I Can't Believe How Bad It Is

Big_Jim59 · Sep 2, 2023

I was reading over the Bushman's Carb Tuning Secrets particularly the section on the pilot jet. I have cleaned many Amal Concentric carburetors and I know all about the embedded pilot jet but I had no idea there was a chamber of horrors back there that can hide trash, gunk and debris. No wonder these things give people hell. I had to clean the Amal Monobloc on my Triumph T100 three times just to get the pilot circut working and the pilot jet is removable and the passage relatively open on that carb!

Has anyone drilled and tapped the off side for a plug or is this surgery better left to an expert?

(Not sure that I don't qualify as expert status due to a lack of skilled individuals)

jamesp · Sep 2, 2023

Big_Jim59 said:
I was reading over the Bushman's Carb Tuning Secrets particularly the section on the pilot jet. I have cleaned many Amal Concentric carburetors and I know all about the embedded pilot jet but I had no idea there was a chamber of horrors back there that can hide trash, gunk and debris. No wonder these things give people hell. I had to clean the Amal Monobloc on my Triumph T100 three times just to get the pilot circut working and the pilot jet is removable and the passage relatively open on that carb!

Has anyone drilled and tapped the off side for a plug or is this surgery better left to an expert?

(Not sure that I don't qualify as expert status due to a lack of skilled individuals)

I have done it, it was not hard. It was a number of years ago, can’t remember all the details, but I can show you pictures and show you what I did. Let me know if you need more info.

baz · Sep 3, 2023

Yes I've done it
Also digging out the welch plug is a good idea so you can clean the mixing chamber thoroughly
You can get replacements from RGM

Big_Jim59 · Sep 3, 2023

jamesp said:
I have done it, it was not hard. It was a number of years ago, can’t remember all the details, but I can show you pictures and show you what I did. Let me know if you need more info.

Did you tap the hole for a plug? I would like to see the pictures. I have a feeling that once trash is behind the pilot jet it will cause nothing but problems down the road.

Yorkie · Sep 3, 2023

Very interested in this thread as I have a set of 930s on my bench that have not been touched since the 70s. I’ve soaked them in carb cleaner than about an hour in the sonic cleaner. I can get carb cleaner to spray through the two downstream holes but I still suspect the cleanliness.

Lineslinger · Sep 3, 2023

Ultrasonic cleaner with compressed air is a great tool, goes where nothing else can.

lazyeye6 · Sep 3, 2023

I drilled the opposite side of the mixing chamber back when I was a newbie vintage Brit bike owner. It was quite easy
and then, with a light on one side you can actually see the orifice and whether or not it is clear. A good idea to clean
entire carb with ultrasonic cleaner and blow out everything with compressed air. I tapped the hole and screwed in a
plug. That carb never gave me any trouble. My mechanical skill set was very limited over 20 years ago.. Easy work.

marshg246 · Sep 3, 2023

Yorkie said:
Very interested in this thread as I have a set of 930s on my bench that have not been touched since the 70s. I’ve soaked them in carb cleaner than about an hour in the sonic cleaner. I can get carb cleaner to spray through the two downstream holes but I still suspect the cleanliness.

A standard carb (not Premier):

The pilot circuit starts near the bottom of the bowl. There is a passage through the bowl to the bottom of the carb body where the fuel must make a greater than 90 degree turn to the passage in the bottom of the carb body heading towards the front of the carb. Once it gets about to the front of the carb, it must make a 90 degree turn towards the center of the carb. Then it goes through the pilot jet and more. A clean pilot jet means nothing if that passage through the bowl and carb body are blocked!

From every opening you must be able to spray and have it come out of the two small holes in the bottom of the throat of the carb. The one from the bowl is generally missed and often blocked,

Yorkie · Sep 3, 2023

marshg246 said:
A standard carb (not Premier):

The pilot circuit starts near the bottom of the bowl. There is a passage through the bowl to the bottom of the carb body where the fuel must make a greater than 90 degree turn to the passage in the bottom of the carb body heading towards the front of the carb. Once it gets about to the front of the carb, it must make a 90 degree turn towards the center of the carb. Then it goes through the pilot jet and more. A clean pilot jet means nothing if that passage through the bowl and carb body are blocked!

From every opening you must be able to spray and have it come out of the two small holes in the bottom of the throat of the carb. The one from the bowl is generally missed and often blocked,

Thanks Greg. I did not know that and will take a look at the carbs on my bench.

Big_Jim59 · Sep 3, 2023

marshg246 said:
A standard carb (not Premier):

The pilot circuit starts near the bottom of the bowl. There is a passage through the bowl to the bottom of the carb body where the fuel must make a greater than 90 degree turn to the passage in the bottom of the carb body heading towards the front of the carb. Once it gets about to the front of the carb, it must make a 90 degree turn towards the center of the carb. Then it goes through the pilot jet and more. A clean pilot jet means nothing if that passage through the bowl and carb body are blocked!

