Are 47 year old carbs worth rebuilding?

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robs ss said:
My 376 monoblocs (1964) came to me with those old pot-metal slides.
I suspect, but don't really know, that brass slides were an owner retrofit.
Click to expand...

Quite, I think I was getting confused with other carbs. Hence I deleted the post, seemingly just as you were replying to it !
 
robs ss said:
I think that earlier carbs (TTs??) came with brass slides
Click to expand...
Still wondering, just as everyone (nearly) agrees that Mikuni's are better than the old concentric's and maybe the new ones too, was there an advantage to the monoblocks in terms of metering fuel from idle on up. Or was the advantage on other terms such as wear characteristics?
 
illf8ed said:
From Wikipedia
The Concentric design with non-protruding float chamber was both slimmer and shorter, enabling easier mounting to two-port cylinder heads. Left or right sided carburettors could be produced from basically the same die-casting, and the design was compatible with up to 40 degrees of downdraught.
Click to expand...
Sorry to have missed your post about the monoblocks. Thanks for the info.
 
From the Amal/Burlen website-

The Amal Mark 1 Concentric carburetter was introduced onto motorcycles during the 1967 model year. A lighter and more compact carburetter than the preceding Monobloc, the name derives from the design of the float chamber around the main jet. It is an arrangement intended to reduce the effect of fuel surge in the float bowl under cornering and transient throttle conditions.
 
Lund bores the carb bodies although not sure if because of wear. More likely due to out or round from over tightening. Just buying new slides may not work on used concentrics if the body is warped.
 
I had the Lund bore and sleeve done to my Concentric 930 mounted on my Triumph Tiger. It worked perfectly
for 15 years and 8000 miles but always leaked fuel until one day the slide began getting stuck and not returning to fully closed. I
guess the body was warped. So I bought and installed a new premier. It works real good.
 
lazyeye6 said:
I had the Lund bore and sleeve done to my Concentric 930 mounted on my Triumph Tiger. It worked perfectly
for 15 years and 8000 miles but always leaked fuel until one day the slide began getting stuck and not returning to fully closed. I
guess the body was warped. So I bought and installed a new premier. It works real good.
Click to expand...
From what I understand if the two flange nuts are tighter too much the carb body will distort. If you have a slide hanging up unscrew these nuts and see if the slide then drops.
 
Joe Schlaberdowski said:
Still wondering, just as everyone (nearly) agrees that Mikuni's are better than the old concentric's and maybe the new ones too, was there an advantage to the monoblocks in terms of metering fuel from idle on up. Or was the advantage on other terms such as wear characteristics?
Click to expand...
On the issue of fuel metering, or rather supply, one of the weak points of the twin monoblocs (on Nortons anyway) is the RH carb is chopped (no float bowl) so the LH carb float bowl has to supply fuel to both carbs.
Can be an issue - for example if the bike was on the side stand the RH cylinder would suffer from apparent low float level.
Probably why my 650ss doesn't have a side stand!
I wonder if this is true of all twin carb, monobloc Norton twins? Probably includes Manxman, 650ss, Atlas and maybe 88ss, 99ss?
Anyone know?
 
robs ss said:
On the issue of fuel metering, or rather supply, one of the weak points of the twin monoblocs (on Nortons anyway) is the RH carb is chopped (no float bowl) so the LH carb float bowl has to supply fuel to both carbs.
Can be an issue - for example if the bike was on the side stand the RH cylinder would suffer from apparent low float level.
Probably why my 650ss doesn't have a side stand!
I wonder if this is true of all twin carb, monobloc Norton twins? Probably includes Manxman, 650ss, Atlas and maybe 88ss, 99ss?
Anyone know?
Click to expand...
The 1966 Matchless Norton I had years ago had monoblocs and both had floats as I recall. This was 1970. Even then people were saying put a Mikuni on it, which I did along with a hot cam (young, inexperienced -- and dumb). But I never got it completely sorted out before the disaster with the cam ring on the Lucas mag. So I can't say it really ran any better after all that.
 
I was thinking of buying some new carbs for my Triumph Bonneville. I have put about 60,000 miles on that bike since about 1981. When I put that bike together I had some spare parts and I put the bike together using the best parts I had. I used the better carbs of the sets I had. After 60,000 miles they were really worn. I had replaced the needle jets a few times but the main problem was the slides were wobbling around all over the place. The bike would still run well but when you slowly opened the throttle the engine would slow down before it speeded up. You had to open the throttle quickly to get by the dead spot. I was used to that it wasn't bad. Last year I found the spare carbs in a box where they had remained un used over the years. They are great, hardly any wear. I put them on my bike (with new gaskets and needle jets) and it runs great again. Of coarse if I had bought new carbs that would have been great to. I think a lot of people who buy new carbs really only need new needle jets. I have never had any problems with pilot jets plugging because I drain the gas out when I am storing the bike.
 
