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Monobloc on a '51 Comet

Desmond

Website User
VOC Member
We recently re-built the upper end of my Comet because it was not possible to purchase a new 90.. piston. It is now standard but may have Mk 2 cams but we re-fitted the 32 mm Concentric and she started beautifully and ran well for 8 miles or so and was then impossible to de-throttle. I tried everything and the same thing always happened when she warmed up. I removed the Concentric completely and a machinist friend has informed me that it is junk so I have re-built and fitted a Monobloc. I only have a 230 main jet in it and I am not sure of the others. After some problems with loose pilot jet etc, we have her running fairly nicely but she is now very difficult to start and, even after 8-10 kicks with tickling and full choke, the plug is dry. I am getting great spark but obviously no initial fuel. In looking at "Forty Years On" there is a good article by R. Swannick on tuning the Monobloc for a Comet and he seemed to recommend 280 Main jet, 106 needle and 30 pilot which looks right but may not really solve her getting fuel for starting. When I tickle her, fuel runs out of the bellmouth. With 8-10 kicks on full choke she should then be flooded. I have dropped the needle down one but that should not be my problem. She started so beautifully on the old Concentric, what should I be looking at now with the Monobloc?
 

Bill Thomas

Well Known and Active Website User
VOC Member
As you say it won't help the starting , But I would get a big main jet in SAP !,
They use a much bigger main jet than a concentric,
What size Monobloc ?.
Maybe put the concentric back on just to prove a point,
Ignition Timing may have slipped ?.
Have you got a good spark, H.T. lead with wire in it, Non Suppressor type plug cap
There are different needles etc for the monobloc, But if you have petrol pouring out it should start.
You didn't stuff a rag down the inlet when you took the old carb' Off ??
 

Desmond

Website User
VOC Member
Bill: Thank you for that. I have just asked my machinist friend to send me down the 270 main jet out of the Concentric that he had examined and said that it was now junk. That will be a big improvement over the present 230 but I am not really working onff the main jet yet as I am still running in the new top end and not really opening up the throttle---head, piston and barrel all new. Don't think my timing has slipped as--when I did get her started--she ran nicely and sounded good. My first problem had been that my pilot jet was not well seated and she was running VERY rich with black smoke and banging in the silencer. When I know exactly where I am going with this Monobloc, I shall order all new jets and needle from Surrey but, with Brexit, everything is taking a long time to get to us in Ireland although a small envelope of jets and bits might come through fairly easily. I think I have the 389 Monobloc (1 1/8") and I need to find out the exact recommendation of jets etc. I have a good HT lead with wire in the centre and a good non suppressor plug cap. I have a beautiful strong blue spark. No, I don't think I have a rag still at the inlet as I have now run the machine a few times. I did think of that one as well. In the Forum I see some reference to the Greg Brillus modification. What is that?
 

vibrac

Well Known and Active Website User
VOC Member
I would back a monoblock against a concentric anyday I have just retimed my Comet with ATD closed at 4 degrees.Thats a first for me, but realistically I have decided recently that 34 or 36 degrees and possible lost of a few MPH is second place to good starting. I have my ear out for pinking nothing yet :)
 

timetraveller

Well Known and Active Website User
VOC Member
The Greg Bruillus modification refers to machining out the rear holes in the lower link to take two, each side, 32 mm OD ball races. This reduces the friction inherent in the Oilite bushes but is best used with a modified steering head and either you can read forty odd pages of that in the forum or you can email me on enw07@btinternet.com for fitting instructions of this modification. If you want more comfort and safety that is the way to go. If you are happy with what you have then you can ignore all that.
 

Bill Thomas

Well Known and Active Website User
VOC Member
If your talking about Greg's Mods with the Carbs, I don't think that was Monoblocs ?.
I played with a monobloc On my Flash spec' Comet and hated it , I rubber mounted it,
Because I have a rear Cyl' Head, And it was Very heavy, I would not do it again !.
Your problem with the Concentric I think was distortion after the flange was overtightened ?,
At some time in it's life.
I can only give you spec' for the Black Prince, !. 1 1/8" Monobloc ,Because the Amal book won't show the Victor, Only 2 made !!, 280 main, But comets like it a bit richer, I would say 300 ?,
30 pilot, Needle 106, Needle position 3, I think that's from the top ?,
The Amal book say the slide 604, I think that means 4 slide ?.

Just looked in Know Thy Beast, It says some people have trouble with the 30 pilot,
And it's fixed by fitting a 25.

I have been told that monoblocs can have sticking float trouble ?, Something to check ?.
I think you have flooding problems, Easy way is switch the petrol OFF ,
As the float level goes down , If it smooths out you know it was too rich ?.

Did you try Conways for the BIG piston ?, I am interested because am thinking one for myself ,
Only dreaming at this point !., But they say they have big barrels !!.
Good Luck, Bill.
 

vibrac

Well Known and Active Website User
VOC Member
Venolia knock out a piston at very reasonable prices they made me a 90 bore methanol piston from an old piston with a plasticene crown and an accurate bore measurement over the Atlantic and back £120 add a few pounds it was five or so years ago but good value.
American know how
 

Desmond

Website User
VOC Member
I would back a monoblock against a concentric anyday I have just retimed my Comet with ATD closed at 4 degrees.Thats a first for me, but realistically I have decided recently that 34 or 36 degrees and possible lost of a few MPH is second place to good starting. I have my ear out for pinking nothing yet :)
Vibrac---Thank you for that input. I think we have the timing right as she always started beautifully on the Concentric after being brought back to standard. She used to kick back a lot under Bob Dunn's modification but I learned to live with that and she ran very well. What guts are you using in your Monobloc?
 