From every opening you must be able to spray and have it come out of the two small holes in the bottom of the throat of the carb. The one from the bowl is generally missed and often blocked,

If you don't know if the passage is clear you can never be sure and I want to be sure. I am going to drill the plug out so I can access the mixing chamber and see through the back side of the pilot jet. I looked in my box and found the BS tap for the air screw thread. I thought "all I need is a spare air or idle screw to act as a plug" but alas, I was looking at my stock of carb parts and all I have is massive bin of Monobloc stuff. I ordered two sets of screws with knurled knobs. This will free up the other screws to act as plugs. I have been wanting these knurled knob adjusters because trying to get a slotted screwdriver on those adjusters, while the engine is running, while I am trying to keep the engine running , while standing on my head has been a source of tuning frustration.

jamesp · Sep 3, 2023

Big_Jim59 said:
Did you tap the hole for a plug? I would like to see the pictures. I have a feeling that once trash is behind the pilot jet it will cause nothing but problems down the road.

The carbs are mounted on the bike, so not able to show the detail I would like to. Basically, I drilled out the plug and then tapped the holes for the allen plugs. If you do not have a set of taps, then first drill out the plugs, then take one carb down to a well equipped hardware store to get the tap and allen plugs. After my plugs were drilled out, I was able to use a thread tap that fit the hole very well without having to do any more drilling. I believe the tap used was a 1/4”, but could have been a 5/16”. Once the threads are put in, a very short allen plug is needed. I recently rebuilt my carbs and was able to run a guitar string through with ease, after removing the idle screw and allen plug. Here is a picture of the underside of the carbs showing the allens in place of what was there before. If you are unable to do it, you can send me your carbs and I will be happy to do it for you.

Big_Jim59 · Sep 3, 2023

jamesp said:
The carbs are mounted on the bike, so not able to show the detail I would like to. Basically, I drilled out the plug and then tapped the holes for the allen plugs. If you do not have a set of taps, then first drill out the plugs, then take one carb down to a well equipped hardware store to get the tap and allen plugs. After my plugs were drilled out, I was able to use a thread tap that fit the hole very well without having to do any more drilling. I believe the tap used was a 1/4”, but could have been a 5/16”. Once the threads are put in, a very short allen plug is needed. I recently rebuilt my carbs and was able to run a guitar string through with ease, after removing the idle screw and allen plub. Here is a picture of the underside of the carbs showing the allens in place of what was there before. If you are unable to do it, you can send me your carbs and I will be happy to do it for you.

View attachment 109590

Thanks a bunch. I am going to drill out the plug and then using my BS tap install threads for a air screw (with the point cut off of course.) I am pretty sure I can do this with a fine steady hand. Drilling carbs is always a bit stressful but if I bugger it up I can always buy Premiers.

bchessell2 · Sep 3, 2023

I did it for a lot of my customers when I resleeved there carbs over the years. Just hold it in a vice. drill out the plug just going through not too deep.Tap #10-32 and just shorten 2 old air mixture screws and use them . You can remove them when needed and look through to see the blockage.Only problem with a commando you will have to remove at least one of the carbs to check.

Big_Jim59 · Sep 4, 2023

bchessell2 said:
I did it for a lot of my customers when I resleeved there carbs over the years. Just hold it in a vice. drill out the plug just going through not too deep.Tap #10-32 and just shorten 2 old air mixture screws and use them . You can remove them when needed and look through to see the blockage.Only problem with a commando you will have to remove at least one of the carbs to check.

I just did this. It worked like a charm. I should have done this 100 years ago. Now I don't have to guess that the pilot jet is clear Now I know. I can spray carb cleaner through one side and watch it spray out the other side. Brilliant. I ran a corisponding BS tap in the hole to match the air and idle screw thread. (I have a couple of new air screws coming next week.)

baz · Sep 4, 2023

Big_Jim59 said:
I just did this. It worked like a charm. I should have done this 100 years ago. Now I don't have to guess that the pilot jet is clear Now I know. I can spray carb cleaner through one side and watch it spray out the other side. Brilliant. I ran a corisponding BS tap in the hole to match the air and idle screw thread. (I have a couple of new air screws coming next week.)

You can use a 5mm cap head screw

Big_Jim59 · Sep 4, 2023

baz said:
You can use a 5mm cap head screw

This is a great option if you do not have the BS tap. Taps in metric are more ubiquitous and easer to obtain. I was just fortunante that I had the tap to clean out the badge screw holes on my T100 tank..

I Can't Believe How Bad It Is

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