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E: Engine Comet Mongrel



Black Flash

Well Known and Active Website User
VOC Member
Well Cyborg,
I don't know anything about prices in Canada, but over here prices for mikuni jets are dirt cheap. On fleebay you can get sets of 10 in rising oder for about 20 U. S. They won't be more expensive in your country.
Safe your time and buy a set, it is very extolling to fiddle around and get the best set up carb around.
 

Cyborg

Well Known and Active Website User
VOC Member
I did find some kits for mains that were cheap and can't imagine they can screw up something as simple as a main jet.... although I would be inclined to measure them. The cheapest kit I could find for primaries was a kit for $85 USD. The single no name ones from Taiwan are cheap, but I need the VM22/210 style and don't know if I would trust anything other than the OEM Mikuni primaries. They can be had for around $6 each, so still thinking drill the 15 primary I have, figure out what is close and then buy 1/2 dozen in that neighbourhood. I read somewhere that the selection of needles and needle jets for that flat slide are limited, so do hope I don't have to spend any time finishing a needle.
 

eglijim

Well Known and Active Website User
VOC Member
I did find some kits for mains that were cheap and can't imagine they can screw up something as simple as a main jet.... although I would be inclined to measure them. The cheapest kit I could find for primaries was a kit for $85 USD. The single no name ones from Taiwan are cheap, but I need the VM22/210 style and don't know if I would trust anything other than the OEM Mikuni primaries. They can be had for around $6 each, so still thinking drill the 15 primary I have, figure out what is close and then buy 1/2 dozen in that neighbourhood. I read somewhere that the selection of needles and needle jets for that flat slide are limited, so do hope I don't have to spend any time finishing a needle.
Purely as a set up exercise do you not have any other carbs from a running bike just to get you off the ground safely before starting with an unknown setup ?. Usually easier and safer to start with a setup on the rich side then work leaner once running.
 

Cyborg

Well Known and Active Website User
VOC Member
Welcome back....
I don't really have anything other than a pre monoblock that is good to go. Room is very tight and I have to make a new manifold or an adapter to change the angle of a stock manifold. You are right about starting rich. As mentioned, the plan was to drill out the primary until I reach a point where it's starting to behave and then buy a few jets. Then I started thinking about the chrome on my nice new pipe. Once I get the primary sorted, if it has drivability problems from the needle and main etc, I'll trailer the bike to a safe location for plug chops. It's deadly riding around here at the best of times. On a side note, I do have one of those colour tune setups that my brother bought back in the dark ages, but I've never actually tried it. Think I'd rather stare at a plug instead of peering though an ancient colour tune. Would feel like staring down the barrel of a gun to see if it was loaded.
 

chrislaun

Well Known and Active Website User
VOC Member
Here's a handy tip if you don't want to blue your pipe, a friend used to do this on his HD and it works !!
I used it on all my custom bikes.
Buy some Sperex VHT exhaust paint (this had found worked the best, mind you it was 30 years ago).
Block one end of the pipe with a potato and spray loads of the paint into the exhaust, keep sloshing it about until it's all sticking, leave it to dry, swap ends with the potato and repeat, do this a couple of times until the inside is well coated, it will give you a heat resistant barrier inside the pipe.
The colour tune will give you an indication of the mixture instantly, whereas with the plug reading you will have to run it at a constant speed and load it "colour" the plug.
 

timetraveller

Well Known and Active Website User
VOC Member
If your colour tune still has all its original bits then they came with a plastic tube and mirror so that one can look into the mirror and not straight down at the flame. Personally I have no worries about using mine and looking straight down at it.
 

chrislaun

Well Known and Active Website User
VOC Member
I've used them for years, my twins both have the extra plug hole so I put the colourtune's in there, I recently forgot to take them out and rode several miles until it dropped on to one, they had only been in finger tight and one came out, not the inside but the whole colourtune had unscrewed itself.
I've never heard of one losing the inside and they've been around a long long time.
 

