Britbike forum Forums British motorcycles in general Bsa gold star forum Changing jets..

With a GP3 1 1/2" carb...I'm wondering what should I expect if I changed to a 330 main jet (now its a 340) and a 108 needle jet (now it's a 109) ... with a GP3 needle ??
DB34 Goldie 1956 in Clubmans trim.
We're getting close to things being nice...

One change at a time. There is considerably more overlap between the jetting circuits than the books would have you believe..

Stock 1 1/2" GP needle jet is a 109, and if your 109 is worn the effect of changing to a 108 would be very noticeable. Even if the 109 is OK, given that the modern unleaded fuels seem to like running leaner than the old leaded gas, the 108 might clean it up nicely. Lift the needle up a notch if you go to the 108, for a start.

The change from 340 to 330 will be noticeable on a 'full-noise' plug chop, on a new plug. A minor change might be felt through the seat (a B33 I raced on petrol years ago was quite sensitive to a change from 330 to 340 or reverse, partly because the extra fuel was being used to cool the thing!), though it won't be far from the ideal. (stock book main jet is 350).

My own bike, on 1 1/2" GP is good, but not great, so I'll be exploring the same sort of changes come northern summer time. I did try the 3GP6 needle last summer, though it was clearly not the right move. Legend has it that if you can get them to run on the (leaner) 3GP6 needle, they are better, but I didn't fiddle with the needle jet or slide to make it work. Assuming it does. (standard silencer - leaner needle usually used with open megaphone).

My engine was running rich at ½ throttle, so I changed from a 109 to a 108 needle jet.

That resultet in a little stumble at low throttle openings, so I changed from a 4 to a 3 slide and that cured it. Unfortunately I can not remember if I tried to change the needle setting before changing the slide.

I will try the same changes as Kerry has made including a go with the 3GP6 needle and see if have any luck with that, but it will have to wait to spring (also because the engine has been taken completely apart).

I'd expect to get away with the 3GP6 needle with a 109 needle jet, but go richer (smaller) in the slide and bigger in the pilot jet (say a 30 to 35) to make it work.

Will try my own medicine one day soon!

Kerry,when you talk of a pilot jet,are you talking about the Air Jet ?.... the jet behind the air passage sealing plug ?...I've taken this out and there's no number stamped on it...

Standard size air jet is 0.125", 1/8" they do come in smaller sizes. A smaller air jet will richen the mixture, think Kerry was quoting Mikuni pilot jets. There is a post of mine titled AMAL GP air jet on the forum, trying to stop my GP running weak at wot

Thanks Dave..yes,mine is 1/8"..
I'm waiting on a new Gp3 needle,a 109 needle jet and a 350 main jet..
I hope that get the carb dramas sorted....Still having them... we're tying hard though

No, actually I mean pilot jet, though just like the Mikuni. The air jet is different (and probably OK).

Here's a Hitchcock's Motorcycles AMAL 3GP parts diagram - I'm talking about item #27 (Hold the mouse on the actual number and it'll open a new little window and offer various sizes betwen 15 and 45):

http://www.hitchcocksmotorcycles.com/partsbooks/pages/974/AMAL_GP2

The air jet (behind the big plug where the float height is checked) is item 25. Hitchcocks don't list specific sizes for this, though AMAL in the UK list them as 80, 100 and 125. The 125 should be the one in the big GP. (The Mikunis have this as well, screwed into the front of the intake, between very, very small and 2.0mm in steps of 0.1mm). http://amalcarb.co.uk/g-p-series/3gp/jets/air-jet.html. The air jet is a useful tool to correct the 'slope' of the fuel graph, but seldom needed to be varied from standard.

Oddly, AMAL don't list varations of pilot jet for the GP, though it is the same item used in the Monobloc, as I recall. (Plus the Hitchcocks part number for the pilot jet is the same part number AMAL uses for the Monobloc pilot jet..) AMAL list all the Monobloc pilot jet sizes. http://amalcarb.co.uk/Monobloc-series/376-series/jets/pilot-jet.html

On the basis that 'all jets flow all the time', we can take advantage of the far more significant overlap of fuelling controls (pilot jet/needle jet, slide, etc) that exists than the books would have you believe, to allow us to clean things up here and there: hopefully, this is how we'll manage to use the 3GP6 needles..

