Igniton & Carburation set up

Good Morning All,


Is this a topic worth starting up or has it all been covered before?


Regards



Eddie

Hi, Ed,
Post what you want to know on both topics , someone is bound to run with it.
Could be a good one!

regards Trevor.

Hi Trevor,

Well i will break it down into two parts and it is more 175 orientated:


Ignition

I undertsand the max degress allowed, is there a base setting or is this all down to how good a pipe you have and your portings and down to your carburation?


Carburation

I see there are a few different options to carb choice providing it is within the rules, is this yet again down to personal choice and what works and yet again is there a base setting to work with to progress from.



Apologies if sparse on a starting point.



Eddie

Hi Edward & Malcolm,
For a 175 motor you should start in the region of 1.30 mm BDC, you will be safe on AVgas 100LL petrol, this is around the 16 degrees before TDC. you should be slightly more retarded on the ignition if you use pump petrol, but I recommend you use Avgas as it is very stable. Beware you do not have too high a compression 12:1 is a good start point, higher and you will restrict rpm. Having a flat ignition is a bit restrictive on power, but very easy to set up!
Carbs are fun! if you are running around 32mm Mikuni (On a TMX style) then start with a 190 main and a 70 powerjet and expect it to be somewhat rich, if not obviously so on first full throttle run, then kill and jet up asap!!
Interestingly if you go then onto a smaller carb you will still have very similar jet sizes, larger carbs and you will need to gradually enlarge the jetting as the suck gets weaker from the engine, and you will struggle to settle your jetting out and it will vary greater through weather changes.
I do prefer Mikuni for no other reason than they have a mega range of spares and carb types and they are easy to obtain. Others may differ! and there is no reason why not! but remember that a carb is a metering device with quirks! so best to stick to one and work the problems out gradually, rather than swapping and changing till you find one that is set up correctly for your engine!
Be careful on jet numbers.... Mikuni have two jet types and the numbers do not match each other, and the numbers definately do not correlate between different manufacturers of carbs!!!
Good luck and there is help out there.
Alan

Hi Alan,

Many thanks yet again for elping with info, we must owe you a pint or three! Be interesting if anybody has any Amal carb opinions although this may have been covered before, i know Derek was kind enough to give us some technical info on these.

Kind Regards



Eddie

Hi, Ed,
There you go , top notch info from Alan , a man who knows a thing or two .
As you asked about Amals i can maybe help as i use one on the w/c engine , very easy to dial in for
clean operating and judgeing from Mark`s performance at Cadwell ,flows as well as any other carb to give
good top end power . Mark picked a good 2nd hand 34mm one this week for just £20 !
The only problem they seem to have is they`re not fashionable, but they do work well .
Keep asking, you`ll get there and enjoy doing so .

regards Trevor

Hi Trevor,

I completely agree, alan is always very useful and doesnt mind helping with info.


Well we have the following, although carb stubs will be the next point:

38m Mikuni TMX (have stub)
MK2 38mm's
MK2 34mm's
MK1 32mm

Various main needle, main & pilot jets for Amals. From reading Alans post again we are probably oversize straight off?


Kind Regards



Eddie


P.S. Alan, we have yet to find anybody we can get avgas from

Hi Ed ,Trevor & Alan,
my recommendation is the 34mm mk2 amal Ed, it may not be the latest in the fashion stakes but for simplicity on setting up they are nice and basic.I agree with Al on the mikuni spares side of things but have found the later flat slide type to be a bit fussy on the low range settings and had to switch back to amal .
The amal spec sheet gives good starting points for jets , needle jets ,slides etc.
Be carefull as 2 and 4 stroke carbs are different the numbers are on the carb bodies.check they have 2 stroke internals at the very least.
Als done loads of testing and as he recommends smaller is more stable but dont choke the motor , i think you would find the mikunis will have a better flow rate than the amals of equal size.remember that unless you can spend hours on the rollling road with someone who knows what they are doing then all your set up time will be on the circuit and that aint cheap.
By the way most of the modern dynos dont have much to do with 2 strokes anymore and wont want your smelly 2 stroke ruining their gas analyser! and thats if they no what they are looking at. The guys i know in the trade only want to do custom fuel maps for 4 stroke crutch rockets ,just done an S1000RR at work 201 BHP at the back wheel ,a real bad boy.
A friend of mine is running a superstock 1000 in the BSB and it also has a similar output .whats funny is these bm motors are standard out the box with just full race exhaust systems and the correct fuel and ignition maps so it proves how crucial the right carburation and ignition is.

