New 106 jets, needle ~ still running rich

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I'd like to keep my 71 Commando as stock as possible so I've still got the original Amals on board. I've replaced the jets and needles with new, authentic AMAL parts but the left side is still running rich. I can't get 30 miles out of it before I have to change the plugs.

Other note is the idle is high at 1750 rpm with idle screw fully out.

Where/what else might be causing this? Float bowl level appears correct. I would have thought wear on the barrel would result in a lean mixture. A new set of AMALS seems a little radical and expensive.

Baffled.

BC
 
Stillreel said:
I'd like to keep my 71 Commando as stock as possible so I've still got the original Amals on board. I've replaced the jets and needles with new, authentic AMAL parts but the left side is still running rich. I can't get 30 miles out of it before I have to change the plugs.

Other note is the idle is high at 1750 rpm with idle screw fully out.

Where/what else might be causing this? Float bowl level appears correct. I would have thought wear on the barrel would result in a lean mixture. A new set of AMALS seems a little radical and expensive.

Baffled.

BC

How many miles on those carbs? Have you done a compression test on the engine recently? What are the choke slides like? Cables for them?
 
Compression good for a 71 @ ~ 160. No choke slides; they're out with cable port sealed.

BC
 
Stillreel said:
Compression good for a 71 @ ~ 160. No choke slides; they're out with cable port sealed.

BC


If you've taken the choke slides out, what's blanking off the choke jet?
 
Choke jet, what are you calling the choke jet? If you mean the choke valve or slide that normally rides in the throttle slide, there is no reason to block that off should the choke valve/slide be removed.
 
Stillreel, when you say idle screw, are you talking about the idle speed screw? The one that is slanted up into the carb body? And are both of them all the way out with no drop in idle? Or do you mean the mixture screws that run straight into the carb bodies?
 
Yes, idle screw is the one at 45 degrees, mixture screw runs flat into the carb. I can tweek the idle up but no matter how far down I drop the idle speed screw, it won't come below 1700 rpm, which is definitely high to my ear.

When adjusting the mixture screw, I use 1/2 turn out (counter-clock wise to add air air) leaning out the mixture but this has no effect until the idling engine falters and/or cuts out. So 1/4 turn back in and still, after 30 miles, plugs look like crap.

Testing the spark plus out side the cylinder and there is a good solid spark (boyer ignition)

Maybe a new set of Amals is the way to go?

BC
 
just a thought

you still have the original slides in those old carbs?

if so, they have been rattling in the carb bores for over 40 years, slowly wearing both slide and bores away

to the point where "unwanted" extra air is being sucked between slide and bore, making correctly diagnosing
problems even more difficult if not impossible

ie: what seems like needle, air screws, and jetting solutions could well be slide to bore causation

example: inability to bring idle down with slide fully down (throttle screws fully out) is a sign of excess air,
which is usually corrected by richening the idle circuit by turning air screws in

if you really want to keep your old carbs, this would well be time to send them out and have a professional
clean up and bores and properly fit new chromed slides?
 
Biscuit said:
Choke jet, what are you calling the choke jet? If you mean the choke valve or slide that normally rides in the throttle slide, there is no reason to block that off should the choke valve/slide be removed.


My mistake, I'm thinking of Mk2s that have a separate choke circuit.
 
1up is right about worn slides and carb bodies being a good possibility, but you could do a bit more troubleshooting first. With the idle speed screws backed all the way out do you know for sure if the slides are both bottoming out in the bodies with a good solid "clunk"? Visually, how do the slides look to you? And can you rattle them around in their bores a lot?
 
+1 on the loose slides, but, if they're okay, it could be broken spring(s) on the AAU. That may or may not cause kickback during start-up, depending on whether or not only one or both are broken.
 
Stop guessing and measure the needles and needle jets.The parallel part of the needle should measure 0.0985" diameter,and that doesn't come fully out of the needle jet until 1/3 throttle (so it controls a lot of your low throttle mixture).
If you haven't got a go/no-go pin gauge to measure the jet ID,buy a number 36 drill and measure both ends of it (or measure a few of them,before you but one).The flutes should be 0.1065" and the shank should be a little under 0.1060".The shank should fit,but the flutes shouldn't.
The cross-drilled holes in the needle jet should measure 0.035".If they're a few thou smaller,it will run richer at about 1/4 throttle.

Measure your slide cutaway,and file one if necessary to make them the same.They're not always what the stamped number says,but they're USUALLY closer to that than any other size.If the slides are worn and rattly,it should only run slightly leaner.
Are both your jet holders the same?There are 2-stroke and 4-stroke jet holders,but the difference is obvious.
 