There are two versions of the 3GP. The old version does not have a pilot jet.

Perhaps that is the one Charlieboy has.

If you have a 25 jet which is the normal jet for a 4 stroke in a Monobloc or Concentric then a 16 thou drill also called a #78 drill will clean it out.

Ahhhh... the picture is clearing ! ... It seems as though I have the 'old version' ie: ,no pilot jet..
The 120 k ride today was pretty good... it did'nt stop with a fouled plug..
Observations were...hesitation at 0 to 1/4 throttle opening,wanting to stall when I came to stop and when I got home,a plug check revealed the electrode to be white/clean and soot around the rim ,riders behind me said that it smelled a bit 'fuely' and a puff of smoke when throttle was given after a gear change..
I've ordered a new 109 needle jet,a GP3 needle and a new float needle in the hope of eventually getting this carburettor problem sorted.
To refresh...we have #4 slide ,109 needle jet,Gp3 needle,340 mainjet,95 octane fuel..
A bit of a pain to have this lovely /restored machine let down by these carb dramas.... we'll persevere though,got to get things right sooner or later.

The GP will have a low speed mixture scre on the right side (on a righ-handed instrument). Is it on the engine side of the main mixing chamber where the slide fits, or the intake side *betwen the bell-mouth and the mixing chamber?

Alternatively, does your GP have an adjuster as shown in the photos here? http://www.racingnorton.co.uk/shop/article_0293%2520316%252F283/Pilot-Adjuster-Needle-for-AMAL-GP1.html?shop_param=cid%3D55%26aid%3D0293%2520316%252F283%26

Glad to hear there's improvement!

What colour 'puff of smoke'? Blue or black?

GP2 mixture screw adjusts air and the GP3 mixture screw adjusts fuel. Usually you adjust this in neutral with a warm engine, try to hold at 2500rpm and adjust in and out untill highest rpm is found, allegedly you should not have to adjust this again unless you alter any other carb settings..

Personally, I'd never heard of a 'GP3' until I came on here, and assumed it's the next series, being the remanufactured versions of the GP2 (where the screw adjusts fuel)

As above, what I know as the 'GP1' had the larger knurled ajuster, controlling air.

Regardless, the versions with the pilot jet and mixture-controlling screw with lock-nut have the need/facility to change pilot jets, unlike the GP1 with the big knurled air screw (which often gets damaged by over tightening). The placement of the screws indicated which version is which.

I stand to be corrected, but I suspect the GP2/version with the removable pilot jet came out after the end of Gold Star production, though they meter fuel a little better, I believe.

Both GP1 & GP3 have large knurled mobs, but I was led to believe that they meter fuel and the GP2 metersair same outcome though.
Did you get my e-mail Kerry ?

GS Dave

You're probably right - maybe the big knurled know does meter furl and not air and the other way around for the GP2! I recall remembering that the placement of the fitting gave the clue - air on the air side, fuel on the fuel (downstream side), but I'd have thought the replacemeble pilot jet metered fuel..must have more of a think before I write (been asleep in front of the TV - family went to the UK today..)

Sorry - no email?

Been out of action for a few days... (gathering strength to deal with the next round of grief in dealing with the GP carb !! ).
It has T3GP1 stamped on it..
A 109 needle jet,a new GP3 needle and a new float jet are on their way..
What I don't understand is puffs of blue/black smoke,the thing smelling 'fuelly' and a general vibe of 'richness'....and yet the centre of the plug is white/clean !....jeez,whats novice tinkere supposed to do ?

As someone else has pointed out, the chemical mix they call "gasoline" these days, no longer gets the plugs to color up well at all...

If the smoke is "blue" or white, I believe thats an oil burning problem?

