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gunner, Nick H, slofut
Total Likes: 12
Original Post (Thread Starter)
#819199 08/08/2020 12:21 PM
by Nick H
Nick H
I'm trying to tune a pair of Concentrics on my Lightning.
Got the float heights correct and set the idle slow and matched on both carbs.
As I turn in the pilot air screw on the left carb the idle speed goes up and up steadily until the screw is all the way in.
The right carb behaves more normally and idle rises and smooths out at 1 and a half turns out.
Turning the screw in will make the mix richer, i know, so is something leaning out this carb like an intake leak?
Liked Replies
#819212 Aug 8th a 02:47 PM
by triton thrasher
triton thrasher
WD40 works for finding air leaks when the engine is idling slowly.
1 member likes this
#819200 Aug 8th a 12:28 PM
by kommando
Air leak or the pilot circuit in compromised, the pilot circuit has both fuel and air galleries so both need to be clear and properly sized to function.

[Linked Image from]
1 member likes this
#819205 Aug 8th a 01:52 PM
by gavin eisler
gavin eisler
If there is a balance pipe fitted, temporarily clamp it off, or plug it. Tuning twin carbs properly with the balance pipe operational is impossible for me . reconnect when idle is good on each side.
What K said about air leaks, check the float bowl is not warped and the gasket is A1.
1 member likes this
#819255 Aug 8th a 07:54 PM
by gunner
Just checking but I think you start with the basics which means cleaning out the carb idle circuit with a tiny #75 drill on a plastic extension tube, old guitar wire or similar, these old amals have a habit of gumming up the circuits especially with modern fuel which needs to be fresh.

Having the left idle screw turned all the way in means the idle circuit on that carb is running on neat fuel with no air, so something is not right.

I don't think using propane in the inlet manifold is safe and even using WD40 will only show that you have an inlet manifold leak which is almost always caused by bowed flanges on the carb caused by over tightening the nuts. Your best bet is to remove the carbs and check the inlet manifolds for bowing, there's an old post Here which explains how to fix.
1 member likes this
#819332 Aug 9th a 08:22 AM
by kommando
The pilot fuel circuit starts with a gallery on the inlet on the opposite side to its air inlet and is plugged, this plug can leak air. For a test smear some silicone goo over the plug.
1 member likes this
#819334 Aug 9th a 09:16 AM
by gavin eisler
gavin eisler
When the pilot air screw is turned in from the optimum idle mix, the revs should fall and the engine will run ruff, 8 stroking at the extreme then stall . Correct position is set to max idle speed then turn in approx 1/8 of a turn.
If you find the revs increasing all the way in something is not right, probably a blocked pilot passage.
If the jet is clear then look hard at the two holes in the venturi floor, fore and aft of the slide lip cutaway on the carb floor, these are bigger than the pilot jet but will still block from oxides if left damp/ with old fuel residue.
You can check the holes in the floor with the float bowl removed, throttle open, plug the pilot fuel port on the bowl flange with one finger, spray WD 40 or carb cleaner in the pilot air port on the lower lip of the bell mouth, you should see two stream of juice coming up through the holes in the venturi floor.
If it passes this test but is still not tuning correctly, then there is stuff in the internal drillings.
I have removed the welch plug for access to these in the past, with the plug removed the true horror of mung/ oxide goop is either revealed or not, Amal sell new Welch plugs, I got away with using a wee drill and a self tapping screw to pull the plug and resealed it with epoxy when clean, if the plug isnt too fecked after removing this works, new plugs are obviously a better idea. new carbs are an even better idea.
1 member likes this
#819335 Aug 9th a 09:24 AM
by gunner
Could be a couple of things like the idle mix screw is distorted or worn and isnt seating fully when screwed in, some air may be bleeding past. Additionally are the choke cables present? If they have been removed the holes can be an air leak. Also check the manifolds for bowing as described.

