mikuni flatslide on piston port puch

hey all

I am a puch owner but not of the moped variety, I am doing a bit of a modern anachronistic race version of a 1950s puch 175cc and am thinking I should try a modern carb on it to see how it improves power throttle resonse etc, im sorry to invade your forum but you guys really know your tuning stuff, i've been reading on here for ages, wish mopeds were legal in aus, I have 2x e50 engines in my shed lol!

I am looking at either a mikuni flatslide 24mm or 28mm, but have seen random posts of people saying the needle jets an whatever else are set up for 4 strokes (175) and I need a 169? Where can I get one?. I know I will have to tune to find the right main jet which is no drama

has anyone run a flatslide on a piston port motor? what setup did they use?

the 175 bike has a 30mm inlet so its probably well suited to the 24mm mikuni, plus its top rpm is around 7-8k max (long stroke, split single design) so I cant see myself needing the benefit of a massive carb. The stock Fischer 22mm is a roundslide.

once I get the crankshaft from austria (2 month lead time and $400) ill be making up some chambers for it...

dont know if its relevant, but

- intake exterior to piston face 60mm

- intake duration 140 deg

thanks in advance!

Re: mikuni flatslide on piston port puch

sorry just found this


any more advice comments abuse welcome



Re: mikuni flatslide on piston port puch


nice work you are on the right track. australia, eh? rad. you may not have mopeds but you do get postie bikes, ford falcons, cortinas, cross flow ford six cylinders....mmmmm you guys get a lot of cool mechanical kit. Not to mention all the stories i hear about 20's and 30's motorcycles finding their way down there to get preserved perfectly in a barn in the outback with perfect weather for no corrosion... dang.

if it were me, i'd probably call my buddy matt who builds and races vintage puchs. I've picked his ear at length about tuning his race bikes, he has a competitive twingle that he built (hardest part is keeping the crankshaft together) and a couple singles.

here is his tech page:


soo you are saying you have a 175 twingle? i've never seen that, just the 250, either way matt has told me that they don't respond to a chamber exhaust the way that a conventional 2 stroke does. he tried all sorts of different sized chambers and settled on a 2' ish pipe with a megaphone muffler at the end for his race bike.

on the carb thing i cant quite remember for the twingle. If it was a single i'd say go with the 28 flatslide, you can run a TM or TMX much bigger than a round slide without loosing low end so there is that. The trick is going to be the cast-in cylinder spigot, those are notorious for breaking off at high rpm.

anyhow, if you call matt he might give you a couple minutes of time, he's really busy and gets a lot of phone calls, but if you are polite and reasonable you should be able to ask him a few quick questions, he's certainly the foremost expert on puch in the states, and probably one of the top guys in the world. especially when it comes to resto-racer type stuff.

Re: mikuni flatslide on piston port puch


thanks a lot for your reply, I know matt fairly well I recently just bought a brand new 175 crankshaft off him!

yeah the problem with the puch is that the exhaust port closes before the transfers!!!! so if you have a standard pipe with a converging / end cone / baffle section then it can potentially cram the charge back down the transfers which can kinda screw up power. to make the chambers work Ill likely have to change the port timing so the transfers close before the exhaust (to allow the supercharging effect)... whether or not that wil be feasible I don't know... lol but its worth a shot...

they used to use megaphones in the race days to good effect, the scavenging was of course beneficial...

what do you mean by the cranks falling apart? haha

the 175 has a bolt on inlet flange, is that what you mean by cylinder spigot or are you talking about the carb flange breaking?

I have a few puch split singles!

1951 150 TL x 2 (one has just been sold)

1951 175 SV - getting it running now

1951 125 SL (twin carb race model)

and some scooters haha but they're all in parts

I might have to email matt as I dont have a phone that can call internationally at the moment! I will ask him in the next message I send him about my crankshaft order!

Re: mikuni flatslide on piston port puch

heres the bikes for interest sake

red one is sold

blue one is the 175 I am gonna paint it and re chrome it etc

Re: mikuni flatslide on piston port puch

Matt Mahloch -- MBM Customs /

That's some awesome stuff, so cool the internet allows for us all to connect. On our mopeds we just run the 24 with the "4 stroke" atomizer in it. It actually runs really well.

Mopeds are illegally Australia, learn something new every day.

Re: mikuni flatslide on piston port puch

they're not illegal, but you have to register it, has to be roadworthy and for it to be roadworthy has to have

front and rear lights, brake light, blinkers, chainguard, high eficiency front and rear brakes etc etc etc etc, noise restrictions.. we could never do what you guys do! it sucks bigtime, I'd love a moped

I have seen that the 24 flatslide runs well with the 4 stroke atomiser if the engine is reed induction, but with piston port I was getting the impression a 169 was required...

I have learnt so much stuff from this forum , I am into fabrication and customising stuff for performance and the 2 stroke knowledge here is insane!

Re: mikuni flatslide on piston port puch

well the 4t/2t atmoizer thing has to do with the progressive-ness of the fuel delivery curve.

if you have flat fuel requirements, as in, the AF ratio doesn't change much with flow rate, the 4t atomizer handles it well. If the AF ratio changes dramatically (low efficiency bottom and midrange, i.e. piston port; high fuel demand at high rpm from pipe) you need the other atomizer.

your engine is pretty radically different from the average piston port, you'll probably be fine with the 4t atomizer, but to be honest you are way off the map for anything we're used to around here.

Re: mikuni flatslide on piston port puch

hey thanks mate!

I will give it a crack, I can always keep or sell the carb on... Ive got a few 1950s carbs I should be able to raise heaps of money by selling them

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