Cheers Nick

Good Morning Nick, Trevor, Alan and anyone else looking in,

Thanks for the info nick, im sure they are all 2 stroke carns but will go through the box again tonight, as for dynos i agree they arent really interested as we have seen before.

I think it would be good for Derek to post into this as a man of amals before and now running a mikuni, what do you think derek as it saves me annoying you on the phone!

Hi Malcolm and all,
Carb set up is always a struggle for the beginner as it includes some elements of the dark art's! but general principles for first start up and getting it going (fairly) quickly as below:

1. Don't start on large carbs... very bad!! and hard work!
2. On first start up remember that the mainjet does not come into play (unless you are very excessively big). You are only working on the needle and the pilot jet and these are the only things you need to concentrate on initially.
3. If you need to keep making the needle richer to make it run without the choke then increase the pilot jet size. Most people believe the pilot only has any influence at tickover NOT TRUE !!! it can have a greater influence than you imagine especially when blipping the throttle and pulling away from a stand still.
4. If the engine continues to wet the plug and die on you when you drop the choke off then lower the needle and consider reducing the pilot. Note: always start with the needle in centre.
5. A zingy noise and feeling when you blip the throttle coupled with a lag of response to the throttle indicates very weak settings... Stop immediately and lift the needle!!!

Once you have the bike running reasonable on blipping the throttle, then and only then, consider doing a run to check out the mainjet. If you are using pump petrol then there are plenty of good quality pictures on the internet of plug readings to help you decide if you are weak or rich, but a good and safe way of dealing with it is to say that if it runs well on full throttle on the first run then you need to jet up for safety!!! MAKE IT RUN RICH DELIBERATELY !!!!!! at least you will know which side of the fence you are on.
Once it is jetted reasonable then get to a dyno and ask for help! a few hours there will tell you a lot about what you need to do with the carb settings.
I can help with jetting the Mikuni's, but I know little of other carbs, but feel free to ask as there are general priciples that are much the same.
Hope it all helps.
Alan

Dear Ed,

I have some track experience of using an Amal Mk1 and 2s on Bantams for a few years.

My first old long stroke 125 first had 28,the second had a 30, later 32mm Mk1, used the standard two stroke needle, 107 needle jet and 270-290 mainjets.
Currently running a 32mm Mk1.5 on my longstroke 175 with 108 needle jet, 900 series 2T needle and 280-320 mains.
This is on cast iron D14 barrel, there ain't enough metal to safely go beyond 32mm. I was told by several old guys there was no such advantage with the standard barrel anyway.

My short stroke 125 originally had a 34mm Mikuni, we are not allowed Jap carbs in our classic class, so I fitted a 32mm Mk2 Amal with a power jet (the inlet port was only 32mm anyway) on the dyno there was no difference in power or torque and it runs well, pulls like a steam train out of corners. Mk2 easy to set up with out dyno or analysers etc. Don't start with power jet kit; take out main, try 108 needle jet with needle in middle slot (if needle jet aint big enough there will not be enough fuel sucked up to make main work any way and you will find changing main makes no difference, but backing off from full throtle, runs better ) and see if it starts to rev upto 1/3 - 1/2 throttle then gases up - needle jet is big enough. If needle jet is too small it will rev out easily to towards full throttle and lean out. Ginger Molloy taught me this (50 years of racing Bultacos at the top of the game) then sort main at full throttle chop runs.
Now have 2D2 needle, 107 needle jet, 210-240 main and 70-75 power jet which is equivalent to 280-320 main on non power jet setup. Again I would start with 108 (NB the Mk2 ones are different to MK1s !) needle jet if trying on 175. There is a 2F2 needle that I think is used on the larger 36+mm Mk2s

Good luck and let us know how you get on.

The longstroke 175 currently has fixed timing PVL with 1.9mm BTDC timing on AvGas, that is my starting point as used on 125 it may well be too advanced, but works well on shorter tighter circuits.

Great minds think alike !
Alan must have been typing at the same time

Absolutely John!
Good info on the Amal side, but I note the principles remain the same! I must say that fine tuning where you get most benefit is where the real art lies! and I am sure you understand that all too well!
Cheers
Alan

Hi, All,
Just a few points which may be of interest , the 2stk version differs from the 4stk on the Amal only by way of the needle
jet height . The 2stk has a shorter jet and the block it screws into is shorter , this is done to create a stronger depression
to pull fuel , remember, the 4stk has a full down stroke to do the same thing. Both are easy and cheap to buy to complete
the convertion, as are a good range of needle jets .
Alan, whats your take on slide cutaway function , has`ht been mentioned thus far and may be of additional assistance to us all ?
Phil Betty is an Amal user , i`m sure he can help as well .
Flat slide carbs are superior to round ones , but they do appear to be trickier to set up reliably for newcomers with limited
experience , so i guess you pays your money and takes your choice !
Dyno testing is fine , it does provide a snapshot of performance there and then but the dynamic world of the race track
is a totally different matter, and you do get a chance of direct comparison with other bikes .

