450 desmo carb

[blethermaskite]

Bill, so well done getting "on the road" again, even if its a bit stuttery.....so I am going to say swap the condenser again, and I would fit the condenser on the ignition coil not on the points plate, I would also fit a proper earth to the engine directly from the frame/battery earth to an engine bolt (one of the two crankcase pinch bolts at the cylinder base is a good point. Are you confident the ignition coil is good? I would swap that too, and to eliminate another gremlin hiding point are you using the standard 45year old ignition switch? they are not great, I would also temporarily just run the points straight from the coil with no electronic assist box. I have been through what you are experiencing with my 350 after it had been laid up for a lot of years, I drove myself angry with the carb but it turned out to be ignition issues.
So my 250 desmo which hadn't been running for 35 years was fired up literally first kick on a second hand 30mm mk1 concentric with no float height adjustment (or even measuring) with a viton tip float needle and I'm pretty sure a white plastic float, jetted at a very general ok ish guess spec, tufnol carb heat spacer 20mm (no rubber mounting)and importantly a proper Amal velocity stack bell mouth (as designed for the racing triumph/bsa triples) I won't use a mk1 concentric without a proper velocity stack. With the standard ignition set up paying attention to all the above and apart from a bit minor jet fine tuning running fine and rideable right away.
I may well not be right on this for your bike but if you do all and check the above at least you know for sure it carburation, one point about carb set up, the recommended procedure is to aim to get the main jet correct first, then needle/needle jet, then slide, then pilot mixture...because the main jet will have a degree of effect on all the other setting (I know only too well how problematic this is with a newly built engine which you need to run in) when its not really great to be trying full throttle blasts to get the main jet right, you just need with advice to guess a suitable jet size preferably on the rich side and work from there putting it right later on. And last point have you a choke slide fitted to the concentric? in my experience not having the choke facility will definitely make starting difficult from cold on these engines, hope this ramble makes sense and of use to you
Cheers,
George

[themoudie]

UPDATE 18/09/20 23:20pm

Aye George, thank you for the additional 'to eliminate' list.

The condenser has been swapped for the new one from the motor factor and mounted in the same position on the coil as the previous one was. That mod was made in 1976 when the original electronic ignition failed and was replaced with Ford Escort points, back plate and a ground down cam with a single lobe.

I have an earth from the engine directly to the alloy plate on which the Boyer box and the Electrex rectifier are mounted beneath the seat and this is also earthed to the frame. There is also an earth wire run from the headlight, back to the coil bracket and then back along the frame spine to the alloy plate earth. The ignition switch is a new key switch from Vehicle Wiring Products and this has a rubber boot fitted to protect all of the connections. All connections are crimped and soldered before either being sheathed by the moulded sleeve and or heat shrink tubing. I haven't tried the disconnect of the Boyer box, but don't think that is giving me problems.

Having just come in from the workshop this is the update on what I have done today. Started the engine that took the usual multiple kicks and spit backs through the carb, after flooding the carb and managed to warm the engine through on a steady throttle at 2,000rpm. Adjusted the carb and achieved a steady fast tickover ~1,500rpm, but then it went walkabout again! Turned the engine off. I have only ever used the short trumpet on the carb as there is insufficient room to fit the race trumpet with the flexibkle mounting. If I mounted the carb to the head studs, then I think I could fit one, but have never tried. The reinforcing gussets bracing the spine tube of the frame to the down tubes onto the rear engine plates get in the way. Main jet in the carb was a 220. Nigel Lacey in perscomms agreed with 220 - 240 main jet as a suitable starting point. I have never had a choke slide fitted to any (2 to date) of my MKI 932 fitted to this engine. A good tickle was always sufficient.

