450 desmo carb

[themoudie]

Aye Graeme, Colin and Jordan,

No updates, but just a wee reply to your queries etc.

I pondered long and hard about purchasing a Mikuni, but in the end was swayed by the frequent and successful use of AMAL MKII's by the racing fraternity, my having plenty of jets that fitted and the fact that they appeared to be relatively simple, easy to tune instruments. Like a lamb!

I too have used worn out AMAL MKI's and Monoblocs and still achieved good running and mpg. But, the old MKI slide has a smaller O.D., greater weight (35 grammes), softer bearing surface than the MKII slides (anodised), so in theory because of it's smaller bearing surface to the body than the MKII slide, it should be more prone to wear between itself and the bore and then tilting in the bore. However, as this is a new carb body and slide, the "running in" period for the slide and bore is taking longer, because of the greater bearing surface, anodised alloy finish and less weight of the slide. I have yet to put a straight edge down the face of the carb bore to see if the slide seat in the venturi base aligns with the bore or is slightly within the bore. This could be a very small machining inaccuracy, but sufficient to allow the engine to draw air around the slide, especially once the carb body expands after the engine has been running for ~4 minutes in the workshop or after a 6 mile canter. That might be a sop to AMAL and some may choke and cry "B******s!". Having got this far "Pink elephants might fly!"

The "stronger" springs are on order and the lead is cut ready to fit inside the bore of the slide, 40 grammes of the stuff and that will bring the overall slide weight up to 90 grammes. That alone may be sufficient to "pile drive" any minute slide seat inaccuracies into submission. I will post further results. Now where is that LeapFrog?

Good health, Bill

[blethermaskite]

Bill, same problem, different machine (but not a bike) so I have a 1934 Morris 8 based post vintage racing car, when I first refurbed the power unit (918cc 4 pot sidevalve)I made up my own inlet/exhaust manifold under the advisement of the late John Crossley of Crossley racing cars who had remarkable success with small Ford sidevalve powered racing cars in the 50s and 60s, in the main John advised me to make up a small bore exhaust header pipe system to increase gas velocity in the pipework producing a significant extraction effect which in turn would help the very poor breathing of the sidevale engine by increasing depression in the cylinder on the inlet stroke....to quote John " best treat a small sidevalve engine a bit like a 2stroke to get the best out of it". Sorry to ramble on I fitted a pair of pre monobloc Amal std carbs on this engine 1" bore used but in as good condition as I could find.......took a bit of time to get them set up nicely but eventually they were sorted and the car ran very nicely on them,.......well until it got fully hot .......and then what happened?....... the revs would not drop below 1750rpm unless I pushed both slides tight to the back of the carb bodies, no matter what I tried I could not fix this problem, all your stuff, fuel levels, air leaks,jetting, stronger springs, weighted slides, nothing made the problem go away, so in an a really ironic sense (for you) I solved the problem by replacing the early Amal carbs with a pair of 1" mk 1 Amal concentrics again used but in very good condition and with new zinc alloy slides. the only conclusion I could come to was that the increased vacuum in the inlet manifold was drawing air and fuel past the closed (slightly worn) slides in the old carbs and the newer less worn concentrics solved it, I know this is not going to help you but just more of a commiseration as I have been there too, keep at it!
Cheers,
George

[themoudie]

UPDATE 17/3/2012 N.B. George posted his insight whilst I was typing this lot out!

I measured up the clearance between the new AMAL 2932 carb body and the new 3.5 slide and a new 3.0 slide, just in case of excessive ovality in the carb body or a slide, allowing air to leak past into the inlet. Whilst there is a positive difference of 0.002" (0.05mm) between the venturi measurement and the side to side measurement, this is within the production tolerances described on the production drawings for the AMAL 2900 series carb. Furthermore, the longer the trial progresses and the hotter the engine becomes, the effect of applying horizontal pressure to the slide reduces, until there appeared to be no effect and reduction in engine rpm at all.

I then cut two discs of 1mm flashing lead, to sit beneath the spring, with a 12mm circle cut out to allow the needle/clip assembly and the throttle cable into their respective positions. This added 30 grammes to the weight of the slide, bringing it up to 80 grammes and preloaded the 'standard' return spring by 2mm. Repeated the running test, blipping the throttle and waiting for the revs to return to ~1,200rpm, whilst the engine temperature built up. Same results as before, so Graeme's prediction, despite my alternative thoughts proved correct. No change!

