Britbike forum Forums British Motorcycle specific Brands Triumph bulletin board Pin gauges for needle jets.

So, I got an education on line about pin gauges/ plug gauges and so on. You guys speak a whole nother language.
Is .1045-.105-.1055 for $3.33 each close enough or do I have to spring $12.79 each for .1048-.105-.1052? I'll need a .106 set also. That's too much money for a probable one time use.
I think I'll call one of the reputable sponsors and ask if they measure them before they send them out.

I use a Mitutoyo 368-022 which measures them all very accurately. Bought second hand on eBay but otherwise not cheap.

The AMAL needle jets are pretty accurate now so it would be cheaper to buy direct or buy known sealed AMAL stock from someone reputable.

With you being so friendly with the good folks at AMAL I am glad that it was you this brought this up with "now" being the pejorative word. Learning to make carburetor parts, on the face of it looks easy, but it is a collection of very subtle machining operations. many of them make little, or no, sense to the typical engineering (machine) shop doing production. Even the good folks at Burlen went through a learning curve trying to reproduce what was done on very specialized equipment when they abandoned existing production equipment in favor of modern CNC machinery. There was a lot to learn, and it took some time to sort things out, but sort they did.

Yes, the ones in the bags, although that makes it impossible for us to check each one before we sell it, are made to a much, much better tolerance than ones some we have seen in the past. IMHO you should insist on AMAL needle jets in the AMAL bag. That said, when I experience something out of the ordinary I still get out my plug gages.
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Desco all you need to check your 105 needle jets is a .105 - zz class plug gage which you can buy from Amazon for 3.33 plus postage. A .105" - (minus) plug gage will measure .1048" and should be a slip fit in your needle jet.
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DD: A Mitutoyo 368-022 has its application but it would not be my first choice for this job. It is also more than a bit expensive, even when bought used. At $1500 Mitutoyo new vs plug gage at $3.33 it isn't practical if you were in the business of manufacturing needle jets. These jets are reamed and if you look at the surface it it is anything, but smooth. It is actually a series of hills and valleys for the length of the orifice. You would have to take a whole bunch of measurements up, and around the orifice, and average them to get an idea of the actual physical i.d. of the hole. A plug gage gives you a much quicker measurement of the actual i.d. for much less cost and it is the diameter we are most concerned about. The humble plug gage is more than up to the job we are asking it to do.

While it may be impractical to dis-assemble your carb periodically to measure the needle jet, a simple calculation after refilling your gas tank is very accurate in predicting needle jet wear. You mileage will really suffer from a worn needle jet and you can catch it early by getting to know what to expect from your mileage.

On the other hand, it almost never happens that a needle jet is not worn when I remove one from a carb. Replacing it at, say, every 5000 miles is not out of the question as a routine maintenance task.

The savings in fuel will pay for it very quickly.

I do have a .1065 pin I made myself on a lathe.(pre unit needle jet) If you're handy you can make your own.

Cheers,
Bill

I've got the tools and the skills to turn a hole in the ground and a pile of lumber into a house but when it comes to metal I'm clueless. I'll buy a couple of gauges for $3.33 each.
Thanks to everyone.

I found some drill rod at a Nuts&Bolts shop in town. Way too long of course, but Mics at .106" and seemed like a good check. Half of the used jets I checked were way bigger than the rod, so I think it helps even if not an actual plug gage.

Something to note is although the AMAL jets are supplied to size, it does not mean they will remain this size when fitted into the jet holder. As even the slightest over tightening can crush the jet taking it below the size required.

On one jet I had, it would measure fine on its own but crushed when fitting, soon as I slackened it off enough to not crush the jet was loose. In this case the problem was caused by the threads in the jet holder being over cut. Fitted a 40 year old jet holder and it cured the issue.

I will add that this was on a set of carbs I bought from Burlen about 7 years ago. Every carb I sell now I check before sending it out, each one gets checked with the plug gauge and so far not one has required adjustment. This doesn't mean though that if over tightened it can't crush the jet because it can and will.

Hi Alan have you got any SU Carbs l need one for a 58 Tbird.Val.

So...what's going to happen to all the improperly sized needle jets? These things aren't exactly a dime a dozen.. And how about the main jets? Are they properly sized? Like car carb jets,I assume the hole drill size,length of bore and lead in angles affect flow rate. This smells of a conspiracy to rid the world of foul old Brit bikes.
Who's gonna buy an old Davenport screw machine and start turning out jets?

The threads in the jet holder MUST be counterbored. There was a production run of these main jet holders made where they were not counterbored. These made their way into carburetors and supplied as spare parts in the after market supply chain.



