Air jet on tm28ss carbs - change them at all?

[Mojoracinguk]

So, those of you who ran premix, seem to be using larger pilots to ensure the motor is still getting life giving juice, when the throttle is shut off and engine is on overrun...   my question is did you install larger air jets to correct the air ratio?

[Mojoracinguk]

OK, seeing as the bike is back to running with an autolube pump, I'm not so worried about the jets getting blocked with oil siting in them.

Airbox has no snorkel. Running 280 mains. I went to a #25 pilot jet and was rich and splutters from 1/4  to about 1/3 throttle.

Dropped to 22 5 and still a little rough but pulls through if you give it more throttle opening.

Revs out beautifully.

My question really is, would stock pilot jets be rich enough?

Or would the next size up, pilot air jet (45 iirc) be a good shout to balance the fuelling?
Really has anyone ever changed the pilot air jet?, if so did it just change the low, throttle opening fuelling?

[m in sc]

if its back on the injector pump why not just run the stock pilot?

[Mojoracinguk]

It's a fair question. I had the bike sieze a while back. My track jetting was a smidge too lean at 1/3 to 1/2 throttle for road. it never caused and issue on track as those throttle openings were not used for miles on end....but on the road that's where you spend a lot of time.
So I'm just being cautious in leaning out, seeing as I just run the motor back in.

I'm really surprised no one messes with the low end jetting except for when running premix.

I assume its rich enough to cope with any mods done to the engine and airbox. So may just pop the std pilots back in.....see what it's like.

I can swap back if needs be to the 22.5 and try the next step up pilot air jet......but can't find solid cases where anyone has used those to correct a low end fuelling issue. Just seems odd.  In some carburettor tuning text, references are made to certain air jets altering the fuelling slope all the way up to main jet and that makes me question.....who has altered the oddball tm28ss pilot air jet?

Apparently not many.

Oh well a good old fashioned tinker and test ride is upon us.

[jools]

If you believe the throttle range 1/3 - 1/2 is responsible for your seizure you should be looking at needle position rather than pilot jet.
Try lifting the needles up one notch - moving the clip down - this assumes you have the correct needles fitted and not some other non-OEM type fitted which could have a different taper.

the use of larger pilots is really only for insurance when the the throttle slide is closed for extended periods, like travelling fast down a long steep gradient using premix, with the throttle closed as the pilot circuit is the only supply of oil to the engine.

The std 3XV compensates by opening the oil pump slightly via the ECU in these situations.

[rz500guy]

Am I reading that wrong on clip? I thought the lower the needle the richer it was. My 3MA had high EGT at partial throttle at 7k to 8k when holding it steady. I tried both dropping the needle and bigger pilot. With bigger pilot I could not get it to even take throttle. Stock one back in. Needle ended up second from top. EGT or AFR gauge to get jetting correct in my opinion. I figured the EGT test was cheaper than a blown engine.

[Mojoracinguk]

Cheers,

Yes I had larger pilots as it was premix. Now it's back on pump.

My plan is to go to the stock pilots and if its clean with the stock pilots, I'll raise the needles (but didn't feel much change last time I did it).

If I can get it to be rough around 1/2 throttle with the needle position changes, I'll dial it back until its  clean and call it a day
Then it'll  be off to the dyno for air/fuel ratios to be checked and tweak jets accordingly.

Still I'm astounded there is nothing on the air jets being altered on these tm28ss carbs 

Thanks

[jools]

Am I reading that wrong on clip? I thought the lower the needle the richer it was. My 3MA had high EGT at partial throttle at 7k to 8k when holding it steady. I tried both dropping the needle and bigger pilot. With bigger pilot I could not get it to even take throttle. Stock one back in. Needle ended up second from top. EGT or AFR gauge to get jetting correct in my opinion. I figured the EGT test was cheaper than a blown engine.

Dropping the needle ( moving the clip up) weakens it relative to the throttle slide as it's sitting further into the nozzle for the same throttle opening.

[James P]

The lack of pilot air screw on most (especially early) versions of the TM28SS is a real pain - it makes things difficult when fine-tuning the idle mixture. Having said that, the pilot jet and pilot air screw (or pilot air jet for the carb in question) should only affect the mixture in the range from closed throttle to very small openings (about one-eighth, I think). The pilot circuit does of course 'work' at all throttle openings, but its effect is very much less influential beyond one-eighth throttle (and is supposed to be negligible beyond one-quarter throttle).
It would make things (a little) easier if we had some relativity between 'number of turns of an air screw' and 'sizes of pilot air jet', i.e. how many sizes smaller pilot air jet equals one turn in of a pilot air screw?
It should be possible to obtain some relativity by calculation. It would be necessary to know (or measure) the taper angle of a pilot air screw (I think most Mikunis are the same) and the size of the aperture/seat it screws into (although for convenience, this latter figure may be estimated). Also knowing the thread pitch of the air screw would enable calculation of change in area of the air inlet passage for one whole turn of the screw (in or out). It should then be a simple matter of comparing the result to the changes in area produced by available increments of pilot air jet (the BS30/97 air jet is available in 0.1mm increments up to 0.8, then 0.05mm increments up to 2.0). The range of adjustment of a pilot air screw is usually three turns out from fully closed.
Practically speaking, in a case where a 20 pilot jet is found too lean and a 22.5 pilot jet is found too rich, it should be possible to use a 20 pilot jet with a smaller-than-standard pilot air jet or a 22.5 pilot jet with a larger-than-standard pilot air jet. However, there will still likely be some trial and error involved because you probably won't know exactly which size pilot air jet to use at first (just as you probably wouldn't know how many fractions of turns to unscrew a pilot air screw for optimum running...but it is quick and easy to find the correct setting using a pilot air screw!).

