plug gap / ignition timing/ carboration

so   what  considerations  are needed when deciding on these factors? ive  been trying to get the bike  to sort of pull away with at least a hint of normality, if at all  i have gone back to my mikuni 34-2 flatslide  carb and im having a nightmare. Ive had the ignition at 1.5/ 1.6/1.7/1.8 btdc and gapped the plugs at various sizes still no good   Ive just tonight gone to a pilot jet at #27.5 from 37.5- 42.5-45-50 in this carb and it is  better. getting the kind of response from the air screw adjustment ive read is required for this carb at 1/8th openings.. but getting really frustrated trying to get it all to come together in a half decent way though. I havnt tried Trevors settings yet on the ignition side( as posted on the avgas thread) as ive been messing with the carb thinking this was the main culprit. But, as ive never had the sound of a lean running engine / rich running engine to compare with  im kind of stumped...   would changes in plug gap/ ignition settings help ? or am i barking up the wrong tree.
Have been looking at the lectron carb info on the net and they seem to be considered the easiest carbs to set up, but there not to easy to find, except in the USA.. any suggestions with regard to this would be greatly appreciated, thanks.

You pull away normally, whats it like at the moment ?

It doesn'tpick up even when i give it loads and slipping the clutch...ill give a better answer tomorrow as im a bit cheesed off right now. Cheers dan

Hi Nigel
just to put another spanner in the works what about inlet track?
I have been setting my bike up with a 186mm track that's from piston face to the start of the bell mouth of the carb   

Evening Nigel, actually its morning now?
Sorry to hear you are having problems, the trouble with diagnoses is that they are very engine specific. You say you have gone back to the 34 Mikuni, what did you use prior to this, and were you experiencing the same trouble? Some flat slides can be, by their very design nature, acutely responsive and a small depression can pick up a lot of fuel at small throttle openings making clean engine response difficult. Your move to the smaller pilot may indicate this, as a punt, drop the needle down as far as you can to limit fuel flow, then note any change. It may be worth remembering that modern Japanese carbs are jetted to run on a reed valve equipped engine, your piston port job is not the same. Reverse pulses from the crankcase, then atmosphere, will pick up fuel from the needle jet and positive pulses then pressurise the float chamber and change fuel height. This can happen half a dozen times per inlet cycle, the reed eliminates this pretty well, as those same pressures open and then close the reeds, ensuring clean running.

Spark plug gap is nearly always determined by the ignition system discharge energy, a poor, weak spark will need a small gap to arc across and fire up the gas, a nice big blue spark means the standard gap will be fine. Check the float level is ok and the floats are floating, we had trouble once that was traced to a leaking float altering the level. If you suspect the timing then advance it to say 3mm btdc, about 25*, if no different you have eliminated that.
As an outside curve ball, does your engine vibrate a lot, if so, the fuel may aerate in the carb and cause problems?
Assuming no mechanical problems such as leaks of air, and the ignition system is fine, with no dodgy connections and a good solid earth and no corrosion to wiring, and so on, then all the evidence might suggest the carb functions are the culprit. Alan has experience of flat slides and their foibles, having a word with him might help. Either way let us know how you cure the fault.

Trevor

Thanks firstly for everyone's advice again its really appreciated. Ive just had a nightmare getting home   so im having a evening off from the bike.

Dan, ive got the plot circuit of the bike set up as i feel it should be now and under no load the engine will respond fairly well to a blip of the throttle, but, when ive tried to move off under load it doesnt respond as it should , even when i get the revs right up and slip the clutch. Ive tried using this carb before and couldnt get the engine to run half decently  but i decided last weekend to fit it and have another go as i  wasnt having much luck with the carb id used before either    I get the feeling that its too sensitive/ not right for the engine set up.

Robbie, yes inlet track length as in the snowy set up, Micky Scutt posted. In his post Mick has stated that they had flooding problems with the carbs they wanted to fit, so maybe this my problem too.


