Maybe we are all confused and I thought the old Monobloc was simple carb to tune...
It is, but is is made a lot easier when we are all talking about the same thing. You introduced something "out of the blue" that was not relevant to the discussion: Is the new Concentric viton/brass needle a direct replacement for the Monobloc's original plastic needle? The answer is Yes!
Is the original Monobloc needle different from the Concentric? Yes. This is why Barry and Geof had to change the specs for the needle to have it work in both carburetors.
Besides not being my first rodeo we have been a factory direct AMAL distributor in the US since 1972. We sell, and support dealers, selling over a thousand AMAL carburetors a year. We have raced, especially drag and road raced, using AMAL carburetors since 1959. You can see one of my Triumph 500's in Racing Triumph's in America. A lot of the tuning specs you see for Mikuni carburetors today were worked out between John Calchio (JRC Engineering) back in the 1970's his is when we manufactured TR6 intake (for JRC) and T120 manifolds for Triumph Mikuni kits.
I run 2 Norton twins, one 99 with the orriginal 1960 376 and an Atlas with a replacement 376 bought 15/20 years ago . I replaced the brass float on the 99 carb with plastic years ago but recently (after the introduction of eth fuel!) went back to brass with a brass needle ,suffered a bit of rich running so now running new alloy needle ,better but not perfect ,slight richness off idle ,had 20 thou turned off the valve as experiment , not really helped, but bike is useable. The Atlas kept flooding on start up ,found the newish plastic float too big and catching on screw casting tunnel if float chamber allowed to get low on fuel. ,Filed off float and casting did not solve problem, fixed by making brass sleeve to fit on float stop in base of chamber to restrict float drop. Probably also restricts max fuel flow thro valve but don't know and no problem yet. Early monos generally NOT fitted with fiber washer under valve, but there were exceptions IE single carb Atlas which needed a raised fuel level for some reason. Eth fuel has a different SG to old gas and seems to mess up SOME settings probably where they were already a bit marginal. Seems to be big difference in fuel quality in uk,, cheap supermarket eth fuel runs well but ruins alloy parts ,tanks and blocks carbs, Top grade Esso (no eth) solves those problems but runs rich mostly (but not always-- probably dishonest practises going on --mixing??). The new (15/20 years ago!) 376 had more slop in the slide than the worn out Concentric it replaced. Lost faith in new stuff around that time. May be better now though. Monos WERE easy to tune ,but I don't think that applies now, Fuel injection systems can cope with wide variety in fuel specs and the suppliers are now taking advantage of that puttng wide range of products in the mix.
I looked in my Monobloc box and found 4 nylon needles, 1 mono and 3 Concentric. They would all have been bought in Birmingham during the 80's from the Brit bike spares shops of the day, C&D Autos, Vale Onslows etc, when I was using a B40 for commuting with a 389 carb.
They measure from tip of dome to tip of needle.
Brass 0.640" (but hard to measure with the soft rubber tip)
Have no idea if they are genuine but they all have the same 3 marks on the 3 flat sides from a chuck matching Tribsauk's needle on the far left. However at the time a lot of OE parts makers were not bothering remaking Brit parts so the part dealers were having parts made at alternative suppliers of which there were a lot in Birmingham.
It shows why I had no issue moving from a Mono needle to a Conc as the nylon are within a +/- 5 thou tolerance band so my fuel level did not change much. The brass one is the largest and outside the +/- 5 thou but no idea what tolerances AMAL worked to.
Does it explain why Tribsauk has 3 longer Mono needles, no it does not. Did AMAL make some longer needles to allow a washer on the needle seat to carb joint ? Is my Mono needle short because it was made with Conc needles and the operator forgot to change the length settings? Or did AMAL shorten all Mono needles when they went to Conc ?
Last edited by kommando; 07/10/1712:42 pm. Reason: added brass dimension
Dont embarrass yourself John, the brass needle was not a roaring success! It has been a bit of an embarrassment really. Would you recommend it?
Opened up the computer this am, read your post and almost wet myself laughing.
I guess I must be in good company as my dealers have bought just under 5,000 brass needles to just under 200 for the aluminum one in the past 4 years. Not only recommend it but use it.
There has been three people who made the Monobloc: IMI AMAL Birmingham, then Grosvenor Works in Tottenham, North London 1993-2003 (with funds from David Holder) laid down new tooling for 276, 289 and Monobloc and now Burlen Fuel Systems from 2003 to present. If you bought a new Monobloc 15 to 20 years ago it was most likely produced by Grosvenor Works as Burlen didn't start making the Monobloc right away.
