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lingeringstin...
Trackday Trickster



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PostPosted: 23:03 - 24 May 2017    Post subject: Two carbs or not two carbs Reply with quote

This is probably going to open a can of worms I'll struggle to digest, but seriously what's the difference between one carb vs two on a smallish engine?

I've always gone for the one carb setup just because it seemed to always work good enough even on an old Honda 750 Four I once had.

I am about to start faffing about with a Chonda 250 engine just for the hell of it and have been wondering if there would really be any difference in performance between one or two carbs.

It's apparently based on a Rebel engine. This one claimed to be out of a quadbike but there's no sign of a reverse gear so I'm not sure. All I know is it's a 253FMM which is a Chinky CMX250.

My own personal thoughts initially are to bung a 30mm single slide pull carb on it and see what happens, but you can get a pair of new Chinese CV carbs for it for about £50 postage and all if they'd be any better.
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Kickstart
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PostPosted: 00:08 - 25 May 2017    Post subject: Reply with quote

Hiya

Advantages are better inlet design, carbs closer to the head, smaller carbs (which tend to work better in other wasy), and a few other minor bits.

However what the actual advantages are on a specific engine might be more limited. And might also be more related to the specific carbs you use and how well you can set them up

All the best

Katy
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Teflon-Mike
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PostPosted: 00:11 - 25 May 2017    Post subject: Reply with quote

At best it's a 20bhp motor, with a 11K rev cieling. You could run the thing quite happily on a single 26mm Mikuni Slide carb ala the CB Two-Fifty.

The single carb manifold for the Benly motors is a bit constrictive; not so much due to the size, but the two tight 90 degree bends in the thing to fit carb close to the head.

Single carbs on a benly do seem to pick up better at lower revs; twns start to come nto thier own whe the revs pick up.... whch they will just about do before running out of puff on the 360 timed motors.

The CV's used on the CMX and the last f the line CB125TD-J.. are a bit of a mightmare. Check the prices for a diagphram kit on Dave Silvers! It's 'bonded' to the slide; so effoff expensve to overhaul, and a liability to strip ad clean! They also de-tuned the TD-J even more than they had the middle era UK spec 'reduced effect' motors to use them; by all acconts they do carburate better and pick up better where a slide can let a motor bog f cracked to wide too quick... but the intake stubs to mount them are just as bludy expensive as the diagphram kits, so I have never been tempted to try!

If you want to give a single carb set up a go; Petemate, based somewhere oxford way had a spare branch manifold from a CD200 off me not long ago, to try on his CM, with a CG carb, that I believe is now surplus to drop him a PM see if he still has it,.

CB Two-Fifty so equipped was barely any less powerful than other 233 series Benly motors, and remarkeably frugal on it.

Alternatvely; Regal-AJS used the Benly motor for a good few years and even evolved a fuel injected water-cooled varient of it! Still struggled to make more than 20bhp... B-U-T.... Offers suggestion why not go hi-tech, and inject it? There is actually a proprietry squrter kit, I recall.. and even a TURBO!

As to the advantages of multiple carb set ups over singles? Technically its down to iduction tract length.

A single carb that can feed a single 250cc pot, can just as easly feed four 250cc pots, if they are 180 timed, so one is on each stroke of the cyle at a time, the peak flow is no more than that for the single; but each pt sucking i turn, means that the flow through the carb is much more constant; when one pot stops sucking from it, another starts, so the flow doesn't have to keep stopping and starting, and you can actually get a better average flow through the carb; meanwhile, you do get much better metering and throttle response from it. Unfortnately, the inlet tract volume, tends to increase to account for cross-conecting manfolding, and the inlet tract is likely to become more restictive, offering more twists and turns between carb and valve. On a higher reving engine, the lack of inlet tract volume, and the less air it needs to get shfting before it starts sucking, gven the very short inlet duration at higher revs, means that the adantages of a shorter, more direct inlet tract start to out-way the advantages of cross-coupling, and a single carb per pot on a muh shorter, smaller volume manifold, will allow much better throttle response and power than a cross-couples single carb set up... but, BIG area of optimisaton, ad can start to get complicated looking at all the exeptions to prove rules, one way or the other.

And on the Benly engines?!?!?!? Well, I'm happy just if the damn thing RUNS!
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stevo as b4
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PostPosted: 11:08 - 25 May 2017    Post subject: Reply with quote

I'd generally rather one very trick kick ass carb like a very tunable flat slide race carb, than a pair of old second hand or crap Chinese carbs that at best would work ok and at worst would be a bastard to ever get to run right.

If the inlet manifold for a single is a nice flowing smooth design and there's space for it, then that's the right answer IMO. Longer inlet tracts are generally beneficial on many road going engines anyway so I doubt there's anything to lose.

