450 desmo carb

[themoudie]

G'day Graeme,

Thank you for the insight into the reasons behind your timing at 0.5°ATDC. I might go back to 0.5°BTDC, but I'll leave well alone for the time being.

As for the flambe recipe on the barbie, I'll pass! My workshop is wood framed, with creosoted board walls and exposed wood roof trusses, but does have an insulated steel sheet roof and a good pot stove in the corner with plenty of plaster board to protect the walls nearby. When firing the 450 up, the doors are open and the dry powder extinguisher is within arms reach.

I use a wee skoosh of WD40 and have found that to work as well on perished inlet rubbers, but this Triumph TSS one is new.

I will re-mount the alloy stub that has no provision for an 'O' ring, with some high temperature gasket and a thin phenolic insulator. Maybe even try to get the Dremel to machine a seat for a Viton 'O' ring for a proper job!

Good health, Bill

[Duccout]

Jesus Bill! Revs sitting at 4000, and blipping the throttle makes no difference?? Are you sure that the ghost of Bob Mac is not haunting your garage? When you get to the bottom of this you will have earned yourself a bottle of special malt!

Good luck and keep us informed,

Colin

[themoudie]

UPDATE 15/3/2021

So, after the last round the inlet stub was remounted as described, but without adding the seat for an 'O' ring and allowed to cure.

Re-checked the ignition timing at 0.5°ATDC, as per Tassie Graeme's suggestion.

Again started up no problem with the following settings:

Main jet: 240
Needle jet: 106
Needle: 2A1 with the clip in the 3rd groove from the top (middle)
Slide: 3 anodised slide
Pilot jet: 25
Air screw: 11/2 turns out, off the seat
Cold start jet: 35

Still had the same problem getting the engine to return to a tickover and once warmed through. Again checked for inlet tract leaks, with no result. Replaced the plug with the Gunson to view the flame colour. This remained a light blue inspite of screwing the air screw in to only half a turn from seating. So, time for an increase in pilot jet size. Replaced the 25 pilot jet with a 35 jet that had provided good colour previously (notes! ) and also returned to a 3.5 anodised slide, as from 1/4 throttle on the flame was yellow, with yellow blobs of burning fuel. With this revised set up I was able to attain a good steady tickover at 1,000 - 1,200rpm and a clean increase in revs, without spitting back or blowing the carb off! Again replaced the plug with the Gunson and could achieve a yellow flame at tickover and then tune this out, with the air screw at ~13/8 turns out from seated. The transition to the slide was more a green than yellow colour flame, with some yellow blobs and at half throttle became more yellow, so it would appear that the needle could be droppeded by at least a notch to the clip in 2nd groove from the top. But, the reluctance to tickover problem remained.

Today, went for some shopping and got lost. After 6 miles, stopped to put some fuel in and the revs sat at 4,000rpm. After putting a splash in the tank, restarted, required a single prime on the cold start and then started on the next attempt, will not start on first kick inspite of being hot! After approx another 10 miles shutting the throttle had no immediate effect unless the revs were reduced by going down the box and using the brakes. Been here before! This behaviour was consistent all the way home for 15 miles.

Rang AMAL! A technician who appeared to know his stuff probed long and hard about the settings, sucked hard when I said I was up to a 35 pilot jet and finally suggested that I should try specifically pushing the slide horizontally, rather than down and see what difference if any this made. there was also a suggestion, after I confirmed that I had leak tested the whole inlet tract from carb to cylinder head flange that maybe I had a leaking valve guide or poorly seating valve. This thought has crossed my mind as the sustained revs only start once the motor has warmed up.

So, had a mug of tea and a 2 mile walk to ponder. Went and started the engine, single prime with the cold start and then away on the next kick with the cold start 'OFF'. Warmed everything up and when the revs wouldn't die, pushed the slide horizontally to ensure a seal between the slide and body, sure enough the revs dropped back to a steady tickover at 1,000 - 1,200rpm. This was/is consistently repeatable. Bear in mind that this trick is only supposed to work on well worn slide/body combinations and this slide/body combination has less than 10 hours running behind it from new!