Took the 'SuperTrap' short megga off the shelf and fitted it, with 16 diffuser discs, as it had last been used 20 odd years ago. Tickled the carb, swung the engine over and away it went. A regular tickover at ~1,000rpm was achieved, with the air screw 1 turn out from seated and the slide screw lifting the slide by 1/2 a turn. Taking the motor to 3,000rpm and then letting the revs drop, the tickover became variable but not as excessively as before and would still spit back and die. So it would appear that there is a weak running problem and the unrestricted Goldie exhaust excaserbates this. I also again felt for fore and aft movement in the carb slide and didn't think it excessive, but noticed lateral movement which, I thought, excessive and hadn't tested for before.

Tonight, I went through a box of Amal MKI 932 bodies and bits from a friend who has Norton Commandos and has replaced the Amals with single Mikunis on a twin manifold (Take the hint! ) Amongst this lot there was an apparently unworn body, with no warping to the stud face and when the new anodised 31/2 cutaway slide was tested in this body there was no fore and aft or lateral movement. So decided to use this ex Commando body to replace my current body.

Transferred the air screw, slide screw, complete float bowl and it's contents, changed the main jet to a 230, along with the slide etc. to the Commando body. Then dismantled the stub mount from the front of the carb to use on the Commando body. The new Amal OEM 'O' ring sealing the face would not remain in the machine groove in the face of the carb flange and either side of the 'O' ring appeared to be wet with fuel. Maybe this isn't made from Viton and the fuel has affected it I don't know! So when fitting the stub flange to the Commando carb, I fitted a new 'O' ring, not from Amal, that I purchased as having been made from Viton. I also added an Amal paper gasket, with a light coating of Hylomar 300 RTV applied using a small artist's brush and bolted the stub and body together.

Mounted the carb and checked that the slide seats on the bottom of the venturi. Adjusted the air screw to 11/2 turns out from seated and the slide screw by 1/2 a turn in from starting to lift the slide.

Tomorrow, I will see if the engine starts any easier and whether I can achieve a steady tickover and clean pick up, with the 'SuperTrap' in place. If I can achieve this then I may try the Goldie again!

Good health, Bill

[Beach Life]

Dan,
If you contact Phil at Road and Race in Australia, he will help you out.

[graeme]

Hello Bill,

Is your fuel tank clean?
Good fuel, about 95 octate. The 98 premium stuff can give you issues. And the cheap stuff is not good.
Are you using any upper cylinder lube or additive ? If yes, don’t until you get it sorted.

Graeme

[Dan C]

Hi Beach Life,
Thanks for that but I'm wondering what he needs to help me with?
I have bought a fair bit of stuff off Phil over the past 30 odd years and at times he helped out with various issues. Good bloke although I've never met him in person!

Hi Bill,
You are leaving no stone unturned that's for sure. I'll be very interested to see if the new carb body makes a difference. I can say that the VHBs (square slides) had a lot of slide/body wear even after having the slides re-coated at great expense....(That was actually an example of Phils work that I wasn't really happy with). The PHF with round slide had no discernible movement in any direction but I couldn't get it to work at idle and for starting.

Very interesting that the exhaust made a difference. And if you think you are too lean then maybe use the other Amal that gives you the option of a bigger pilot jet?

Also all 3 carbs I tried (VHB, PHF and Mikuni VM) had 2.5 slides. I think this means the cutaway at the rear of the slide is 2.5 mm high and so 2.5 on a Mikuni is the same as 2.5 on a Dellorto. (If I'm wrong someone will advise!). 2.5 cutaways seem to be fairly standard recommendation for the singles.
So if 3.5 on the Amal means the same thing it will be leaner. If you have a slide with a lower cutaway it would be worth trying.

Not sure if I made it clear but in this thread but I think in a PM I mentioned to you that I first suspected ignition issues and so fitted a Sachse 6v system on the recommendation of Steve Craven. It was expensive but made no difference to the problem at the time. But since installation I can say it has performed fine. What amazed me was 4 days after I ordered it it was in my post box. Very efficient these Germans!

Cheers
Dan

[Bevel bob]

If you are using a Boyer or Pazon ignition booster (which made a huge difference to my 250) then you need to remove the points condenser completely. I would raise the fuel level to the point of it flooding and then reduce by a couple of milimeters. Its the only way to ensure a good idle and starting, especially if you have a hot cam or big carb.

