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how to make your d1 go as it should have gone
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ferguson
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August 19, 2016 - 11:19 pm
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as requested by bsadave

not how to make it go 70+mph but more like 60mph.60 is quite feasible without too much aggro and cost-basically putting your bantam back together as it should have left the factory-if a bit more quality control had been in effect.bantam's left the factory with quite a variety of issues than can be corrected.2t's performance is directly linked to piston position in relation to the ports-and small discrepances can either make your bantam a slug or a flyer!i've measured mountains of bantam bits in my youth[all in the quest for speed] and they can vary by quite a bit-some con rods are 0.5mm longer than others,so are some pistons.the balance of the crank can be brillant to awful.

first off the carb.the slide can cause problems.pin throttle open and look down it[off the bike].lots of no5 slides dont quite go up far enough and leave .5 to 1mm hanging down in the way-at the rear of the slide-so potential full throttle isn't! easily sorted, just file the correct amount of the TOP of the slide until full throttle is achieved.next look down the rear of the carb.nice round hole that butts up to the inlet flange.shame its about 2mm smaller than the hole its butting up tocant-look.just reach in with your dremel/die grinder and radius the edge-just 1mm will do fine to stop the eddy currents.now look at the carb stub on the engine.over 75% will have the hole drilled off centre.if yours is one of the 75% just scribe a line round the edge to make the hole central and gently remove excess meat till done.dont need to go far into the 'hole' just taper it in. so now max air flow possiblelaugh

next-head off and piston down to bottom dead centre.the base of the port 'should' be just level with the top of the piston.chances are are that the piston is masking it a wee bit and you have two options that work.first you can get out your dremel again and remove a wee bit of your piston crown.just 1mm lower in the area of the port will make a big difference and not interfere with the integrity of the piston.the other solution is to pack up the barrel.just a 1mm gasket will help instead of the normal thin gasket.however if you do that the barrel top should have approx 1/2 mm machined off to compensate.now raise piston to top dead centre.the inlet is controlled by the piston skirt and sure enough on the d1 the piston is masking the port.look down the inlet stub and you will normally be looking at the bottom of the piston.reach in with a felt tip pen and mark it -then out with the dremel/file etc and remove that bit.it isn't a huge lump out of your piston-just a little bite so again no integrity problems with the piston.

you will have noted i'm not telling you to alter ports etc.just make sure all edges nicely radiused.they are pretty rough inside and smoothing them does help if you have the patience.next use a base gasket to see how well the cutouts aline.probably the odd step at the very least.trim up the base gasket to make a template and radius any steps you find.just a gradual taper is all you need to help the airflow.

now the head-unfortunately some machining required however its only a little job and most engineering firms will not charge lots for it but mate with a lathe or milling machine is vunderbar.basically remove 2.5mm off the base of the head will give you approx 9.something compression and a bit of fire in your bantam's belly.once 2.5mm removed measure the diameter of the dome left.it was 52mm and will now be a bit less.it needs opening back up to 52mm!.however now is your chance to put in a squish band .just get your mate/firm to machine the dome edge out to 52 but do it leaving a 1mm deep step.would show you with a drawing/photo but totally incapable of doing that on a computer.just imagine a ww2 english helmet shape.straight/flat at edge-then dome then straight/flat at other edge and set into the dome 1mm deep-width of the 1mm deep flat bit not crucial.normally works out at 2 or 3mm.but must be 52 mm across absolute minimum.

if mate with lathe/milling machine seriously good one can set the head at an angle and then remove the 2.5mm.so head sloping front to back.so end up with head tilted down at front.then put it back to flat to machine in the squish band.end result is large squish band at front and tiny one at rear.you get a much better combustion with the angled spark plugs d1's use and more importantly the exhaust port area runs much cooler due to the better gas flow of the large area of squish band at that end of the engine and also less likely to pink too.so put it all back together and put bit of plastercine on head[thin strip right across and torque down the head.turn engine over once and check plastercine for clearance-should have at least 25thou -if less add a thin base gasket until at least 25thou-that will help the exhaust port as well.

