Hot Rodding a 500cc 2 Stroke

[Bioflex]

I've a 500cc 2 stroke (a 96 model Maico) which for all intents and purposes compares to a CR500.
Though this years Maico is supposed to have a powervale, my barrel is off an earlier bike making it powervalve free.

The bike will only be ridden occaisonly so a practical, smooth machine is not what I am after, rather maximum top end power, ripsnorting 4 stroke killer.

The mods I am doing so far are, replacing the 38mm Bing (which is sadly smaller than my TM125's carby!) with a 44mm Mikuni. The reed valve will be changed for a V Force 3, the cylinder ported as well as the compression raised.
The bike will likely end up with an aftermarket pipe and if possible I'll change the ignition (as the spark does look kinds weak). That's about all that's been decided at the moment.

If anyone has any advice on mods or if there is anyone here who has owned a modified Cr500 I'd be interested to hear your thoughts.

Without sounding like a nob, cost of mods isn't an issue. I suppose I would draw the line if running on race gas becomes a necessity and the engine requires rebuilds ridiculously often.

[bearorso]

A higher lever of a practical, smooth power is what will give you a 450 4t killer. Unless you're a duner or race HC.

Make it all top end, and you will get nowhere fast. I'm not setting out to denigrate your riding ability, but even the best riders in the world would get nowhere with just a top end oriented 500. If you can go genuinely fast , for any length of time, on a bike such as you propose, get thee to a factory team, now.

Mikunis are difficult things to deal with. You'd be much better off with a 41.7mm Keihin.

It's not hard to get a lot more power than a 450 4t with a 500cc 2t, smoothing out the power delivery is what will get you places.

How old is the barrel? - I'm fairly sure the 86 and onwards  water-coolers had the diaphragm PV. Perhaps you've an AC barrel conversion - I don't think I ever saw a factory PV'd AC barrel.

What made the 490s/500s so good was their usability. Not that they had the most power - Honda spent quite a few years making the CR500 more ridable.

[2T Institute]

To add to the above, even at a world level factory riders were asking HRC for the CR 500 to be detuned.  What smokes 450's is predictable traction , not having to square off corners and 'wait' until you have the bike pointed straight before you open the throttle. Unless your going Motard or beach racing you don't want all top end. I built a CR480 that is used for dirt track, on a road or somewhere it can find traction 450's just get filled in. On a dirt track it is a right handfull to get off the line , but goes from bog last to first in the length of the straight and drum front is overheated after first hairpin. On the occasions the track conditions are right it's unbeatable.

[SachsGS]

First thing I'd do is get the correct powervalve barrel for your 96.You'll notice an increase in power right away and then I'd do things that are reversible,carb and silencer to start with.

[opfermanmotors]

I'm fairly sure the 86 and onwards  water-coolers had the diaphragm PV.

The 500 Maicos did not get powervalve until ~88.  The 1986 did not have powervalve on the 500 models, only the 250s.

[Bioflex]

Cheers for the advice guys.
I should have made it clear the reason I am after pure top end power is that I have a few big bores, this one will be reserved for beach riding and possibly a bit of motard racing where bottom end isn't a factor.
I already have a 44 Mikuni hence why this was the choice, if it doesn't work then a Keihin could be used. 44mm doesn't seem too big given how much airflow these have, when you consider some 125's run 39mm carbies these days.

It remains to be seen though.

Sachs, having had a few Maico's can you confirm the powervalve models work better than the earlier non PV one's?
If that's the case I'll make that a priority before anything else is done.

[2T Institute]

125's have that size carb because they have more rpm, therefore can keep intake speed up. I have proved you can get just as much HP from a smaller carb with better low end and throttle response. Don't get air 'flow'( I hate that word) and intake velocity mixed up.

A air cooled bike will still work very hard in beach racing, a good paddle tyre makes a hell of a lot of grip to. My advice would be to just go in stages, there are plenty of things to do to a engine at add performance before making it a top end screamer.

[SachsGS]

The powervalve Maicos are quite a bit more powerful then the non powervalved versions.I've noticed that my 380 feels virtually as powerful as my 500 wide open (bear in mind the 380 revs a lot higher).The 380 is still smooth and torquey off pipe so if the 500 could be ported to mimic the 380 you would have a revvy,albeit still rideable, monster of a 500 on your hands.Both my 380 and 500 are powervalved.

[opfermanmotors]

I'll trade you a Honda CR500 for that 96 Maico 500.

[GlennC]

I'll trade you a Honda CR500 for that 96 Maico 500.

Bioflex,
You should consider this offer, Lots of people making big power with the CR500's, Plus the primary drive is not made of granola flakes.

