steve+cub Posted January 23, 2006 Report Share Posted January 23, 2006 Having recently fitted a new 22mm Amal concentric carb to my cub, to try to cure low rev stalling in sections I am still having problems curing a flat spot which happens if i snap open the throttle from idle. i called Surrey Cycles for advice they suggested fitting a brass slide with a bigger cutaway which i fitted today. i took the Cub for a run on road to warm it up and then tried to trickle along at idle occasionally bliping the trottle to lighten the front flat spot is still there - again only if the throttle is snapped open, if the throttle is opened slower it revs cleanly all the way through the range. Do you think this may be an ignition problem, I am using a Boyer microdigital ignition kit and black box? Quote Link to comment Share on other sites More sharing options...
brian r Posted January 23, 2006 Report Share Posted January 23, 2006 I don't know anything about cubs (except I want one) but your probably right with the carb problem. Seems most four strokes have a problem with off idle coughing. The air needs to be sped up to atomize properly. A small carb works and sometimes retarding the timing a little. For something easy turn you idle screw up some. Quote Link to comment Share on other sites More sharing options...
old trials fanatic Posted January 23, 2006 Report Share Posted January 23, 2006 I wonder what you mean by "snaping the throttle"? remember 1. it's a four stroke. 2. the engine has to accelerate the heavy flywheels. 3. It's a Amal carb not a CV unit or a Mikuni (although the above also applies to a Mikuni) As Scotty used to say "Ye canna change the laws of physics". You cant expect an old cub to behave like a modern 2 stroke. Now this may or may not be the case but i was just wondering what you were expecting? I dont know your previous trials experience but riding a pre65 bike takes a radical modifying of your technique. These bikes respond to more of a constant throttle, momentum is all, and you cant just "snap" the throttle open and expect the bike to behave like a modern one let alone a 2 stroke. These comments are meant helpfully not as a criticism and it may be an idea to try your bike back to back with an already well fettled cub. Also what sort of cam is fitted? what sort of piston? which head as this will have a bearing as will valve size and therefore inlet gas speed and therefore atomization? Have you got a thermal barrier fitted as my cub was a bugger to carburate without one. Also mine was very critical on inlet tract length, detonation problem, and also air filter flow rate, more of a problem at high revs but did affect "spitting back" at low throttle openings till i increased the size and flow of the pipe connecting the filter to the carb. Finally silencer ability to flow gas freely at low revs can sometimes cause excessive back pressure that isnt such a problem at higher revs due to increased flow. Cubs are notoriously hard to "set up" and a lot of the local guys fit the carb from a TL125 and report it solves a lot of their problems. Just an observation. Good luck with the cub. Quote Link to comment Share on other sites More sharing options...
g4321 Posted January 23, 2006 Report Share Posted January 23, 2006 Have a look at the 'competetive cubs' that exixt in your area - if you are unlucky enough to live South of the Border you are likely to find that ex Honda 125 22mm or even 20mm Keihan(sp) carbs are likely to be order of the day - since there is little to force the use of true P65 kit - (carbs either from TL or CB 125 single I think). There are some riders using Delorto carbs of a similar size. If you are lucky enough to live in Scotland or want to compete in the Scottish P65then the above carbs are not popular with the organisers, that leaves the Concentric (or in Johns case being too mean to buy one, an old Monobloc), the Concentric is known to give problems as you describe on the cub - the Monobloc - as John quite rightly says is likely to be worn out! Some of the top riders - Mick Grant (yes the ex road racer) being the instigator have been using Villiers carbs with good results - these are a simple carb but are only 19mm in size (unless modified) and do reduce the top end power. Quote Link to comment Share on other sites More sharing options...
woody Posted January 23, 2006 Report Share Posted January 23, 2006 You may be worrying about a problem that won't affect you in sections. I snap the throttle open on my C15 to kill the engine which it does quite effectively as it's quicker than going for the kill switch (that I haven't yet fitted anyway...) In sections however, I never have to open the throttle that quick that the engine cough stalls. I'd wager you are less than likely to ride pre65 sections where you have to yank the throttle open that hard so possibly the situation may not arise in sections. As someone has already said though, depends on what you mean by 'snap' and how hard you're pulling it (stifle the giggles please) and ideally you need to compare the same action on someone else's bike that you know performs well. I've been told that the villiers carbs people are using, as fitted to Mick's Honda - sorry cub are reworked on the inside. A bit like fiddle forks perhaps. I've no first hand knowledge on this but I was told by someone who I believe. Mick's bike certainly isn't lacking in top end power, or anywhere else for that matter. Quote Link to comment Share on other sites More sharing options...
totalshell Posted January 24, 2006 Report Share Posted January 24, 2006 I have a decent enough cub, at the moment it has the tl honda carb on it and although the guys are right in saying that most of the trials dont involve snapping open the throttle your carb should work as you would want it too at all times. (mine does, that only leaves my riding as the weak link..) I would only fit my Amal for an event that demands it,By coincidence I checked out a top riders bike on Sunday at the trial and he was running a jap carb yet always rides the amal for Scotland, speaks volumes... the popular choice for off the shelf availability seems to be the dellorto at the moment in our club. If you stick with the Amal I would wind up the tick over a fraction and see what difference that makes and as you've now got a spare slide why not get the file out and play with the opening Quote Link to comment Share on other sites More sharing options...