timing will need advancing  2.8mm 5.8mm btdc normally about in the zone (see ** Please log in to view **).should be fine on the stator plate but elongate the 3 slots if necessary.run it on the 'better' petrol of the two green varieties normally on sale-or acquire some 100 octane avgas if really want to fly[but not necessary].last but by no means least-be very tempted to hand over your crank to a 2t expert and get it balanced.i know an ex race mechanic who has built tt winning engines and next time my crank see's daylight its going to him.the difference from bad to good is amazing-and on a bantam will release a noticeable amount of more oomph.

i've got reams of notes from building silly bantam's in my youth but next step involves changing port profiles etc and not for the faint hearted but i have plans for a really effective expansion chamber if anyone wants that step further without radically changing their engine

ferguson

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cocorico
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August 20, 2016 - 8:20 am
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Great stuff there ferguson.

How about compiling your notes and publishing them in pdf format for the greater benefit of us all?

I'm sure one of us would be prepared to do the 'computer stuff' for you...thinking_gif

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ferguson
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August 20, 2016 - 11:18 am
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got two note books of successes and failures-destroyed a few barrels looking for that extra oomphwhistle.to give you an idea of how variable the internals can be-worked on a friends bike a few weeks ago.at bdc the exhaust port was masked by the piston by about 1mm.but on my own the piston is at least 2mm below the exhaust port!-the bikes are a year apart in age. very variable on the barrel castings methinks.his ports are also about 5% bigger than mine too but my bike is quicker.where they are more important than exact size[both engines still standard with no tweaking].

bit i missed out-stuffer plates/compression plates-whatever you want to call them.if running a standard bike then in or out makes little difference but the more you tweak it the more effect can be felt.stuffer plates dont directly affect the bhp but seem to affect the torque instead[no idea on the science].we used to make up thick alloy ones.as fat as can fit in the crankcases and they definitely give the engine more low to midrange oomph.

will plough through the note books.my solitary typing finger could sort some of it but drawings on computers totally beyond me!sorry

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Mags 1
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August 20, 2016 - 11:47 am
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To get drawings on here you could use a photocopy of an ordinary drawing and just publish here.

Or you could use the 'pencil' on Microsoft paint facility, save it and publish here, same thing basically.

I'm sure there are lots of other ways too.

Four now on the road and at least several in bits.

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Hans Kreuzen
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August 20, 2016 - 11:53 am
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ferguson said
will plough through the note books.my solitary typing finger could sort some of it but drawings on computers totally beyond me!sorry  

Hi Ferguson

Very interesting reading and much appreciated,

Maybe you could take a picture of your drawings and load them onto the forumdunno

1950 D1 plunger for daily use, Concourse 1948 D1 rigid, Black 1953 D1 plunger nearly done and a 1949 rigid D1 survivor.

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ferguson
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August 20, 2016 - 12:56 pm
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you have no conception of how much of a techy luddite i amrofl-microsoft paint-brain gone dead just thinking about it.would not even know if on my computer-or where to look.refuse to answer mobile phones cos always «censored» it up and they never do what i want them to do.digital camera's far too complicated for idiot me.can manage an email/google search and join a forum and that is the full extent of my techy capabilities.if some one gave me a phone to text on i would not have a clue!

ferguson

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Anderzander
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August 20, 2016 - 12:56 pm
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Or just take pictures of one of your heads ? 

60mph sounds too much from tidying the ports and fiddling with the head though! 🙂

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Anderzander
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August 20, 2016 - 12:58 pm
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We posted at the same time :). That's my photo idea out the window then 😀