Trevor, after knackering up my last barrel, ive got another barrel with the snowy set up.  I did previously use a 28mm  rd ypvs 350 power jet carb, with the power jet blanked off. I did have the old engine running as well as it ever did with this on, but on this new set up i couldnt get it to work as well as before  and i also decided to try with the power jet pipe refitted back on line . I couldnt get the power jet to operate correctly to use it. I had looked on- line previously and read these carbs were difficult to get working properly with the power jet being the problem, which is why i had  blanked it off before, with a larger main jet to compensate for this.
So out came the mikuni 34mm flatslide. I have re-read how to set up and felt maybe i could give  it another shot,armed with this knowledge i have been driving myself up the wall. Pilot jets come right down to get some half decent throttle responce  while static, but under load the engine just doesnt want to rev out/pick up.So ive also tried to move the ignition timing around between 1.5 -- 1.8mm btdc.
I will have a go over the weekend again with suggestions with the 34mm  and see if i can get it rideable..

thanks again to all ..   

Hi Nigel
just let you know the snowy Engine is a clone of my engine or should I now say Michaels
our engine runs on a 38mm but I know when they were setting the snowy engine they could not get it run on a 38mm
so that's why its on a 36mm carb
you could try putting some spaces in the inlet track also have you pumped up the engine to check for air leaks?

Robbie, yes i did leak test it when i put it all back together and after i tried to run it with the 28mm carb and there were no leaks , I will test again though, you never know. looks like maybe the size of carb and inlet tract are whats giving me grief. I will be trying again saturday..thanks again

Hi Nigel I has similar problems while using my iron barrel that I to made similar to the snowy sizes, lots of revs no power, came to the conclusion that the iron barrels restricted transfers could mot keep up with exhaust and inlet ports I'd done, alloy barrel with the same ports but better transfers fixed it immediately

Dont you just hate it when you write a reply and it just disappears before your eyes   I suppose there must be a time limit set somewhere to adjust .  

Here we go again.. shortend version  


Tried playing with 34mm , gave up.. bolted on 28mm removed 220#main jet
installed 185# main
got bike to pull away without the BLLAARS and cutting out by raising needle to the max
setting, this gave me some amount of adjustment on the pilot circuit as at the middle setting the air screw was right in..
tried the bike around the back of the house ( ASK THE NEIGHBOURS..!!) and got it to pull off with the usual high revs and slipping the clutch, but it felt like it was ready to fly  Sadly i dont have enough space to get it really moving but it was responding in a more positive way.Re-attached power  jet pipe but could not convince my self that it was actually working around wot. Pretty sure a 185 #main is too small without this,Noticed a fair bit of fuel cloud exiting the rear of the carb whist reving up but im sure this is no more than before.. maybe the inlet tract needs looking at  

looks like Mallory race is a good two day test opportunity....  

Power jet kit.. probably a good investment though 

Nigel,
I`m guessing the 28mm Mikuni carb is a round slide but the miss-behaving 34mm is a flat slide?
So, is it a diameter problem or a slide/design problem. Trying a 34mm round slide, like the primitive Amal Mk 2 that is on my w/c engine, might be an option worth trying, if only for elimination purposes?

Trevor

Hi Nigel
been thinking over the weekend if you have the same set up as me runs fine on the 38mm lectron set up for this weekend powerjet 20. needle 2.4 at 47mm inlet track 186mm avgas 25.1 ing 70"your find when you get the bike nice and crisp and sounding good it will not pull away the 175 engine seams to like it well rich on the bottom and sound crap and very woolie but you should fined you can pull away
what are your plans for Mallory ? are you racing? would be willing to bring a lectron carb for you to try and help you set it up . and bring you a bike so you would get a ride
Robbie

Robbie, I'll be at Mallory with my bike
i will use the race days as testing
see what it goes like and take it from there.Thanks for offer of help and carb,maybe we could look at the carb if needs be. I had been kindly offerd the use of the same type of carb (lectron) funnily enough on sunday, a spare bike to ride could be handy, but id be worried if i broke it,   How did you come up with the tract lenght? trial and error or calculation?