[themoudie]

Good morning Gentlemen,

Bright, fresh morning, not a cloud in the sky and yet to fire the old bird up. However, I will just answer a few questions that your suggestions raise.

Fuel tank is as clean as a whistle, 95 octane pump fuel, with ~9.5:1 compression ratio. No Avgas in the tank or UCL but with the first tank that went through a 50:1 mix of quality 2stroke oil in the fuel, well agitated.

Amal slides are Imperial measure and for each whole number = 1/16" in the height of the slide 'cutaway', so the 31/2 slide that I am using = 5.556mm. A number 3 (3/16" or 4.7625mm) that was originally fitted when I first used an Amal MKI in 1977 proved to rich and in spite of changing exhausts the 31/2 has always performed well.

Thought about a Sachse ignition system, as my Electrex alternator is the model before an integral ignition system was offered, but on speaking with Nigel he advised that he had had two 450 engines that had 'issues' when using the 12 volt system, yet the 6v on smaller engines had been satisfactory. I suspect, as did he that the alternative ignition advance curves had proved tempting to the owners and an inappropriate curve was selected, too much advance at the top end of the range! Messy!

I have dug out the Boyer fitting instructions and it specifically says in the last two sentences of the first paragraph of the 'Notes'; "The unit can be used with engines having no distributor, but one unit per contact breaker is required, this applies to most motorcycle engines. The standard fitting instructions can be used but the condenser must remain across the contact breaker not the ignition coil." This is backed up by Fig.5 on page 2 of the instruction sheet that shows the condenser remaining wired across the points after the fitting of the inductive discharge unit. This was the same query that the auto electrician raised with me when I asked if they had any condensers in stock. " Why would you need one, the points have no current and are just acting as an electrical 'make and break' switch, rather than a magnetic field or infra-red beam?" I was tempted to disconnect the condensor, but haven't spoken directly with Boyer to find out "Why?" it needs to remain in the circuit. Job for Monday morning!

The 'StayUp' float has been used along with a brass 'Viton' tipped float needle, rather than the aluminium type (maybe less prone to vibration ) and the float level has been raised to be level with the top of the float bowl casting.

Off to see if it will start and then go for a canter, all being well! The day is too good to miss and if it misbehaves the Yamaha SRX 600 XTE engined monoshock will be out.

Good health, Bill

[themoudie]

UPDATE 19/09/2020 23:45pm

Well, that didn't work! Refused to run at all with the apparently better fitting MKI body, with no lateral or back and fore play with the anodised slide. Regardless of slide screw and air screw positions. Liberal tickler flooding produced a short burst, less than 5 seconds, at best and then deathly hush! After an hour I consigned the bike to the back of the workshop and went for a chilled solo run of the Unclassified roads for 100 miles on the SRX600.

I'll see what Sunday brings with the MKII! Or maybe even the original MKI body again! I must be barking!

Monday could be Mikuni day! After Dan's experience with one operating satisfactorily on the 350 at the 20° downdraft angle and another on a 20° angle manifold, one of which I have, on his 450, I could be persuaded.

Good health, Bill

[themoudie]

Morning George,

Wee question. How are you mounting the carbs if you are using variable thickness Tufnol spacers between the head and the carb?

Different length studs from the head casting or cap screws in to the head casting?

Thank you for your time.

Good health, Bill

[Duccout]

I feel for you Bill. That was strange that the bike would not run with the different carb body. What I hate about carburretion problems is that they are the same as ignition problems, so you are never sure that what you are changing is barking up the wrong tree!

What you really need is to borrow a carb from another bike that is running perfectly, so you can be absolutely certain that the carb is the problem. What happens if you buy a new Mikuni and the bike still does not run properly? How will you know if the Mikuni jetting is at fault, or if it is an ignition fault all along?

Funny thing is, years ago when I had my Sebring, I replaced the tiny Dell'Orto with a 30mm Amal Monobloc that I had laying around, and it ran perfectly and that carb stayed on the bike for the rest of the years that I had it.