Author Topic: Carb Adjustment  (Read 1103 times)

Offline tommo903

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Carb Adjustment
« on: 04.07. 2017 09:49 »
Having now got my A7SS running, I took it out at the weekend for a few happy miles and found that when warmed up, it was revving too high when I pulled up at roundabouts, junctions, etc. Anyway, got the bike home and found that backing off the throttle stop screw alone made no difference at all but turning the air screw clockwise (richer) brought the tick-over down to a nice steady level. As I had the throttle screw fully backed off, I needed to do a bit of adjustment between the two and managed to achieve an acceptable tick over level with the throttle stop screw under some tension at least. I can't recall exactly where air screw was set at this stage, but it was certainly less than a full turn out (is this a problem?) instead of the 1.5 turns when I started. By the way, I had also pulled the plugs when I first got back and both looked pretty spot on...a nice light brown colour. Went for another run...no problems with the tick over but when I re-checked the plugs on my return, both were, not surprisingly, quite black.

So in terms of adjustment, I'm wondering what the consensus is...should I concentrate on changing main jet size or dropping the needle a notch? The carb is the original 376/15 which I have fully cleaned and replaced all internal parts with new items at standard settings. I have also checked the float level (plastic tube method) and it's just the tiniest bit below the pip, so I would consider acceptable.

Your thoughts appreciated...thanks.
Steve

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Online Klaus

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Re: Carb Adjustment
« Reply #1 on: 04.07. 2017 10:05 »
Hi Steve,

for this carb the setting is a 30 pilot jet  3,5 cut away 270 main jet and needle position is 3

With the years of use carbs, went the richer way by worn out the needle jet and needle.

For a first try you can lower the needle one noth.

Spend a little money for a new needle and needle jet and have fun.

cheers Klaus
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Offline tommo903

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Re: Carb Adjustment
« Reply #2 on: 04.07. 2017 10:16 »
Thanks Klaus...I have already changed the needle jet and needle with new and the carb has been set up as standard.
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Offline duTch

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Re: Carb Adjustment
« Reply #3 on: 04.07. 2017 11:02 »
 
I'm absolutely no expert with carbs, but will suggest at this stage not touching anything other than maybe going down a size on the pilot jet and see what happens...?
 I've had similar stuff and been up and down with the pilot to no great avail....but then I had a maggy issue too, so may have contributed, but haven't checked the plugs since the maggy rebuild- but is running much better and still only about one turn out at best idle....
 I think a lot depends on your riding conditions/style/stuff, and what stage of slide opening yo're on to get the plug reads. As in previous threads, when not doing a lot of 'highway-speed' riding, and even when I do at ~100Km/h (~60.25Mph), the twisty grip is only about 1/4ish open, so I figure the main jet doesn't even start to engage...

 Forgot to post this and klaus jumped in
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Offline Slippery Sam

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Re: Carb Adjustment
« Reply #4 on: 04.07. 2017 12:43 »
Tommo - worth a quick look through the Amal tuning guide.  It's pretty good at explaining what parts of the carb have what effect.  Depending on your throttle openings, different jets and the needle come in to play and it describes how to determine which way to go if adjustments are needed.
I appreciate that if you went a run and the plugs blackened up under normal riding it could be a number of things (as you're using a range of throttle openings) - but one thing that jumped out at me in your original post was the adjustment of the air screw to reduce the tick over.  My understanding of the tickover mixture adjustment (and it applies to nearly all carbs not just your Amal in question) is that - you should adjust the idle mixture for maximum tick over and reduce the revs by backing off the throttle stop.  By reducing the tickover using the air screw you're effectively upsetting the mixture to a point where it's not running at it's most efficient (in this case richening it till it starts to die a little).  it sometimes takes a couple of adjustments but worth doing it the right way round if you're having carb problems (gets a potential variable out of the road).
A test to check which area of the carb is rich (as evidenced by the black plugs) is to clean the plugs and run it on idle for a bit (no more than 1/8th throttle) - that would check if it's the idle circuits that are rich or not (I suspect it could be rich if you have the air screw pretty far in).  By doing the idle test - you are keeping, main jets, needles, cut outs and slides out of the test.
Thereafter - if that shows idle mixture is good you might want to try a chop test (you'll need to read up on that) - that will test the open throttle settings better.

PS - my old adage is - if it was running good before you touched it - then it's whatever you just did that's screwed it up.  That's why it's always better to do one job at a time.

good luck and let us know how you get on.

Cheers Raymond.
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Online JulianS

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Re: Carb Adjustment
« Reply #5 on: 04.07. 2017 13:00 »
With the throttle screw back right off the slide should completely close the carb and engine should stop.

If it does not stop then the cause could be simply that there is not enough slack in the throttle cable to allow slide to follow the throttle stop all the way down, or the body is distorted or that there is an air leak.

Air leak could be due to distorted flange or poor slide to body fit.

The carb gasket sets have a washer which fits under the float needle seat of the "new" monoblocs which are recessed to accept it. Fit the washer on an original carb, which was designed for metal to metal joint you will raise float level.

Last year I fitted new jet needle and needle jet on my A10 with parts made by W******. Could not get it tuned properly and had to junk them and go for genuine Amal, it then ran fine.
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Online Topdad

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Re: Carb Adjustment
« Reply #6 on: 04.07. 2017 14:26 »
Julian beat me to it. I'd check what effect turning the handle bars as ,the cable may be tight somewhere .also do you have any inline adjusters if so make sure that when at rest the slide is fully down on throttle stop .
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Offline tommo903

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Re: Carb Adjustment
« Reply #7 on: 04.07. 2017 18:06 »
Lots of good info there...many thanks to you all. In the short term, I will check the throttle cable slack...you may just have hit the nail on the head there Julian!

Steve
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Offline tommo903

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Re: Carb Adjustment
« Reply #8 on: 05.07. 2017 08:41 »
Well, I checked the throttle cable last night, slackened it off a tad and yes, what a difference! The engine now reacts to the throttle stop screw as I back it off...just as it should. I also re-set the air screw back to 1.5 turns out and let the engine warm up and it ticked over really nicely. I know it was one of those simple things that I should have checked, but hey ho, sometimes you miss the obvious!
Anyway, now that's doing what it should be doing, I can now start looking at the carb tuning in a bit more detail.

Appreciate the help..thanks!
Steve
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