Desmond

Website User
VOC Member
The Greg Bruillus modification refers to machining out the rear holes in the lower link to take two, each side, 32 mm OD ball races. This reduces the friction inherent in the Oilite bushes but is best used with a modified steering head and either you can read forty odd pages of that in the forum or you can email me on enw07@btinternet.com for fitting instructions of this modification. If you want more comfort and safety that is the way to go. If you are happy with what you have then you can ignore all that.
Timetraveller--Thank you for that. I mistook what I read and thought he was talking about Monoblocs.
 

Desmond

Website User
VOC Member
Venolia knock out a piston at very reasonable prices they made me a 90 bore methanol piston from an old piston with a plasticene crown and an accurate bore measurement over the Atlantic and back £120 add a few pounds it was five or so years ago but good value.
American know how
Vibrac:--I contacted everyone I knew about the availability of 90 mm pistons-even the person there in the UK that used to have 20 at a time made up by Omega. He was not doing it anymore. I went on the Forum but did not get any reply so I scrapped the modified head and barrel and went back to standard and this is where I am now!!!!
 

Desmond

Website User
VOC Member
If your talking about Greg's Mods with the Carbs, I don't think that was Monoblocs ?.
I played with a monobloc On my Flash spec' Comet and hated it , I rubber mounted it,
Because I have a rear Cyl' Head, And it was Very heavy, I would not do it again !.
Your problem with the Concentric I think was distortion after the flange was overtightened ?,
At some time in it's life.
I can only give you spec' for the Black Prince, !. 1 1/8" Monobloc ,Because the Amal book won't show the Victor, Only 2 made !!, 280 main, But comets like it a bit richer, I would say 300 ?,
30 pilot, Needle 106, Needle position 3, I think that's from the top ?,
The Amal book say the slide 604, I think that means 4 slide ?.

Just looked in Know Thy Beast, It says some people have trouble with the 30 pilot,
And it's fixed by fitting a 25.

I have been told that monoblocs can have sticking float trouble ?, Something to check ?.
I think you have flooding problems, Easy way is switch the petrol OFF ,
As the float level goes down , If it smooths out you know it was too rich ?.

Did you try Conways for the BIG piston ?, I am interested because am thinking one for myself ,
Only dreaming at this point !., But they say they have big barrels !!.
Good Luck, Bill.
Bill:---Ah! now that is great guidance. I did just get a note from Mike at Surrey Cycles and he aimed me at a sticking slide due to a possible flange problem and close inspection of the Concentric has now shown up a hairline crack. It had to be the slide but I could not identify that or why. I'll work with your recommendations on the guts of the Monobloc. I do not have Know thy Beast but I do have Forty Years On as well as Richardson and others. The only info I could get on Monoblocs was for Rapide and Shadows and we are likely to be a bit different---as you say.
I have a bored out Nikasil barrel for the 90 mm that you might get from Venolia like Vibrac did. Someone else now has the modified head but we might be able to get it back as well if you were interested. Bear in mind that you need to modify the crankcase for this barrel. That is why I sent the bike off to a Vincent machinist friend for rebuilding so that he could machine a modification of the crankcase back to take the standard barrel.--------Desmond
 

Bill Thomas

Well Known and Active Website User
VOC Member
I found once that the last flat of doing up the flange bolts, Would distort the body,
And make the slide stick,
I think I just ground the flange flat, On a bit of glass and wetanddry paper,
Or something like, A simple fix !.

Thanks for the offer, I have a lot banging around in my head at the moment,
I am trying to Stroke a Comet !,
Just lost Confidence, Getting Old and Daft !,
I have been thinking of buying C/Cases already bored,
But didn't like the Nikasil type, Conways say they can do normal barrels ?.
 

Bill Thomas

Well Known and Active Website User
VOC Member
Desmond, If you had it going as a Big engine,
What happened to the old piston, Did it nip up ?.
Cheers Bill.
 

vibrac

Well Known and Active Website User
VOC Member
Vibrac---Thank you for that input. I think we have the timing right as she always started beautifully on the Concentric after being brought back to standard. She used to kick back a lot under Bob Dunn's modification but I learned to live with that and she ran very well. What guts are you using in your Monobloc?
Without taking it apart I cant say what I have in the Monoblock whatever it is is a good starter as its been on at least 3 of my bikes without alteration
 

oexing

Well Known and Active Website User
VOC Member
Seems Desmond got two problems, a distorted Concentric, and in the Monobloc a blocked pilot jet. A dry spark plug will tell, no fuel at kicking speed, the pilot jet is critical for kicking, no other components responsible here. Also check for air leaks on the carb flange or stub . The slide cutout comes only next for picking up from idle speed.

Vic
 

Bill Thomas

Well Known and Active Website User
VOC Member
I had to put weaker slides in my Bikes when we went ethanol,
Or I wetted plugs after a 5 to 10 miles.
Then my Plug Threads went loose and had to helicoil on my Special, Not Happy !.
But after having the Bike for 55 years,
And 10 years racing that engine, Maybe not Too Bad !!.
 

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