With EFI I'd always prefer ITB's and close to the inlet valve injectors, unless it's a siamesed head where a single throttle body and wet manifold is far superior.
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lingeringstin...
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PostPosted: 13:07 - 25 May 2017    Post subject: one Reply with quote

Right, that's all pretty much as I thought. I've always leaned to single carb setups and I do rather distrust a set of cheap Chinese CV carbs so I'm going to go with a 30mm flatslide just because that's what I've got at the moment. It might be a tad too big but clever jetting will sort that out. I've run carbs up to 44mm (alcohol) on engines with much smaller inlets and made it work.

I ran an inch and a half SU on my MZ for like 20 years and that was technically too big for the 30mm intake. I converted it to slide pull instead of CV and found it worked great with the only niggle being that you ran out of throttle response at about 3/4 open as that's all the engine could suck in but it wasn't ever a real problem. It just acted like a quick action throttle and I didn't really use the last quarter turn. So I don't think I'll have any problem getting a 30mm to work on this 233 engine.

I've got a couple of different single carb inlets for it. I think one is off a Honda Rebel 250 engine with the carb right in the middle and the other one is a quadbike intake that angles the carb off to one side if need be and looks like the most likely culprit. If neither of them are suitable I'll make one out of PVC pipe and stick the carb out the side if I have to. I'm sure it will work.

Having looked at the engine I think there's some scope for tweaking. You can rebore and stick in some 185 pistons or get a 300 top end and skim the head to increase compression. Might be able to squeeze a few more horsepower out of it. Seems to be a robust enough engine design. I've come across wild tales of a secret fancy-pants cam that is impossible to get but I probably wouldn't bother. I've seen a picture of an AJS 350 engine that looks almost identical to this 250 so maybe...
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stinkwheel
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PostPosted: 15:15 - 25 May 2017    Post subject: Reply with quote

I know you like to make you life difficult so why not fit two carbs of differing types?

Have a 2-1-2 manifold. Have a tiny slide carb controlling idle and up to about 1/3 throttle opening then have a big CV carb with no idle circuit that kicks in at 1/3 to full throttle.

In town, you can trickle about on the diddy carb, sipping fuel with a stable and responsive idle, crack it open and it can snort a load of fuel to get out of trouble fast. Out of town, you can have it tuned for best response at wide open throttle.

They had this system on my MZ baghira. It had twice as many carbs as cylinders. Worked well, you could chug over and around loose and uneven surfaces keeping it on the slide carb and it was a little pussy-cat. Then crack the throttle wide open and hear it gulping down the entire contents of the airbox in three revolutions, squatting down and heading for the horizon.
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lingeringstin...
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PostPosted: 17:14 - 25 May 2017    Post subject: counting carbs Reply with quote

I had actually thought about that very thing in the past. Once I heard about the carb setup on your bike I've often mused that it was a really good idea. It would sort of be like the old V8 cars we had when I was a teenager that were fitted with Quadrajet or something carbs. You'd go around on the two-barrel bit until you stomped on it and then the other two bigger barrels would kick in. Worked quite well on old cars, no reason why it wouldn't work on a bike.

And it's not that hard to do. I figure a T-shaped single inlet with a little carb on one end and a bigger carb on the other end would work. I could see fitting a CG carb on one end and an SU on the other with a spring loaded lever thing that started the SU going once the CG carb was almost full open. I don't think it would be that hard to design.

However, the simpler solution is probably a carb with a power jet. Never had one of them before.
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Teflon-Mike
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PostPosted: 19:47 - 25 May 2017    Post subject: Reply with quote

Trying to tune the Benly for power is a route to madness.... after the MZ, I suspect you are already there, so there may be room for optimism, or who cares.. welcome to the asylum! B-u-t, from my prodding and poking....

The head is actually pretty useful 'as is'. It has a Hemi combustion chamber, and little scope to increase valve sizes much if any without getting ultra engene-ius with ali-weld and milling.

The inherent restriction in the Benly motor is the cam timing and compression ratio. Hottest cam they used in them was the ultra-rare 309 profile used in the CB125T, which on 41x43 bore/stroke, achieved about 17bhp, making it one of the most powerful of the Benly family.

The 53mm bore motors; the CD&CM200 and all the 250's; ran a lot closer to square, with a lot less revs; typically topping out at 10K, where the CB125T could rev to over 14K; the 200's actually making less than the screamer 125's at about 15bhp, the 250's only about 19-20.

Looking at the head, knowing it's good for 20ish bhp in the bigger bore motors as 'stock', there's little scope to find any real gains; the ports are a bit meh, especially the exhaust, but they are pretty big, and there's not a lot of metal in there to start messing! And without tackling the cam-timing and compression ratio, it's unlikely to reveal any significant gains... and there's even less to be found upstream i carbs & manifolding.