If the smoke is black, that one is a fuel problem.

Yeah, get your new needle and jet " emulsion tube " installed, and see if the 'old ones' were indeed worn out.

I've written about jets before so this is just an abbreviated post about them. Briefly, I have two types of bore micrometers with accuracy and resolution better than 0.00005" (i.e. a half of a ten-thou.) for measuring the ID of needle jets, and two bench micrometers with even better accuracy and resolution for measuring the OD of needles.

I have eight .109 needle jets for GP carburetors, and it turns out all of them have IDs within 0.00005" of each other (all are NOS "original vintage" Amals). However, this is in contrast to NOS .106 jets of which I have 22. Of these, 2 are one jet size too small, and 3 are nearly one jet size too large. That is, 23% of these NOS needle jets are off by a full jet size. I would have to have more .109 jets to measure to determine whether AMAL applied more careful manufacturing and quality control to GP components, or whether I was just lucky with these eight.

I only have five GP needles to measure but they range from 0.0991" to 0.1005", i.e. the equivalent of 1.5 jet sizes. The smallest of these is an aftermarket needle, but even excluding it the other four vary by nearly 3/4 of a jet size (this is consistent with the variation I find with NOS needles for Monoblocs and Concentrics).

Again, the above results are on original AMAL production components from the classic era, not anything currently being manufactured. I haven't measured any of those so have no way of knowing at this point how good they are. But, it's worth keeping my measurements in mind as you go through the process of getting your jetting worked out. If there is as much variation with new production -- which, again, I have no way of knowing without measuring them -- as with old, one could easily install a new needle and jet and find the carburetor even richer than it had been with the "worn" components they replaced. Because of these variations, the situation is even worse when jetting a twin. Because I have the necessary micrometers, and a large stock of needles and jets, I hand select each replacement before I install it.

I guess the logic then is, if it seems rich, try leaner, regardless of what the numbers on the jets are telling you.

What plug are you running again?

Further on the issue of jets, I measured the flow of twelve '500' main jets on my flow bench. The good news is 75% of them flowed the same to within one jet size. The bad news is, 25% of them flowed up to 3-1/2 jet sizes too much or two little (i.e. at the low end the flow was equivalent to a '465' and at the high end a '535').

Jeez Magnetoman...inconsistant sizing in jets....now I'm REALLY depressed ... ;-) ..I can only hope for success when the new parts arrive..
B6ES at the moment Kerry ...waiting on a B7ES from the bike shop,maybe even an 8 a bit later on.

Add this to the list of reasons why working on old British bikes builds character...

What these measurements on jets mean is, if your bike is running rich and the next marked size smaller seems to make it run even richer it may not necessarily mean you're going crazy. It's also why you have to change only one thing at a time and retest since you shouldn't completely rely on how things are marked.

B6 is pretty warm - the B7 might give more meaningful results (colour) and I'd invest in a B8 to have on the shelf..

This is a possible first draft of one section of what may or may not expand into the carburetor equivalent of my Bosch ZEV magneto restoration thread.

The following describes how I measure the bore of needle jets. Since such measurements are needed with higher accuracy than 0.0001", and since that's a lot of zeros to keep track of, in the following I'll give such dimension in micro-inches (0.0001" = 100 micro-inches).

There are a couple of ways to see how much precision is needed for these measurements. Although AMAL spaced their needle jets by 0.001", experience tells us that wear of even half that is sufficient to seriously degrade carburetion. The fact that Mikuni spaced their needle jets by 0.0002" (200 micro-inches) is a good indication of how much wear (or manufacturing error) is tolerable before a noticeable change in performance occurs.

Another indication is that moving an AMAL needle up or down by one slot (1/16") changes the mixture by 10%. This is enough to cause a noticeable change in performance in the midrange, and Mikuni spaces their slots by 1 mm, giving a 6% change in mixture. Considering a perfect 0.1000"-diameter AMAL GP needle and 0.1090"-diameter needle jet as a baseline, the annular area between the two is 3.693x10^-4 square inches. Hence, the mixture will become 6% richer if the jet wears to 0.10951", i.e. increases its diameter by 510 micro-inches (or if the needle's diameter decreases by this amount).

