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lingeringstin... |
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lingeringstin... Trackday Trickster
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Kickstart |
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Kickstart The Oracle
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Teflon-Mike |
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Teflon-Mike tl;dr
Joined: 01 Jun 2010 Karma :
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Posted: 00:11 - 25 May 2017 Post subject: |
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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! ____________________ My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?' |
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stevo as b4 |
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stevo as b4 World Chat Champion
Joined: 17 Jul 2003 Karma :
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lingeringstin... |
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lingeringstin... Trackday Trickster
Joined: 01 May 2014 Karma :
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stinkwheel |
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stinkwheel Bovine Proctologist
Joined: 12 Jul 2004 Karma :
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Posted: 15:15 - 25 May 2017 Post subject: |
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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. ____________________ “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. |
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lingeringstin... |
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lingeringstin... Trackday Trickster
Joined: 01 May 2014 Karma :
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Teflon-Mike |
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Teflon-Mike tl;dr
Joined: 01 Jun 2010 Karma :
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Posted: 19:47 - 25 May 2017 Post subject: |
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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? ____________________ My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?' |
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wristjob |
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wristjob World Chat Champion
Joined: 30 Sep 2005 Karma :
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Rogerborg |
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Rogerborg nimbA
Joined: 26 Oct 2010 Karma :
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Posted: 21:07 - 25 May 2017 Post subject: |
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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. ____________________ Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike |
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Commuter_Tim |
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Commuter_Tim World Chat Champion
Joined: 09 May 2013 Karma :
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Shaft |
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Shaft World Chat Champion
Joined: 27 Dec 2010 Karma :
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Posted: 23:13 - 25 May 2017 Post subject: |
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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? |
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. ____________________ Things get better with age; I'm close to being magnificent........
20 RE Interceptor, 83 Z1100A3, 83 GS650 Katana
WooHoo, I'm a Man Point Millionaire! https://www.bikechatforums.com/viewtopic.php?t=234035 |
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stinkwheel |
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stinkwheel Bovine Proctologist
Joined: 12 Jul 2004 Karma :
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Posted: 01:01 - 26 May 2017 Post subject: |
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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? |
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. ____________________ “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. |
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lingeringstin... |
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lingeringstin... Trackday Trickster
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Snod Blatter |
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Snod Blatter Crazy Courier
Joined: 21 Nov 2014 Karma :
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lingeringstin... |
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lingeringstin... Trackday Trickster
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Old Thread Alert!
The last post was made 6 years, 337 days ago. Instead of replying here, would creating a new thread be more useful? |
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