Some very useful tips have been posted but no one has yet stated the obvious

IF you have fast idle first check you have enough free play in the throttle cables to allow the slides to fully close ( tight cables may be causing fast idle )
then check for air leaks balance tube and manifold o rings / gaskets are often the cause of air leaks

As for the rich running issue it could be the wrong combination of needle jet and needle jet holder , if your carb parts have been altered by a previous owner it is difficult to
know weather the jet holders are 2 stroke or 4 stroke as the only difference is the stand height ( which affects the needle position in the jet tube )
a quick check is to remove the main jet with the throttle needle set on the middle clip position it should stick out the jet holder 5mm

if the needle height looks good and you have the correct 4 stroke needle jets / needles fitted look to float height and main jets
 
I'm going to try lowering the needle on notch (currently at centre) this weekend. See if that helps. Thanks all for the input but wear on barrels and slide would indicate to me more air into the mixture and thus lean running. My issue is too rich.

Another ugly possibility is value guide wear it seems to me to be more of a carb issue.

BTW: I've go the Boyer installed.

Thanks again.
 
It' not over. Hold the float itself up to a light to see if there is gasoline inside of it. Float needles should be changed out too.
 
kiwi said:
Some very useful tips have been posted but no one has yet stated the obvious

IF you have fast idle first check you have enough free play in the throttle cables to allow the slides to fully close ( tight cables may be causing fast idle )
then check for air leaks balance tube and manifold o rings / gaskets are often the cause of air leaks

As for the rich running issue it could be the wrong combination of needle jet and needle jet holder , if your carb parts have been altered by a previous owner it is difficult to
know weather the jet holders are 2 stroke or 4 stroke as the only difference is the stand height ( which affects the needle position in the jet tube )
a quick check is to remove the main jet with the throttle needle set on the middle clip position it should stick out the jet holder 5mm

if the needle height looks good and you have the correct 4 stroke needle jets / needles fitted look to float height and main jets

+1

The only thing which could cause a fast idle is an open throttle slide - if adjusting the screw has no effect it may be because it isn't touching it?
(sorry if that sounded patronising, BTW - I didn't mean it to :oops: )
Easy check is to wind the throttle fully open, apply MKI earlobe to vicinity of carb and release the throttle - if you don't hear a sharp clack from both sides, there's the problem.
It's also a good way to check if the slides are lifting and dropping together if a carb balancer isn't available.

The cables can jump out of the holders at the splitter and may not be easy to see.

The rich running may be easier to pin down once the idle issue has been resolved, but the 2-stroke jet issue has caught me out in the past - they have the same numbers as the 4-stroke :roll: This continues to baffle a very good friend of mine, even after it was pointed out...
But as he prefers Triumphs to Nortons I don't push the issue ;)
 
Andy yes both will be marked .106

But the 2 stroke and 4 stroke needle jets can be easily identified

2 stroke has the metering restriction at the top of the jet and no air hole drilled in the hexagon ( thick wall )

4 stroke has the needle restriction in the threaded portion of the jet with air hole drilled in hexagon ( thin wall )

the jet holder on the four stroke measures 9.6mm from main jet seat to the top of the hex where it meets the upper body

not sure what the 2 stroke stand height measures as I do not have one
 
Disclaimer.
It is sometimes hard to offer help thinking you might insult someone. It is really not practical to search someones posts just to determine their skill level and history.

If the idle screw is out, you will surely not be able to tune the pilot curcuit. The best thing to do is start from the start. I have had to do this many times for tuning tends to be somewhat sequential.

First is to bottom the slides and make sure they come off the bottom exactly together through cable adjustments and allowing for an appropriate amount of cable play.

While feeling the slide, turn the idle screw (not the air screw(don't confuse these)) till you feel it just connect to the bottom of the slide. Then give each side a couple of equal turns to give yourself a base setting.

Then set the air screw to the bottom and turn it out equally a couple turns, again to give a base setting.

I could go on but I feel this is the starting point. These setting and more procedures are outlined in the previous link given.

All that being said, we hope both sides will act and respond the same, but of course this is not always the case. It is not uncommon to have needle clips in different positions. Pilot/air curcuits clog up without notice and need to be re-examined.

I might think the your rich running and high idle may be linked to a bad slide position but this also begs for the tuning sequence to be performed rather than throwing boogers at the wall and hoping for one to stick. But then again the issues may be unrelated. This again is a good reason to go back to the basics and validate the tuning sequence and isolate the problem.
 
There are a couple of other things you can check for a rich running carburetor.

1. Ensure that the spring clip that holds the jet needle is seated properly on the top of the slide. It may be that the top of the slide has a ding or foreign material stopping it from seating correctly. It is even possible for the spring clip to ride up into the slides return spring.

2. Make sure that the drilled air passageway that goes directly to the needle jet is clear. There is a chance that an insect could have made a nest in that passageway which can cause a very rich mixture. Don't laugh, that actually happened to me once! It was on a carburetor that I had recently cleaned. However, I did not get around to putting the air filter on for awhile leaving it exposed for enough time for a little mud dauber wasp to build its muddy nest.
 
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