"lazyeye6 said: ↑
I had the Lund bore and sleeve done to my Concentric 930 mounted on my Triumph Tiger. It worked perfectly
for 15 years and 8000 miles but always leaked fuel until one day the slide began getting stuck and not returning to fully closed. I
guess the body was warped. So I bought and installed a new premier. It works real good."

illf8ed said:
From what I understand if the two flange nuts are tighter too much the carb body will distort. If you have a slide hanging up unscrew these nuts and see if the slide then drops.
Click to expand...

You are most correct. Triumph, to their credit, designed in a fat little "O" Ring between the nut and the carb body to compensate for
this. No, the slides still bound up after the nuts were loosened.
 
robs ss said:
On the issue of fuel metering, or rather supply, one of the weak points of the twin monoblocs (on Nortons anyway) is the RH carb is chopped (no float bowl) so the LH carb float bowl has to supply fuel to both carbs.
Can be an issue - for example if the bike was on the side stand the RH cylinder would suffer from apparent low float level.
Probably why my 650ss doesn't have a side stand!
I wonder if this is true of all twin carb, monobloc Norton twins? Probably includes Manxman, 650ss, Atlas and maybe 88ss, 99ss?
Anyone know?
Click to expand...

True that the float level changes when the bike is leaned, but only if the bike is stationery, or not moving. At speed, in a turn, the fuel level remains exactly in the same geometric relationship within the float bowl, as it is when level.

Pilots will understand this effect .... The physics of an aircraft in a turn is the same as a motorcycle in a turn. The turn and bank indicator in an aircraft is a glass tube shaped in an arc with a ball inside the tube. The ball rests at the low point of the arc in level flight. In a turn, the ball is higher than the low point, but in the same geometric relationship as it is in level flight.

Another example ..... If you were in the cabin of an aircraft in level flight, and had a glass of water on the seat back drop down table, the surface of water in the glass would be parallel to the aircraft floor, and the earth's horizon. In a turn, the table surface would be parallel to the aircraft floor, but at the bank angle to the horizon. What will the surface of the water in the glass be? Ans: at the same geometric relationship within the glass as it was in level flight .... that is parallel to the aircraft floor.

As robs ss states, the principal deficiency of dual Monoblocs, with a chopped float bowl on one, is fuel delivery. In essence, two carbs have to be supplied by ONE needle and seat. The typical seat orifice is 0.125 inch and this is not large enough to satisfy the appetite of a hot 650 engine, or a warm 750.

My Atlas gets around this problem with a 0.200 inch seat orifice, with an appropriately sized needle.

Slick
 
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texasSlick said:
My Atlas gets around this problem with a 0.200 inch seat orifice, with an appropriately sized needle.

Slick
Click to expand...
Where did you get the seat - or did you modify?
Does the stock (Viton tipped) needle work with it?

Rob

ps: sorry for going off thread - will stop now
 
lazyeye6 said:
No, the slides still bound up after the nuts were loosened.
Click to expand...
I found this on my monoblocs and so measured the bores which were indeed oval.
The fix was to use a vise, with soft timber to protect the carb, then carefully squeeze until the ovality is about opposite of what it was like. It took me several goes the get it right.
When it was right the new anodised slides ran up and down like new.
Worth a try!
Very careful with flange nut (or in my case - screw) tightening.

Rob
 
robs ss said:
Where did you get the seat - or did you modify?
Does the stock (Viton tipped) needle work with it?

Rob

ps: sorry for going off thread - will stop now
Click to expand...

Strangely enough, my Atlas came with an oversized needle and seat, when bought new from my dealer. I suspect the bike was a special build for Berliner's evaluation, and Berliner installed the after market needle and seat. These were made by a small company. Webco, or Webbco, now defunct.

For (sorry also for going off thread) more info on fuel flow vs. needle orifice size, see:

https://www.accessnorton.com/NortonCommando/flow-measurements-on-ewarts-fuel-tap.21225/

Edit: Amal makes an alcohol needle, part no. 622/279, which has a 0.200 inch dia. at the base of the cone. One could in theory bore out a standard seat to 0.156 or 0.188 dia, and lap in the needle.
Slick
 
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illf8ed said:
From what I understand if the two flange nuts are tighter too much the carb body will distort. If you have a slide hanging up unscrew these nuts and see if the slide then drops.
Click to expand...
I was told the torque pressure on these 4 nuts should not exceed 2 lbs. of. That's why I use Nylocks so they don't want to back off.
 
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