Carburettor jetting for low octane fuel?

Thanks to David and George for responses, and apologies for tardy reply -- I have limited time for bike finagling at present â?The discussion has gone in two directions that I hadn't anticipated in my original post (i) what's my specific carb problem? (ii) the rationale for increasing jet size when retarding ignition timingwhereas my original concern was (iii) the implications for modern owners of the factory's 1970s view of "low octane" fuel.It's convenient to deal with these in reverse order: (iii) The Rider's Manual (undated, but presumably no earlier than 1973, given the reference to 8.9:1 heads*) recommends a 13.6 per cent increase in main jet (250 instead of 220).This (and the attendant timing adjustment) strikes me as a sizeable change, particularly since what was regarded as low octane in 1973 is now higher than standard unleaded. That being so, and if one takes the Manual seriously, should not advice to modern owners take this into account?But if one refers to (e.g.) the Amal web-site and asks it to specify jets for a Commando it offers 220/230, standard/Combat. Similarly, the instructions for Pazon ignitions give the standard recommendations for timing, and this despite the fact that the quality of modern fuel is specifically given as a reason for fitting one of their systems. (ii) I understand the logic of retarding ignition to ward off the danger of detonation, but I'm not clear about George's argument about increasing main jet size as a consequence. I see that the combustion products will have less time to transfer their heat energy to piston motion, and hence will be hotter at the moment the exhaust valve opens. But how does a richer mixture help? After all, richening the mixture is quite literally adding more fuel to the fire â? (i) My problem is as follows: on acquiring my 1972 750 it was a very easy starter and would quickly settle into a slow steady tick-over.On the other hand it displayed classic symptoms of excessive richness -- "hunting" on modest throttle openings, such that one constantly felt obliges to either open or close the throttle, and fuel consumption was as low as 25 mpg.Furthermore, opening the throttle firmly would produce a very smart response, accelerating strongly up to an indicated 180 (on the kph speedo . . . )Dismantling the carbs revealed

(a) worn slides and needle jets (and hence presumably worn needles) -- picture of needle attached; for some reason attempting to add two pictures causes the message to disappear, so I'll post a picture of one of the slides in a second post

(b) needles with the clip in the bottom, in other words, richest, notch

(c) 240 main jets.The latter would be on the rich side for a Combat, but it's not clear that my bike is one: engine number is 211xxx and it has 32 mmm carbs and the close head-to-barrel finning, but against this there's no evidence that the barrel was ever painted black. Unfortunately there's no sign of the RH identification stamp above the RH exhaust rocker cover, and I haven't had time to remove the head steady to look for the "C" stamp (the mark of Cain?).As a temporary measure I restored the needles to the middle notch and this did make some improvement to the hunting problem, but fuel consumption was still dramatically poor.Having now replaced the worn parts with new (using Amal's anodised slides), and fitting new 220 jets it's apparent that the bike is now running very weak. The obvious response would be to raise the needles and fit 230 jets.However, I've yet to try this out, since on the test run with the new carb parts it also became apparent that the bottom oil tank mounting was detached. Further, the actual oil has now decanted itself into the crankcase . . . convenient when it comes to removing the oil tank to investigate its mounting!Further comments on any of (i) to (iii) warmly welcomed,Julian* In my original post I should have pointed out that my suggestion that the Manual referred to 9.5:1 heads was an *interpretation* of a typographical oddity; the actual heading of the relevant column is "9.?:1", that is, a decimal point followed by the printer's character for 1/2. But referring to the NOC on-line technical notes, I see that the nearest reported compression ratio is 9.3, so what the Manual actually has in mind is unclear.

And here's one of my worn slides . . .

David

It will be interesting to hear something quantitative about slide/body clearance and wear. Subjectively, my old slides were distinctly loose, but the new anodised ones don't have any perceptible play. This suggests that the wear was to the slides and not the carb bodies -- a relief, given the price of the latter.

An further question that occurs is to what extent is wear at the slides (presumably tending to weaken the mixture) offset by wear at the needle/needle jets (presumably tending to richen the mixture)? Or is erratic mixture the principal problem?

At long last (see separate threads on oil tank/battery platform/air-filter backplate issues) I'm able to report on my carb set-up saga.

1. restoring the needles to the bottom notch (i.e. richening) has cured the weakness resulting from running with the new needles on the middle notch
2. a relatively gentle excursion to the Pioneer Run (75 mph cruising on M25/M23, return via B-roads through Ashdown Forest and Biggin Hill) shows fuel consumption better than 50 mpg, which is the sort of level I would hope for.

Notes

1. I also went up to 230 mains; subsequently, as a precaution, I've re-installed the 240s that the bike came with
2. the above run also brought to light that while the previous consumption was definitely poor it wasn't as bad as I had thought. It turns out that although the speedometer is calibrated in kph, the odometer reports miles -- so rather than 25 mpg the bike was actually doing about 40 mpg; decidedly unimpressive, but not atrocious

David

Thanks for heads up; I did in fact do this -- thought I'd mentioned it in a previous post. Given the state of the needles (see pic in an earlier post) I took it for granted that the needle jets would be knackered too.

I've upped the main jets as cheap insurance against getting carried away before I get a chance to do a proper plug chop ...

Previously david_evans wrote:

Hi Julian, You say you have replaced the needles, have you also replaced the needle jets? This might improve consumption even more and as the jets are brass, they may wear more than the needles. If you get a tell tale puff of black smoke when accelerating that would indicate richness. while the main jets do come into play at low throttle settings, most of the running is reliant on slide cutaway, needle and needle jet. In my experience with Amals. a 3.5 cutaway will give spitting back on acceleration on a 750 so your 3's should be ok.