Carb adjusting, idle circuit

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CR125Honda

CR125Honda

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75 MKIII
Amal 930’s

Cleaned both carbs.
Confirmed carb cleaner spray came thru each of the idle circuit fuel holes in the venturi. Holes are located behind and in front of the front half of the slide, where it closes.


When running, adjusting air bleed screws doesn’t make any difference.
Also, I couldn’t back the idle screw out enough on one side to make it’s cylinder die.
Could get it to a very low idle, 800~900. But no lower.

Thinking maybe the crank breather line (going from the crankcase to “T” junction to oil tank and air filter box and finally to a nipple on each intake manifold may be acting as an intake leak??

No leaks at manifold(s) to head junction.

Haven’t tried disconnecting that line a capping of the two nipples on the intake manifold.

Thoughts?

Any suggestions on what I should be looking for?

Ken
 
By spraying through the air intake holes, all you establish is that air can pass along the idle circuit. You’re not ascertaining the fuel flow. Air without fuel won’t help.

By spraying through the fuel circuit (not sure if you’ve done this) all you establish is that it’s not fully blocked. Even in Amal’s, jets are finely machined metering devices, a partial blockage will play havoc with fuel metering.

The pilot jet meters fuel flow. Blockages in the pilot jet are a very common cause of issues.

Bottom line, my money is firmly on blocked, or partially blocked pilot jets.
 
Pilot jets are only fully clean when probed or drilled out with the correct size. 25 pilot is best cleaned with a 16 thou drill.
 
CR125Honda said:
Thinking maybe the crank breather line (going from the crankcase to “T” junction to oil tank and air filter box and finally to a nipple on each intake manifold may be acting as an intake leak??
Click to expand...

It's unusual for the oil separator, line and balance pipe T-piece to still be there as it was often removed and replaced with a one-piece tube between the inlet stubs.
 
CR125Honda said:
75 MKIII
Amal 930’s

Cleaned both carbs.
Confirmed carb cleaner spray came thru each of the idle circuit fuel holes in the venturi. Holes are located behind and in front of the front half of the slide, where it closes.


When running, adjusting air bleed screws doesn’t make any difference.
Also, I couldn’t back the idle screw out enough on one side to make it’s cylinder die.
Could get it to a very low idle, 800~900. But no lower.

Thinking maybe the crank breather line (going from the crankcase to “T” junction to oil tank and air filter box and finally to a nipple on each intake manifold may be acting as an intake leak??

No leaks at manifold(s) to head junction.

Haven’t tried disconnecting that line a capping of the two nipples on the intake manifold.

Thoughts?

Any suggestions on what I should be looking for?

Ken
Click to expand...

I'm not familiar with the specifics of the MKIII, but on earlier models the nipples on the intake manifolds were simply jumpered with a balance tube (hose). A leak there would definitely give you high idle that can't be compensated for. If the air screws have no effect then either you're sucking intake air somewhere or as others have said the fuel flow is restricted.
 
Thanks all. Will check on the numbers on the Amal’s. Thought they were 930’s.

Will take more pics to help describe the breather system too.

Need to do more research on the carb to locate the pilot jet.

Does the screw meter air or fuel?

Hopefully the manual will break it down. Going back to the workshop in a few hours.
 
Carb’s are 932’s. Thanks.
Here’s a pic of the breather junction and line to intake manifolds.
Wondering why this set up like this. The top hose is tapped into the oil tank. The oil tank has another breather hose coming from the crankcase. Non-regulated breather air between the carb and engine? I could see tapping into the back side of the carb but were they worried about emissions on motorcycles back then?

Still researching the rest of the pilot system. Hopefully will have time to get at the jets later.

Carb adjusting, idle circuit


Carb adjusting, idle circuit
 
Last edited by a moderator:
I have always found this diagram helpful.
I think your current set up is correct as stock.

Carb adjusting, idle circuit
 
CR125Honda said:
Wondering why this set up like this.....

.....I could see tapping into the back side of the carb but were they worried about emissions on motorcycles back then?
Click to expand...

Yes, that is what it's for.
 
try blocking off the contraption and either use a connect the two manifold stubs together or block off the stubs by themselves. allow no air leak into the manifolds. If this doesn't change things, buy a new set of premiers with a #19 pilot and a 107 needle jet. Trying to get an old pair of Amals to work is just misery and aggravation, especially when you have to synchronize them. In that first 1/4 throttle opening you have the pilot jet, air screw, slide, and needle jet all converging and it makes dialing them in very tough...and that is where you are riding at least 80% of the time. I wish Amal made a 34MM, I would use it as a single carb on a Norton. A single carb make life much easier.
 
LAB’s link to Bushman’s is spot on. IMO

So, drill the inside side of the pilot circuit, remove the pilot/air screw, and look through for daylight. Based on the symptoms you describe you will not see daylight. Probe with a fine wire until you see daylight. Tap the hole you drilled and seal it up with a screw and gas resistant sealant. Tricky job because you have to avoid drilling too deep and same risk during tapping operation. Use or make a bottoming tap. Bushman’s describes procedure

The fact that you get spouts out of those holes proves nothing about the clogged or unclogged status of the jet. This is because it sits in the middle between your two points of carb spray access

As for those who denigrate these carbs, they have not tried the above and If slides/ bodies are tight the 930 series are excellent and easy to tune
 
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If your carbs are OEM fitted parts, you may want to considering replacing them. If the carbs were subject to multiple (although it only takes one) long term seasonal storage without being drained and blown out, you may want to consider replacing them.

The design and materials used to produce the OEM fitted Amal Concentrics appear to me to be a General Motors grade study in how to cost-reduce; new Monoblocks are more expensive then the Amal Premiers...

Running a #78 drill bit into the idle jet through the hole exposed when the air screw is removed, and through the orifice in the bore may save your bacon, but commencing an escrow account for a new set, or a single Mikuni conversion kit will take the sting out of your next round of carburetor induced frustration.

All the best.
 
Thank you all. Much to go on. The help is very much appreciated. Bound and determined to get this right. Taking the next week off. Will report with another update then.
Ken
 
Looked at the pilot jets last night. Plugged big time.

Another quick question on the 932.
One of them has a loose “tickler”.
The housing that goes through the body that gets pinned is loose and is worn.
The pin is missing.
I see rebuild / conversion kits but don’t see the roll pin in the kit.
Do I need to get my own pin or?
Any tips or trick on this repair?

https://www.ebay.com/itm/3822141737...3a1e26616b0a9c44e01d0c7ffd2194a&ul_noapp=true
 
Last edited:
Your link does not work for me, last year I bought part 622/172 Tickler conversion set direct from Burlen, . and that included the roll pin, spring and new extended tickler
 
CR125Honda said:
The housing that goes through the body that gets pinned is loose and is worn.
The pin is missing.
I see rebuild / conversion kits but don’t see the roll pin in the kit.
Click to expand...

There isn't a roll pin to hold the body, only the long roll pin shown in the link (and below). What you see below is the tickler assembly. The small hole in the tube is a vent:

Carb adjusting, idle circuit
 
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