Carb Adjustment

Russ,

It would be good if you provided a history of your bike. It makes a difference to the experts who have the knowledge to advise you.

The baseline for the air screw is 1-1/2 turns out. I can tell you that, behind the air screw is a tiny jet that measures .016". It must be clear for the bike to start and idle properly. A #78 drill bit is just the thing to clear that jet. Some recommend an old guitar string or a bread tie but the drill bit is better. You can order them online for cheap.

There are also two tiny holes at the bottom of the throat of the carb on either side of where the slide contacts the bottom and toward the intake side. These must also be clear.

Go to this link to learn all about your carb.

http://www.jba.bc.ca/Bushmans Carb Tuning.html

You could have other issues as well such as a warped float bowl and others but my money is on a clogged pilot jet.

Henry

Go to your local music shop and buy a .016" guitar string.
Straighten it for about 3 inches and file the end so that there are no burrs.
Remove the horizontal mixture screw.
Check the round rubber seal.

I use a LED penlight so that the jet can be seen.
Insert the wire into the jet hole as far as it will go (about 1.5" as Pete says).
Turn it and push it in and out a couple of times.
Spray some carb cleaner into the jet, the spray can be seen exiting the small holes in the carb body (ventura).

Now you know the jet is clear, screw the adjuster-screw fully in and unscrew 1 1/2 turns.

Turn the throttle-stop screw in until the slider begins to rise.

Start the motor and adjust the throttle-stop screw for a fast tickover (1000+ish rpm).
Now turn the air/mixture screw IN (slowly) until the engine begins to stutter, then unscrew for nice, even tickover.
You might like to mark it with a felt-tip.
Repeat the above by unscrewing it OUT and screw it back in.
The `sweet` point is half-way between the stutter points.

Adjust the throttle-stop screw for the desired tick-over. (about 1000-ish or less rpm)

I don't want to hijack this thread too much but if I could for a moment ask a jetting question to Caulky and others. My bike is a '70 T100C. I just put a brand new Amal Premium carb on (best thing I've done for the old girl yet by far). It is all setup with the stock slide, main, pilot and needle. Starts and idles lovely, but seems a tiny bit lean at about 1/4+/- throttle. Needle is already in the highest position available.

I say a "tiny" bit lean, as once it is good and hot, it runs sweet, but for the 1st 5-10 mins of running it needs the choke to be closed, if temps are 70F or lower, to take any substantial throttle without banging and popping. Choke on and it goes away, which to be confirmed a lean condition rather than rich. Float height is right on spec.

Since needle is already all the way up, the only way I can think to add more fuel would be a larger main, or raise float height...aside from a richer needle which I'm not sure is even available? Suggestions?

OK, I've pretty much formed a plan of action, involving checking the sparks, making sure the battery is properly charged and cleaning the pilot jet as Caulky described. Just got a few last questions before I set to work...

I can't find anything in the workshop manual about the torque settings for putting spark plugs back in. Any suggestions?

Battery is showing 12.17 volts. Too low?

I have some guitar string arriving tomorrow - do I definitely need the carb cleaner too or is it worth my while poking the jet anyway?

Doesn't clearing the jet with it in situ just push gunk back into the fuel passageway where it will block up the jet again? Sorry, bit confused on this point.

Regards,
Russ

Russ, 12.1V is no good at all for E.I.
Recheck the ignition timing, most important!
When I begin to try to start the engine after a overhaul or major service, I charge the battery (maintenance charger for a day or so).
The voltage is over 13V...this is just the surface charge which drops to 12.6V [fully charged] after a kick-over or two.
[6 cells at 2.1V each]

The more it is kicked, the more the voltage will drop, if it doesn`t fire up.
Get a couple of new Champion N3? plugs.
Charge the battery and with the plugs out, tape both to somewhere bare metal (frame/earth) next to each other. (not near the plug holes and fuel OFF)
Voltmeter across the battery.
Ignition on, kick-over.
Both plugs should have nice blue sparks, at the same time.
Yellow or intermittant sparks are no good and the plugs will soot-up quickly, causing further misfiring or a stall.
Watch the battery voltage...less than about 12.4V needs a re-charge.
I put the charger on (fuse out) between adjustments/fiddling, until the led goes off.

I use a ring spanner 11/16? until the plugs bite, then a quick/light tighten to crush the washer.
Plugs do not need to be very tight.
Electrics now sorted.

Now fuel delivery.
2 1/2 turns out makes the mixture very weak...too much air.
Do a fuel flow check by unscrewing the carb drain plug. (fuel tap off)
Use a clean container to catch the fuel, throw it away.
Turn the fuel tap/s on.
You should get 300ml or more per minute.
Check for air leaks around the carb joints.
Check the tappet gaps.
How old is the fuel?
If its more than a few months old, drain the tank and use new stuff.
The ethanol attracts water which is denser than the fuel.
Check the tappet gaps.
Clean the air filter.
I dont have chokes on my bonnie, just the ticklers.
Have you got an aluminium float needle?

Update and conclusions based on all the info supplied so far:

I have points not electronic ignition. So battery voltage probably not a factor here.

Cleaned the pilot jet with guitar wire as instructed - drained the float bowl, bits of crud came out. Fired it back up, idling much better but still a little erratic.

