Further carburation questions...

Nige W
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Further carburation questions...

Postby Nige W » Sat Aug 25, 2018 5:24 pm

Hi All.

I put a previous post on regarding excessive fuel consumption (which is still a symptom) but I have further symptoms/characteristics of this particular bike which may help with a diagnosis - (though still think a brand new carb might address everything).

Today I went out on a run. The bike started firtst kick before setting off (petrol on, tickle carb, three kicks, choke on, ignition on - fires up easily). However, I stopped for five mins 20 miles on and, on trying to re-start, it just wouldn't have it. I removed plugs, kicked it over, re-fit plugs and still no joy (though a lovely loud explosion in one silencer!) Eventually re-started with a bump downhill and then ran fine. If the bike stands for half an hour or so then it fires up straight away (without any choke). Whilst running, if I do put any choke on at all it dies.

A couple of things I've noticed - idle level rises quite a bit when engine heated up, and no futher adjustment available on setting of slide height (so fully seated). Pick up/acceleration from closed throttle is perky, but there seems to be a spot at part throttle where it doesn't want to rev any more/deliver more revs, however I noticed that if I wind the throttle open more it picks up again and delivers more power and revs out.

I'm not a carb expert by any means, though the black plugs and the fact it falters with any choke at all once running would indicate a rich state. However, the higher idle when hot could indicate a lean one.

Any ideas please?

Nige

bh.sprint
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Re: Further carburation questions...

Postby bh.sprint » Sat Aug 25, 2018 8:43 pm

Hello,

When putting the Mikuni carb on the Arrow it started OK cold and warm but did not start easily when hot ( after about 25 miles ). The problem was the pilot jet which was too large. Dropping down two sizes cured the problem. The engine also had a heavy note to the tick-over with a smokey exhaust from the silencers ).
I have come across problems with Amal carbs where the pilot circuit is blocked or the pilot jet does not set properly.
At what throttle opening does the lack of response appear ?

Brian H.

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Re: Further carburation questions...

Postby ken-drink » Wed Aug 29, 2018 1:57 pm

i had similar probs with my 1960 leader, gave up with the monblock carb and fitted something else, which was made by JRC engineering in the USA, needed a couple of mods, but now runs well, starts fine hot or cold and will do approx 50mpg at the mo as it is still running in and running a bit more oil than needed, once i have enough miles on it i will lean it off to 32:1 and put slightly smaller jets in which should improve the mpg a bit more.
see my earlier thread memberlist.php?mode=viewprofile&u=2518
Ken

Popester David
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Re: Further carburation questions...

Postby Popester David » Mon Sep 10, 2018 10:48 am

Any one come across this problem? Bike can be started and ticks over evenly, revs responsively but soon as I put it into gear and try to drive away engine dies completely. Almost like fuel starvation. However, tickle button indicates there is plenty of fuel in float chamber.

bh.sprint
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Re: Further carburation questions...

Postby bh.sprint » Mon Sep 10, 2018 8:32 pm

Hi ,

Does the engine have standard port timings/sizes ie. have they been altered /made bigger ?

And - does the carb have the recommended jets etc.

The reason I ask is because when I put the alloy cylinders, which had bigger ports, on the Arrow it would not pull away very well, it turned out to be the needle jet which needed to be bigger ie the mixture in the middle range was too weak. Opening the throttle wider brought the main jet into play so the bike would run but not on mid range throttle settings.

Brian H.

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Re: Further carburation questions...

Postby Popester David » Mon Sep 10, 2018 9:25 pm

Thanks Brian.

Everything standard on the bike and it was running perfect on a 30 mile run to Knebworth. Just wouldn't get me home :-( Back home now courtesy of RAC recovery ;-) Best regards, David

bh.sprint
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Re: Further carburation questions...

Postby bh.sprint » Tue Sep 11, 2018 5:54 pm

Hello David,

The only other item I can think of is a blockage in the carb primary circuit/jets, which means a good clean/blow out of the passages in the carb. body.
I've heard of one carb body where the primary jet would not seat properly, perhaps due to not being threaded deep enough during manufacture.

Brian H.

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AndyEames
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Re: Further carburation questions...

Postby AndyEames » Tue Sep 11, 2018 9:23 pm

Might be of interest : I was attempting to assemble 1 good carb ( 375 monobloc ) from 2 for a friend. One of the bodies had a 4 stroke emulsion tube ( the brass tube that protrudes into the venturi) i.e. it has a 'square' top instead of being cut away on the engine side. The cut away version is , I believe, the correct type for a 2 stroke. Could this be the problem ? Also 2 drillings opposite each other across 2 flats on the needle jet is said to aid acceleration.
Andy

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Roger James
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Re: Further carburation questions...

Postby Roger James » Wed Sep 12, 2018 2:37 pm

As a point of interest, the very early production Leaders had a flat top emulsion tube fitted and a different size main jet to suit, Ariel did change to the angled emulsion tube that I've been informed are used on a 2 Stroke engine, that said, the 389 monobloc carb that is fitted to the Salt Arrow Production racer has a flat top emulsion tube fitted (with a 570 main jet to feed the porting work carried out on the engine), this has been on the engine for around 5 years now and has not given any running problems.
A few years ago I had a 376/277 carb for a Sports Arrow refurbished by a well known person in that line of business (the bore was machined to eradicate general wear and score marks and the throttle valve re sleeved to suit, however it never performed very well no matter how many times I stripped it down, checked for cleanliness and any other problems it never improved, in the end suspecting that the was an internal blockage or restriction I fitting another carb that restored performance of the engine, more or less proving that the refurb carb was at fault.
One possible course of action whenever a carb is suspected of being at fault is to take one off a running engine that is known to be working o.k. and fit that to the engine you are trying to get running properly, I appreciate of course not everyone has a second bike with the same carb fitted that could be used, this is where our club comes into its own and getting to know fellow enthusiasts who live nearby who might be able to help out with the temporary loan of a carb.
Jess

bh.sprint
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Re: Further carburation questions...

Postby bh.sprint » Wed Sep 12, 2018 8:08 pm

Hello,

These two pages from a 1960's Motorcycle magazine may help.

Brian H.
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