450 desmo carb

[Jordan]

That rubber tube was a good discovery.
It's odd that it has such a bend, as the Triumph engine is almost vertical.

[themoudie]

Aye Jordan,

The 1982-1983 Triumph TSS 750 was manufactured using the Nourish 4 valve head that has a steeper downdraft inlet angle than the usual 2 valve heads of the foregoing Triumphs. The TSS also used the Amal MKII 2934 34mm carbs on the UK market machines. The export models for the USA used Bing carburettors and this was due to limited clearance problems within the frame of the Amal MKII bodies, with the movement of the rubber mounted engine.

So these inlet rubbers were manufactured with this intended kink, it appears, so that Amal MKII 2900 series carburettors could be made to work within the confines of the frame and the steep downdraft inlet angle of the Nourish 4 valve head used on the TSS engine.

The inlet rubbers have been unavailable for many years until a batch was manufactured by a Triumph spares specialist.

Whilst there use might improve the performance of Amal MKII carbs and Mikuni carbs that have an aversion to steep downdraft angles, which appear to create fuel level problems in the carburettor float bowl. My experience with a Amal MKII carb on a Ducati single has not been exactly joyous! And I still cannot explain why!

Admittedly, my cylinder head is not 'standard' having been ported by John Wittman back in 1977, but running 'standard' 450 sized valve heads. Using a Piper 173A cam and a flat topped 450 piston, exhausting into a 38mm ID exhaust pipe and a gutted Gold Star exhaust with a single disc baffle, with a single 28mm hole. This is still too small a hole according to the formula and calculations I have made, but until I can get my hands on a larger sheet metal cutter to take the hole out to 32mm - 33mm, the constriction is going to have to remain.

Whilst on the dynomometer, I am told, a Amal MKII shows the best flow rates, when compared with other manufacturers carbs, on Ducati single race engines. I do not have the luxury of access to a dyno, spare cash if a full blown dyno run grenades my engine and iy is not a race engine, I look for mid range tractability, rather than top end speed, so I have reverted to the tried and trusted Amal MKI 932 32mm carb.

Good health, Bill

[Jordan]

Amal Mk1 Concentrics certainly are compact.
The Mk2 was available as a smooth bore type, which might have improved performance a bit.

[themoudie]

Still not cured!

Was going to go and get some shopping. Turned on the fuel, couple of tickles, primed the cylinder, brought it over TDC, turned the ignition 'ON' and gave the swinging kick, fired up and was holding it at the 1,500 - 2,000 rpm warm up, spluttered then died. Bit cold! Another tickle, single prime kick, ease over TDC, big swing kick, followed by a loud report and the MKI 932 swinging from the throttle cable and the inlet trumpet split in two against the frame. Blown the carb off the inlet rubber, AGAIN!

I cannot for the life of me explain these explosions, unless there is a stray spark coming from the Boyer Induction Disharge Unit (IDU). So, I have written to Boyer to see if they have any ideas.

And I thought that I was winning!

Good health, Bill

[Duccout]

Hi Bill,

All I can say is I haven't a clue. I think that it is time for a wee dram!

[Bevel bob]

That sounds exciting !. My first thought was too advanced ignition and wrong valve timing, but I'm sure you are right , an extra unwanted spark at exactly the wrong moment. If its too much trouble I'll straight swap you a reliable low milage 150 mph Honda that sounds just like a big Ducati.

[themoudie]

Aye Duccout and Bob,

Thank you for the chuckles! The dram is usually an Islay malt or something smooth from Speyside, but I'll enjoy even a Lidl malt, which is surprisingly good. As for the offer of a 150mph missile (VTR?), I don't know if I have the gonads! Having only ever had singles up to a 660 Yam single, I'm not sure if I would cope! I have a friend who insists that his KTM 1090 is really a big softie and another who runs a KTM 1290 Super Duke R and that is definitely only for the brave. I know the throttle works both ways! Then there is all that ABS, TC, EI, GPS! I can't even get a set of points to work!

As for the too advanced spark, 0.5°BTDC is what I'm running at present and I'm not getting huge kickbacks on the kickstarter, whilst the motor runs really nicely, once under way. The last wee 10 mile canter was very smooth, good colour on the plug nose, no spitting back through the carb or popping and banging in the exhaust. Checked the cam timing for the 3rd time and all within 3° of the predicted timings and therefore not a problem according to Nigel Lacey, who supplied it.

Next way to go is to disconnect the IDU and see if we get the same problem. Also thinking that a choke slide in the MKI 932 might help keep things running when cold, so less chance of having to go for the re-start. I'll see and let you know.

Good health, Bill

[graeme]

You mentioned Boyer,,,,

Boyer had a habit of destroying sprag bearings (clutches) on electric start Mk3 Commandos when the voltage wasn’t 12v the ignition would advance on start.
Don’t know what model Boyer it was, just a thought?

Graeme

[Bevel bob]

Good guess Bill, a VTR 1000 , And you are right, all too much. At least it has carbs . Go back to points.

[blethermaskite]

Bill,
I think I would be suspicious of the boyer ignition, have you enough bits lying about to do a temporary convert to coil and points? I am running Lucas rita ignition on the 350 Mk 3, I am no expert in electronic ignition but I believe the Lucas system has a wasted spark on the exhaust stroke? the reason I mention this is the 350 will occasionally go off with a bang in the inlet tract , due to my clutch having a tendency to stick if I have the fuel on and kick the engine over to free (the clutch) the instant I turn on the ignition any mixture in the cylinder will fire and if the piston is on the inlet stroke I will get a bang through the carb......(mine can't blow off because it is solid mounted).......just my thoughts on what might be happening to yours.
Cheers,
George