engine advice needed

[lingeringstin...]

A hundred miles on hilly, windswept roads pulling a trailer full of camping gear going to a rally has definitely shown me that I need more power. I have what's basically a Honda CD250U copy engine (or Rebel 250). It's little more than a CB125T with slightly bigger pistons and slightly longer con rods. I think it was simply Honda's design attempt to compete in the cheap 250 commuter market by "beefing up" a 125 engine they already had to keep production costs low. I can think of no other reason for making such an abomination.

The 250 engine has the same exact porting and valves as the 125 and there's nothing that can be done about that. Consequently the 125 was something like 15hp and the 250 was only like 17hp, hardly worth the effort. I've done all I can on a limited basis to squeeze another couple of hp out of the 250 engine but it's just fucking miserable so I was snooping about for potential replacements that wouldn't be too difficult to shoehorn into my frame and started looking at the ZS175FMM single pot "thumper" engines as an easy replacement.

I was pondering the 75mm piston size and suddenly it occurred to me that I have a 360 degree crank in my 250 twin. That means both pistons go up and down at the same time, it's just that they fire alternately so that what you get is like a 125 "pop" with every revolution of the engine, which is pretty feeble. It's like a 125 two stroke without the benefit of an expansion chamber. Think BSA Bantam and that's pretty much what this engine power is like.

The 175FMM engines have a single 75mm piston to get a 300-ish cc displacement of a bit over 20hp.

HOWEVER...

It occurred to me that since both of my pistons go up and down at the same time why couldn't I just saw the cam in half and turn it around and weld it so both of my pistons fired at the same time becoming basically the same thing as a big single? That should mean my two 54mm pistons (I use YBR125 pistons in my engines) would make 108cc combined which the online cc calculator says is practically the same cc's as a 500 engine even though two 125 pistons should equal a 250. But a 250 single piston is only like 65mm and I would have 108mm to play with, so WTF?

Now I know there's a lot of other shite to consider like carb shenanigans but that doesn't bother me. But basically doesn't two small pistons do the same thing as one bigger one? Or am I missing something?

I know from previous experience that if you use a 125 crank in one of these twin engines with a pair of 54mm pistons in a shortened barrel you get a better engine, so a short stroke "dual piston 500" should be a blast. But am I not thinking this through correctly? I mean, would I REALLY get a vast improvement in power by firing both pistons at once instead of one after the other? It all hurts my brain to think about it.

[Easy-X]

There some sentimental attachment to this bike that we should know about? Seems like an awful lot of work vs. getting a bike with a bigger engine to start with
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Royal Enfield Continental GT 535, Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[lingeringstin...]

No, it's just cheaper to do it my way. I already know what the rest of the bike is made out of because I've built every bit of it so I'm reasonably content it will get me about reliably. Also, I've had so much ass time on this thing I know exactly what it's going to behave like in any situation, and I've cobbled it over the years to "fit" me like a glove. I can comfortably ride it any distance until I need to stop for petrol. If I got another bike it would mean the whole arseache all over again of tailoring it to suit me. Plus I've got a load of spares, so I'm still trying to improve on what I got.

[stinkwheel]

It'll be exactly the same displacement. The way it is with your standard engine, you're displacing 125cc every 360 degrees. If you "big bang" it (not exactly the same as a big bang motor but the term split single is already in use for something very different), it'll displace 250cc in one 360 degrees and nothing in the next 360 degrees.

I don't know what sort of calculator you're using but displacement is half the bore squared, times pi, times the stroke. (all in cm, not mm to get cc)

So for your 54mm piston with your 53mm stroke you get 121.3cc per cylinder. Total displacement of 242.7cc.

If you use a 125 crank which has a 43.2mm stroke with a 54mm piston you get 98.9cc per cylinder, a total of 197.8cc. You also have a massively oversquare engine (A CD250 and CD125 engine are exaclty square as they come from the factory) which should produce peak torque at higher revs at the cost of lower rev torque. So it'll pull better when spanking its arse but will feel anaemic when the revs drop.

It'll probably produce much the same power output if it doesn't shake itself to bits, almost certainly less than a single of the same capacity due to increased piston surface area contact.

Weird but ok, give it a try. I'd imagine it'll vibrate like hell. Biggest thing will be how you rejoin the cam. As in clocking it accurately, maintaining concentricity then joining the halves together strongly enough they don't split but without using so much heat it ruins the hardening. It would take some precision machining.