Mixing and matchg Benly bits is frustrating in the extreme.

The small-bore motor's run 41x43 dimensions; the big bore motors 53x53, with some annoying differences in things like gudgeon pin sizes.

Best cam to use in one, and the only real start to any significant tuning gains is to use the 309 'full-power' 125T cam.. but that is timed for a 180 crank motor; most of them, 125's and 233's are 360-timed.... and that cam is like rocking horse poop to get hold of! But, does tell how much is lost in the valve timing on the other motors.

Other impediment is the compression ratio, due to almost flat top pistons, which is dropped from over 10:1 on the 125's down to 9:1 on most 233 variants, and I believe even less on the Chinky derivatives to meet emission controls.

Looking for small gains on these motors; there really is bugger all to be found without digging into the bowls of the motor.

Uppng the C/R is a likely place to start; but with a Hemi Head, that's rather 'thin', scope for skimming isn't great and higher crown slugs are the order of the day... and gains there will still be limited by the soft cam-timing.

I did clock a 309 Cam and a 'reduced effect' 125 cam, and compare to a Chinky TD-J example; there's actually bugger all difference in the lift; so takng some metal off the base circle, or reaming the rockers eccentric to get a tad more isn't likely to help much; actual duration wasn't any different either; big difference was in dwell, they get the valves open a lot faster and hold'em open a lot longer, which means adding metal to the lobe and re-profiling to get that opening ramp back....

I dumped a take-out 200 into one of the super-dreams; and ran it on the Super-Dream twin-carbs, and twin circuit ignition, off a single pick-up. Went pretty well; certainly had a chunk more bottom end than the 125; but exaggerated by lower gearing in the 4-speed box. Didn't really delver any more up-top, and was loath to rev any higher, and was no faster.

Silly ideas based around the older CB200 'sloper' motor; that ran 41x55mm bore/stroke, were sort of shelved when Sowie got the bludi-guzzi. The Bank-Manager, in his role as psyco-therapist dispelled the greater excesses of trying to build a Silver-Super-Dream-Turbo, trying to ape the 'factory' styling of the super-maggot, with Chinky blower-kit!!! 'Best' Benly I have built up from accumulated parts, including 309 cam, from a broken T and TD-C motor.. or at least I hope its the 'best'.. it's waiting me to finish a chassis for it!

BUT conclusion of my musings, is just how frustrating these ruddy things are... and without getting very creative with the ali-weld, in the big-bore motor's, exploiting that low compression ratio for forced induction, is probably about the only way to find any significant gains from the damn things!

Best feature the have is probably the miserly MPG... which suggests a completely different approach and direction... Alcohol or LPG perhaps?
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wristjob
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PostPosted: 20:09 - 25 May 2017    Post subject: Re: counting carbs Reply with quote

lingeringstink wrote:
I had actually thought about that very thing in the past. Once I heard about the carb setup on your bike I've often mused that it was a really good idea. It would sort of be like the old V8 cars we had when I was a teenager that were fitted with Quadrajet or something carbs. You'd go around on the two-barrel bit until you stomped on it and then the other two bigger barrels would kick in. Worked quite well on old cars, no reason why it wouldn't work on a bike.

And it's not that hard to do. I figure a T-shaped single inlet with a little carb on one end and a bigger carb on the other end would work. I could see fitting a CG carb on one end and an SU on the other with a spring loaded lever thing that started the SU going once the CG carb was almost full open. I don't think it would be that hard to design.

However, the simpler solution is probably a carb with a power jet. Never had one of them before.

progressive chokes are a tried and tested way of getting an engine to run better at at low revs then high revs.and it is sort of like having 2 carbs(pie sandwich)
on such a small engine is it needed? i doubt it.
what are you going to gain from the added complexity?
any real world power?
if you really want to get into it then making your own fi setup is going to be more realistic.
and in these days of cheap electronics makes more sense than "see what happens"
either way ,if you are doing it for fun then get on with it and enjoy the technical exercise and win or lose you had some fun.
if you want more power then bigger engine all day.
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Rogerborg
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PostPosted: 21:07 - 25 May 2017    Post subject: Reply with quote

stinkwheel wrote:
Have a tiny slide carb controlling idle and up to about 1/3 throttle opening then have a big CV carb with no idle circuit that kicks in at 1/3 to full throttle.

Hmm, now that I think about it, my Volvo 34-something had twin carbs. The throttle opened the butterfly on one, and then t'other.

Of course, I wired them together so that they opened at the same time, wrecking performance. But on the upside, it used more fuel.
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Commuter_Tim
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PostPosted: 21:26 - 25 May 2017    Post subject: Reply with quote

stinkwheel wrote:
I know you like to make you life difficult so why not fit two carbs of differing types?...