After a series of unfortunate deaths relating to slides sticking, one right here in Salem Mass. at Eastern Cycle Salvage, AMAL increased the clearance on the slide to a point where only the real hamfisted mechanic could bend the carb body enough to get it to stick. People confused this with poor quality, but with certain adjustments they made to the body to compensate for some additional air, you wouldn't know the difference. As much as .004" clearance is within spec.
I read a lot here about the bike running good one day, and not another. Sometimes for no apparent reason it will change during the day. It will always be that way! The atmosphere that plays an important part of how these carburetor work is always changing. If the barometric pressure changes from a front passing or you are experiencing a change in elevation how the bike runs will change. Cold air contains more oxygen. Warm air less. The atmosphere at sea level contains more oxygen, at 5,000 feet less. All will change how much fuel the carburetor will supply and thus how the engine responds. The only way for it not to do that is if you installed an computer engine management system and sensor to provide all of the data. How well the engine performs in these changing conditions is made worse if you don't have the tuning right. If it is near to being to lean, or rich, environmental changes will be more evident.
Have you ever checked the orifice of the needle jet for size?
And any of the "cures" related to the aluminum needle and going on about needle weight, with the Concentric would not apply to the Monobloc. The Concentric is a bottom feed, while the Monobloc is top feed.
I have the carb back on the bike. Some day I may file a bit more of the lead off of the needle because I have to push the tickler all the way down to flood the carb. Or I might buy a new needle seat holder and new needle. But my next question is where do I find the instruction on how to tune a Monobloc? The low speed jet works best when fully closed and I know that isn't right.
Please do not believe anything I write. I am a hack but I like to guess the right answers.
I have the carb back on the bike. Some day I may file a bit more of the lead off of the needle because I have to push the tickler all the way down to flood the carb. Or I might buy a new needle seat holder and new needle. But my next question is where do I find the instruction on how to tune a monobloc? The low speed jet works best when fully closed and I know that isn't right.
I understand thrifty, honestly I do. However, stop mucking about and spend a couple of dollars on correct replacement parts so that you are not always second guessing whether the tuning or operational issue you have is or is not connected to the last bodge you made.
Right now the AMAL Premier Concentric carb is the ticket. All sorts of upgrades from the original and still under 200$. What a deal. The monoblock will be a bit more and just more of the same you already have. However, after saying that, I have to say that I have a monoblock on my '57 Tbird and it runs damn near perfect. I'm having some float needle issues right now that i may have to solve with a new needle seat/banjo fitting. But I get about 60mpg, the thing starts easily and idles like no other Triumph. At this point, I'm leaving well enough alone. I run the brass float and setting the level is easy as you can just wrap a small piece of brass sheet around the abutment that contacts the needle. I run a viton tipped brass needle. I made a fitting for the gland nut for checking float level with the clear plastic tube. On Thunderbirds, the idle jet nut sits right up against the coil and makes removing the nut a real chore. My Monoblock is an original one from that period with a hard chromed slide replacing the worn out cast one. I find the Monoblocks to be good carbs when sorted. You find them with distorted bodies, stripped threads, and worn bodies and slides often. Find a good body and start from there. All the bits are available and you can build yourself and good carb from them.
Bikes 1974 Commando 1985 Honda Nighthawk 650 1957 Thunderbird/T110 "Flying Tiger" Antique Fans: Loads of Emersons (Two six wingers) plus gyros and orbiters.
I broke down and bought a brand new Monobloc 376. I'm done trying to figure out what other mistakes the previous owners had made. The old one will be listed on eBay some day. But the question for today is about the choke slide. This carb isn't for a Triumph, it is for a Norton 500cc single cylinder. It didn't have a choke. You set the compression release and retard the spark and hold down the tickler for 5 seconds and give it a kick. No choke. It never had one. The choke slide was not in the old Monobloc. The hole where the cable would go was capped off. Do I want to remove the choke slide from the new Monobloc and cap the cable hole? Or will this cause a vortex in the hysteresis and have the flay-rod come askew on the treadle or some other bad joo joo? Or should I hook up a choke cable and just never use it?
Please do not believe anything I write. I am a hack but I like to guess the right answers.
1) If you never needed a choke, why would you want one now? 2) So, you are going pass off your troubles on some poor, unsuspecting SOB on eBay thus perpetuating the reason I won't buy a roll of toilet paper on flea bay. Still can't do a smiley face. : ) Hey, that's pretty good and a first.
1968 T120R 1972 T120RV Any advice given is without a warranty expressed or implied.