I wonder how Allen Millyard does it?...

Oh, it appears you chop it in half with an angle grinder on your lawn then rejoin it using an old gudgeon pin as a dowel.
https://youtu.be/7myfnvhl_r4?si=Kt_V2_lwVfiGg0Lg

Regarding carbs, I found a CB125T ran perfectly well on a single, standard carb instead of the twin carbs. It was producing the correct fuel:air mix. I know this is different though because that was only sucking through one pot at a time. You probably want a carb with a choke surface area equal to the combined inlet port surface areas.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Easy-X]

Rebore the cylinders, bigger pistons, hot cam, retiming, carb re-jetting... what are we reckoning here: at most you might rinse 25hp out of the engine?

And then you find the clutch slipping

Could you squeeze in the engine from a Rebel 400 or Superdream CB400N?
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Royal Enfield Continental GT 535, Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[stinkwheel]

Occurrs to me that if you're going to go to the effort of cutting and rejoining a cam, it would stand more chance of gaining power if you messed with the valve timing. I'm sure the valve timing specs for the K-series CB250s or early T-series CB125s are out there somewhere which would probably be a good starting point (although bore and stroke are different).

Re-profiling would be trickier because I bet they also have more lift and duration but the hardening on them is pretty thin.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[sickpup]

CD250 had twin carbs and would do 70-80mph with a bit over 20BHP and 5 gears as opposed to most of the family having only 4.
Not really anything you can do to make the engine faster, it was the pinnacle of the Benly engines and was surprisingly good, good enough that they mainly sold to Police forces over here.

If you want a faster bike, buy a faster bike, whatever you do you will be beaten by the gearing and if you over rev you'll drop a valve or two.

[WD Forte]

[stinkwheel]

You should fit an MZ engine. I'd imagine a rotax 500 single would fit too.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[lingeringstin...]

Yeah I sort of figured there would be no difference in power regardless of where or when the pistons fired per rotation of the engine. I've found absolutely no difference between using a 180 crank or a 360 in the same engine.

But weirdly when I used the online cc calculator to figure two pistons firing at once (54 + 54 = 108) and figured the stroke of 53 it came out with that almost 500cc number which sort of threw me. I was just going by the fact that a 300-ish single used about a 75mm piston so a pair of 54mm pistons firing at the same time like a single would in theory be 108cc. I guess the calculator can't handle that sort of weirdness or something because my mind said it should still be a 250 but the calculator called it a 500.

These engines are robust but they do have their limitations. I've already got the muchly uprated clutch (more springs and more plates) so there's no problems there and the gearbox seems to be actually pretty sturdy but there are no engine modifications you can do to a 250. The biggest bore you can run is 56mm and although you can run slightly higher compression pistons it doesn't really add up to much.

There aren't any "hot" cams to be had for these engines any more. Once upon a time you could get a better cam in like the days of the CD185 but they stopped making them ages ago and now EVERY replacement cam for the 125-250 Benly style engines is exactly the same, meaning they're basically 125 cams.

Same with the valves. They are fitted so close together in the head it's impossible to modify it for bigger valves so you're stuck with basically 125 valves no matter which engine you use. I can slightly increase compression by fitting a 125 head to 250 barrels but it's a very minor upgrade.

The inlet ports are impossible to fiddle so you have a choice of either 19mm inlets on the 125 head or 22mm inlets on the 150-250 heads. I've run both and found no performance difference between the 19 and the 22.

Being such small ports means your choice of carbs is limited too. I read somewhere that the power difference between a single and dual carbs was about one horsepower and that's been my findings as well. I run a single carb because it just adds complication and weight to run two of them for no performance gain.

The carb I use at the moment is a screamer but it's tiddly. It's a PZ27 carb with a 125 jet and a 24mm carb needle (shorter and pointier). No air filter as such, just a pod filter with the paper element burned out of it so it's basically just a screen that keeps rocks and twigs out .

In the lower gears it revs like stink so I know the carb works fine, it's just that at around 20hp you can knock off about 5mph pulling a trailer, knock off another 10mph going up a serious hill, and knock off another 10mph if you're doing it into the wind. Then if you have to slow down for some reason like going round uphill corners you NEVER regain the momentum again so you can find yourself stuck in some lower gear with the throttle pinned wide open just creeping along until you get over the hill.

