Carburettor size??

This season so far I have gone from a 36mm to a 34mm and will be trying a 32mm at Cadwell...
Anyone else 'playing with carbs' ??

every night when i have my dinner lol

So what benefit can you see from the smaller carb Mike ?

Now I have similar port timings to 002, albeit piston controlled inlet; I was going to fit the 36mm Lectron I got from you and and a similar chamber. The ex LH engine had a 34mm Mikuni on a 32mm inlet, but on the dyno CMRR eligible Mk2 Amal 32mm worked just as well.

Is it the greater vacuum/flow with the narrower venturi or better dispersion/ less outer laminar flow ?

Still staring at the BTW/TM 175 top end on the bench, awaiting 4 speed gears for bottom end.

Interestingly, my current two stroke tuner, who quickly saw the Banatm foibles and idiosyncracies; was horrified at the thought of a 36 - 38mm carb on the 175 ! He wants to put Devcon in the inlet to make it 34mm to start with, despite me quoting youse experiences.

Seems theories go round in circles every decade ?

John you (or your man) are dead right! and it seems to be 10 years not 8 or 6 !

With 001 36mm very poor response to throttle but fast....
34mm better throttle response out of turns and away from harpins nearly as fast flat out...
32mm .... ? don't know yet!

Brian White runs 38mm on his sprinter and Tom I think runs a 36 or 34. Tom's Bantam is reed valve, though, and I believe reed valve engines don't require or respond well with big carbs? Of course in our sport we're not inconvenienced by changes in direction so we're on the pipe all the time. In fact, as I remember, Tom's sprinter will only turn one way at the end of the course, due to his humungous and rather low expansion chamber.

On my bikes, albeit non-Bantams, I've always been in the smaller carb camp: good throttle response, easy starting and more straightforward to set up. On my 100cc sprinter my fastest speed (but not my quickest time) was with a 26mm carb, rather than the 28mm instrument that I used later.

I happen to think some of it is psychological, hence the old American addage: 'If big is good, then bigger is gooder.' Ain't necessarily so!

Hi ,All
Don`t know if it helps , but Mark won his first championship using a 30mm Amal , and his subsequent two using a 32mm Amal .
The larger the choke dia, the higher can be the pressure ratio in the duct , and , so increase maximum flow rate at high rpm .
However, this will always be to the detriment of clean carburation , air /fuel control ratio, air flow rate , and transition to , the
bottom of the power band, at lower rpm .
It`s been said a thousand times , bigger is not nesessarily better ! Getting around a circuit in the shortest elapsed time won`t always
mean top speed , clean carburation and superior acceleration are probably more important, at least that was always Mark`s contention .

Does this mean that the search for the Bantam ,Holy Grail, is set to continue ?

regards Trevor.

when i started back last year, my bike had the original carb, a 30mm amal mk11, bored to 34mm - but the throttle stuck open, I entertained the marshals at mansfield, and frightened myself silly.

so we asked for advice on here, after some considerable debate, some people saying NO not a mikuni, others saying a Lectron/or dont get a mikuni, we got a mikuni, 30.0mm - after some gremlins, and a trip to Alans, with a power jet kit fitted, a new set of jets, we had it carburating a lot cleaner,

it will pulled from nothing, in the interest of obtaining a little more power we lifted the barrel for more port open timings, and its still very tractable and still pull into the powerband without any slipping required. So is this due to the smaller diameter bore / INCREASED air SPEED /FLOW /PRESURE the smaller venture. ot better control with a quality carb/jet/ metering system. ?

it has phernominal fuel consumption too (low).

so where do we look now for improvments, compression ratio / a new pipe / ports / ignition ?

"where would you look for more power", and why ?

regds Derek

everyone will always tell you differant things. i agree with Trevor, bigger is not always better. as i've said in another post, josie runs a 30 ml 4 stoke mk 2 amal with no power jet conected, but she still flies for a dinosoar. your last statement is where it's at.commbine all those things to suit the way you ride and you'll get the best out of your package ..... eventualy!

I say that for any given set up of your engine, ie: fixed exhaust, unchanged barrel, the same compression ratio etc. then you will not get any improvement in maximum power if you fit larger carbs. For no other reason than the engine will only provide a fixed amount of "suck" to the inlet.
However... that means if you go larger then the airflow becomes slow at low engine speeds and so your powerband width suffers.

All carbs have an efficient internal air speed at which they work well, so the trick to getting not just good power and clean air/fuel ratios across all throttle openings (improving your powerband as well) is to not over carb, and not undercarb and spend a lot of time sorting the jets.
Also there is a point of restriction at the other end of the carb sizes where you really are holding back the air flow so too small will kill the power for sure.

Note all carbs have different air flow characteristics which must be worked on to get the best from your finite amount of "suck". That is why you see lots of wierd and wonderful valves on my carbs! not to improve power at all, but just to get the best of what I have whilst I solve the anomalies that I find.
Don't use big carbs for a three speed motor... you will lack powerband and struggle to get clean mixture strength, and it will change drastically with the weather!
Hope this helps,
Alan

great info from alan

spot on as it worked for my engine "smaller is good" - "same as cranks" I feel.

regds Derek.

so from reading there is no standard set up but down to other factors and whats suits best?