The air screw was originally about 1/2 a turn out or maybe less. Must have been a very rich mix. Any special reason why this would be? It was tuned by an experienced shop so no reason why it would have been done badly. Anyway, after I cleaned it all out I set it to 1-1/2 turns out and then adjusted the throttle stop to get the idling speed right. The idling revs dip and rise across a 500 rev range so I had to set the throttle stop such that it was wavering between 1000 and 1500 revs otherwise it dipped too low at times and would stall.

Obviously I still have more to do. You've all mentioned other checks I can do which I will do to pre-empt any further problems: sparks, points, carb gaskets etc. There is crud visible in the bottom of the reserve fuel pipe and in the bottom of the fuel tank. Looks like metal filings! My thought is that this is not an immediate problem unless I run out of fuel and have to switch to the reserve line but I probably ought to get this cleared fairly soon. My bet (as was your consensus) is that my immediate idling problem was due to clogged pilot jet. Other little symptoms make me think the carb needs reconditioning, and now I understand a lot more about it, I could do with taking it back to the local guy and have him show me a bit more about tuning in practice. But it's running!

Thanks for all your help. This may seem like basic stuff to you guys, but I am working up to a 6 month road trip next year and my hope is that I will be confident enough with basic maintenance to take this bike. Your help has got me off to a great start.

Follow-up question: am I right that beyond about 1/4 throttle, the fuel/air is drawn in by a different route, not via the pilot jet?

Russ

russellw said:

Update and conclusions based on all the info supplied so far:

The air screw was originally about 1/2 a turn out or maybe less. Must have been a very rich mix. Any special reason why this would be? It was tuned by an experienced shop so no reason why it would have been done badly. Anyway, after I cleaned it all out I set it to 1-1/2 turns out and then adjusted the throttle stop to get the idling speed right. The idling revs dip and rise across a 500 rev range so I had to set the throttle stop such that it was wavering between 1000 and 1500 revs otherwise it dipped too low at times and would stall.

Russ

Click to expand...

Hi Russ,
The air screw adjustment should be in the range of 1-1/2 to 2-1/2 turns and 1/2 a turn will be fuel rich.

Try this method:

Turn the throttle stop screw in so that the idle rpm is 20% higher than normal and adjust the air screw slowly left or right until the highest rpm is obtained.

Then bring the idle back down and do the same adjustment for highest rpm and back off just slightly on the lean side.

If your carbs don't respond you may need to try again with the filters off and see if there's a difference.

Lightly turn in the air screw until it bottoms out and then back off to 1.5 turns as a starting point.

After 3 turns and no change the spring will lose the ability to control the adjustability of the screw.

Hope this makes sense. . .
Naturally the bike needs to be warmed up and a new set of plugs will help to see what's going on.
My own bike likes to idle at 1000 rpm so I set it at about 1200 to do the initial adjustment.
Having two carbs makes it a bit difficult to judge the exact 'sweet-point', but some tuners like to err on the side of rich vs. lean.

russellw said:

The air screw was originally about 1/2 a turn out or maybe less. Must have been a very rich mix. Any special reason why this would be? It was tuned by an experienced shop so no reason why it would have been done badly.

Click to expand...

No,it was probably lean because the blocked jet wasn't allowing enough idle fuel.They've closed the air screw to compensate.
The air screw is like a hole in the side of a drinking straw,but it's adjustable.

One of the fixed holes near the front of the slide does a similar thing when the throttle is closed.As you open the throttle more,it gets enough vacuum to become a discharge hole and allow more fuel.All these holes and the pilot jet orifice need to be in correct proportions to work properly.

The idle circuit doesn't have much effect above 1/8 throttle,but even at full throttle about 1% of the fuel is supplied thriugh the idle jet.

Above idle,fuel starts coming out of the needle jet.Needle jet size is critical from idle to about 3/8 throttle.Needle jets do wear,and are worn out at about 10,000 miles.0.0005" wear will cause richness.

The throttle cutaway has an effect on the amount of suction on the needle jet.Cutaway is most effective at just under 1/4 throttle.

Above 1/3 throttle,the tapered part of the needle comes out of the needle jet to allow more fuel.Needle clip position will have a big effect from 1/3 throttle to 3/4 throttle.

Above 3/4 throttle,the main jet stops too much fuel coming through the needle jet.Main jet size doesn't have much effect below 3/4 throttle.Below 1/2 throttle,you don't even need a main jet.

It sounds like your idle variation is an advance curve problem.The ignition timing is probably not staying retarded at idle,because of weak springs on the AAU.If you're getting full advance at less than 2000 rpm,that's probably the case.You could also test this by locking the AAU in the retarded position;pull the centrebolt and put a 7/16" washer against the AAU cam,then tighten the original washer and bolt against it.

You can't ride the bike with the timing locked retarded,but you could test the idle.

I would replace one spring on the AAU,if you can get Lucas part number 54417992.This is supposedly the only spring that is supplied thesedays,and it's heavier than the original spring.Sometimes called "HD" spring (heavy duty spring).You can just tighten the original remaining spring (about 0.015").

That should give you an advance curve that starts advancing at about 1200 rpm-1400 rpm,and reaches full advance at about 3500 rpm.

If you've still got original Lucas points,just clean them.Don't change them for low-quality modern replacements.