ET: Engine (Twin) Mikuni carbs

[Ian Savage]

To bring my original question kicking and screaming back to the fore.
Thanks for all the comments and pics, also to John S and Brian C for direct contacts.
As normal for Vin owners I took in all the info and then went and did my own thing! I used an A61AS for the right side reserve tap and A77AS with an extra pipe silver soldered in, both attached to the taps with A22s (thanks Brian). I think it's worked out quite well.

 

[vibrac]

Very neat

 

[Bill Thomas]

Well I posed the question to Allens the Mikuni Gurus lets see what they say I would like to use the Mikuni if I can its my first choice.. as light as a feather to use and its carburation is superb....
I of course have some fall back there is a perfectly operating 34mm Mikuni but its a bit large
My second choice would be an old 276 I dont have a good one at the moment and a new one is silly money.
There are the two Monoblocks off the Alphabet twin that raced at Cadwell thats now sporting two 1-1/4 GP's we havent taken on the brake yet so these were back up, but after buying the GPs I cant see us going back.
so that makes Monoblocks my No three choice
Finally there is my stash of TT carbs but they need close inspection to find the unworn combinations and I would have to butcher a slide to get a tickover
I will struggle on with the Mikuni first to try and find the problem it is very strange I seem to have exhausted all the normal causes and I have not encountered the problem before

I would think you need a rubber type tip, Has the big one got one to try.

 

[Robert Watson]

OK Ian ---- hijacking you own thread back to its original intent will soon be banned if you keep this up, try not to do it again please and let us get on with our diversions..........................


My original rubbers on the flat slides didn't last too long and like others I reverted to a piece of reinforced hose that was the right size. When we did our big Europe trip in 2010 somewhere in the south of the UK in the rain it started running very badly and then on one cylinder. I managed to find a bit of cover in one of Tim's favourite petrol stations (supermarket) where I found one of the carbs hanging about on its cable and fuel line. One end of the hose seriously deteriorated, and fuel spilling everywhere. I managed to change the hose end for end and clamp it all back together, and carried on.
On rolling into Lubeck Germany a few weeks later, as we pulled into our evening accommodations it came off again, this time unusable. Out with the VOC "I need help" book (In fact Reinhard Augustine) and within an hour or two up turned Andreas Wagner (RIP). He duly traipsed me around to an industrial supply house where we found some absolutely perfect oil hose. I asked for enough to make 4 pieces, perhaps a foot. "By the meter only" I was told. I agreed to buy a meter but only if they would cut it a foot long! We were packed FULL. I changed the disintegrated one that evening and later on the other. Speculation from members on hearing this tale immediately blamed the fuel, and the fuel and of course the shite fuel. I did point out the the failed hose was if I recall a water suction hose and that it had been on the bike for many years and some 60 or 70 thousand miles and that I was really quite pleased with the service it had given. I now have two spares still sitting on the shelf.

 

[brian gains]

reading between the lines I should expect any manifold hose to have a limited life span due to the cursed ethel.
Spent some time as per Vibrac post trying to align carb (float bowl ) as level as possible to at least have a fighting chance that the floats and needle rise vertically rather than fighting against introduced yaw.
I still have faith in the Mikunis but in my particular situation just stuffed for space.
Ian, you are streets ahead of me with what looks like a very tidy set up.
I shall look into replacing inlet manifold hose but am concerned about not having a stepped internal diameter and the fuel charge not having smooth flow. Are there enough dimeter options with the hose that an inner sleeve could be glued in or otherwise turn up an ali' sleeve, no doubt ethel would eat one in nylon/polycarbon.

