Author Topic: Problem with first road run.  (Read 1592 times)

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Offline Chris54

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Problem with first road run.
« on: 19 Jul 2015 at 20:33 »
Hi All
I have just had my first road run on my 1914 2 3/4, couple of teething problems to sort out but it went so well, after nearly 2 years of work i am so pleased with the result.

One thing that i am concerned with is the carb, Amac, the engine starts and runs with very little opening of the throttle slide, the air slide seems to be the dominant one, open a little to start and then use as throttle, when hot and traveling in high cog with the air slide fully open all the throttle slide seems to do is make the engine sound louder, yes the speed stays the same just the engine note gets louder, close the throttle down and it gets quiet again.

when i stopped i checked the throttle slide position, about 3mm above the spray bar, starts and runs in this position.

checked the float level, seems OK, just below spray bat, jet clear (28), inlet manifold for air leaks, timing, valve gaps, all seem OK and it starts and runs well, no black smoke so not to rich, only pops back if the decompressor is used,

Any suggestions that will help are needed.

What size should the jet be,

Had one thought, swap the cables over and use the air slide as the throttle, too easy but not right, want it right.

await your comments.

Regards, Chris

Offline cardan

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Re: Problem with first road run.
« Reply #1 on: 19 Jul 2015 at 22:47 »
Hi Chris,
Early carburettors are not automatic: at each engine speed and load the optimum settings for throttle and air are different. Even a perfectly rebuilt carburettor would required both the throttle and air levers to be adjusted as conditions change. You should think of this as one of the joys of riding a veteran motorcycle.
That said, for high speed running - say 50kmh (30mph)  - the engine should take something like full air. If not, adjust the main jet until it does.
Triple check for air leaks in the manifold joints as these won't help, and your efforts to understand what the motor wants will be in vain if your carburation is being dominated by the vagueries introduced by air leaks.
Also check that your slides are a reasonable fit in the throttle body. If they are worn running can be erratic.
Otherwise, I'd expect the levers to be moved more-or-less together to produce what modern riders call acceleration. On a veteran, just pick a lazy speed to travel at, and enjoy adjusting your mixture so that the bike is running "just so".
Well done on the rebuild.
Leon

Offline graeme

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Re: Problem with first road run.
« Reply #2 on: 20 Jul 2015 at 00:15 »
Leon talking sense as usual!
Can I also add that an easy way to adjust the jet size is to fill it up with solder, then drill a smallish hole in it and check how it runs, then progressively open up the hole with a jet reamer or number drills. If the jet is too small a size to start with (though it doesn't sound like it to me with the symptoms described), you open it up with progressively larger number drills

Offline Chris54

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Re: Problem with first road run.
« Reply #3 on: 20 Jul 2015 at 05:44 »
Hi Leon and Graeme

As usual, a lot of sense coming from down under.

I will check inlet for leaks again, belt and braces with more PTFE, may even turn up a couple of seals.

with that done i will recheck the running, i have a No 33 jet so may try that to see what difference it makes.

After that its just as Leon so rightly put it, On a veteran "enjoy adjusting"

Many thanks

Chris

Offline psjc

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Re: Problem with first road run.
« Reply #4 on: 07 Apr 2017 at 10:38 »
Having trailed a cloud of blue smoke to Brighton and filled up with petrol before the obligatory coffee stop I am wondering whether my wonderful new 1914 2.75 HP with clutch has an incorrect float level setting. So takes carburettor apart (AMAC with heating jacket etc and No 27 main jet installed). Now in most of my vehicles the float level is set a smidgeon (between 1-2 mm) below the upper surface of the main jet. Ten minutes with wife's kitchen scales and a crude drawing suggests that the 11.5 gm float would need to be about 10 mm submerged to close the inlet valve. Problem is that this 10 mm is way above the top of said jet.

Finally the question! Does this AMAC carb have a float level set above the top of the main jet and if so where should it be please? If the float level setting is just below the sprayer bar as I now suspect what does the main jet do exactly except perhaps to snub the fuel flow into the chamber below the sprayer bar?

A secondary but no less pressing? question. Where can I get a new brake block for the rear brake. The original is made from rubber with laminated cord and is seriously past its best. If I get some block rubber can I machine out the dovetail locking bar and if so any tips on feeds and speeds etc?

Any help/guidance appreciated.

Shaun Crofton

Offline cardan

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Re: Problem with first road run.
« Reply #5 on: 07 Apr 2017 at 13:02 »

Hi Shaun,

The veteran AMAC is a bit weird: the jet is submerged and serves as a metering valve but not (as on later carbs) a sprayer. Instead the sprayer is a row of little holes along the top of the ridge in the throttle body under the slide. I'll attach a couple of articles to give you the idea.

If the carb does not drip when the fuel is on, the bike is on its wheels and the motor is not running, then the fuel level is not too high.

If the bike is running rich (black smoke and high fuel consumption), the air slide should be opened fully. If it's rich with the air lever open, then the jet is too big. If you have a set of number drills you can measure the jet size, or use a tapered needle (carburettor or darning!) that goes part way into the jet and measure its diameter at the point where it sticks with a micrometer. Find a bit of fuse wire say 10% the jet size, feed it through the jet and wind it around so it doesn't get lost. Go for a ride and see if things are better.

Good luck!

Leon


Offline graeme

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Re: Problem with first road run.
« Reply #6 on: 07 Apr 2017 at 23:15 »
Great advice as usual Leon.

Brake block material can be purchased from the VMCC. I can't give advice on lathe speeds, I have just cut and filed it into the required shape for my veterans

Offline psjc

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Re: Problem with first road run.
« Reply #7 on: 08 Apr 2017 at 12:04 »
Many thanks to Leon and Graeme for the advice which will now be implemented I hope. Having done the VCCA car rally in Tasmania last November I drove the Renault AX back to Adelaide from Melbourne and had intended to thank Leon personally for all his sound advice to the general community but as ever time intervened and the weather was not what I needed in an old open car!

Again many thanks and hope to do a bike rally in September so may see all the famed Australian Douglas,

Shaun Crofton
London

 

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