I also took my knackered MKII No.3 brass/chrome slide, a MKII No.3 anodised alloy slide, a MKI No.3.5 anodised alloy slide and an old worn MKI No.3.5 'pot metal' slide and weighed them individually on a plastic letter scale that I have. Not to Nigel's standard, but good enough for bush/forestry engineering! The weights were:

MKII No.3 brass/chrome slide = 75grammes
MKII No.3 anodised alloy slide = 50grammes
MKI No.3.5 anodised alloy slide = 60grammes
MKI No.3.5 'pot metal' slide = 85grammes

This would indicate that the lighter, "less wear prone slides", are in fact more susceptable to fluttering in the breeze in the carb body when faced with the intake pulse forces generated by a lusty single. Hence the use of weighted/heavier slides and "stronger" slide return springs to try and retain control of the slide.

AMAL have back in stock MKII brass/chrome slides, but will they be heavy enough, say 100grammes?

AMAL do not make a "stronger" slide return spring, so what are the alternatives that others may use?

Or should I be lifting the head as well to inspect the valve guide and inlet valve areas, for minute cracks etc that might open up as engine components expand with the increase in temperature?

I am inclined to the former two first.

Thank you for your time and good health, Bill

[LaceyDucati]

Bill

Amal used to make stronger springs, It's a fairly simple spring, so something generic should fit. I'll maybe look at my spare springs, to see if I have any reference.

Regarding slide weight, do you recall my tip about adding lead weights inside the slide under the spring?

You should be able to hear a leaking valve through the open carb easing the piston up on the kickstart. Failing that it should show up with a compression test. If the guide was leaking to any extent, I would have thought there would be significant smoke.

Regards Nigel

[LaceyDucati]

Quick search on google...

https://www.rgmnorton.co.uk/buy/mk2-car ... y_1545.htm

https://www.trialsbits.co.uk/product_in ... ts_id=1770

[Duccout]

Hi Bill,

I have never experienced the problem that you have with the slide being lifted off the throttle stop, in spite of running some pretty worn engines! If the valve was seating so poorly would the engine start? And as for air leaks down the guide, surely the rubber seal would keep this under control? I just cannot see that inlet suction at tick-over would be enough to overcome the strength of the throttle return spring and the weight of the slide.

So what we do know is that the engine is producing enough suction to pull the slide open and increase the revs; does this point to fuel getting into the inlet tract and increasing the revs, or would fuel/air mixture be required to do that? If air was coming down the inlet guide, would it require fuel to increase the revs or would the engine simply refuse to tick-over due to a weak mixture?

Colin

[themoudie]

Aye Nigel,

Thank you for the double barrel advice. I haven't forgotten the lead under the spring trick and I have just bagged a couple of the "heavy duty" springs from R.G.M.. Hopefully, this should cure things so that I won't need to purchase a brass/chrome slide. I'll see and keep the results posted here.

As for the leaking valve/guide etc. I don't buy either. I'm 95Kg and can stand on the kickstart and there is NO SMOKE from the exhaust, especially light grey oil smoke.

I just hope that I don't have a dodgy body casting!

AMAL don't openly list the stronger spring, but it does appear if you enter the part number that is: 2036/075 Throttle Slide Spring - Strong

Aye Colin,

The slide is not being lifted off the base of the carb venturi, but not sealing at the front of the slide, hence the horizontal pressure to the rear of the slide in the test. The stronger spring ensures that the slide does not "float about" and the extra weight of the lead ensures the slide has greater stability against the intake pulses. If the latter doesn't work, then there is the brass/chrome slide that AMAL have produced new stock of and that was unavailable when I purchased the carburettor.

So, thank you both for your help and interest.

Good health, Bill

[graeme]

Hello Bill,

Do I recall you purchased a Mikuni ?


Re the Amal,,,, If the slide is moving horizontally, how will a stronger spring pushing down fix that?
If the slide is loose in the bore, seems like the carb body is faulty or the slide is worn?

I'm not against Amals but ,,,,,,,, never heard of the issues you are having before.
I've used worn out PHF32's and VM Mikuni's and never had your problems.

Good Luck

Regards
Graeme

[Duccout]

Hmmmm, you would think that if the slide was so loose in the bore that it allowed enough air to pass to cause the revs to rise, then there must be a fault in the carb. Bill, it does this with both the Mk 1 and the Mk 11?

[Jordan]

I have brand new never used original series Amal Concentric Mk1 carbs, with loose slides.
I guessed they deliberately made them that way, and compensate for the extra air they pass, with appropriate fuel settings?
After all, in use some air always goes through the carb.
A reason why they are made so loose could be because the body of the Amals (before Mk2) is not very resistant to deformation, being thin and without much external buttressing features. So generous clearance avoids jamming problems.
Just my guess.