To the ignorant, this all sounds too good to be true.
If as efficient as it sounds, is there a reason all bikes didn't end up with such an arrangement?
Did FI get introduced before most bikes adapted?

EDIT: Did I miss sarcasm about the efficacy of such a setup I wonder? Thinking
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Shaft
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PostPosted: 23:13 - 25 May 2017    Post subject: Reply with quote

Commuter_Tim wrote:
stinkwheel wrote:
I know you like to make you life difficult so why not fit two carbs of differing types?...


To the ignorant, this all sounds too good to be true.
If as efficient as it sounds, is there a reason all bikes didn't end up with such an arrangement?
Did FI get introduced before most bikes adapted?

EDIT: Did I miss sarcasm about the efficacy of such a setup I wonder? Thinking


My XL600 has a twin carb set up and it works very well, one side looks after idle to around half throttle, then the other side kicks in to give max thrust.

As has been mentioned, it used to be quite common with cars, single body with two unequal chokes, usually geared with a quadrant, so as the smaller choke reached the end of it's performance threshold, the larger started opening to deliver full fueling through to max power.

The idea is you aren't using masses of fuel when you don't need it at slow speeds.
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stinkwheel
Bovine Proctologist



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PostPosted: 01:01 - 26 May 2017    Post subject: Reply with quote

Commuter_Tim wrote:


To the ignorant, this all sounds too good to be true.
If as efficient as it sounds, is there a reason all bikes didn't end up with such an arrangement?
Did FI get introduced before most bikes adapted?

EDIT: Did I miss sarcasm about the efficacy of such a setup I wonder? Thinking


It worked fantastically well. Used almost no fuel if you kept it on the slide carb and it spent a lot if its time in town on wide open throttle on a small carb rather than the usual compromise situation of small throttle openings on a large carb.

I'd imagine getting it set up to work well initially is a form of voodoo though. It also means extra complexity and cost compared to having a single CV carb with an idle circuit.

Multiply this by four and it's not a huge leap to realise why it didn't take off on sportsbikes.
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I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.
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lingeringstin...
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PostPosted: 02:17 - 27 May 2017    Post subject: one Reply with quote

Having looked at all the online diagrams I could find it does really look like there's bugger all that can be sensibly done to a 253FMM engine that would make any real difference in performance. The expense and hassles of any meaningful enhancement are just not justified, so I'm going to keep it simple. In fact, the easiest thing to do is fit the 300 top end and be done with it. As it's a popular quadbike engine bits are plentiful.

I will make it a single carb setup and I'd love to use an inch and a quarter SU just for the looks of it but that might be too much to ask as it's near as dammit 31mm. Maybe an inch and an eighth SU if I can even find any to play with since they're all "vintage" expensive these days. I do have a 30mm flatslide that should just about work with clever jetting but it'll probably wreck the frugal mileage these things are supposed to get. I'll most likely be running a 2 into 1 exhaust. The fact is that a nice little 26mm carb is probably ideal for this softly tuned motor and no doubt after several months of frustration with the 30mm I'll probably cave in and buy one anyway.

NOW the big discussion is...

CV, flatslide or what?





Anybody know what a Jikov is like?
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Snod Blatter
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PostPosted: 12:01 - 28 May 2017    Post subject: Reply with quote

Sorry to throw the idea of twin carbs into the mix again but I can report that a mate's CD250U (twin carbs) is significantly faster than my Two Fifty (single carb, stupid manifold) on the motorway.

If you're looking at fitting pod filters (and surely you are because the airbox isn't going to fit the new carb(s)) don't go with CVs, the movement of the open air can disturb/unsettle the diaphragm and it'll be a bugger to get running right. I'd be looking at Mikuni TMs or similar.
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lingeringstin...
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PostPosted: 01:01 - 02 Jun 2017    Post subject: no airbox Reply with quote

The twin carbs idea is tempting but I can't fit an airbox on my bike so stock CV carbs are probably out. The only carbs I have at the moment to play with are various SU carbs or three sizes of two stroke flatslide carbs in 30mm, 32mm and 34mm. I can probably change the needle in one of the two stroke carbs for a four stroke needle and just about make that run right.

I'd rather not get another 30mm flatslide as I think that would be overkill since this engine is supposed to run a single 26mm like a CG125 carb or a pair of 26mm CV carbs. I figure for the moment I might be able to get the single 30mm to run without much expense or faffing about.

I'm not that fussed about top speed as this engine is supposed to be good for about 70mph and with my lack of suspension, appalling handling characteristics and antique drum brakes that's probably good enough. I prefer to try to squeeze mega mileage out of it as I do a fair bit of cross country journeys and this engine in a Honda Rebel is supposed to get about 70mpg with the single carb which I can live with.
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