Eddie

Speaking to Scuttie the other night about the difference between my Lectrons and his?
Ian's are high velocity types - necked in the middle (venturi) where as mine are straight through.
With lack of availibility of Lectron I am trying Dellorto's on my bikes now...

HI ALL

ON CARBS - I have to have accuired a 36mm lectron with a power jet and some spares ! a birthday present from a former bantam racer.
we are not intending using this at present. But will be trying it on the dyno.

so where are we upto 2011 so far been on the dyno a few time found a few smaller items for minor improvments, we had ok meeting at mallory, and done a practice day testing gearing and clutch parts, we do not use steel plates on the current bike and have tried solid bronze and sintered bronze, however doe to the weight of the bronze when it gets hot it slipps, even with 18 springs ? issues comprssion ration /new ignition on the way, possibly use latter in the year dependant on time available.

We want reliability over speed and power so, are only looking to reduce stresses, paying particular attention to friction, on the crank and piston, we have allowed more clearance on the skirt for the piston, and identified we have a limitationn in the exhaust system, but only by chance a small piece of decon fell out of the exhaust port and it pulled another 2 hp and another 600 rpm ?? wow, we have returned the piece but this will be removed for cadwell, we have also put a small cordal shaped hole high on the rear on the piston skirt this has reduced the btm end torque but added volume in top end, but not making any extra horses.

Iam looking to finish all the races ? and get a bit closer to James, my target guy for this year, Ihope he's not going any quicker butwish him to have better luck with the gears and is sorted.

so for us this year reliability is paramount, no point in looking for more power with the current setup due to some limitations, we have found.


For the future - edd's bike is looking good hopefuly we will soon have a complete bike and he can make his appearence.

For me my future is to continue development with my old engine, - for this ? we are to make a new barrel, if only we could get an air -cooled barrel to fit the cases it would be far less of an issue and a lot lighter, we had decided initially to build a new water colled barrel, modelled on the old one, but some minor design differences, the work involved is unberleavable, I can not believe I went to these lengths to get it all correct, so much welding, stress distortion machining, re machining heatreatment, blimey a mission, and for what ?

we have been avoiding fitting an air cooled barrel as a result of the complexity of clamping it to the crancases,

Andy Moulden did this succesfully on a crancase reed VALVE motor, the same as ours, but his suds were closer together than we had done, my engine was designed to allow VERY LARGE transfer PORT VOLUME, so the suds are very wide apart, and all four studs ARE on different centres, SO ITS NOT AN EASY ONE, Ican not see how to get an air cooled barrel on it, with out casting a special ? , so we were forced to consider just the water cooled route again. but in the vain to try again I would like anyone to feel free to take a look at it, i'm brining it to cadwell,
to see if anyone can see how it could be done, modify a barrel air cooled barrels Brian white does,

but I will not be moving the studs, the cases are very accurate, I jig bored them on a newall borer, whilst in the aerospace industry, the whole thing is very precise, the cases strenthened internally, so moving the studs is a no no, be easier to start again!.

So I will bring the complete engine barrel to cadwell, strip off the barrel, to show anyone who wishes to take a look, or comment on how this could be done with an air cooled barrel, without moving the studs, I will be vey happy for any one to take a look, and listen to any ideas or thoughts.

Regards Derek

Mike,Derek & all,

interesting ideas on carbs ,my approach has always been what carb size works best for you the rider ,for me i ran 34mm carbs on a tz350g with good results so tend to go along the smaller carb is sometimes better. However the water cooled ex carl bike came to me with a 32mm mikuni tmx this just would not work for me ,i tried every needle ,pilot ,main combination and was still not happy.It now sits in the box dormant.
It now has a 34mm mk 2 amal , yes its prehistoric but strangley works well so my theory is carb design as well as size also has a big influence on a motor.
Derek ,please put me in the queue to look at your watercooled motor,dont know if i will have any ideas but saw some pictures of it on the forum and it looks great .
Good luck to all runners over the weekend, fingers crossed on the weather.

Cheers Nick

we identified we have been going the wrong way with jetting, engines the first i believed had an air leek, so changed it after it seized twice, before it distorted itself, the later engine a piston ported, ran ok in the scratch race but was not pulling properly, so we reduced the jetting again the wrong move, this also overheated and took out the big end timing side shaft looks like a rebore is in order, so we have a lot to do before the next meeting.

My clutch and gearbox drive changes appear to have worked great, as I made far better starts and the bike pulled top gear well as previously it would not, perhaps another reason carburation has become much more critical we must try and get it right.

regards Derek

Sounds like a good weekend at Cadwell.

So firstly Mike, How did the 32mm carb work ?

and Derek, was the better starts with the 14T engine sprocket ?

yes john 15t

but with snell gears top gear is still very low = to 15x50

and the phenomenal fuel consumption has now been identified as down to being too week and its destroyed engine is the result. ?

Derek