 

[BigEd]

Reading between the lines I should expect any manifold hose to have a limited life span due to the cursed ethyl.
Spent some time as per Vibrac post trying to align carb (float bowl ) as level as possible to at least have a fighting chance that the floats and needle rise vertically rather than fighting against introduced yaw.
I still have faith in the Mikunis but in my particular situation just stuffed for space.
Ian, you are streets ahead of me with what looks like a very tidy set up.
I shall look into replacing inlet manifold hose but am concerned about not having a stepped internal diameter and the fuel charge not having smooth flow. Are there enough diameter options with the hose that an inner sleeve could be glued in or otherwise turn up an ali' sleeve, no doubt ethyl would eat one in nylon/polycarbonate?

I have a pair of 28 mm Mikuni carbs on my Rapide. The material and build quality are first class and far better than most but not all Amal instruments I have used on bikes in the past. The Amal GP and TT carbs that I have had on various bikes over the years were much better quality. I am led to believe that replica Amal units built more recently are better made but don't have experience with these.
Interestingly the only problem I have had with them is that the rear carb very occasionally floods until given a tap. This has happened maybe four or five times in ten years of use. I'm going to check the float guides, etc as mentioned in earlier posts.
I have two quite bulky plastic fuel filters fitted so hosing up is a bit untidy but works OK. Ian's installation if nice and neat. I should tidy mine up but realise that once I've got something to work OK it often stays as it is. I'll have another look at it .... sometime.
I joined the carbs to the inlet stubs using homemade adapters, black rubber radiator hose and Jubilee clips. I got the hose from somewhere like Halfords or maybe a motor factor and just looked at and measured a few until I found one the correct diameter, cut the bends off and used some of the straight portions. I still have enough left to cut two new pieces. I might do this as the first two pieces have been on since 2008 and have done 50,000+ miles. Perhaps they don't make them out of the same material now.

 

[vibrac]

Bens BMW hoses I cut a bit off are silicone hoses from SAMCO I also used some red ones for my Scott radiator

 

[Ian Savage]

I think we Vin owners suffer from the belief that things last forever. We search for 'better' or 'different' solutions after some part fails after decades or 10's of thousands miles. Everything has a service life it just reached the end of it, so replace it. Once a spares man always a spares man!
Oh bu##er you've made me go off my own topic!

 

[Little Honda]

Well this is a rouge unit I have now examined the needle under a glass, checked the float height (15mm from gasket top face (where gasket would sit) to lever tip (28mm VM)
checked the lever pivot hole and the pivot pin, no binding, cleaned out the float holes, checked the pins lifted them up an down either with a light touch and then with a flick they always drop down
Held carb in hand turned on petrol it leaks in a steady stream from 10 degrees each side way and twenty odd front to back.as well as its normal horizontal level
filled up bowl with petrol floats bob up like it was a fishing day on a smooth lake
petrol is Esso premium this months.fitted a filter but its all as clean as can be.not much of a head in the tank
I am a bit lost now, I have a 34mm somewhere I will see if I can replace the immaculate valve assembly I already have with another immaculate one!
This is the first time I have had any problems with a Mikuni (and I have had a lot) and this one has been on for two years I remember it worked straight from the box

From memory, I assume, that yr angles side- and forward are the operating limits, as claimed by Mikuni.
It´s not the carb angle in a solo bike, as long, as it´s fitted as it should. It may be due to the slow hardening
process of the rubber connector, which hardens by time, temperature changes and fuel used. On all V4- Hondas their rubber fittings harden to brittleness within 30.000miles. You cannot refit their 4 carbs onto
these old hard rubber studs without warming them up to soften timewise for refitting.
Imagine, how the fuel surface looks inside yr carb, while you cannot recognize anything in yr rear mirror!
Open the throttle a bit less, and no leaks - if not anything fell apart inside yr carb!

 

[vibrac]

The angles I quoted were the angles at which the fuel still poured out at the same rate as horizontally which is its normal position as I said above I dont have rubber fitting I have a silicone hose one which shows no use after two years
the Mikuni is getting one last test on the bike I have increased the float level to 20mm and then if that fails its the monoblock I cant waste good riding weather I will set up a bench test for the Mikuni later on its been a good carb and i dont want to say its beaten me