The thread on the needle jet ends short of the shoulder. In an effort to save, what must have been thousands of main jet holders, new needle jets were made where they undercut the end of the threads. These also made there way into caruretors and into the after market supply chain.



You want both the jets to have a chamfer. Notice one of these jets is chamfered and the other isn't As HB mentioned, this will effect flow.



As Alan explained above, over tighten you needle jet and the orifice gets smaller. While this shows what happen when you over tighten the needle jet. The results are even more graphic when you see that you cannot put a .104" plug gage into your .106" needle jet. When over tightened the orifice really closes up.

While most of us can understand that changing from a .106" to a .105" or .107" orifice will cause the bike to run very lean or very rich, it escapes me why people have so much trouble understanding the importance of getting the size of the needle jet right when it isn't made to the correct size to begin with or the orifice is changed during installation.


A similar thing happened to a run of MKII carb bodies where the thread for the main het holder in the body was not counter bored. When you screwed in the main jet holder it would bind about one turn from being tight. Of course this didn't stop people from forcing it home. And as you would expect they broke the carb body casting.

Unfortunately not, I only have the AMAL account. Apart from Burlen I am not sure who has SU accounts??



Very nice a clear explanation John

Well HB, with at least 10 companies around the World, including here in the USA (which several of our dealers have had trouble with), making these jets there are a lot more of them in the market place than you can believe. As I have been saying for years, all this has been a consumer nightmare. One which Burlen has tried to address as best they can.

To make it worse, several of the manufacturers are actually stamping AMAL on their jets. Main jets are a similar story. Of course none of this matters if you don't care how your motorcycle runs!

Of course a pin gauge won't measure a worn needle jet. And my Mitutoyo cost about $200 second hand - a useful device for checking these jets.

Incidentally I have set of accurate Swiss reamers from Toolex here in the UK which enables me to make:-
0.1048
0.1052
0.1056
0.1060
0.1064
0.1068 needle jets.

I ream them held in a thread so hopefully they are pretty accurate and consistent when installed. And NO, I definitely don't stamp AMAL on them! Lol.

DD, we haven't got passed spending $3.33 for a plug gage, let alone $200 for a second hand micrometer, and $15 each for a set of decimal reamers. Might be good for guys like us who are interested in all this, but all this seems to fall on deaf ears when we start talking about it. To me this is all ancient history having learned all this some 40 years ago. A lot of it from Barry Johnston.

To most of the people who read this it is the first time they have seen anything about it because of all of the popular literature talk about main jets and slide cutaways. The literature assumes that the needle jet is .106" and it didn't cross their minds that someone would be selling needle jets marked .106 that were .104" to .110" or more.

As a practical matter the carburetor will not work with any smaller than a .105" needle jet.

For the average mechanic or home tuner the plug gage will serve better than a micrometer. It certainly can easily check for an oversized new one and while it wont tell you just how worn an old jet is it will certainly confirm that it is worn and needs to be replaced. In the end we are tuning a fairly insensitive instrument, not splitting atoms.

JH, I didn't realise there was a Forum for splitting atoms. I must go have a look sometime. :-)

Yes you are of course right in that the carbs are relatively straightforward and basic. However, the more recent ability to monitor exhaust gasses gives us the opportunity to look and see (and therefore adjust settings) far more precisely than hitherto at what the engine fuelling is doing right across the range.

Admittedly I have yet to get any bikes on a rolling road but I'm sure it will be informative when I do.

I'm loving this thread as I've been chasing perfect jetting for over a year now and did discover that even new needle jets were different sizes.
I couldn't get good running with 106's jets so I went down to 105's and still has calibration issues.
I also found my jet holders were different lengths which affects the jet height in relation to the needle which complicated things even more.
In the end I worked with 105's which were still a little big on my setup (928's on a T100R ) as they were 0.0055" new and eventually found good setup by opening the cross drillings in the needle jets until the mixture was just right.
Opening the cross drillings gives an extremely fine adjustment on the needle jets and I found in the real world it does actually affect mixture right through the whole rev range and not just at the lower end as someone did suggest to me and it even does have some effect on the full bore mixture.
I opened them from 0.034" to 0.038" in 2 thou steps till I got it spot on and the carburation is as perfect now as I will ever get it.
Even finer adjustment is possible as you can just open the cross drillings up one side at a time rather than going right through with the jet drill.
I did do some slide tuning as well, skimming the base of the slides to increasing vacuum signal over the jets for sharper snap throttle response rather than just fitting smaller cutaway slides which works very well indeed for me.
My plugs and piston tops now burn a perfect mid chocolate brown and she's making enough power to embarrass my riding mates 1000cc Harley on acceleration and leave it for dead on top end (I use his bike as a comparative performance dyno)
The performance difference between an average jetting setup and a really well calibrated one is totally worth the effort and time invested and the difference is quite tangible on the road.
I gleaned the basic info for this task from this and the Rat forum so I thought I'd share my results and conclusions here for everyone else's benefit as it can be a long frustrating road getting the jetting right on these old machines, especially with the poor quality parts we sometimes get supplied with and the clues available here are invaluable.