Worth also checking whether there is a screw in idle mix jet fitted, normally there isn't and all you see is a pressed in jet on the underside. It may be one carb does have the screw in idle jet whilst the other doesnt, so you may have different idle jets.

Since you have twin carb's, its worth checking that the slides are synchronised. When the throttle is closed you should be able to hear both slides 'clack' against the throttle stops at the same time. Some use lolly sticks under the slides and then adjust the throttle stops so the sticks are equal in height. An even better option is to use vacuum gauges to see how each carb is behaving.
1 member likes this
#819378 Aug 9th a 06:12 PM
by Nick H
Nick H
Problem solved.
Took the carb off and put the mounting flange on glass. I could get an .008" feeler gauge under one side. Other side was flatter. Made up a device to bend the flange back with a bolt through the carb (crude version of method used by JH in gunners link above). It helped the bowing of the flange some but I didn't want to break anything so didn't put a lot of force on the tool.
Remounted the carb adding a thin o-ring to the gasket which I've heard is a no-no but this is the thin o-ring which would never seal on it's own.
Anyway, pilot air screw behaving normally now.

I have also removed the welch plug on a Concentric. It's like popping a zit - I couldn't resist. I had bought replacement plugs from Amal. I mutilated the ones I did, there was no saving them.

Float height:

My experience with Concentrics consists of 4 carbs. The two on my BSA now with Stay-Up floats and two older (no float drain) with original floats.
In both cases the float (fuel) level was too low when I got them. But the needle seats were already as low as they could go. I have heard other people complain of this and I don't think it's a case of the needle being too long..
With the Stay-Ups you can bend the tangs a little but I found that if you go too far, this can put an angle on the tangs to the needle that can cause the needle to stick in the seat and generally messes up the smooth operation of the float.
And what do you do with the original floats where you can't bend the tangs?

My solution was to heat the carb and remove the float needle seats and grind a bit off the bottom of them. I forget how much, maybe a tenth inch. Then re-heat the carb and push the modified seat back down as far as it can go again. Now my fuel height is perfect and my Stay up tangs aren't bent to extreme angles. Helped the older carbs also.

Has anyone else tried this?
1 member likes this
#819450 Aug 10th a 07:51 AM
by gavin eisler
gavin eisler
Float needle valves, originals were white plastic, later came brass tipped with viton, later still alloy tipped with viton. The latest alloy types work best, the plastic types either work well or dont seal properly, the brass types seal well but are heavy and need less fuel in the bowl to seal. What type have you got?
I have never messed with the needle seat, IMO its not a big deal, so long as both floats sit at the same height.
Seems to be an American thing to do. There is an easy experiment that shows how little float height matters.
Forget to switch on fuel, ride away, notice anything, yup, the bike runs fine until the bowls are empty, float height not so critical.
1 member likes this
#819382 Aug 9th a 06:22 PM
by gunner
Great news glad you got it working, I have heard reports of others changing the float seat height as you describe, so noting for the future in-case I have issues.
1 member likes this
#819465 Aug 10th a 12:32 PM
by RF Whatley
RF Whatley
THE most over-looked part of the entire system, and the place I always begin is the balance hose running between the 2 intake tracks. Those hoses are always cracked and leak air because people will use any old hose. They're not thinking about all the fuel and heat in that location.

I always start with American made 1/4" ID fuel hose that complies with the SAE J30 spec. If you'll grease the intake nipples, then the 1/4" ID hose will slide right on and STAY in place. If you'll take the time and properly fit the correct length hose, then you won't need any fuel line clamps. That's the way they were from the factory: tight-fitting black hose with no clamps.

Hop this helps.
1 member likes this
#819471 Aug 10th a 01:19 PM
by Bob Buchanan
Bob Buchanan
Just to add a bit on cleaning the idle circuit. Instead of removing welch plug, I have drilled lead plug in intake, used drill bits by hand to clear lime up to jet back, push shank of #78 drill (1/16") thru jet. Tap hole to 8/32" and plug with allen screw ( loctite) .
1 member likes this
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