Stop Tuning Your Carb

Stop mucking with your carbs. Just stop.

Most people would instantly laugh at the suggestion of applying an arm cast to cure a stomach ache, yet if you hang around the motorcycle community for a while you’ll find that it happens every day. Except in this case, applying an arm cast is mucking with the carbs. The poor, humble carburetor is an incredibly misunderstood beast, and unfortunately it is blamed far too often for things way outside its control. A quick scroll through your favorite classifieds will yield result after result with “needs carbs cleaned” or “carbs need adjusted” or “ran great before, probably needs carbs balanced.” This last one is particularly entertaining to read when the advertisement is for a single cylinder motorcycle. This is not an article on carburetors. There are far too many of those already. Instead, this is an article on everything else.

It is true that stale gas and debris can gunk up a carb and cause problems. It is true that carbs must be tuned and adjusted to suit a particular motorcycle and environment. Far too often though, people turn to carbs first before taking a look at the big picture. Perhaps it is because of all the forum posts on how to adjust them. Perhaps it is because “carburetor” is a buzzword that most people have heard before, and associate with poor-running engines. Far too often though, people spend hours or worse mucking around with changing jets when the problem has nothing to do with fuel delivery at all.

Let’s take a step back, and revisit Engines 101 for a moment. Yes, I know that everyone reading this ‘already knows this’ but humor me and read on. Engines require three basic things to run. Compression, Fuel, and Fire. Here’s a little secret that people seemingly haven’t figured out yet: fuel isn’t the only one of these three things that can break.

No amount of carb tuning will fix valve problems, or ignition problems, or a holed piston. So stop trying to tune bikes with other problems. So what is the proper diagnostic order? There isn’t one answer to this question, and different mechanics will approach things differently, but the important thing is that we check ALL of the systems. Here’s an order I tend to use:

First stop is the combustion chamber. Compression. Compression checks are the easiest checks, so I tend to do them first. It’s worth talking a bit about compression checks though. There are actually two main tests that can be done here: cranking compression checks, and leak-down tests. Cranking compression checks are a decent start, as they tell you if someone decided to set off a stick of dynamite in your cylinder, but beyond that they start to fall apart. These are the checks where we pull out a spark plug, screw a gauge in, hold the throttle wide open simply because the internet said to, and then roast our starter as we watch the numbers tick up. We then run off to the internet where we post about what our gauge said, and ask everyone else if it sounds right, to which someone posts a poor quality jpeg of a service manual page. Then we either celebrate because the numbers match, or cry because they are wrong. Somewhere in this process we completely forgot what we were trying to accomplish, and ultimately we have a pile of numbers that don’t even tell us much.

How about we break this process down a bit. We are checking the engines ability to pressurize the top of the cylinder. This is good, but it leaves as many questions as it answers. If the number comes out to be exactly what the service manual says, it is usually a good sign that everything is okay, but if it is low you have no idea why. Is it just because the engine is cold? Where is the air going if it’s not making pressure? This is where the second test comes in. The differential pressure test, or colloquially the “leak-down test.” In this test we use an air compressor to pump air into the sparkplug hole through a very specific metered orifice and see how much air is leaving. I’ll leave the test procedures for a different article, but the important part here is that if the air is escaping, we have a way to figure out where it is going. Barring major catastrophic mechanical damage, there are basically two places the air could be going. Air could be escaping through the valves, or past the rings on the piston. We can figure this out by listening. As we pump air in to the cylinder we can listen to figure out where it is coming out. If you hear air rushing out the exhaust pipe, you know that it is leaking at an exhaust valve. Rushing out the intake, and you know it is an intake valve. Rushing out the crankcase vents and the dip stick? Rings. Here’s another little secret: engines can continue to run even with a TON of piston ring blow-by. To blow up another myth, blow-by leading to slightly lower compression does NOT noticeably lower power output. If you don’t believe me, you can read about a series of tests done to aircraft engines by Continental. Dynos don’t lie, and it’s hard to argue with aircraft engine manufacturers, regardless of what that old motorcycle mechanic told your uncle that one time. So if your engine just has some blow-by into your crankcase, that might be something to address and get fixed, but it is not likely to be the reason your engine is running poorly. If the air is going past the valves though, you have a problem. I’ll let you read elsewhere on fixing that.

Another amazing check to do at this point in the game is to take a look into the cylinder with a borescope. This is a little camera that you can drop down the spark plug hole. There are cheap versions of these available online for the cost of a six pack of beer. Buy one. This allows you to actually get a visual look at the condition of the inside of your cylinders. While you already have your spark plugs out for that compression check, just drop a camera down and look around. A look at the cylinder walls can show wear. Is there cross-hatching still visible? Horrible vertical scoring? Now turn that camera around and look up at the head. See any nasty stuff? Cracks? How about those valves? They should look symmetric and have heat signatures that look like perfect bulls-eyes. If they look asymmetric that can be a hint of a problem. This can tell you things that 10 years ago would have required pulling the head.

So now that we know our compression is good, move on to spark. A very very basic test is told by every shade-tree mechanic to ever live. Hold that spark plug tip against the block, kick the bike over and watch for a spark. If you’re lucky, you saw a spark and didn’t zap the shit out of yourself. That test doesn’t tell us a ton though. We know nothing of the quality of the spark, condition of the entire spark system, or the timing. A simple gap type spark tester can give a better indication of spark strength. This is because the gap is longer than that of a spark plug, and requires a higher voltage to spark. This tends to help show if there is a weak spark scenario. If you have no spark at all, simple multimeter tests on the bench can usually tell you if a coil has failed. On newer bikes, the timing is all digitally controlled, so you have to largely just trust, but on the older bikes you have points to contend with. Pull off that points cover and take a look. Do the points look relatively clean? Check the points gaps. Grab that trusty timing light and take a quick check to make sure your timing is on target. This whole check process takes less time than changing a pilot jet once you’ve done it a few times. If you see issues here, fix them. If there are no issues, move on.

We have now checked timing and compression. The last thing to consider checking before moving on to that carb that you desperately want to tear apart is valve clearance. Some bikes have hydraulic lash adjusters that eliminate this check, but that’s less common. HORRIBLY wrong valve clearance issues can show up in a leak-down test, but just because the leak-down test came out okay doesn’t mean that the clearance is okay. On some bikes this is very easy to check, on others it can be a bit of a hassle. If it is an easy check, don’t pass go, don’t collect $200, just check it. If the bike is such that checking is a particular hassle, you can consider things like age of the bike the last time it was checked. If the bike is running poorly, and it hasn’t been checked in a while, suck it up, get out your feeler gauges, and pull that valve cover.

Now if you made it this far and everything looked okay, you are allowed to move on to tuning your carb. I know that process seemed painful, but I promise it isn’t as bad as it sounds. And if you just suck it up and check everything first it will make your life better when you go to tune the carbs I promise.

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