jets-- which I can't imagine is very easy. So after all that porting, and increased speed/rpms, you'll really need more fuel. I'd go with what paul says.

Re: Moby porting

Yeah, a dellorto is definitely on my list.

I'm thinking of an sha 15:15 if I can find one on the cheap.

maybe a 14:12, only cause they're easier to find used.

Re: Moby porting

Hey Paul, what's the name of that stuff you used to seal up the internals of the crank case when you matched the case halves? And where can I find some?

I searched and came up empty. And I don't really feel like taking the whole variator and magneto assemblies apart just to get the case halves apart.

If all goes well, this weekend I'm going over with my dad to see a friend of his who has his own machine shop to see if he could do some porting for me :D.

I plan to have everything done pretty much by the wiki guide, except widen the exhaust ports by 1mm on each side.

Plus bore and match the intake.

It seems I've caught the french fever.

Maybe when I'm done, I can accept the offer made by a cocky co-worker of mine to a race vs. his scooter (Chinese I believe).

He asks me about my moped from time to time, and I try to engage in a conversation but then realize, "wait, this dude rides a scooter. He doesn't know shit about working on his own shit." ...but I digress.

Re: Moby porting

It's such a nice day, I couldn't let it go to waste....

Gotta love them Dremels.

I used some silly putty I had to keep the shavings from entering the crank case. that worked surprisingly well. Only problem is that silly putty tends to "melt" or "ooze", so it's best to work in a cool space.

I still need to get in touch with my dad's friend with the machine shop to do the cylinder/ head/ intake machining. I hope it won't cost too much, I just spent $60 on tires & tubes + shipping.


Re: Moby porting

Paul, how many mm's did you raise the moby's exhaust, or better yet, what is it's timing in degrees?

Re: Moby porting

nice work. putting together a very similar modified stock 50v. case matched bottom ends, ported as per wiki specs very carefully. piston skirt chopped... all the basic stuff.

havent milled head.. 3mm seems like a shit lot but if you dont have head slapping issues that sounds great.

port map _before_ porting is attached. do the math and you'll see what it'll look like after.

thinking the 16 sha is going to be too big even with a G3, might go down to a 14 just to try it. i also hear you can pop the jet holder/emulsion tube out of a 14:12 sha and put it into a 15+ to get the progression of a 14:12 but the top end of a 15+. anyone got any input on that?

this is all very generic old-tech moby mods but i hope to get some huxpa out of em.


Re: Moby porting

RJ make sure you post results after running!

Re: Moby porting

_RJ make sure you post results after running!_

Definitely will do.

I'm still on the fence if I should go for a high-dollar new Dellorto 15:15 or just opt for a cheapo 14:12 and possibly bore it to a 14:14... possibly.


Re: Moby porting

15mm will rob bottom end unless you do a lot to combat it on a stock cylinder even ported. Find a friends old 14.12 and you will be happier.

Re: Moby porting

Seriously? I thought the 15:15 was like "the standard" when upgrading to an SHA?

Well that potentially changes everything...

Re: Moby porting

Just because it is 'the standard' doesn't mean it is the best option!

Re: Moby porting

ok well thats that then. id still like to try switching out emulsion tubes but even if that doesnt do me any good ill just run a 14:12 straight up.

Re: Moby porting

Jerry Franek /

Steve, I've mounted 3 SHA 15's on bone stock Mobys, no performance improvement, just louder.

Re: Moby porting

On that same note I've ridden JesseS's stock (HEAVILY) ported moby with a 15 dellorto and the G3 exhaust that did a bit over 45mph for BBQ14, I guess he later fixed a few things with it and had it clocked at 50mph.

Re: Moby porting

well the top end i believe, im just curious how that bike did on the low.

to be honest with you, jerry, ive done maaaany dellorto swaps to a lot of different mobies and its been a mixed bag. sometimes i get better performance, other times decrease, usually just the same. i think the "norm" for the moby dellorto swap needs to be 14:12 or 14:14.

Re: Moby porting

I pretty much hate SHA carbs on kitted or modded bikes on any occassion or setup. That motor that I was running finally lost all compression and was the saddest thing ever. My last trip on it was 100 miles long by myself when I moved from Louisville to Cincy and it used an entire tank of gas in about 45 miles. I'll rebuild it faster but it's going to be a minute.

The best carb for a moby is still the stock carb. Runs better than any SHA will ever ever. And you can still do over 40 on it. With all stock parts. And an airbox. Fact.

Re: Moby porting

yup! and the look on people's faces when they catch up to you and see you're still running a gurtner is priceless.

Re: Moby porting

Hmm. I have a nice clean Gurtner here. I may take you up on this challenge and see if it runs better.

Re: Moby porting

Even better is that you can all your friends old gurtners that they throw to the side cause, well, extra parts are always good. Way better throttle progression as long as you clean all the passages out. Oh and a lack of jets, but we have tricks for that.

Re: Moby porting

Jesse, since you know more about this than anyone else I know... I'm building my second 50V and I'm going to use the 12mm Gurtner AR2 from my first 50V on it (to replace the 10mm it had). Porting the cylinder to match the specs in the wiki (3mm up exhaust, 3mm down and wider intake, wider transfers, case matched) When it comes to adjusting the jetting am I going to have to solder and drill a spare jet, or are there other options?

I've not got a lot of experience with SHAs, but I put a 15:15 SHA on my first 50V and I don't like it... no low end at all and not much better top end. It's a basically stock bike with a 30mph cylinder and ported/smoothed bendy intake. Got a 14:12 in a box I'm going to replace it with to see if I like it better...

Re: Moby porting

this all makes sense... and i agree for the most part. not going to use a gurtner though, cause i dont feel like drilling jets.

however... what about a small PHBG for a modified stock setup? worth it? like a 15 or 16 to get the best of both worlds.. tuneable low end, screamin top end

but yea.. your average sha is a dogggg on a stock moby.

Re: Moby porting

Thing with the small PHGB is you have to get the large intake for it, but really that's not a big deal, it's just hard to fit it under there, you can do it though.

Word Iain, haven't talked to you in a long time. I seriously put down the shit in a post on moby tuning about 3 weeks ago for porting a stock cylinder but I can't find the post. Owell, it's out there somewhere waiting to be found again, like treasure.

Re: Moby porting

_Owell, it’s out there somewhere waiting to be found again, like_ treasure

Re: Moby porting

FWIW my Gurtner experiment was a total failure. Speed with Dellorto 14.12 - 31mph. Speed with Gurtner, 240cc jet, 25. I broke my Dellorto banjo in the process so now I am stuck with mediocrity...

Re: Moby porting


I just got my parts back from the machinist the other day. He did a damn good job. I've still got some finishing to do with the dremel, but he did the brunt of the work.




New/ old intake gasket for comparison:

I've got some work to do on the intake:

I'm quite pumped to get this thing back together.

Re: Moby porting

dude ditch that stock intake and get a 15mm one from quarter kick or treats or someone.

And holy fuck. I've never seen anyone actually modify the cylinder head as recommended! That is sick! and cheating since you had a machinist do it ;)

The intake looks perfect, but there is a lot of work left to do for the exhaust.

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

Login or Create Account