Hard Starting Tips

by: Rick Sieman

Bikes have changed dramatically over the last three decades. Back then, we rode very crude two strokes and cranky old British four strokes, and many of those bikes were very hard to start.

Nowadays, we have very sophisticated two strokes with all sorts of trick exhaust ports, and four strokes that are lighter than the two strokes we used to ride. Ain't life grand?

Well, whey then, with all this blazing technology, do we still have hard starting bikes? Especially the big-bore bikes, and it doesn't seem to make much difference if they're two stroke or four.

HOW THINGS WORK
Get your engine running, remove the air filter boot and put your palm over the carb air inlet. You'll feel the rush of air through the carb and into the engine and you'll notice that it tries to pull your hand firmly against the air inlet opening. The closer you get your hand to the opening, the more the air is blocked by your hand, and the more suction there is. If you allow your hand to close off the opening completely, the engine will quit running.

What this proves is that any obstruction in the air passage of an engine that is rotating will cause a rather large suction, or vacuum, between the obstruction and the engine. Carburetor designers make use of that suction to cause a process called starting enrichment, which helps your bike start easier when it's cold.

STARTING CIRCUIT
Typically, most carbs incorporate a separate little carburetor built right into the main carburetor. The separate one is called a starting carburetor. It has an air inlet on the side of the bell mouth at the front door of the carb. From the starting air inlet, air flows through a little passage in the body of the carb and passes by its own private fuel jet leading from the fuel bowl. There, the airflow joins with fuel flow through the jet, and both air and fuel continue flowing through a small passage toward the backside of the carb.

The starting carburetor dumps its fuel-air mixture into the main air passage at a point behind the throttle slide. If you look in there, you'll see a small hole that does that.

Somewhere in the starting carburetor passage will be a valve, which is just a round plug, or plunger, that blocks off the passage. The operation of this plug is simple: When it plugs up the passage, nothing can happen. When the plug is pulled up to open the passage, stuff can move through.

This plug in the starting carb operated by either a control lever on the handlebar, or a little lever on the carb, which is commonly called the choke.
The thing that makes air flow into the starting carburetor and makes fuel jump out of the bowl to join the airflow, is the high vacuum, or suction, because you are cranking the engine. When you operate the kickstarter (or hit the electric button if you're lucky) the piston moves up and down, trying to pump air, but it can't get very much, so the vacuum is high.

What makes high vacuum, is the fact that you are cranking the engine with the throttle slide closed.

DON'T TOUCH THAT THING, MISTER!
When you're kicking over an engine, leave the throttle completely closed until the engine starts. If you can't resist opening the throttle with a spastic reaction every time your kicking foot moves down, try putting your right hand in your pocket, or scratch your butt. Whatever it takes. Especially if you're trying to start a big four stroke.

Why does cracking the throttle screw things up? If you insist on opening the throttle, the vacuum behind it disappears and the starting carb will not work.

INSIDE THE CHOKE
A choke is a mechanical slide that lives up in the top of the carb and can be moved down into the main air passage ahead of the throttle slide. This is exactly like putting a part of your hand over the carb opening, except it is much more accurate. Most chokes have a small "door" held closed by a spring. When you crank the engine over with the choke closed, the door opens exactly just the right amount to let the correct amount of air in to the engine for starting.

Normal start up procedure is this: activate the choke; boot the kickstarter with some authority, do not touch the throttle and the engine should start. As the engine warms up, gradually lift up the choke. Do it too soon, and the engine might stall. Leave the choke on too long, and you might foul a plug.
When the choke plate is "working" the air path, the area behind it has high vacuum. Fuel will spray up out of the needle jet and/or pilot jet and enrich the mixture for starting.

How long you leave the choke depends a lot on the temperature. If it's a cold day, you might have to leave it on for several minutes. On a warm day, 20 or 30 seconds might be enough. You'll have to experiment and learn how your particular bike best responds.

OLDER BIKES
If you have an older bike, many of them were equipped with a tickler button, a small plunger on the outside of the carb. Lots of Bings and Amals had this so-called feature, and some models of Bing have both a tickler and a starting carb.

The way the tickler worked was thus: you depress the plunger and it moves the float down, allowing gas to slobber into the carb. Not very subtle, but it certainly did enrichen the bike for starting. The only problem was that it often over-did it, and flooded out the bike. Not much fun.

