450 desmo carb

[LaceyDucati]

Bill

Wrong way with the plug, a B8HS is a colder plug....but as the engine is not that high a compression, I maybe wrong....

Regards Nigel

[Duccout]

I listened to Bill and went out today, as there was a fairly dry line on most of the roads, but the low sun was horrendous - where I could I shaded my eyes with my left hand, but could barely see with the reflection off the road, and nobody else seemed to see me either! I had one moment when a car almost pulled out on me: 'idiot' I thought to myself, then as the road changed direction into some shade I saw that my indicator was on!

Colin

[George]

I also listened to Bill and had a 12 miles ride round local roads. All within pushing distance from home to start with. First impressions are good maybe running a little rich but will leave for now, better rich than weak till I get more time on it. Suspension is firm, brakes work, no oil leaks yet. Will check it over tonight when it cools down.

[themoudie]

UPDATE 17/12/2020 17:41pm

Having read through my notes and considered Nigel's comments of last night and this morning, I set too, again this morning.

Warm up the engine and then change the plug for a NGK B6HS that is the recommended plug for the 450 MKIII, with an electrode gap of 0.024" (0.6mm). The idle seemed to be smoother, but the bike still required an initial priming cycle with the cold start in operation, before starting with the cold start 'OFF' on the next starting attempt, with a 'HOT' engine.

I then removed the NGK B6HS and inserted a Gunson Colourtune 500 spark plug, before starting the engine and viewing the flame colour. The colour at idle (~1,100rpm) was the recommended "Bunsen blue" (BB) and as the throttle was slowly opened this changed to a BB, with yellow (Y) 'blobs' of un-emulsified fuel showing very clearly. As the revs were increased these disappeared, and between a quarter and a third open throttle the flame was a constant Y, on a steady throttle. I deduced from this that the pilot jet size 35 and the airscrew at one and a half turns out from seated was producing a good running mixture, but maybe marginaly weak for initial starting. Hence the need to use the cold start for an initial priming cycle. Any adjustment to enrich this setting, using the air screw, causes the engine to take a long time to return to the idle, it appears the fuel level in the fuel bowl is still 'leaking' through in the form of 'blobs' of un-emulsified fuel and keeping the revs above the ~1,100rpm.

The 'blobs' of burning fuel, I think, indicate that the downdraft angle (20°) is affecting the fuel level in the fuel bowl and the vibration of the engine exacerbates the formation of these 'blobs' that are drawn into the engine.

As the throttle is opened further, to the third open throttle position, so the flame turned a constant and brighter Y. I deduced that this was caused by my having used a 107 pilot jet and the needle clip in the 2nd groove from the top. I then reverted to a 106 needle jet and the needle clip in the 5th from top groove. This made the flame still Y at a third throttle, but not as bright as before. I then moved the needle clip to the 4th groove from the top and re-ran the test. This time the flame colour was closer to a BB colour than a Y and I decided to leave this setting until I get a chance to road test.

I have yet to now change the position of the pilot jet from the fuel bowl to the carb body. This may help remove the 'blobs' from the fuel mixture and I think that coming back to a 260 main jet would also help.

I am a bit frightened of putting the engine on a dyno, not sure that the technicians will know what they are doing if the computer won't "Plug and Play" the 'Power Commander'! Comments welcome!

Thank you, Bill

[themoudie]

Aye George and Colin, very pleased that you managed a canter today.

These bikes require 110% concentration in todays urban/suburban road conditions, keeps the brain VERY active!

Wee dram and the feet up now and you'll be able to spin a yarn next year!

Good health, Bill

[Duccout]

Bill, what were the old terms we used to use for sorting out carburattion? Flying by the seat of the pants; suck it and see? I think that you are doing fine.

Have a wee dram for me.

Colin

[themoudie]

UPDATE 18/12/2020 20:30pm

Out for another plug chop today. gutters and water everywhere and the forecast for heavy rain by 11:00am. That rain materialised as I put the bike in the workshop after a hose down.

Started no problem on the cold start alone after an initial prime. However, I was too quick to knock the cold start off and there then ensued a 10 minute awkward SOD session, no go with cold start and no go with a prime. I think that I must move the 35 pilot jet from the float bowl, to the carb body, to see if this will sort this out as the flame colour looks fine, once the engine is heated through idle and starting is no problem. Not sure about going to a 40 pilot jet, as I am not sure if I could lean it off enough with the air screw, but I may have to do that. Then make an iniitial check with the Gunson plug.

Out on the road, the engine picks up and runs strongly, but if running on a slightly backed off quarter to half throttle opening there is a surging that wasn't apparent before and I suspect that this is a slightly weak mixture. So, I'll drop the needle clip down the needle to the 5th groove, with the 106 needle jet.

