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E: Engine 1951 Comet Carburation

Chris Launders

Well Known and Active Website User
VOC Member
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.
 

greg brillus

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
VOC Forum Moderator
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 :oops:) assumed it was an Egli. Oops.
 

Bill Thomas

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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.
 

Glenliman

Well Known and Active Website User
VOC Member
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
 

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Glenliman

Well Known and Active Website User
VOC Member
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
They will run quite well on worn slides as long as one replaces the needle jets now and then.
I had very worn Concentrics on the Rapide. It started to burn a tremendous amount of fuel, so I installed new Premieres on it, problem solved.
Afterward it occured that clapped out slides should lean things out, not make the bike run rich. The poor mileage wasn't due to worn slides, it was due to worn out needle jets.
A few years later I had the same scenario with a Norton twin. That time I just replaced the needle jets and fuel economy went from 35 mpg back to 50-55 where it normally is. And the bike runs fine with quite loose slides.

Glen
 

Bill Thomas

Well Known and Active Website User
VOC Member
I have had bad Tepid starting for years, Not with the Dellorto, But that needs choke, With the fast idle.
I have found they need more fuel !!, Concentrics, Mk1 and 2 so Flooding or choke with the Mk2 Just for a few seconds then put the choke off.
Being a Mechanic all these years, I could not let myself flood a warm engine, But that is what I have found,
Maybe the petrol ??.
 

Chris Launders

Well Known and Active Website User
VOC Member
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.
I has been the same since I built it almost 20 years ago, with both Amal MK2s and the Mikuni's, it has a twin points set up and Dyna coils , I even had twin plugs in at one time, I think the main problem is that as it's a chopped engine the engine only turns over once on the kickstart and isn't drawing enough fuel through the cold start system of those carbs, whereas I can flood the MK1s with the tickler.
 

Roslyn

Well Known and Active Website User
VOC Member
Yes Nulli, when they developed the Premier carbs Burlen have eliminated the main weaknesses of the Concentrics, that is relatively high wear rate of the throttle valve and body and pilot orifice almost impossible to clean out when clogged up.
As they are on the market for at least ten years now I thought everybody in our little world was aware of their existence!
Thanks anyway Nulli.
My point is that every time someone enquire about carburetor problems the ultimate solution for some seems to be: "fit Mikuni".
I just want to counteract this!
Fit a new Amal instrument since they are available (lucky us) and virtually indistinguishable from a period one. The carb problems will be cured (unless they are actually ignition ones!) and the machine will still look and perform like an old British Bike!
But this is just my opinion, in the end it's up to the owner to do what he like with his machine, of course.
I ride an old 1950 Speed Twin sprung hub and does well for a old bike not bad at 80-90 mph have done over 100 mph on it but not often as the front single sided 7" brakes are not that good at stopping at that speed but that was pushing it bit not bad preformance for old british bike they can tweaked
 

greg brillus

Well Known and Active Website User
VOC Member
If you are using a bike with an AMC type gearbox you generally need to position the engine in a different spot for kick starting, that is just easing it over TDC and then kicking will not start very well. I start with the k/s lever at 90 degrees to the ground and then ease it over TDC and keep turning it further to the bottom of the kicking stroke.........then kick it with full effort and it will start much better. The kick start stroke versus mainshaft turn ratio must be different.........Pretty sure on a stock twin the mainshaft turns about 5/6 th's of a full turn.
 
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erik

Well Known and Active Website User
VOC Member
This is where the new pilot jet lives.On the other side of the carb it is as it was ,the airscrew! Erik
 

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