It doesn't matter how you gear the bike it's the horsepower that dictates how fast you're going to go. On my "Yorkshire gearing" front sprocket I have five useable gears but much less top end, and on my "normal" sprocket for daily use I have four gears and an overdrive for cruising but you can't pull a trailer worth a damn on that one.

Basically I'm good with the general performance when NOT pulling the weight of camping gear and going up hills. The bike is fine for general use and being tax exempt and low insurance it's very cheap to run. Even on my lowered "Yorkshire gearing" sprocket I got about a hundred miles per gallon so that's pretty amazing. The only real complaint I have is that uphill climb with camping gear into the howling gale and pissing rain. If you let the bike slow to below 37mph you're doomed until you can get over the brow.

I think what I'm really looking for is temporary boost that I only need to use in certain situations, and that probably means looking at laughing gas again. I dabbled a bit in the past on my two stroke engine but it was usually a disaster so I don't know if I want to go to all that complication again. Maybe I'll delve into the murky world of designing a turbo I can call into action at the push of a button or something.

The plot thickens.

[lingeringstin...]

[quote="stinkwheel"]You should fit an MZ engine. I'd imagine a rotax 500 single would fit too.[/quote]

I had forgotten about those MZs with the Rotax engines. I used to want to get a Rotax engine to replace the two stroke because it's Hell to find decent MZ engine bits these days. The repro stuff is even more miserable quality than the originals were and everything is "vintage" expensive now.

[lingeringstin...]

OH FOR FUCKITY FUCKS SAKE!!!!


I was perusing info about those MT350 and 500 engines thinking that looked like a very good choice for my needs, and then something caught my eye.

The fucking sprocket is on the wrong side of the engine!

My bike being an MZ frame USED to have the chain on that side but when I converted to the Benly abomination clone I hacked the bike up and rejiggered things for the chain to be on the other side. It wasn't just a matter of swapping the wheel over, there was all sorts of fuckwittery I had to do like build a new rear brake lever system and shit.

I still like the Rotax idea though. Seems like a sensible engine for my needs.

[blurredman]

[virus]

the rotax they used in the mt350 will run fine on 87 octane or even lower too, which would suit your penchant for filling your tank with anything vaguely flammable
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own: 81 xs1100g...
owned: 85 rat CG (sold), 91 GS500e (stolen), 84 gsx400f (scrapped), 81 z250 (siezed, siezed, scrapped), 83 cb250rs (sold), 84 gpz750r ratfighter (killed) 84gpz400 (sold), '80 cb650 ratfighter (wrote off) 95gsx6/12f ratfighter (killed) 91 xj900 (sold)
stinkwheel Well I just had my hands up a pigs fanny. Which makes your concerns pale into insignificance.

[stinkwheel]

The military bikes are also famously badly geared. They wanted gearing that would do road and off road riding and in classic military fashion, picked gearing that does neither very well.

Anecdotally, from watching them at many rallies, owners also seem to spend a lot of time repeatedly kicking and swearing at them any time they want to go anywhere.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[lingeringstin...]

But here's something I discovered about these Benly-style engines.

If you take a 250 Benly style engine and put a 125 crank in it, and then shorten the 250 barrels to suit and get some pistons that fit (I use YX140 pistons) you basically get a short stroke 250 which goes like the clappers. So why did the bog standard 250 have longer con rods anyway?

My theory is that as a short stroke engine it only works out to something around 200cc on paper even though it goes better than a 250, but at the time 250cc commuter bikes were all the rage and my theory is the CD250U was just made to "be" a 250 to generate sales even though a 200 like I described would have been a better engine.

I rather liked the 202 I made but I sold it to somebody to put on a go-kart. I think I'm going to have to make another one now and keep it for myself.

[stinkwheel]

It doesn't have a peppy short-stroke engine for the same reason it doesn't have a 60bhp 2-stroke engine or a DOHC inline 4 (both of which they were making at the time). It's a dull, economic, easy to ride commuter bike. Practically the definition of a mundane Universal Japanese Motorcycle. They made it with a square engine precisely because it's so middle of the road.

They made it with a 250cc engine because when that series of bikes first came out, you could ride up to 250cc on L-plates so that's what people wanted. Same reason as learner bikes are now almost all 125cc, they make 100 and 110cc bikes but people in the UK don't buy them.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[lingeringstin...]

[quote="stinkwheel"]It doesn't have a peppy short-stroke engine for the same reason it doesn't have a 60bhp 2-stroke engine or a DOHC inline 4 (both of which they were making at the time). It's a dull, economic, easy to ride commuter bike. Practically the definition of a mundane Universal Japanese Motorcycle. They made it with a square engine precisely because it's so middle of the road.