FWIW I check the plugs after each run (a light touch with a soft toothbrush will remove the soft black carbon from any idling and reveal the underlying actual plug reading) and get the best reading straight from the piston tops by looking directly at them through the plug hole using a 5mm high brighness led on the end of a twin core wire inserted into the cylinder.
With modern fuels I find the piston top reading far more relevant to what is actually going on in there than trying to read spark plugs and you can see any detonation straight away,

I'm also thinking that oversized needle jets can be rescued from the scrap bin by careful use of the cross drill hole enlargement at no more than the cost of a few small jet drills


davy

I'm a bit late to this thread because I just returned from a trip. But, I wanted to point out that the territory of plug gages and bore micrometers for measuring needle jets has been addressed before at:

http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=519624#Post519624

plus other posts earlier and later in that thread.

The Starrett micrometer discussed there functions the same as the Mitutoyo 368-022 mentioned above. However, in addition to micrometers I also have plug gages covering the necessary range. If I only were interested in making my bikes run well, and not also interested in the measurements themselves, plug gages would nicely take care of that task. Given how critical the needle jet is to a well-running motorcycle, that they wear out, that dodgy replacements are in the supply system, and how inexpensive a plug gage is (which will be used again every ~5k miles as worn jets need replacement), I can't understand why anyone would insist on not having one.

So my question is if you buy new jets and they measure oversize what can you do about it?
It costs me almost as much in postage to return faulty parts as it does to buy them as my nearest parts dealer is 1800km away.

davy

I would think it unlikely that a needle jet would measure over size and if it was you hadn't bought it from Burlen.

On the note of expensive micrometers to measure ovality in a brass jet. Wouldn't it just be cheaper to buy a handful of new needle jets from Burlen and change them more regularly? A £3 pin gauge for checking size on new installation and go from there. A new jet which has been reemed to size from blank ( blank being .100") shouldn't ( I may be proved wrong) have any ovality from new.

Allan,we have to assume the jets are made by accurate machining and not on a worn out WW2 Davenport screw machine

Production factories often have huge scrap piles of parts that failed quality control.When the pile is low,is that because the quality is better ,or is money tight and quality control workers were laid off???

Never assume anything

Whilst in the Isle of Man ( great roads to try jetting) I found I have been fouling plugs at long durations of 1/4 throttle ( top end is unbelievable with rpm) any way. This was with the small port head and using 106 needle jets.

I brought with me my carb kit and just happen to have 3 unmodified 105 neede jets.

1 is brand new and grips the plug gauge as per johns photo. The other two had more tollerence although gripped as the new one did when they were new ( maybe 2500 miles ago) anyway as they still are less than 106 they are fitted but the bike is still a little rich... But not fouling the plugs up.

I'll order a batch of new 105's but I also have some emission friendly needles which will lean the mixture out further.

Just shows the wear on the needle jets!

Allan, I too run 105 needle jets but cured the slight richness by opening the cross drillings up (I went from 34 to 38 thou eventually)

davy

As I say my 105's need replacing but again may be too rich still.

I did open a set of 105's upto 0.039" ( which was the closest larger size I had over the stock .035") but although it ran it did make it a little too lean. Although it did still run ( still got those jets) I need to invest in a set of tiny drill bits. They come in a big kit for about £60 - which on hind sight isn't too expensive. More investing required....

It's amazing the impact that bleed hole has on the jet.

I had the same the same experience Allan, I actually used a 1mm drill (39 thou but found it drilled almost to 40mm which was too much)
I went down to a 38mm on the next jet which was perfect.
The beauty of it is you can just open one side at time for an even finer adjustment if your that keen.
Thing is most blokes don't really understand just how big a range the needle jet has influence over.


davy

Davy:
That is the chart for the Mikuni VM and is Not applicable to the AMAL. Note that the tip of the needle in a VM never clears the needle jet orifice and the one in the AMAL comes completely clear of the needle jet orifice just above 3/4 throttle. Thus the needle still is in the picture at full throttle with the VM and has nothing to do with flow at full throttle on the AMAL. Also the relationship of the straight portion of the needle to the needle jet orifice varies between the two. Tuning an AMAL using that chart can give one headaches.
John