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Carburettor jetting for low octane fuel?

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Carburettor jetting for low octane fuel?

Posted by Julian Wells at October 31. 2016

I've been wrestling with the carbs on my 750 and have come across the following in the Riders' Manual for 750 models with engine no. 220000 onwards (part no. 064135):

=============

ALTERATIONS TO BASIC RECOMMENDED IGNITION AND CARBURETTOR SETTINGS MUST BE MADE TO OPERATE SUCCESS­FULLY IN COUNTRIES WHERE NORMAL RECOMMENDED FUELS ARE NOT AVAILABLE

Compression ratio

8.9:1

9.5:1

std

alt

std

alt

Octane rating

99

96

100

97

Main jet

220

250

220

250

Ignition setting

28

26

28

26

Ignition settings given are maximum ignition advance (stroboscope) @ 3000 r.p.rn. A small power loss is to be expected when grade fuels and associated ignition and carburettor changes are utilised.

=============

The manual assumes 32mm carbs with standard settings of no.3 slides and 106 needle jets, needle in the middle notch.

 

I've seen nothing comparable in any other source (nor indeed have I seen the accompanying recommendation about retarding the ignition.

Given the octane ratings of currently-available petrol, this suggests we should all be implementing a significant increase in main jet size.

Comments sought ...

Re: Carburettor jetting for low octane fuel?

Posted by david_evans at October 31. 2016

You say "wrestling with your carbs" what exactly is the problem with your carbs? My 750 seems to rattle like pinking but once it is warmed up it's quiet. Only noticed it when I went out without ear plugsSmile

Re: Carburettor jetting for low octane fuel?

Posted by george_farenden at October 31. 2016

Hi,

Whilst I can't comment on your particular bike, the rationale of increasing the main jet size along with the retarded ignition is to keep the exhaust gas temperature sensible. When you retard the ignition timing, the combustion temperature increases, and hence the exhaust gas gets hotter, which can give the exhaust valves/seats a much harder time and also blue the chrome on the downpipes.

Enrichening the air -fuel ratio (ie. the bigger main jets) effectively cools the exhaust gas down again. "Temperature control" by this method is used on all petrol cars/bikes nowadays as catalysts and turbochargers don't like getting too hot.

Richer mixtures can also help with 'pinking' under load, again because the richer mixture burns slower.

Realistically, you would have to judge for yourself how often and for how long you ride at high revs and full throttle for this to be an issue for you.

Hope that this helps,

George.

Re: Carburettor jetting for low octane fuel?

Posted by Julian Wells at November 06. 2016

Thanks to David and George for responses, and apologies for tardy reply -- I have limited time for bike finagling at present …

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 timing

whereas 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 …

(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 "9.½:1", 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.

Attachments

Re: Carburettor jetting for low octane fuel?

Posted by Julian Wells at November 06. 2016

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

Attachments

Re: Carburettor jetting for low octane fuel?

Posted by david_evans at November 07. 2016

Whilst your main jet will start to take effect from about 1/2 throttle, the main benefit will be at the full throttle area so needle and needle jet along with slide cutaway will come into play in the "normal" running range. 25 mpg is pretty poor. Plug colour is difficult to use as a barometer with unleaded fuel and most Commandos run rich at low throttle settings to give a cough free pick up. I have tried most variations of carb on my Commando. It might be worth measuring the slide bore and establishing the level of wear in the body. I think 0.004" is standard but I'll go and play with my measuring stuff.

Re: Carburettor jetting for low octane fuel?

Posted by Julian Wells at November 08. 2016

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?

Re: Carburettor jetting for low octane fuel?

Posted by Julian Wells at April 15. 2017

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

Re: Carburettor jetting for low octane fuel?

Posted by david_evans at April 15. 2017

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.

Re: Carburettor jetting for low octane fuel?

Posted by Julian Wells at April 15. 2017

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.

Re: Carburettor jetting for low octane fuel?

Posted by Gordon Johnston at April 16. 2017

One little aside. A rich mixture burns faster than a weak mixture, effectively advancing the ignition.

The current problem is that petrol now has a considerably lower octane rating than when our bikes were built. The method of measuring octane rating has changed, so things are actually rather worse than simply comparing octane numbers will suggest.

Trying to run a CR as high as 9.5:1 is now rather fraught. You would certainly need to retard the ignition a couple of degrees to help compensate but pre-ignition or detonation is still a distinct possibility.

We are now looking back to the days when 7.5:1 was considered high compression.

As for mixture, with increasing ethanol amounts in the fuel, an increase in main jet size is no bad idea to compensate. If you look at bikes running on methanol, they are very high compression ratio (ethanol/methanol have good anti-knock properties) but an enormous main jet. The advice was to take the main jet and run a 1/8" drill through it. Mpg figures are rather alarming.

Re: Carburettor jetting for low octane fuel?

Posted by david_evans at April 16. 2017

I normally run on standard unleaded but thought, as I was doing some two up riding to the Prescott hill climb, I would treat it to some 97 octane. The bike went quite well and strangely didn't leak any oil despite getting a good thrashing. 97 octane is quite a lot more expensive than the normal cat piss though.

Re: Carburettor jetting for low octane fuel?

Posted by Gordon Johnston at April 16. 2017

I would suggest it's worth paying the extra for the premium cat's piss.

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