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ferguson
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August 20, 2016 - 2:12 pm
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'60mph sounds too much' oh ye of little faith in your bantam. 60 is quite easy and feasible.just doing the basic skim of the head increases your compression by over 25% which makes a startling difference. if you do it on the angle -tipping it forwards the effect is even better as non central spark plugs lead to very uneven fuel burn[inefficient].on normal d1 head the 'fire' starts at the plug and spreads across the head-comparatively slowlly.worse than that as the fuel ignites it sends out a pressure wave which compresses the unburnt fuel on the far side of the head[front] which can cause pinking cos front of engine round the exhaust area the hottest bit of the combustion chamber.not only that some will be unburnt and just expelled out the exhaust port as the fuel burn is slow.with a large squish band at front the fuel is forced into middle of head and gets quicker more efficient burn=more horsepower!not only that as it isn't pressured up by the pressure wave from the fuel burn it doesn't run so hot so your beloved bantam is under less stress and lasts longer.if you tidy up all the little flaws in production the effects are noticeable-especially as in such a low state of tune every little bit helps proportionally.take a 250 single putting out 15bhp.has some production flaws-just like your bantam-and it looses 1bhp as a result.still putting out 14 bhp and you would hardly notice the difference.same flaws in your 4.5bhp bantam produce the same loss in horsepower 1 bhp.that missing from your bantam is a nearly 25% drop in power compared to a 6ish% drop in power for your 250 single.result being little bantam's just love being tweaked and respond very well having such a low baseline in the first place.

ferguson

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BSAdave
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August 20, 2016 - 4:15 pm
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Thank you for the information, when I get back from Holland I will be giving this ago. thumbs-up

I could agree with you, but then we would both be wrong

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ferguson
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August 20, 2016 - 6:40 pm
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one thing you must realize is that all bantam's just a wee bit different.was giving instruction over the phone to a friend of mine on how to tweak it and within ten minutes he was back on the phone-'but i can't take 2.5mm of the shoulder of the head cos its only got a 2mm shoulder'.one has no idea of what's been done to them over the last 50 years or so-or what they looked like when they came out of the factory.his might have been a dodgy casting that needed more machining up-or the machine operator had a sense of humour and was sneaking in a quick bantam just because he could.or a previous owner could have been playing.just go with the flow a bit.basically just have it machined down till a small shoulder left so can get a good head to barrel seal[i normally dont use head gaskets at all on bantams-just a smear of copper slip].that particular bike turned out well.he put a dellorto carb on it and it flew BUT he was back on the phone just after it was built.he had got a bad misfire at high revs and was confused.i suggested all the normal problems -carb/timing/electrics.he swore blind his points were perfect  but when he changed them problem solved. turned out they were perfect but also ancient and the spring had gone soft.with his new found revs they were being pushed beyond their operating tolerances.he was getting points bounce!once all sorted and settings worked out that bike could touch 40mph in 2nd and a shade over 65mph in 3rd.probably not possible with a tired worn 361 carb.he could have gone a bit quicker if he had put on the fishtail exhaust i gave him but he thought it a bit too noisycry and kept the torpedo he originally had.

ferguson

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Anderzander
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August 20, 2016 - 10:31 pm
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I'll give this a go too. I did some port work on my D1 and detailed it in a thread on here - but then switched to a D3 top end. I'll have a look at the ports on that and pick up a spare head to play with.

Did you ever trying boring the carb stub out at all, to open it up along its whole length ?

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ferguson
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August 20, 2016 - 11:58 pm
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never bothered to bore one out as stll got the problem of stub od and finding a decent[big] carb to fit-so just cut it off and brazed on a bigger orifice.did the same to another and put a flange on instead.used to put rather larger than standard amal concentric on cos were readily available and very cheap 2nd hand.if you go down the cut it off route take the opportunity to extend it by about an inch as this improves the torque.absolutely no point boring it if going to use 361 cos hole allready bigger than carb bore

ferguson

p.s.if really going for it then time to fit expansion chamberclap

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bart
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August 21, 2016 - 8:54 am
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and then some brakes..?

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BonesCDI
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August 21, 2016 - 10:01 am
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Hi Ferguson,

Thanks for posting the info.

I have done basically everything you have said on my D3, plus a couple of more items. I found all this info on the net somewhere in an old motorcycling magazine article, that had been posted which gave specs for "fast touring" and racing.

I can get 60mph out of my D3, but generally cruise around on 3/4 throttle. I do have a CDI on my bike as well.

I would also probably add that if a stock reproduction exhaust is to fitted to the bike, then consider de-restricting it by drilling additional holes or bending the internal tabs out of the way a bit.

regards,

Bones

Running and project bikes from 1912 -2005..........She hasn't said stop yet.........