BigEd

Well Known and Active Website User
VOC Member
VOC Forum Moderator
Welcome back....
I don't really have anything other than a pre monoblock that is good to go. Room is very tight and I have to make a new manifold or an adapter to change the angle of a stock manifold. You are right about starting rich. As mentioned, the plan was to drill out the primary until I reach a point where it's starting to behave and then buy a few jets. Then I started thinking about the chrome on my nice new pipe. Once I get the primary sorted, if it has drivability problems from the needle and main etc, I'll trailer the bike to a safe location for plug chops. It's deadly riding around here at the best of times. On a side note, I do have one of those colour tune setups that my brother bought back in the dark ages, but I've never actually tried it. Think I'd rather stare at a plug instead of peering though an ancient colour tune. Would feel like staring down the barrel of a gun to see if it was loaded.
The plug colour is not necessarily a good indication of mixture these days with various additives and percentages of alcohol in petrol.
I've never heard of a Colourtune exploding and it is probably easier to get an idea of what is happening in the combustion chamber stationery in the workshop than riding up and down a road.
 

eglijim

Well Known and Active Website User
VOC Member
Welcome back....
I don't really have anything other than a pre monoblock that is good to go. Room is very tight and I have to make a new manifold or an adapter to change the angle of a stock manifold. You are right about starting rich. As mentioned, the plan was to drill out the primary until I reach a point where it's starting to behave and then buy a few jets. Then I started thinking about the chrome on my nice new pipe. Once I get the primary sorted, if it has driveability problems from the needle and main etc, I'll trailer the bike to a safe location for plug chops. It's deadly riding around here at the best of times. On a side note, I do have one of those colour tune setups that my brother bought back in the dark ages, but I've never actually tried it. Think I'd rather stare at a plug instead of peering though an ancient colour tune. Would feel like staring down the barrel of a gun to see if it was loaded.
Thanks, been very tied up for the last few months in the fairly tortuous sale of some business property (retiring). No chance of begging/borrowing a carb from a local club member for a day or two just to establish how it runs. Always found plug chopping on the open road tricky especially on a twin, never takes long before some kind soul complains. Fine tuning the main jets became much easier after i bought a datalogging digital speedo which lets you time the interval between two preset speeds (say 60/100 ) in one gear.
 

vibrac

Well Known and Active Website User
VOC Member
In these circumstances welding a M8 NUT onto a hole in the back of an exhaust pipe finding a local rolling road and sticking a lamba sensor in seems to be the best if not the cheapest option last time we did it on the comet we went down on jet size adjusted a few screws and nuts and found 5 bhp and we thought she'd was already going well.....
 

eglijim

Well Known and Active Website User
VOC Member
In these circumstances welding a M8 NUT onto a hole in the back of an exhaust pipe finding a local rolling road and sticking a lamba sensor in seems to be the best if not the cheapest option last time we did it on the comet we went down on jet size adjusted a few screws and nuts and found 5 bhp and we thought she'd was already going well.....
By rolling road do you mean a set up which allows you to run on load ie a dyno so you can hold a throttle opening >
 

Cyborg

Well Known and Active Website User
VOC Member
If your colour tune still has all its original bits then they came with a plastic tube and mirror so that one can look into the mirror and not straight down at the flame. Personally I have no worries about using mine and looking straight down at it.
I think this colortune was produced before they invented mirrors. The shotgun thing was just my warped sense of humour, but... with a slight ring of truth. I could rig up a dental mirror or maybe a borescope so I could hide in another room. The little bottle of ammonium acetate solution B.P. has morphed into some sort of primordial slime, so I'll have to get some sort of cleaning agent for the "heat resistant borosilicate glass dome". If its glass then I assume my jug of Acetone / MEK mix would do the job. There is what appears to be an air pocket around the centre electrode, but it doesn't extend down to the base and doesn't look like and sort of cracking has occurred.

colortune.jpg colortune 2.jpg colortune 3.jpg
 

Cyborg

Well Known and Active Website User
VOC Member
In these circumstances welding a M8 NUT onto a hole in the back of an exhaust pipe finding a local rolling road and sticking a lamba sensor in seems to be the best if not the cheapest option last time we did it on the comet we went down on jet size adjusted a few screws and nuts and found 5 bhp and we thought she'd was already going well.....
You keep trying to inject logic and sound advise into this equation.