bostit Posted January 24, 2006 Report Share Posted January 24, 2006 I've got the same characteristic on my Cub I recon the dreaded cough has cost me a needless five every time I've ridden it to the point where I haven't touched it for ten months now because it just irritates me too much. I've tried lots of things to cure it with little success. My bike runs Boyer and my mates bike with PVL doesn't have the problem anywhere near as much (non when the Delorto is fitted) but it runs lower gearing etc, etc. I run a slow action Domino throttle, have sleaved the carb inlet to 19mm, fitted rubber inlet pipe to reduce heat transfer and lengthened the inlet length and got very trick jets, atomiser and slide fitted. The result is better but not right. The carb now keeps very cold so no vapourising problem but when I go down hill with the throttle closed it will be very rich when I open the throttle, so no cough but still no clean carburation. I think the answer on my bike is PVL and lower the gearing so you don't drop the engine speed too low. The other option is to talk to Martin Adams at Serco and get the credit card ready! Quote Link to comment Share on other sites More sharing options...
trialsrfun Posted January 24, 2006 Report Share Posted January 24, 2006 I am following this with interest as I am building a Cub at the moment, OTF mentions the air filter flow rate can make a difference so which air filter is the best. Some people fit the gauze cone type others the alloy Miller one with the paper element filter (I think this is a car filter cut in half) but surely unless there is a decent splashguard around it the paper type will get wet and give trouble so what filter do we fit to a trials cub. Steve+Cub sorry I am unable to help good luck with the bike. Quote Link to comment Share on other sites More sharing options...
steve+cub Posted January 24, 2006 Author Report Share Posted January 24, 2006 (edited) wouldnt have said you were no help, i am running a SM filter maybe i should try a k&N cone type, althout I think OTF and Big Johns comments about modifying my riding might be more realistic, I have been riding dodgy old Montesa twinshocks and Gas Gas's for too long! I dont know much about the internals of the Cubs engine it has points on side cases a square barrell and a round barrell head !!? it did have a 24mm concentric on ot, but beyond that i don't know its history, I bought it from a dealer in Bicester - bit of a blind date Edited January 24, 2006 by Steve+Cub Quote Link to comment Share on other sites More sharing options...
bostit Posted January 24, 2006 Report Share Posted January 24, 2006 Tried the K&N(S&B actually)filter over the Sammy one, no real gain with the running but its smaller and neater (in my opinion). I think the riding style thing is relevent to a point, but it depends on what type of trials you use it in, some of the stuff round here in the midlands is very tight and nagery which is why everyone uses fiddle Bantams. Quote Link to comment Share on other sites More sharing options...
steve+cub Posted January 24, 2006 Author Report Share Posted January 24, 2006 Its difficult to know what the sections i will be riding are like cos I am completely new to competitive trials (on bikes). i went along to the IDMCC trial at Ardersier two weeks ago and we were trying to do an Intermediate course, tight nadgery, sectiolns in a pine forest, lots of steep sandy ascents and descents with tree roots (and trees) the boys on modern acrobatic bikes were hopping their way around the sections while i was taking loops to get a line, where i found i had problems most was if i throttled off at the top of a climb to keep the front end down, then came back on the throttle quickly to regain momentum the Cub would falter or stall, like wise if i came downhill, slowed to get the line for a sharp turn or climb, the motor would cut out or stutter, meaning a dab or a 5 Quote Link to comment Share on other sites More sharing options...
totalshell Posted January 24, 2006 Report Share Posted January 24, 2006 Good thread so far steve and i'm sure we've all learnt something I have the k+N cone type filter on mine, with no splash guard ( no oil tank either for that matter), ive never oiled it or any thing ( in 18 months) i take it off every weekend after the trial and pressure washer and give it a wipe out never anything other than a very very little condensation in it. I know you' ve peobably spent the best part of a ton on the carb so far but the dellorto's do work well almost straight out of the box, certainly good enough to win http://yorksclassictrials.co.uk/results.htm August 7th class K won convincingly by a cub with a brand new dellorto ( and a decent rider) Quote Link to comment Share on other sites More sharing options...
bostit Posted January 24, 2006 Report Share Posted January 24, 2006 i was taking loops to get a line, where i found i had problems most was if i throttled off at the top of a climb to keep the front end down, then came back on the throttle quickly to regain momentum the Cub would falter or stall, like wise if i came downhill, slowed to get the line for a sharp turn or climb, the motor would cut out or stutter, meaning a dab or a 5 I think this defines the difference between a 40 year old 199cc Cub and modern bikes, if you've got the Cub lined up it will go where you want it, if not, they don't always come in to get you out of trouble. Skilled riders won't understand this because they are where they want to be, the rest of us aren't! 250cc Cubs seem to have the 'go' to solve this and give you a choice of gear other than first. Quote Link to comment Share on other sites More sharing options...
stuhrc Posted January 24, 2006 Report Share Posted January 24, 2006 Have to agree with OTF & BJ..... I've come from modern 2-strokes and started riding my dad's cub. Tormented him for ages because in the garden I'd whack the throttle open from nothing & she'd cough & splutter. Fitted a Mikuni & it improved it but for Scotland we have to run the Amal which has the flat spot off the bottom ..... Have to admit, in proper trials conditions I've never had a problem with it, great little bike, doesn't handle, doesn't stop but as Big John knows, well able to flatten Pipeline!!! Even tied with Mick Andrews in '03 !!!! I'd recommend altering your riding style a bit, try let it flow instead of on & off!!! Quote Link to comment Share on other sites More sharing options...
steve+cub Posted January 24, 2006 Author Report Share Posted January 24, 2006 i must admit I spent a bit of time on one particular section that i had taken a 5 trying it over again and with a bit of practice i could clean it ithout a stutter from the motor by feathering the clutch a bit and keeping the revs steady - I think i'll have to get out to the woods more Quote Link to comment Share on other sites More sharing options...
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