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British Bikes / Re: Troubleshooting a JAP Special
« Last post by Rockburner on May 29, 2026, 09:03:36 PM »The Amal parts arrived today, so I snuck out into the workshop again! 
Gaskets, float bowl, stainless allen screws, stay-up kit, and a nice mesh "trumpet"
So I started assembling the thing.
The "stay-up" kit is mostly material changes: the float is a military spec material that won't sink, and the mounting tangs are steel (the original part is entirely plastic), so you can easily adjust the float height just by tweaking the tang (phnar!). The float needle is Viton tipped.
I was also going through all my Amal stuff, tidying it up and separating it out into relevant groups. I decided to pack up all the Monobloc stuff into a box to be put into store. I also had a load of random jets and needles from other carb types, so I put those safely away too.
The banjo from the Concentric was a tap rusty, so it got a good brassing.
The single feed fitting from the Monobloc fits the Concentric just fine, so I didn't need to buy another.
I have quite a few main jets.....
230 - 310, and a couple of outliers (140, 360). I've decided to start with a 260 in the Concentric so I can then work up or down.
Mostly there:
Unfortunately it turns out the sodding needle clip is knackered - it's actually broken across the important bit, so I need to order a new one.
You can "just" make out the break.
So I then turned back to the investigation of the timing chest.
First I dropped the push rods out.
The idea being that I can see if the noise happens without any interference from the valve springs.
Like this I can hold the rod in place to provide "some" resistance.
Being a numpty, I accidentally undid that blanking bolt in the timing chest cover, DAGNABBIT!
Bloomin' thing.
So - to make it a little more secure, I added a springwasher
My investigations were pretty low in results tbh, I still can't quite figure it out, but the old exhaust side cam follower IS a little loose on the spindle, and the roller wheel IS a little loose on it's bushing.... so I may just have to bite the bullet and get another one. Whether that slackness is enought to cause the clicking noise, I'm not sure, but its certainly a slackness that shouldn't be there. It could be that with the valve spring resistance, it's causing movement of the cam follower that is making the clack noises.
There was a concern that the spindle itself might be worn, but the new cam follower seems solid on the spindle, so I think that's ok.
I also took some measurements of the push rods so I can look into buying some stainless tubeing to make new ones.
[/url]IMG_20260529_191941_407 by James Bridge-Butler, on Flickr
I think 1/32" wall thickness (info supplied from the Guru, who had a pair without caps fitted) is a mite thin, so I may try to get slightly thicker tubing if available.
The push rod ends are soldered on, so that's going to be a fun job swapping them over... yet more opportunity for conflagations!
Next time I try the timing chest I'm going to dribble some oil onto the cam follower spindle and the rest of the area, it could be that a layer of R is all it needs?
Gaskets, float bowl, stainless allen screws, stay-up kit, and a nice mesh "trumpet"
So I started assembling the thing.
The "stay-up" kit is mostly material changes: the float is a military spec material that won't sink, and the mounting tangs are steel (the original part is entirely plastic), so you can easily adjust the float height just by tweaking the tang (phnar!). The float needle is Viton tipped.
I was also going through all my Amal stuff, tidying it up and separating it out into relevant groups. I decided to pack up all the Monobloc stuff into a box to be put into store. I also had a load of random jets and needles from other carb types, so I put those safely away too.
The banjo from the Concentric was a tap rusty, so it got a good brassing.
The single feed fitting from the Monobloc fits the Concentric just fine, so I didn't need to buy another.
I have quite a few main jets.....
230 - 310, and a couple of outliers (140, 360). I've decided to start with a 260 in the Concentric so I can then work up or down.
Mostly there:
Unfortunately it turns out the sodding needle clip is knackered - it's actually broken across the important bit, so I need to order a new one.
You can "just" make out the break.
So I then turned back to the investigation of the timing chest.
First I dropped the push rods out.
The idea being that I can see if the noise happens without any interference from the valve springs.
Like this I can hold the rod in place to provide "some" resistance.
Being a numpty, I accidentally undid that blanking bolt in the timing chest cover, DAGNABBIT!
Bloomin' thing.
So - to make it a little more secure, I added a springwasher
My investigations were pretty low in results tbh, I still can't quite figure it out, but the old exhaust side cam follower IS a little loose on the spindle, and the roller wheel IS a little loose on it's bushing.... so I may just have to bite the bullet and get another one. Whether that slackness is enought to cause the clicking noise, I'm not sure, but its certainly a slackness that shouldn't be there. It could be that with the valve spring resistance, it's causing movement of the cam follower that is making the clack noises.
There was a concern that the spindle itself might be worn, but the new cam follower seems solid on the spindle, so I think that's ok.
I also took some measurements of the push rods so I can look into buying some stainless tubeing to make new ones.
[/url]IMG_20260529_191941_407 by James Bridge-Butler, on FlickrI think 1/32" wall thickness (info supplied from the Guru, who had a pair without caps fitted) is a mite thin, so I may try to get slightly thicker tubing if available.
The push rod ends are soldered on, so that's going to be a fun job swapping them over... yet more opportunity for conflagations!
Next time I try the timing chest I'm going to dribble some oil onto the cam follower spindle and the rest of the area, it could be that a layer of R is all it needs?