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Anderzander
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August 21, 2016 - 10:28 am
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Hi Bones 

Could you share any more details of what you did on the D3 head? What kind of squish band you cut and perhaps how you timed your CDI?  When mine was a D1, I don't remember how fast it was - but it was slow, too slow to be safely used in traffic I felt. With the D3 top end it runs to 40mph on the flat - so 60 would be an enormous improvement.

Stephen

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ferguson
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August 21, 2016 - 11:20 am
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well done bones-backin the 60's lotsa people used to tweak their bantam's and there is a lot of information out there if willing to look hard enough-old motorcycling magazines etc.

anderzander-was looking at a 361 carb this morning and it has no venturi[to the uninitiated a venturi is a taper in the bore which causes the fuel air flow to speed up in the carb and the 361-coming from the dark ages doesn't have one]therefore if wanting your bantam to look 'right' but go well then the engine side of the carb could be drilled out to nearer match the hole in the d1 head inlet stub.had the calipers on it and could take off nearly 1/2mm in total without compromizing the slide.doesn't seem a lot but would give about a 3% increase in available air mix to the engine.if tuning a modern engine and found a way to increase air flow by 3% it would be a serious cause for celebration-normally improvements like that would cost you hundreds of pounds-not the price of a good drill bit and 10 minutes work!

ferguson

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BonesCDI
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August 21, 2016 - 12:00 pm
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Yes Ferguson exactly!

I did the same on my Barrel. Hole was off centre as you have already said, so into the milling machine and bored out the inlet stub so it was concentric and the same diameter as the carb. Then blended the hole into the inlet port tract.

I spent a day truing the crank only to find out that the flywheels were slightly different diameter, so skimmed the outside diameters in the lathe. If someone was to hold a gun to my head and ask the single most critical thing to get performance from a bantam I personally thing the crank is it.

The head has a squish band exactly as Ferguson has desrcribed and nearly everything else in the engine is exactly as Ferguson has said. I also gas sealed the stuffer plates to the flywheels. How.........aviation grade fuel tank sealer.

An old bloke who has recently passed put me onto it and used to braze his plates on. He also got bantams to go nearly 100mph.

My bike has done 2500 miles since I have rebuilt it. The only thing I have done is change clutch springs as they slipped when I revved 2nd. It still slips sometimes in 2nd.

The CDI is obviously my own which is set as per instructions. 

I couldn't agree more that the Bantam engine really lends itself to be "tricked" up without compromising engine integrity. But I also think that you cant do certain things in isolation. It must be done as a collective, but only if the initial engine components are in excellent order such as bearings, piston fit etc. 

Bones

Running and project bikes from 1912 -2005..........She hasn't said stop yet.........

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MOSOGER
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August 21, 2016 - 12:15 pm
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Hi I read this thread with great interest,, it's a bit tesstikcal for me.

90% of Bantam owners are over the moon if the engine goes "Bang" wow let alone does 100 miles an hour. (me included) thumbs-up

If it goes round the block @20 MPH it's classed as a Success.  

And 60 MPH on a Bantam D1 is frightening, cant-lookI think you would need a parachute and an anchor to stop you.fingers-crossed

But as I said Very Interesting reading. tip-my-hat

Bob.

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ferguson
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August 21, 2016 - 11:12 pm
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talking of parachute and anchors-one of the quickest d1 engines i ever heard of lapped the tt course at an AVERAGE speed of 75mph it peaked at 106 mph down the mountain going through the speed trap.that engine had your bog standard cast iron barrel.bantam crank/3spd gear box etc.quite an amazing feat for an engine designed to just reach 50mph on a good day.owner did admit he was overrevving it down the mountain.talking of revs-the crank is good for 10,000rpm -briefly-.racers used to reckon 9200 was a sensible limit.i doubt mine has seen over 7000rpm in its 64 years of life and that only in first gear-so plenty of scope for more speed if you so desire.

mine as said completely standard and will just about hit 50mph.have no desires to tweak this engine but i might just acquire a spare   wink

ferguson

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