I have another, but old and used 2932 carb that had a siezed cold start jet, the float bowl has been butchered to remove it and from which the chrome/brass 3.0 slide was also butchered to try and make a 3.5 slide profile. I received this as a gift! There appeared to be nothing amiss with the carb body. So, I took all the parts from the new carb and built up the old carb body, with them. With this set up there were minor adjustments required to the air screw and the slide screw, to achieve a steady ~1,000 - 1,200 rpm tickover and I checked the mixture by using the Gunson flame view plug, as in previous runs. However, the results were the same. Once the, engine temperature was at running levels, blipping the throttle resulted in the revs staying up at 3,000 - 4,000rpm range. Horizontal pressure on the slide, in this carb body, made no reduction in the rpm and testing for air leaks, again proved fruitless. Even if after stopping the engine, at working temperature and then trying to re-start it, a single kick with the cold start plunger raised is required, before lowering the plunger and kicking over again to start. To add final insult to injury the carb was blown off the spigot, again!

The mystery continues, 3 carb bodies, 1 x AMAL 932, 2 x AMAL 2932, with all of them once the engine temperature rises to working levels there appears to be sufficient draw through the carb inlet to maintain a constant 3,000 - 4,000rpm range, with the slide on the base of the venturi. If the revs are reduced by momentarily blocking the venturi beneath the slide, the revs drop to ~1,000 - 1,200 rpm and the motor will idle very steadily and pick up cleanly when the throttle is either gradually or snapped open. But then the venturi effect appears takes over.

Reduce or increase the length of the inlet tract?

Stick a K&N filter over the inlet, rather than the short bellmouth?

My induction dynamic physics and mathmatics are zero, so where to now?

Good health, Bill

[themoudie]

Aye George,

SNAP!

However, I'm acheiving this with a new carb!

So, how do I gag the bugger? That AMAL 2932, Witty ported head, Piper 173A cam and a gutted Gold Star silencer, with a single plate baffle with a hole of 28mm ID appears to be working for the bottom and mid range. Even the gutted Gold Star silencer, with no baffle and a 45mm outlet was producing the same symptoms.

Thank you George for your interest and time.

Good health, Bill

[graeme]

Bill, do you know any priests that can perform exorcisms ?

Graeme

[themoudie]

G'day Graeme,

No!

This is one of those made by man and so can be sorted by man, problems!

Some fine stainless steel mesh over the bell mouth, or lengthen the distance between the face of the head inlet and the needle centre and also the distance between the needle centre and the edge of the bellmouth.

I will post further info.

Good health, Bill

[graeme]

When exactly does the carb blow off from back fire ?

The only way I can think that would happen is if there is flame present in the manifold ??? How ?

Inlet valve timing overlap,,
Gas flowing back from the exhaust and entering the inlet, valve timing ? Cam ?
Spark at the wrong time ? Is the ignition wandering ?

Things that make you go Hmmm ????

graeme

[Jordan]

What if the mixture is so lean and slow-burning, it is still flaming when the inlet valve opens?

[Duccout]

Is the back-fire due to fuel building up in the inlet port, then igniting? If so, what would that point to? Valve timing out? But Bill has checked and re-checked.......

[themoudie]

Good morning Graeme, Jordan and Colin,

Thank you for your thoughts, I suspect that due to the specific engine intake flow characteristics that I have accidently created, along with the increased inlet valve timing overlap of the Piper 173A cam, compared to the OEM MKIII 450 cam's valve overlap, is causing me the backfire problem.

When the engine is hot and I have stopped it by turning the ignition 'OFF', if I wish to re-start the engine, it requires a kick with the cold start activated and will fire once, but not run. I then de-activate the cold start, kick the engine over in the usual way and it runs; straight up to 3,000 - 4,000rpm because I have had to open the throttle to catch the motor. Then because of the inlet dynamics, I momentarily need to obstruct the airflow beneath the slide that is at rest on the floor of the venturi, so that the engine revs drop and then stabalise at 1,000 - 1,200rpm. SIMPLES!

Andy at SCR did say that he thought that increasing the inlet tract length between the cylinder head inlet stub face and the carb needle centre and the overall inlet tract length could be the answer. But, by how much and without cutting the frame to bits, is the question?

Will lengthening the inlet tract make the old bird more like a 'DYSON Cyclone' vacuum cleaner than it is now, or will it enable the slide to control the intake flow dynamics? More "suck it and see" to follow.

Good health, Bill