What size pilot jets for AMAL 930

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Stillreel

Stillreel

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I think I've found out why the machine is running rich. The PO didn't install the pilot jets. What's even more remarkable is that the Commando ran at all.

The galleries are clear but I see that I have a choice of pilot jet sizes to choose from. I have the standard 622/122/106 needle jet and the 622/124 needle.

Suggestions here?

Thanks

BC

71 Commando
 
The carbs don't use pilot jets unless someone has drilled out the pilot bush. If you look closely you'll probably see that the area where the pilot jets screw in has been staked to prevent someone from mistakenly fitting them.
 
An example of the dimpled former pilot jet location. Note the internal threads.
Another check is to remove the idle mixture screw and look in the hole with a strong light and observe the pressed in pilot jet.
This is the jet that a wire cleans out any solid build-up.
If an older carb used a screwed-in jet, then this internal mixture screw hole would be larger.
What size pilot jets for AMAL 930
 
debby said:
The carbs don't use pilot jets unless someone has drilled out the pilot bush.
Click to expand...

I'll take a look and see if I can see the bush within the air screw opening.

Thanks

BC
 
I love these jetting threads. So many think they can do the same as the other guy, but there are many things at work here that need to be considered. Elevation is one important thing. How the engine runs is first and foremost. In my experience you must do the WOT plug test on "your" engine to get the right info...or exhaust gas analysis if you have someone who can do it for you.

Transfer from pilot to main is critical...jet sizing is the key to this. Smooth transition from idle to 1/8 throttle and into the main fuel delivery. But this is based on how the engine runs and plug color, not the size the other guy has in his carb(s). Yes talking to others can get you close, but you have to tune to your engine.

It's also best to buy a range of jet sizes so you can make changes and see and hear the results.

I've found that sometimes even the standard jets need "tweeking" so I bought a hobby drill set that allows me to open jets between the standard sizes.

Don't forget spark plug heat range in this equation. If you get close changing plug up or down can make a big difference...just be careful about lean burn with a hotter plug...better off a little rich than too lean.
 
From what you've said, your problem lies in adjusting the mid-throttle jettings. Your main jets are only used when the throttle is wide open. If you've got the needle clips in the centre groove and the bike runs without coughing, when you lower the needles one notch it should cough. If it doesn't, your needle jet is too big. With petrol, the adjustment required is very fine, so you need to find out what needle jets you can buy. Don't try making your own from brass hex using number drills if you are using petrol (I do that with methanol). My feeling is that you are better off using Mikuni needles than the Amal ones. They are the same dia. and if you compare where the shoulder is in relation to the grooves, you can get the right length. The adjustment the Mikuni needles provide is much more precise. Amal stuff tends to be more 'one size fits all'. Jet for the cold weather, and if it warms up the jetting will richen, and the motor won't cough when you ride the bike. I think we should do some research on the function of the pilot jet. I believe its function in a two stroke relates to when the throttle is suddenly shut off from high speed.- It is not something I've ever found to be important on a four stroke motor. As far as spark plug heat range is concerned, I'd use the next hotter plug to normal when adjusting the main jets, then fit the colder plug for daily use . Leaning off on the needle won't give you a problem, a lean main jet can, so do a plug chop. I don't usually try to set midrange mixture by looking at spark plugs, a most important thing is to keep the mains so that the black ring is always present on the porcelain, right down inside the plug. If you fit your richer main jets when adjusting the mid-range, you won't get confused about what is happening, you can get them right later.
 
A good thing to do, is take a carburetor off the bike, and look down the bell mouth while winding the throttle slowly open and think about what jets are doing the fuel and air metering. If you get your head straight on that, then you can be systematic in your tuning.
This might help:

http://www.iwt.com.au/mikunicarb.htm
 
Stillreel said:
I think I've found out why the machine is running rich.
The galleries are clear but I see that I have a choice of pilot jet sizes to choose from. I have the standard 622/122/106 needle jet and the 622/124 needle.
Suggestions here?
71 Commando
Click to expand...

What notch are the needles set on ?
And how have you determined its running too rich ?

You could simply try dropping the needles a notch.
Being aware that going too lean could cause it to get hot....

How is your idle ?
Amals set the idle by adjusting the AIR SUPPLY, in conjunction with setting the slide heights, so if its idling too rich this can affect a lot of the low speed running.
 
acotrel said:
From what you've said, your problem lies in adjusting the mid-throttle jettings. Your main jets are only used when the throttle is wide open. If you've got the needle clips in the centre groove and the bike runs without coughing, when you lower the needles one notch it should cough. If it doesn't, your needle jet is too big. With petrol, the adjustment required is very fine, so you need to find out what needle jets you can buy. Don't try making your own from brass hex using number drills if you are using petrol (I do that with methanol). My feeling is that you are better off using Mikuni needles than the Amal ones. They are the same dia. and if you compare where the shoulder is in relation to the grooves, you can get the right length. The adjustment the Mikuni needles provide is much more precise. Amal stuff tends to be more 'one size fits all'. Jet for the cold weather, and if it warms up the jetting will richen, and the motor won't cough when you ride the bike. I think we should do some research on the function of the pilot jet. I believe its function in a two stroke relates to when the throttle is suddenly shut off from high speed.- It is not something I've ever found to be important on a four stroke motor. As far as spark plug heat range is concerned, I'd use the next hotter plug to normal when adjusting the main jets, then fit the colder plug for daily use . Leaning off on the needle won't give you a problem, a lean main jet can, so do a plug chop. I don't usually try to set midrange mixture by looking at spark plugs, a most important thing is to keep the mains so that the black ring is always present on the porcelain, right down inside the plug. If you fit your richer main jets when adjusting the mid-range, you won't get confused about what is happening, you can get them right later.
Click to expand...