Trevor, yes the smaller carb is an old round slide, maybe on the small side possibly. I think that i should have tried harder with this initially , but i had hoped i could sort this bigger flat slide to work.As ive put the 185#main jet in the smaller carb 28mm and raised the needle to its highest setting to get it to pull away/get going.
I have found some more info on this smaller carb and it seems that because it has no needle/pipe pertruding into the centre of the rear of the carb from the power jet, it needs the engine to be up near 10,000 revs to get it to start working  so im going to get a power jet kit which has the needle/pipe set up and hopefully this will sort that out. Should i be concerned about the amount of fuel vapour coming out the rear of the carb?

thanks, nigel.

Hi Nigel,
The 350 Yam peaks at 9,600 rpm, so if the power jet functioned at above 10,000 then it would practically never operate, which seems implausible? Power jets have their ardent devotees and I have used them and subsequently discarded them, the gain is not worth the hassle, just my experience and opinion.
But, the RS wont rev above 12,300 without the electronic p-j switching off! Bear in mind that the highest velocity through the carb. body is along the centre line, so you must balance throttle slide position against the height of the outlet tube to gain the optimum depression acting on the tube for the revs you want.

Forget about trying to calculate a suitable inlet tract length, there are an infinite number of variables to consider and each will vary for temperature and rpm, and a constantly changing case volume, and that's just for starters.

The vapour blowback is normal for piston ported race engines and is a function of the pressure pulses traveling from the case to atmosphere and back again. Normally the longer the timing the worse this is. Strange thing is the reverse exiting mixture does so as a single straight slug, but when entering the bell mouth is a mushroom shape observing the contour of the bell mouth. If you arrive at an optimum tract length for peak rpm this phenomenon disappears.

As for carb. bore size, 30mm is quite capable of providing for 30 hp, Mark won his first championship using a 30mm Amal, so don`t be too put off by a smaller one. Some of Mark`s competitors were running 38mm jobs but still couldn`t beat him. It ain`t the size so much as what you can do with your equipment that counts!

Trevor

Hello Nigel,
Just a thought:- !st on your iron barrel, what angle of transfer port to get the timing you require? It could be that the angle is too steep therefore diverting straight out of the exhaust port. 2nd, are you using synthetic oil in the gearbox? if so clutch slip guaranteed all revs no go. Just a thought.

Les 2012.

Evening, Les the angle of transfers are pretty much standard, which is not great but we will see what it does soon. Ive not got any kind of special oil in the box, should be reasonable on the clutch slip. I was a bit concerned when dansofield said he had problems with his iron barrel  though , which disappeared when he went with the alloy option.

Trevor, thanks for reply, im hoping the mikuni power jet will make up for the smallish main jet thats in the carb. Though Robbie has said that his bike isnt particularly crisp when reving up so maybe im in for an eyeopener when i finally get to get it on the track again.  Ive just knocked up a short (30mm) inlet pipe for this 28mm to get the tract at 186mm. i recon Robbies engine must be pushed right forward as this length on my bike makes the carb  very close to frame loop.   

bye for now...

Hi Nigel
I have to put a angle on the inlet track so the carb misses the down tube on the frame

Nigel,
       Looking at my 186 notes again I see that the inlet tract length was 15.1cm from piston the bell-mouth entry and the main-jet was 45mm from the bell-mouth entry. Overall much shorter than is favoured today?

Another interesting fact from back in the era of piston port race engines concerns the Yamaha TZ350, which for one cylinder has a comparable capacity to the bigger Bantam. Those tuners hoping to gain on the competition and opted for a short bodied carb. discovered that a huge hole appeared in the mid range torque for only a tiny top end gain. Yamaha are not stupid and the use of a big, long bodied Mikuni VM was a considered choice for a specific reason. No other company managed to get piston port race engines to perform as successfully as Yamaha did, a lot of which was down to getting the inlet side functioning efficiently.

