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Concentric Mk1 Fuel Starvation

b.l.hayward@tesco.net

Website User
Non-VOC Member
BACKGROUND:Just finished the rebuild on my twin (basket case so was not running previously) and have fuel problems (I think). I have no previous experience of Concentrics Mk1's so don't know if they have any foibles or age problems! Have used 276's, Monoblocs, TT's and GP's and have NEVER had the problem below. Bought the Concentrics from a club member several years ago and have dismantled and cleaned them prior to fitting but have not renewed anything.
SYMPTOMS: Turned on the juice and did the usual carb tickle (3-4 pushes) and started kicking - nothing. This went on for a while with nothing happening - not even a cough! Tickled the carbs some more (actually until petrol started to drip from the float bowls) and it fired up, ran for a bit on one and a half cylinders and then settled down to life as a 500 single! If I blipped the throttle it seemed that both cut in. Felt the pipes and No2 was hot and No1 was cool. Tickled the carbs again and found the float bowl for No1 was empty (ie the tickler did not make contact with the float bowl until I tapped it quite hard up and down against the stops). Fired it up again, ran as a twin for a while and then went back to being a 500 - But with lots of spitting and coughing from No1. If I tickled No1 whilst it was running, this stopped and it chimed in. Occasionally, No1 would be the hot pipe and No2 the cool one! So it seems to me that petrol is not flowing easily into the float bowls, somehow - any thoughts other than I wouldn't use Mk1's!
OTHER INFO: I have fitted new Spares Co fuel lines and taps and the mag has been rebuilt and retested ok (I originally thought I had an intermittent spark causing the problem but I don't) - so I think I am left with a petrol feed problem which should be so simple to fix it's driving me round the bend!
 

Henry Martini

Well Known and Active Website User
Non-VOC Member
Concentric MK 1 Fuel starvation

Hi, As a quick guess it sounds like you need to look and check the float levels but I cant tell you what they should be.
Regards, Henry Martini
 

methamon

Well Known and Active Website User
Non-VOC Member
Concentricity!!!!!!!!!!!!!!!

Hmmmmmmmmmmm. What about; Turn off both petrol taps. If the bowls are fitted with drains, drain the bowls, alternatively remove bowls and drain / dry out; replace & re-connect bowl banjo bolt. Turn on petrol do not touch ticklers, leave for a minute. Turn off petrol, remove bowl banjo bolts and tie pipes out of the way. Both bowls should be full of petrol. Carefully remove bowl screws whilst suppporting the bowl and see if the bowl is indeed filled with petrol. Repeat for the other bowl. If one of the bowls either doesn't have enough or any petrol, possible causes are (not enough fuel in tank - are your carbs fed from one tap each or linked?) blocked or obstructed pipes, blocked filter at banjo, blocked banjo blockage above float needle or as the other gent mentioned incorrectly set float height but I recollect that the floats are plastic and the needle prongs are non adjustable?:)
 

overthehill

Active Website User
Non-VOC Member
i run mk1 concentrics and have been very happy with them - I also have to tickle them until petrol comes out before starting from cold so this is normal.

It sounds like a sticking float which only drops down when you press the tickler.
have you checked that the gasket between the bowl and the body is not catching on the float?
also check the needle valve as the rubber tip can perish - although this usually results in flooding it could cause it to stick closed.
If you run standard fuel lines you do need both taps open in my experience - better safe than sorry anyway - the last thing you want to do is starve one pot of fuel while overtaking a GSXR down Glen Coe.:eek:
 

sidmadrid

Well Known and Active Website User
VOC Member
Had similar symptoms some time ago after a rebuild.
My mk1 concentrics were fitted with brass float needles with Viton tips, after much head scratching I concluded that the brass needle valves were too heavy, the fuel was always shut off. Depressing the tickler pushed the float down and allowed fuel into the float chamber, but as soon as you took your finger off the the tickler the fuel was cut off, no matter what the level of fuel in the chamber. {Hope you under stood that.}
Anyway I fitted nylon float needles and problem solved. sid
 

ksbundy

Active Website User
VOC Member
Concentric carbs

There is an article called "Bushmans Carb Tuning secrets" on a website called www.jba.bc.ca It explains the operation of Amal concentric carbs very well and also includes information on setting the float heights.
Hope this helps
KS
 

biggles

Website User
Non-VOC Member
Hi.
Thanks found it this time good article. I am however not overly fond of Mk 1s
As they don't last very long especially if there is no air cleaner fitted.
I note however that Surrey Cycles now supply a plated brass slide for them which should go some way to overcoming the problem.
Cheers.
Biggles.
 

mercurycrest

Well Known and Active Website User
VOC Member
Hi.


I note however that Surrey Cycles now supply a plated brass slide for them which should go some way to overcoming the problem.
Cheers.
Biggles.