The conclusion from the above two paragraphs is that we need instruments that measure bores with at least 4-5x higher accuracy than the 200 micro-inch spacing of Mikuni needle jets, i.e. 50 micro-inches.

The next photograph shows my Starrett bore micrometer in a stand I made for it, along with the two ring gages I keep in its box for calibrating it. Note that both of these specify their ID to 0.00005" (i.e. 50 micro-inches). In this case the micrometer can be seen to be reading a little over 0.106". The vernier lines on this micrometer are at 100 micro-inches, and the position of the spindle can be interpolated to at least half that, i.e. 50 micro-inches. Hence, this micrometer has the necessary accuracy for this use.



The next photograph is of the measuring end of this micrometer, showing it to be a split cylinder of length 0.11". As can be seen, this is about half the depth of the bore in an AMAL needle jet. The micrometer screw expands the cylinder until it makes contact and the diameter is then read off the spindle.



To say something more about the precision and accuracy of this micrometer, I mentioned two calibrated ring gages above. The Diatest set discussed below has two additional ones in the range covered by this micrometer: 0.1000" and 0.1150", both specified as being accurate to 35 micro-inches. By calibrating the micrometer with any of these four ring gages and then measuring the other three I've determined the absolute accuracy of this micrometer across the range of interest for AMAL needle jets (i.e. 0.105"-0.110") is no worse than 35 micro-inches. This is at least 5x better than that needed to determine a variation in needle jet bore -- either due to wear, or to manufacturing tolerance -- that would be sufficient to give a 6% change in mixture.

The next photograph shows my Diatest split ball bore gage. In this instrument the jet is placed on the platform and the handle at the right raises it to insert the jet in the gage. Note that the smallest division on the dial indicator is 50 micro-inches. Like the Starrett, a ring gage is used to calibrate the unit. However, with this instrument the dial is "zeroed" using the ring gage and the diameter determined by the amount of deviation from that calibration point (e.g. if set on 0 using the 0.10845" ring gage, a perfect 0.109" AMAL GP needle jet would read +0.00055").



My Diatest set covers a larger total range than the Starrett, but to do so requires eight split-ball tips each of total range ~0.020". It happens that two of these tips overlap in the range of interest (0.091"-0.110" and 0.105"-0.126"), allowing an additional check on absolute accuracy. Like the Starrett, using all combinations of ring gages and tips shows that the Diatest reads correctly to an absolute accuracy no worse than 35 micro-inches.

The next photograph is an enlargement of the tip of the Diatest's split ball gage. The OD of the tip is compressed as it enters the jet forcing the central rod upwards to press against the dial indicator.



The next photograph compares the tips of the Diatest (top) and Starrett (bottom), showing that these two units make their measurements by averaging over different lengths.



This is seen more clearly in the following enlargement.



Comparing these two, since the length of the cylinder on the Starrett is 0.11", this means the Diatest averages over only ~0.015". Operating the lever on the Diatest in combination with a separate dial indicator to register the position of the platform makes it easy to make measurements of the internal diameter of a jet as a function of position to within ~0.010" if desired. In this way roughness and/or taper can be determined with quite high resolution along the entire depth of the bore.

The nice thing about having these instruments is I don't have to rely on the manufacturer's markings. As I wrote in a post yesterday, 23% of the '106' jets I measured were off in size by as much as 800 micro-inches. Both of these micrometers are quick to set up and they have let me sort my jets into compartments labeled in increments of 50 micro-inches. Even if the jets are within "reasonable" tolerance, if one marked '109', that actually is 0.10905", is a teensy bit too lean, I can install a 0.10910". Or, if I do some work and later the bike is running too lean I know to look for an air leak rather than frustrate myself swapping in needle jets marked with larger numbers, that may or may not actually be larger.