E: Engine 1951 Comet Carburation

[Chris Launders]

I don't usually comment on carbs because I have been rubbish at setting them up but I have Mikuni's on my Norvin and Mk 1 Concentrics on my "C" twin and Norton Atlas, altering the settings on a concentric is relatively easy and have now got them something like but there seems to me to be an extra layer of complexity with the Mikuni because of the many different needles and I'm reluctant to even touch them.

If I was fitting new carbs I would try the Premiers because they seem to me to be even easier to adjust and now having overcome the rapid wear problem an even better option.

A question for Mikuni users, what are yours like for starting, the Norvin always has needed three or four kicks (it did when I had Amal MK2s on as well and it's on coil ignition) whereas the "C" twin with the MK1s and BTH mag once tickled starts first kick, the Norton's the same.

 

[ericg]

Tis a NorVin Big Ed, beautifully done, silly man !
bananaman.

Thanks Marcus! Take care.

 

[greg brillus]

like all later designs of carburetors either for cars or bikes, the manufacturers made them run more efficient and lean.......even the stock TT/ XT 500 Yamaha's could be a bitch to start if you didn't use the choke. Problem with most bikes is you tend to open the throttle as you come down on the kick start lever and this definitely does not help with later type carby's especially on a hot motor. The bypass chokes on these later carb's is really just a tinny carb on its own.......so the enrichening feature of the "Tickler" is lost. These are things you need to remember when using either Mk 2 concentrics or Mikunis on early engines, where they had not only a tickler but an "Air dam" for the choke device..........both of these can readily over-choke an engine weather hot or cold. Of course the good old Dellorto is really the winner on account of it's accelerator pump that directs a fine jet of fuel down the mouth of the carb, and this works very well at enrichening the mixture for starting...... But the main function here was to overcome a huge flat spot for an engine sporting a pair of 40 mm PHM's that ordinarily ran perfectly with a pair of 32 mm ones.

 

[Chris Launders]

I have a pair of 32mm Dellorto's I've considered fitting in place of the Mikuni's but no idea where to start with the jetting.

 

[BigEd]

Tis a NorVin Big Ed, beautifully done, silly man !
bananaman.

Dear Marcus,
I was looking at the small picture of Eric on a bike in his avatar. It looked shiny with tele-forks so (wrongly ) assumed it was an Egli. Oops.

 

[Bill Thomas]

I have a pair of 32mm Dellorto's I've considered fitting in place of the Mikuni's but no idea where to start with the jetting.

With the main jet Chris, I think you would be looking at 140 or 150 ?? a lot lower than amals !!,
I got some ideas by looking at what Guzzi's use.
I think I am only about 165 on 36mm jobs.
On mine I also blocked off the pumps, They made it too rich when I put smaller pistons in.

 

[Roslyn]

New Pre Monobloc and Monobloc are available from Burlen and it's not because you've had a couple of bad experiences with probably old clapped out ones that all the new ones are crap. This year only I've fitted about 8 bikes with new Amals and not a complaint from my customers so far!
I have no car or modern bike, my only means of transport are my old Brits, they are ALL ridden intensively, rain, snow or shine with Amals, no modern clutches, disc brakes, modern forks, etc.
Performances are just what they should be for old machines.
If I'd want modern performance and behavior I'd just purchase a modern vehicle.
Ok, in my eyes Miks are very, very uglies.

I run two old triumphs (Val Page 1936 Mk5, 1950 Speed Twin on worn out pre monoblock carbs plus a 1957 Thunderbird with a Sinners Union and they run and start well and tick over like swiss watches and it all about starting and usng the bikes daily

 

[Roslyn]

I don't usually comment on carbs because I have been rubbish at setting them up but I have Mikuni's on my Norvin and Mk 1 Concentrics on my "C" twin and Norton Atlas, altering the settings on a concentric is relatively easy and have now got them something like but there seems to me to be an extra layer of complexity with the Mikuni because of the many different needles and I'm reluctant to even touch them.

If I was fitting new carbs I would try the Premiers because they seem to me to be even easier to adjust and now having overcome the rapid wear problem an even better option.

A question for Mikuni users, what are yours like for starting, the Norvin always has needed three or four kicks (it did when I had Amal MK2s on as well and it's on coil ignition) whereas the "C" twin with the MK1s and BTH mag once tickled starts first kick, the Norton's the same.

Erm Have you put fresh fuel in the tank as this modern stuff goes off? and when you check you have not flushed any muck in the carb and clean the jets and check if the float is working as should as most problems are down to the float not working right.

 

[Roslyn]

Hello all,
I have a pre monobloc fitted to my comet ( series C) which gives rather an erratic tickover when it’s very warm. I would prefer to fit something more modern in the carb dept and wonder what advice members could offer. Amal concentric, Mikuni, Delortto ? I do have good results with the concentric on a Venom but that’s a different animal.
Regards.
David

One other thing check the points HT leads plugs etc if the bike has been stood for a long periods even if stored in the dry as they do get damp and corroded.

 

[Monkeypants]

An observation regarding Mikuni VM equipped Vincents- after many thousands of miles touring alongside Vincents using VMs, it's undeniable that those bikes are tricky on tepid restart.
There are a bunch of these here on the West coast and so far every bike I've observed that is fitted with VMs, including my own, has this problem. So there's often a whole lotta kicking going on after lunch on restart of the VM equipped bikes. Not always, but you just never know. Cold start is generally good as is hot restart, tepid, not so much, although the problem is intermittent.
The other issue with the Mikunis is sticking floats. I've had two new sets on the bike and considerable problems with sticking floats, both sets. I've heard other riders complain of the same.This is with inline fuel filters fitted, so no gunk is getting to the carbs.
Other than that, the VMs work well, nice idle, seem to give very good fuel economy for most.
The Amal Premieres on the other Rapide work as well as the Mikunis for running and do not have the tepid restart problem.
Here is a photo of the anodized slide at 15,000 miles. The carb bore is polishing nicely.
I don't think I'll be wearing these carbs out!

Glen