Giving Mikuni advice for Amal carbs is about as useful as selling refrigerators to eskimos ?
How long since you've ridden a petrol engine on the road.
Going a bit lean in a petrol 4 stroke doesn't make it 'cough', it just gets hot.
Maybe too many methanol fumes ??
 
If the mixture screws are happy at about 1-1/2 turns from bottomed,the pilot jet is about the right size.If the was no pilot jet fitted,it wouldn't be happy at 1-1/2 turns.
Richness is usualy caused by a worn needle jet,and it will be mainly rich between idle and 1/3 throttle.
0.0005" wear= rich,and they're always worn out.
 
Rohan, If you believe you have your needles set to the correct height on your petrol fuelled motor, what happens w he n you drop the m one notch? Does the motor behave the exactly same after the change, and I'm not talking about combustion temperature. If it becomes too lean, it must give a response other than destruction, otherwise how can you tune it without doing damage. Plugs don't change colour q uickly enough to be used as the indicator. Tuning a round slide Mikuni, is exaclty the same as tuning an Amal up t o at least Mk2. The early mikunis were exact copies of small Amals of that times except that the adjustment provided was better. Except for possibly the later pre-power jet or flat slide units, Mikunis still use the same needle jet and needle set-up as an Amal monobloc up to Mk2. The exceptions are in the pilot jet, low running arrangement (lower than one quarter throttle) which allow mikunis to be more effective on two stroke motors, by giving better throttle response at small throttle openings. It means you don't have to be so careful in feeding the throttle on - irrelevant to four stroke motors, where you can whack the throttle opening without getting a stall.
The information in that link about the effects which occur at various throttle openings on a Mikunis apply equally to Amals with only very minor differences..
 
Rohan, I rode petrol fuelled British bikes for the first 15 years of my motorcycling life, and I've used the old standard leaded fuels and the two common grades of Avgas. Jetting for methanol to get the max, is no easier than jetting for petrol , you can opt out , get sloppy and don't even bother to lean it out, then tolerate a fair bit slower bike. A fuel blockage on a methanol fuelled two stroke is interesting, they go fastest just before they seize and bend the crank, a speed higher than you ever get by normal tuning. What you guys are into are bikes for use on public roads, why aren't you using the manufacturers' recommended settings and OEM carburetors ?
 
X-File, I think you've got the right answer on this. If dropping the needles one notch doesn't indicate lean , the jets are probably worn. I consulted another friend on this and he says that petrol motors still cough when the mid-range carburetion is too lean.
 
acotrel said:
What you guys are into are bikes for use on public roads, why aren't you using the manufacturers' recommended settings and OEM carburetors ?
Click to expand...

This question here IS about stock Amals - or am I missing something here. ?
You talking Miks and methanol in the same thread is just pure red herrings - and likely to lead folks up the garden path ??

I've experimented with needles in Amals in Commandos - my bike came with different settings in each carb, for no good reason that I could see. !
(Wasn't new, obviously).
Never got a cough out of it. Nor seen/heard this mentioned anywhere else before.
Engine will get plenty hot if you go too lean though.
Although it will cough - when its on its last gasp, running out of fuel !!!

Perhaps we have discussed this before, but you can give engines way more advance and lean the fuel right out, and get a bit more power.
But it will get HOT, and go bang after not very long probably...
Ask airplane folk - they have throttle mixture lever right beside them.
But they also have exhaust gas temp and cyl head temp gauges (EGT and CHT), and can see when their engine goes up into the red zones.
DANGER DANGER DANGER your engine is about to go bang.

Not something most folk would like to explore ?
Except on a dyno, where you can keep a close eye on what is happening.
And back off from such dangerous settings.

P.S. That suggestion about worn jets and needles sounds good.
Doesn't take much wear for the mixture to richen up, noticeably.
And a lot of town running happens here, to wear things faster.
 
My Combat with 932 MK 1 Concentrics has been ticking over a bit lumpy, so I figured I need to clean the Pilot jets out, that's when I discovered one had fallen out, so I do need to know the size of the pilot jet which I cannot find answered in this thread.
My carbs were new old stock which do indeed have the jets screwed into the bottom of the carbs.
 
Thanks Needing,
I dont know where it disappeared too,, I expect when I took the float bowls off an tipped the petrol away it went along with the petrol.
 
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