They made it with a 250cc engine because when that series of bikes first came out, you could ride up to 250cc on L-plates so that's what people wanted. Same reason as learner bikes are now almost all 125cc, they make 100 and 110cc bikes but people in the UK don't buy them.[/quote]



Ah yes, the market dictates what gets made. I'd forgotten about the 250 L-plate rule at the time. And back then boring little commuter bikes were being touted as ideal city transport so they didn't have to focus on any kind of performance just to ferry somebody to work through busy streets. They just had to work even in the hands of morons. About the only thing you can really say for these engines is there's no way to blow them up. Too under tuned to fly apart but I do like the hundred miles a gallon thing.

I'm probably just going to make another short stroke "250" using a 125 crank, which as I recall actually came out to something like 202cc if you consider two 56mm pistons and whatever the 125 stroke is. That's been the best combination I've found with these limited ingredients.

You can press the crank together as a 180 or 360 and I've used both, the 125 cam being for 180 and the 250 cam for the 360. The lobes on either cam are exactly the same size as are the valves in the heads. Didn't find any real difference other than simpler wiring for the 360.

The low end skidaddle was much better with the shorter stroke which is probably better for my purposes anyway. I hate screaming for high revs on these engines. They don't particularly like it.

Can't really fit bigger pistons unless you use a 300 quadbike top end (Quadzilla) but they're water cooled so you have to start adding all the complication and weight of water cooling which might defeat the purpose for so little gain in CC's. However, I haven't made a short stroke bored out water cooled 300 yet. I'll have to do some thinking and see if that's a viable option. I don't know how much you can bore out the water cooled 300 top end.

The question is, does the head REALLY need to be water cooled or can I get away with just water cooling the barrels and using a 125 head for more compression?

[stinkwheel]

People still find ways of blowing them up. Never cleaning the internal gauze oil strainer seems to be the main one.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Nobby the Bastard]

The only question that comes to mind is 'Why the fuck go to that sort of bother over a fucking Benley engine?'
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trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Triumph Sprint ST 1050

[A100man]

[Nobby the Bastard]

Yeah but they usually make them bigger and not smaller.
____________________
trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Triumph Sprint ST 1050

[lingeringstin...]

[quote="stinkwheel"]People still find ways of blowing them up. Never cleaning the internal gauze oil strainer seems to be the main one.[/quote]

Yeah that's an issue. They don't hold much oil in the first place and you need to change it often and make sure that wee little screen on the oil pump is clean, but I'm used to that. Also I use needle roller bearing instead of the Honda type ones on the cam. One of my Chonda copy engine cases even has the attachment gubbins for an external oil cooler which I thought could be rigged for an external oil filter instead since these engines don't get very hot unless they're covered up with quadbike plastics.

[lingeringstin...]

[quote="Nobby the Bastard"]The only question that comes to mind is 'Why the fuck go to that sort of bother over a fucking Benley engine?'[/quote]

Mainly because I've got loads of spares and practically everything fits each other whether it's 125 or 250. Cheap as chips to faff about with. For instance I recently bought a brand new cam from China for like £20 to replace a sadly worn Benly one I was using. Exact same thing as the Benly one but a fraction of the price and gearboxes, cranks and clutch gubbins are similarly priced.

From my experience in the recent years the quality of Chonda copy Benly bits is as good as anything Honda ever made now. I've got I think three complete engines in the garage made of about 50/50 HondaBenly/ChondaBongly and it all works because the Chinky ones are exact copies of the old Honda ones so everything bolts together apart from one or two very minor differences that don't actually matter.

But mainly it's just a lot of fun to mess about seeing what can be done on the cheap. Where's the fun in fitting a sensible and reliable CBR6 engine to my frame when I can get arse deep in stupid shit instead?

[lingeringstin...]

In other news, I've just discovered the "Regal Raptor DD350" engines (AKA Emgrand Raptor DD 350, etc.) that look like the top end of which just MIGHT fit on a CD250U bottom end. I'm currently scrutinizing pics of gaskets and things but it looks promising. I still have a sneaky idea about running a water cooled barrel with an air cooled head.

If I could wheedle out the 250 engine cases to take the 350 top end and use a 125 short stroke crank this could be interesting, but I've got a few things to check into before I grab the wood file and get out the hammers.