Terrific advice so far , keep posting guys ,regards Trevor

I don´t want to knock Gardner too much but with Andy´s 250 I had the experience of doing plug chops with a Gardner man at Brands on the Wednesday where we replaced the needle and on the following Sunday at Snetterton with the ABS really flying and passing everything on the Old Norwich Straight it siezed its piston in a big way -- putting a hole thro´ the top....
...
"TOO WEAK!" as the actress said to the bishop, "Never try running too weak!" You hear that Ed?! -- Alan has already told you -- stay rich for starters.

Recent pic I saw of Dave Hunter in one of his championship race days the fairing had a Gardner Carbs advert on it -- are Gardner carbs allowed in the Bantam formula?*

Good luck young man**!

Cheers!

* yeah yeah! I know! -- I can look....
**I can say that `cos the whole world´s younger than me....

Hi Alan, Trevor, John S, John B,


Thank you all very much for providing so much information and advice, this is actually going to really help with the build and may answer a few questions on the hobby projects too.


john, you are as only old as you feel, people my age think im old before my time for liking classic bikes.


Eddie.


p.s i owe you all a beer!

To go back to Trevor's question on the slide cutaway's....

I have found that they have a "similar" effect to raising/lowering the needle, dependent of course on having a smaller /larger cutaway, but with an inbetween needle position effect, if that makes sense! and all this takes place on smaller throttle openings, the wider you open the throttle the less overall change it makes.
Never forget that there is always a possibility that you may need to change the jet tube at an early stage if you get to extremes of settings or needle sizes. However I have always found (to date) that the standard needle and jet tube of the Mikuni's have served me well and have been difficult to get away from when trying to improve the overall balance of the jetting.
An effect worth mentioning is the two stroke tendency to "carry" settings from lower in the range, this was hinted at in John's post, mainly as it takes time for the mixture going round the crankcases to clear out and adjust to the throttle openings that you have, hence john's leaning out at the top end of the rev range on full throttle.
Worth watching out for this taking place as long circuits can catch you out!
I hope these post's help the new starters?
Alan

It takes only 1.06ccs TEL per litre to change the octane rating of a Premium petrol into twice its Anti-Knock Rating -- called `Performance Rating´ -- which can be `200´ to `300´ according to the base fuel type.

PS -- I forgot to say the detonation can still be there even if you don´t hear the `pinking´ and the engine always goes its best just before detonation destruction.

Good luck!

As Far as i know you cant just pop to an airfield and buy it but i may be completely wrong.




Eddie

UK laws may well be different.

In NZ we can go to registered vendor at airfields with an "approved" container and buy AvGas or some service stations near race circuits sell same from drums as "RaceGas" and sign a declaration that is not being put in a road vehicle/being used for off road racing. It is relatively cheap compared to Race Gas from motorsport outlets as there is much less tax on aviation fuel.

You can tell by my latter statements that I know very little about carburration but I do know a bit about calories, exhaust efficiency and combustion....

I struggled with weight for decades before Bantam racing and I am still struggling generations later....

Edward I would start at 11:1 compression(.040" squish), 1.3mm btdc, a 34mm carb and run
Shell V power at 20:1 petrol/oil ratio(Castrol TTS- Power1 Two-stroke).....

But the choice is yours?

Regards Mike

Also a note on the bantam rules:

Only Petrol or Avgas 100LL mixture to be used.

The advantage is the Avgas is very stable, and doesn't melt petrol tanks (speak to Chris Bennion on that one!)
Disadvantage, price and availability, as some airfields are very funny about supplying, and you have to get them used to you!

I agree with Mike if you decide to use normal pump petrol!
Cheers,
Alan

John methanol is not allowed to be used in the bantam class -

Reason being -

1 Safety- Its 'orrible' and burns with invisible flame.
2 Will melt fibre glass tanks (new alloy tanks are costly)
3 Cost -Methanol is not cheap and you use 4x as much
(intead of carting 4gallons of petrol to race meetings you
would need 16g - how safe is that!!)

Bantam racers circa 1996-98 decided this....