There are two options for looking at what happens along the length the inlet duct. One is acoustic wave motion, the other is mass movement of the mixture itself, and that is the Helmholtz-frequency. Some of the factors that affect this are; a bigger case volume lowers H-f, a longer duct reduces H-f , a larger duct/carb diameter increases H-f. So, some of the symptoms you are experiencing Nigel could have their causes explained in this way. The difference Dan found by swapping the iron barrel for the alloy one and finding an improvement could be explained by an increase in case volume with the larger transfer duct volumes he included. Thereby, the resonant frequency was changed and moved to a different cycle.

Bell-mouth profile plays a part in reducing reverse flow spit back, the clear winner is an ellipse with a roll back on the lip, very few carbs have this optimal profile and commercial considerations always outweigh those of all else. The design maxim seems to be; short and fat, where the diameter is around twice that of the carb. bore size. From this its not difficult to appreciate that intrusion into the large, hemispherical breathing zone of the bell mouth is going to have an effect. No frame tubes, fairing brackets, exhaust body and all other extraneous items should be banished, especially hot pipes!
Hot air entering the engine will increase the speed of sound of wave action and reduce charge density, both of which will mess up any attempt to find a calculated tract length.

Trevor

Trevor, i had thought maybe that the inlet tract with the carb i have would probably mean that the length and bore of the tract would be different. With the carb virtually stuck right on the back of the barrel the length was around 145mm from piston to bellmouth. I have now a stub (offset away from exhaust )which  helps get the length up to what Robbie/Michael are using, so i  can experiment a little with smaller spacers. But ultimatly what ever gets me around the track will put a smile on my  face  . Thanks for your help 

pic of carb

Congratulations! Lovely pics you post Robbie.

Just a quick flip off the subject: Is it my ancient eyes or do you have a long wheelbase?
In comparison, Icarus-1 (on here) `looks´ quite short....It was 52" +or-1/2" or so....

Speaking orf exhaust sound -- your rush out for The Parade at Lydden in 2005 -- sounded as if the engine was working really well. If you´d been a tiny bit slower that crazy ambulance would have clobbered you!!

Heck! that´s nearly 9 years ago -- time I shut up and crept into the corner for a kip....

Vjeers!

JayBee (for John-Boy!)....

Hi John
yes well spotted yes this is my bike with a 50" wheelbase and michaels is 49" and now they fit in the van lol
this is the first outing of my new welsh engine and the engine you saw at lydden in 2005 is the one in Michaels bike this year now doing all that winning   

Had to make another little inlet pipe as the other didnt quite kick enough to clear the rear of the frame loop with a bit to spare. Carb is around 175mm from inner bellmouth to piston and ive made up some thinner  spacers to extend up to 186mm, which i can experiment with.

Something stupid -- perhaps? Probably no help with your problem Nigel yet may well be contributory to the bother....

From the way I see the picture you have two of the Two-Bolt-Flanges, each looking -- to me -- rather thin.
Even with a gasket ... have been know to develope an air leak.

Had a lot of trouble at one time (in Army workshops many yonks ago) with two bolt, carburettor flanges. It was that the Two-Bolt-Flanges -- bowed in the middle. I hope you won´t think me pickingly facetious by suggesting a test to see if either of those two-bolt-flanges have air leaks.....???

With the engine running and warmed-up squirt a jet of thin-oil at the upper and lower curved surfaces of the flanges. If a leak exists -- even a small one -- the exhaust ímmediately exudes clouds of blue smoke.

Cheers!

Yes john, on this second attempt, i made the engine side end with thicker metal, as i did notice the flange flexed enough to bother me.. the other end i left the same as it seemed to not move with the rubber inlet stub and carb bolted up. But although its leak free now it may well leak when vibrating. So... perhaps i should beef both ends up just in case. I had tried to keep it light , but maybe with it being only 30 odd mm long it probably wouldnt make much difference. Thanks for advice  

i havnt started the bike up yet, giving the nieghbours a break