Hi Biggles,
You wouldn't have a website for them would You? I tried to Google
Surrey Cycles UK from here and got pages full of petal cycle shops. Sounds like plated brass slides are just what I need for Wifey's Outfit.
Thanks, John
 

b.l.hayward@tesco.net

Website User
Non-VOC Member
:) Thought it was about time I updated on the thread...Still no absolute solution, BUT the twin runs ok when not on pilot - still seems to tick over periodically on 1.5 cylinders 'till I pick it up on the throttle (have played with pilot air setting and throttle stop according to KTB). But what I HAVE found is that if I remove the pilot jet from time to time the hole in the jet seems to be blocked with a small,spongy blob the size of a pinhead - a bit like Evostik that has gone off in petrol. Now I had my tank fully refurb'd....tank sealant? I also took out a petrol tap and a larger blob of the stuff was wrapped around the top of the thread.This is my current thinking so I am hoping that the passage of time will eventually stop disolving off the inside of the tank which I know was coverd with seal during the refurb.!
 

b'knighted

Well Known and Active Website User
VOC Member
impossible to flood with brass float needle

Had similar symptoms some time ago after a rebuild.
My mk1 concentrics were fitted with brass float needles with Viton tips, after much head scratching I concluded that the brass needle valves were too heavy, the fuel was always shut off. Depressing the tickler pushed the float down and allowed fuel into the float chamber, but as soon as you took your finger off the the tickler the fuel was cut off, no matter what the level of fuel in the chamber. {Hope you under stood that.}
Anyway I fitted nylon float needles and problem solved. sid

It became unworthwhile trying to start my Comet a while ago. I have just recommissioned it with a resleeved barrel and low expansion piston. It has been extremely hard to start and changing to a plastic float needle produced an overflowing carb. For MoT yesterday I turned the petrol on 2 hours before I wanted to use it. After kicking for quarter of an hour I made a clip to hold the tickler down but it still wouldn't flood so I changed float chamber/float and needle to use the complete plastic needle set up. It left a big puddle on the floor but did become startable. By the time I got to the MoT it seemed to have settled to working OK and today it is behaving exactly as it should. Turn petrol on - tickle for two or three seconds (until flooding visible) - Start first or second kick. I have ridden it every day this week and this tip from Sidmadrid seems to have turned it into a rideable bike. Many thanks! Ian
 

roy the mechanic

Well Known and Active Website User
VOC Member
fuel starvation/concentric

You seem to have tried a lot of "answers" now try a silly[simple one] make sure that the breather hole in the fuel cap is not blocked with polish or other detritus. It's surprising how many folks get caught-out on this one. Roy.
 

mr.hutch

Well Known and Active Website User
VOC Member
:) Thought it was about time I updated on the thread...Still no absolute solution, BUT the twin runs ok when not on pilot - still seems to tick over periodically on 1.5 cylinders 'till I pick it up on the throttle (have played with pilot air setting and throttle stop according to KTB). But what I HAVE found is that if I remove the pilot jet from time to time the hole in the jet seems to be blocked with a small,spongy blob the size of a pinhead - a bit like Evostik that has gone off in petrol. Now I had my tank fully refurb'd....tank sealant? I also took out a petrol tap and a larger blob of the stuff was wrapped around the top of the thread.This is my current thinking so I am hoping that the passage of time will eventually stop disolving off the inside of the tank which I know was coverd with seal during the refurb.!

I think you have probably solved the problem yourself, :)
I've had similar in the past and all down to minute bits of "gunge" blocking small fuel-ways, however I'm not sure about the "dissolving off", you refer to, as all the petrol sealing compounds I've ever used and there have some different makes, in different bikes, all needed to "cure" before adding petrol, the times varied, by ambient tmperature and actual application, I would not have expected any "bits falling off" afterwards, I wonder if there's something there worth looking at, as such small items can be major problems, untill removed.
Anyway, I've been running Mk1's on my outfit for fifteen years quite happily, without any major problem, although I did fit K&N filters at the original rebuild, I still think it was a wise choice, as there is very little wear on the slides, compared to the ones I replaced,
go carefully.
 

tractorman414

Well Known and Active Website User
VOC Member
concentrics transformed

I know this is an old thread, but I came across it when searching info on Mk 1 concentrics. I fitted 32 mm carbs to my Shadow, some years ago, after Bob Dunn had ported the heads and flanged adaptors, but I never got it to run well. I had other problems, resultant of fitting Carrillo rods and not balancing the crank [32% balance factor, the vibration was evil], however, that solved and the bike back together, I carried out the mods referred to in both the "Bushmans carb tuning secrets" http://www.jba.bc.ca/Bushmans Carb Tuning.html
and "performanceindian". http://www.performanceindian.com/VinAmalMods.html

The bike is truly transformed.

If you don’t want to read the full details, I would briefly summerise.

Set the float levels correctly
install interchangeable pilot jets in the roof of the float chamber
drill out the fixed pilot jet. [Easier from fuel side, so fit blanking screw opposite air screw]
cut away the back of the spray tube

I used the basic setting recommended by performanceindian

Bernard Jones
Dorset Section
 

Bill Thomas

Well Known and Active Website User
VOC Member
Carbs/Vibration

Hello Bernard,
Thanks for the tips on carbs, I didn't know I had a problem but I might have after grinding bits off them, !! I can't resist a tuning mod'.
But what about Vibration, My bikes have always had that problem, They have always had at least 9 to 1 pistons I know 7s are better but I like the extra grunt !, It didn't matter too much when I was young but with a bit of arthritis in the hands It's a Pain just hanging on !.
Can you tell us who fixed it and how much.

Cheers Bill.
 

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