The foregoing is all very well, but one must decide whether it is actually necessary. If one is certain that a standard size pilot jet is too lean, just keep fitting larger pilot jets until richness is found...and then decide whether to use the larger or smaller size. I converted my own 2AW/2XT bitsa TZR (air intake system all standard) to premix 10 years ago and had seen suggestions to increase the pilot jet size. However, I found 22.5 pilot jets too rich so just reverted to the standard size.
My general experience with my own bikes is that significant jetting changes are usually only necessary for changes to the air intake system. If one fits a different air box and/or filter arrangement, jetting changes will almost certainly be necessary...for all carburettor throttle openings. Accordingly, if changing the air intake system on a TZR, it would be wise to convert the carbs to Formula 3 specification in order to vastly simplify the setting-up in the one-quarter to three-quarter throttle range (you'd still be stuck with pilot air jets though).

Using premix, I am mindful of Jools' advice concerning long descents with closed throttle...so I always occasionally blip the throttle and/or briefly open the choke/starter in such situations.

Regards,
James

[James P]

It would make things (a little) easier if we had some relativity between 'number of turns of an air screw' and 'sizes of pilot air jet', i.e. how many sizes smaller pilot air jet equals one turn in of a pilot air screw?

In an attempt to 'put my money where my mouth is', I have made some measurements and done some calculations:

I have examined pilot air screws in TM24, TM28, TM28SS (3XV type) and TM34 carbs. In each case, the screw taper, screw thread and air passage diameter (where the air screw taper enters) all appear identical. I think that the measurements I made are all fairly accurate, but they are certainly not precision. The quantities I used for calculations are:

Air screw taper - 8 degrees (total included angle 16 degrees)
Air screw thread - M6x0.75
Air passage inside diameter where air screw taper enters - 2.5mm

It appears that the flat end of the taper emerges from the air passage when the screw is two-and-a-half turns out. Unscrewing the air screw a further half turn may still decrease the restriction in the air passage, but more detailed inspection would be required to determine whether the effect would be the same for each of the four types of carb I looked at.

Each half turn out of the air screw (between zero and two turns) gives an increase in area of approximately 0.4sqmm. I say 'approximately' because the increase becomes progressively less as the number of half turns out increases. As an example; From fully closed to one half turn out gives 0.407sqmm increase, whereas from one-and-a-half turns to two turns out gives 0.355sqmm increase.

In comparison, changing a 1.4 pilot air jet for a 1.2 pilot air jet gives a decrease in area of 0.408sqmm. Changing a 1.4 pilot air jet for a 1.6 pilot air jet gives an increase of 0.472sqmm.

With the foregoing in mind, it might be said (as a very rough approximation) that replacing a 1.4 pilot air jet with a 1.2 pilot air jet is the same as turning a pilot air screw half a turn further in...and that replacing a 1.4 pilot air jet with a 1.6 pilot air jet is the same as turning a pilot air screw almost half a turn further out.

Hopefully this may be of some use to anyone contemplating fine-tuning of the pilot circuit on a 'pilot-air-screwless' TM28SS. Naturally, if anyone notices any flaws in my logic, measurement or calculation, please let me know!

Regards,
James

[Mojoracinguk]

That's really good work there!

Although I'll never have an aircrew on my carbs, I find it comforting to know the scale of change going from a 1.2 to a 1.4.

I bought a 1.45 Air jetas it was the next size up to try. So in screw terms it would equate to 1/8th of a turn.....that sounds feasible as I tune the old twostroke strimmers in 1/8th turns 😅

Getting back to conclude my side of this thread, after much reading and trialling I conclude pilots really do only work just off throttle. Any further and their effects are dwarfed by the other components controlling the larger fuel circuits.

So I'm sticking with my 22.5 pilots and no need to change from std air jets, as it starts so easily and ticks over nicely.

My rough 1/3 throttle running was solved fairly easily in the end. I started stripping the carbs to change jets, then thought i remebered changing the needle positions at some point but was not impressed by the difference it made. I was sure I would have put them back to std but my mind  could not recall it. So yes I stripped the needles out and lo and behold.....clip was 3rd groove from the top 

Put it back to std and boom, it runs beautifully again 

I really should just keep a log book of my meddling. It would save countless hours of faffing later on 

Although, through my incompetence we now have JamesP's work to look back on if needed.