STARTED AND IDLING
The idle system on all conventional carburetors is exactly like the starting carb, except it doesn't have a little plug in it to turn it on and off. It works anytime the throttle slide is closed, or nearly closed, and there is sufficient vacuum behind the slide to pull idle mixture out through a small hole, which is also just behind the slide.

Sometimes there are two holes to deliver idle mixture, one just behind the back edge of the throttle slide, and one just under it. As the throttle slide is lifted up, the one under it also starts discharging mixture. The one under it is often called a bypass orifice. Sometimes the two are called primary and secondary idle discharge orifices.

The idle circuit is from an air hole in the bell mouth, past a screw that sticks into the air passage to adjust the amount of airflow, past a special jet that draws fuel from the float bowl, and then through the discharge hole or holes in the main bore of the carb.

The screw that sticks into the idle air passage is usually called the pilot airscrew or the idle airscrew. The jet that limits fuel flow into the idle circuit is usually called the pilot jet or idle jet. Sometimes the airscrew and associated jet are called slow- running screw and jet.

The idle mixture is controlled by three adjustments:

• There is a throttle stop screw, which goes into the side of the carb right beside the throttle slide, and it limits how much the slide can close. The slide is never fully closed, and some air flows into the engine even when your release the throttle all the way and hear it "click" as it bottoms out.
• The size of the pilot jet. This can be change to richer or leaner sizes as needed.
  
• You adjust the throttle stop screw for the idle speed you want and the idle airscrew for mixture strength at that idle speed. These two controls interact and a good adjustment procedure is as follows:

Turn the idle speed up a little higher than you want it to be, using the throttle stop screw. Then turn the idle air or pilot airscrew inwards until the engine starts running a bit rough. Turning this screw inwards makes the idle mixture richer, and when the engine begins to stumble, it's because the mixture is now too rich.

Now you can start backing the screw out, and keep track of the half-turns of the screw as you do it. The engine should speed up as you back the idle airscrew out and then start slowing down as you continue backing it out. When it starts running rough again, stop backing it out.

Count the number of half-turns you made while backing the screw out from run-poorly to run-poorly. Set the idle airscrew halfway between these extremes or at a nearby setting where the engine runs fastest. Usually it will be the same setting.

Now you can back out the throttle stop screw to reduce idle speed to where you want it. Then recheck the idle airscrew adjustment and reset to the place where the engine idles fastest or smoothest.

WHAT IF THE ADJUSTMENTS DON'T WORK?
This is when you might have to change that pilot jet. The pilot (idle jet) should be the right size to allow you to perform the idle adjustment described above. But sometimes you get one that is way too rich, or too lean.

If you're turning the idle airscrew inward, it should be making the mixture too rich. If the engine speed just keeps on increasing until finally you have the screw turned all the way in, you can figure that the idle jet is too small. Put one in that's the next size larger. If that doesn't do it, get still another size larger.

Now, suppose you are backing out the idle airscrew and the engine just keeps running faster and faster until finally the screw falls out of the carb. It should have been too lean long before you got the idle air screw all the way out, so you can figure that the idle jet is too rich and is letting too much fuel through. Install a leaner (smaller numbered) jet.

Most shop manuals tell you to turn the idle air screw all the way in and then back it out a specified number of turns for a good idle. Usually this is around one or two turns out. If you can't get a good idle with this setting, or close to it, the idle jet is the wrong size. If you can get a good idle anywhere in the adjustment range of the screw, most riders think that's all they need to do. Wrong! Read on to find out why.

HERE'S WHY!
The idle system does more than just idle. Remember, the reason for this article is t enable easier and better starting. But a correct idle will also affect overall performance. When you're opening the throttle by lifting the slide, the main system is supposed to take over and the idle system is supposed to gradually cease delivery of mixture.

The transition between these two systems is critical to good performance when you first crack the throttle, and on up to about one-eighth open.
There are two factors that aid transition from the idle system to the main system. One is the setting of the idle system itself. In fact, some tuners set idle as just described and then test throttle response by opening the throttle.

If the setting of the idle air screw is not around one or two turns out - or whatever the manual says - it's likely the jet is the wrong size and the throttle response coming off idle will suffer. You are better off changing idle jet sizes until you can get the idle airscrew setting where it belongs.

NOW WHAT?
OK, you now have your idle/pilot/starting system dialed in. Your last step should be to install a fresh spark plug of the correct heat range. Your bike should now start so easy that it'll surprise you. And that ain't half bad.