Excellent suggestion.. thinking I would still have to get it into the ballpark first or it would eat up a lot of time on the dyno. My way of thinking, the flat slide will want a smaller primary than a Mikuni round slide, so not sure how closely I can rely on what works on them (round slide).
 

Cyborg

Well Known and Active Website User
VOC Member
The plug colour is not necessarily a good indication of mixture these days with various additives and percentages of alcohol in petrol.
I've never heard of a Colourtune exploding and it is probably easier to get an idea of what is happening in the combustion chamber stationery in the workshop than riding up and down a road.
I will give the colortune a try. As for the plug chop, I haven't done it in a long time, so not sure how the additives come into play, but (don't want to sound pretentious) we can still buy ethanol free fuel that works and relatively speaking I live out in the sticks, so a quiet road is only 10 minutes away.
My last episode with jetting was interesting. Who knew you couldn't hear a siren over the racket coming out of 3 unmuffled expansion chambers.
 

Cyborg

Well Known and Active Website User
VOC Member
Thanks, been very tied up for the last few months in the fairly tortuous sale of some business property (retiring). No chance of begging/borrowing a carb from a local club member for a day or two just to establish how it runs. Always found plug chopping on the open road tricky especially on a twin, never takes long before some kind soul complains. Fine tuning the main jets became much easier after i bought a datalogging digital speedo which lets you time the interval between two preset speeds (say 60/100 ) in one gear.
Yes there is likely someone who would take pity and lend me one. Still not sure how much that would help me with jetting the primary, but at least I could make sure everything was running as it should from a mechanical/timing etc point of view, so I don't end up chasing moonbeams.
 

BigEd

Well Known and Active Website User
VOC Member
VOC Forum Moderator
I think this colortune was produced before they invented mirrors. The shotgun thing was just my warped sense of humour, but... with a slight ring of truth. I could rig up a dental mirror or maybe a borescope so I could hide in another room. The little bottle of ammonium acetate solution B.P. has morphed into some sort of primordial slime, so I'll have to get some sort of cleaning agent for the "heat resistant borosilicate glass dome". If its glass then I assume my jug of Acetone / MEK mix would do the job. There is what appears to be an air pocket around the centre electrode, but it doesn't extend down to the base and doesn't look like and sort of cracking has occurred.

View attachment 24784 View attachment 24785 View attachment 24786
Your Colourtune isn't quite the same as mine. The one (actually two) I have doesn't have the insulator bit covered in glass. The insulated part and middle electrode come through a glass piece that sits in the metal body.
 

Robert Watson

Well Known and Active Website User
VOC Member
I used a sniffer up the tailpipe on a local comet that was running very rich. Marked the throttle openings on the twist grip and took if for a ride, rich here, very rich here, etc.. After about three or four trips "around the block" followed by jets changes (only one at a time) we got it very close. It requires 12V to power it up. When you get the mongrel running we could try it if you want......

I tried it on the TTR but with a too long too small exh pipe and could not get a reading. I suspect that there was too much turbulence at the end on the pipe. It now has a bigger shorter pipe and goes waaaaaay better but haven't had the chance to take it to a track again for testing...
 

oexing

Well Known and Active Website User
VOC Member
A lambda sensor will definitely save you a lot of guesswork concerning all carb components. The sensor plus M 18x1.5 mm stainless thread and plug is easily found at Ebay, same with a little digi voltmeter as shown on the BMW damper knob, sensor under the gear box. 0.50 Volt is perfect lean, up to 0.70 V OK, above is too rich. For 6 V systems you carry a 12 V battery to supply the sensor heating, four wire type, for bikes so you can do real road tests ! You will be amazed how far from perfect a stock carb can be on a classic.

Vic
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