Cheers, Bill

[themoudie]

UPDATE 19/02/2021

Ordered a couple of Triumph TTS inlet rubbers, as they are moulded so that the carburettor is held level when mounted. The idea being to reduce the downdraft angle and the flow of neat fuel down the inlet tract. They are a 'no modification' fitment using an Amal 2900 series carb and a 32mm bore inlet, 40mm O.D. stub on the head of the 450.

Set the carb up with 240 Main Jet, 3.5 slide, 2A1 needle with the clip in the middle notch (3rd from the top of the needle), 25 pilot jet, in the float bowl, 11/2 turns out from the seat for the air screw and 11/2 turns in from initial lift of the slide off the venturi for the idle screw.

Having lain dormant since Xmas, took the plug out and cranked the motor over for a good minute, to get the oil round the motor. Put the plug back in, turned the fuel on, used the cold start for two priming kicks, turned the ignition on, eased the motor over TDC and gave it a kick and away we went. Fast idle for about 30 seconds then died. Tried the procedure again and fired up, again at a first idle. This time caught the throttle and knocked off the cold start, then allowed the motor to warm through steadily. Once warmed through a couple of minor tweaks to the air screw and throttle screw and had a good 1,000rpm tickover, with a clean pick up to 4,000rpm. However, as the motor became hotter, so the idle became more unstable. Blipping the throttle to 4,000rpm does not always settle the revs back to 1,000rpm, so turned it off.

Took the plug out and there was fuel on the plug and quite black! Because the old Amal MKI 932 that used to be fitted had the cast in 20 pilot jet and worked well, I thought I should try the 20 pilot jet in the MKII 2932 to try and sort out this uneven idle and surging. So, replaced the 25 pilot with the 20 and re-assembled the bottom of the carb.

Fuel on, single priming kick, turn the ignition 'ON', eased over TDC and BANG, blew the ruddy carb and TSS inlet rubber clean off the inlet spigot. Fuel everywhere and I have the pot belly keeping my back warm in the corner! Fortunately, nothing caught light and I managed to mop up the fuel and open the workshop doors.

You've got to laugh, if you didn't you'd weep!

I suspect that the Boyer Induction Discharge Unit is to blame with a "spare" spark, for the explosions. So, will have to disconnect that for the next test.

As for the variable idle and reluctance to return to the idle from a blipped throttle, I'm stumped. I was hoping that a more level fuel level using the TSS cranked inlet rubber would have cured the idle variation, but apparently not.

May be the MKI 932 on the cranked inlet might prove a better bet?

Thank you for your time. Good health, Bill

[Duccout]

Jesus Bill! I'm guessing that fuel had collected in the inlet tract and when you kicked it over it ignited and the explosion blew the carb off. Other than that I haven't a clue, except maybe the cold-start is not closing properly, or the float needle is not sealing.

Keep us informed, we need all the drama we can get at the moment!

Colin

[themoudie]

UPDATE 22/02/2021

After the last explosion! I again checked the valve timing, just in case, for the third time and was +-2°, with 1mm lift (0.040") and the working clearance on the published opening and closing degrees.

Then had another look at the points. At present the only points readily available have a pressed in plastic bush (nylon?) in the pivoting arm and these do not necessarily enable the two contacts to meet with parallel faces or entirely face to face! In the worst scenario the arm fouls the points retaining screw. So, I had to fettle an arm from one set of points to fit the third points base I had opened. The points bases are a pressing and do not always lie flush on the base plate. At between £8-00 and £18-00 +VAT + P&P per set, I don't think "fettling" should be necessary! Mounted the points and timed the spark, after the usual faff I managed to set it at 0.5°BTDC.

Took the old Amal MKI 932 that has been rebuilt with a new anodised 3.5 slide, new U1 needle, new 106 needle jet, 220 main jet, cast in pilot jet (20?) and mounted it on the TSS carb inlet rubber. Adjusted the air screw to 1.5 turns out from seated and the throttle slide screw 1.5 turns in from starting to lift the slide off the venturi base.

Lightly tickled the carb, a couple of priming kicks, eased over TDC, ignition 'ON' and a good swing and I had a running motor Bit weak on the pilot/air screw so screwed it in to 7/8 out from seated and adjusted the idle to a steady ~1,000rpm. No unexplained increase in revs that refuse to fall, no violent kick backs!

Out for a wee trial run and after warming through the SAE40, I put about 12 miles on some back roads and took it up to 60mph+ on a quiet straight and the motor pulled strongly through all the gears. Took the plug out on returning home and the colour looks OK to my untutored eye. Some more fine tuning, maybe.

So, 17 months, 3 different ignition system set ups, 3 carburettor set ups and 3 exhausts later, I might finally be heading in the right direction.

Amal MKII 2900 series carbs remain a mystery, as do HPI CDI ignition systems and if anybody knows of some properly made points for singles, please let us know.

Thank you to everybody who has made suggestions and spent their precious time trying to help. I am indebted to Nigel Lacey for being a sounding board.

Good health, Bill