[opfermanmotors]

Where do you live Bioflex?

It's low hour CR500, completely rebuilt.

Resleeved to stock bore
All new bearings / Seals
New side case / water pump
New Hinston Clutch
New Connecting Rod

Flat Slide Kehin Carb.  It's 86, which is the most powerful stock CR500 and the year anyone who wants to grab a CR500 engine wants. It is stock so you can mod the crap out of it.    Front disc brake and rear drum, but you could upgrade the rear to 87 to get rear disc.  It's an 80s honda, which are the best looking of the bunch which is also why I got it.

Pro Circuit pipe.







It's got some compression:
CR500 Starting (RE: KX 500)

So, seriously think about it I'm in the Pacific North West

[Bioflex]

No offense Opferman, but I would never swap the Maico for a CR500.
Technically your machine could be better than the Maico, parts would be cheaper and hot up bits far more easily accessed.

I bought it cause as far as I know there is only 1 in Australia (only 2 imported in the first place back in 96) and rare and exotic machines are a bit of a passion of mine. There would be less than 10 Maicos in the state of Aus that I live and if so, I have half of them.

2T institute, surely total airflow and therefore power is ultimately limited by intake size on an engine with a carb so small? I am no engineer but all my experience on piston and rotary car engines has been that increasing the rpm which an engine makes peak torque at will mean more power (assuming torque can be maintained.

Why do 125's have more rpm? How much of this is due to the 500's being so undercarbed and general port timing and structure being so limited? I do appreciate that a 500 is well beyond the so called "optimum" size for pistons, however many V8's with near 1 litre pistons rev to over 9,000 rps - drag motors and such.
Sure all of that inertia means that total rpm will be limited compared to a 125 but IMO the 500's are so underdeveloped as people figured additional power just wasn't necessary - rather than too hard to get.

I am certainly not suggesting that one could ever make 4 times the HP from a 500 single compared to a 125, but surely a carb 27% larger (which is the difference between total area on a 39mm and 44mm) in total flow wouldn't be too much for an engine 4 times as big.

Has anyone had any experience really bumping up carby size on a big bore 2 stroke?
I'd like to hear more about your example of getting more power with a smaller carby as you seem to know your stuff.

[opfermanmotors]

No offense Opferman, but I would never swap the Maico for a CR500.

I wouldn't either but I would swap  CR500 for  Maico, can't hurt to try!  Ya, I figured that it would be easier to access mod information and parts if you just wanted a Machine and I could take that Maico off your hands

[SachsGS]

Internal combustion engines are nothing more then airpumps and the more air (oxygen) they pump,the more torque they develop (look at the torque figures of turbocharged diesel engines). Horsepower is a function of rpm - to get your 35 hp out of a 125 two stroke you spin them up to 12,000 rpm.This is the achillies heel of modern 4s,they have to be spun to very high speeds to develop acceptable power.

Traditionally piston speed limited engine rpm (in the olden days it was 4000 fpm)and so shorter stroke 125s revved higher then 250s,which revved higher then 500s.The problem with two cycle engines is fuel charge losses out the exhaust port.Expansion chambers help mitigate these losses but because the dimensions of an expansion chamber are fixed so too is the rpm range over which they are effective - this is where the two stroke "powerband" comes from.It is easier to handle a 125 coming on pipe at a much higher rpm then a 500,for example.

Direct injection, if it ever becomes a reality,would greatly reduce part throttle fuel charge losses and would therefore increase low rpm torque production in two stroke engines (see Super Hunky article).  

[2T Institute]

Your average V8 drag motor doesn't last that long at those rpms, all come down to what rpm the engine spends most of it's time in.HP is a by product of torque( tq being the work being done and hp being the rate at which the work is being done) X rpm so raising the rpm ceiling is a easy way of increasing hp.
A 125 is a beautiful engine it has a 'square' bore to stroke ratio, which delivers the best port time/area possiblities.500's are not at all 'limited' the only thing that limits them is piston speed9determined by stroke), a 80 odd mm piston can only be turned over so fast.
Why you don't see sky high power from a 500 is the rear tyre, the more power the less tyre life.More power is easy to get  getting it to the ground is, without uncontrolable wheelspin.
 I know that after a small modification a 39mm carb will outperform a stock 44 mm carb in every respect. I know a fellow that insisted an going from 28mm to 34mm carbs on a 350 road racer. The 34's were sluggish, had no torque and difficult to tune. After much ado he tried the stock 28's more power everywhere, dead easy to jet and throttle response through the roof with no loss of top end.