® Diagnostic Tips

Carburetors get a bad rap. They gum up so easily they frequently are the cause of performance issues. However, even when not the problem many think them the problem. And frankly, carburetors are just too accessible to grasping hands as targets for tuning angst.

Too much is expected of carburetors really, particularly in an age of nearly flawless fuel injection. Even the most expertly coddled and adjusted carburetor falls far short of fuel injection's ease of starting and warmup. Add to this the fact that powersports vehicle carburetors sit more, more is demanded of them functionally, and they must cope with more variables than do the carburetors on other types of engines, and you can see why carburetors are over-represented in performance troubles.

All the other stuff

I always say that the carburetor is like the icing on the cake. If what's underneath is not solid, the icing isn't going to make up for it. The same with the carburetor. If the ignition system falls short in any respect, whether timing, spark plug condition, plug wire tightness, or loss of voltage due to dirty connections, the carburetor can't shine under those conditions. Or if the valve clearances are tight, vacuum leaks are present, or an aftermarket exhaust is either too restrictive or is unexpectedly unrestrictive, again, the carburetor isn't going to magically compensate. So due diligene is required. You must make sure all the ingredients in your cake are what they should be before the very last part, the icing, is added. Here are some areas to consider. Print this list out and go over the items on a problem machine.


  • Gas that has been sitting over the winter needs to be changed out, it is weakened and will result in misfires.
  • Gas that has water in it will result in misfires.
  • Gas that has gone too long without fuel stabilizer will start to clog carburetor circuits, most notably the idle circuits.

Fuel tank

  • Poor tank venting due to clogged vent leans carburetion.
  • Blockages from rust or a deteriorated pour-in liner or petcock corrosion leans carburetion, often intermittently.
  • Vacuum petcocks get worn diaphragms that slowly lean out carburetion on some models and suddenly richen it on other models.
  • A fuel valve (petcock) that is blocked from debris or corrosion, or is improperly rebuilt, will starve the engine of fuel.
  • Sometimes a petcock that isn't flowing correctly can be turned to "prime" or "reserve" and at least temporarily you can confirm whether the petcock is the problem.

Fuel pump

  • A failing pump can lower fuel pressure, leaning carburetion. This can often be difficult to detect, so pressure testing during use is recommended.

Fuel lines

  • Clogged or exccessively small fuel filters result in lean issues.
  • Kinked fuel lines starve the engine for fuel.
  • Poor quality fuel lines chunk internally, sending fuel-blocking debris into the fuel system, resulting in leanness.
  • Attaching the fuel hoses to the carburetor vent nipple(s) instead of the fuel nipple(s) will have frustrating results.

Air filters

  • Poor quality aftermarket air filters such as foam pods restrict air intake, causing mid to high rpm issues.
  • Paper air filters past their changing intervals, even though visibly not very dirty, can restrict airflow, resulting in high speed misfires.
  • Many pod filter designs have an abrupt edge that upsets air flow
  • Switching to pod air filters without compensating for the freer air flow severely leans carburetion.
  • Even going to a gauze type replacement air filter element leans carburetion.
  • Removing the air filter box lid leans out carburetion.

Carburetor vent hoses

  • Bowl vent hoses not properly routed restrict venting, leading to leanness.
  • Overflow hoses that hang too far below the engine can develop venturi effect and starve the engine for fuel.

Carburetor manfolds

  • The o-rings or gaskets on metal manifolds need replacing whenever carburetors are rebuilt, to avoid vacuum leaks.
  • Rubber manifolds that have cracks, are shrunken so badly they cannot be tightened properly, or are so hard they won't seal against the carburetor spigots properly need to be replaced,mas they create vacuum leaks.

Throttle and choke cables

  • Improperly connected cables affect how carburetors work, particularly idle rpm and idle-down.
  • It's a bad idea to remove cables from the carbs by simply removing the cable bracket. That will tempt you to simply resintall the bracket. Uh uh. You need to install the cables individually so that they can be adjusted individually for proper slack.
  • Choke cables on some models if misadjusted create hanging idle issues.

Carburetor installation

  • Carburetors that have been hammered on, banged on, or pushed on using cabinet clamps may have their linkages knocked out of whack, resulting on starting and idling issues.
  • Rough handling on CBX carburetors during installation can cause their choke linkage to become disconnected.

Valves, cams, and cylinders

  • Too tight clearances greatly reduces cylinder compression. Some engines that have poor valve sealing by their design can benefit greatly from adjusting their valve clearances to the larger end of the range.
  • A badly worn cam chain retards valve timing which lowers cylinder compression.
  • A loosely adjusted camshaft belt affects ignition timing on GL1000 Gold Wings.
  • Badly worn cams starve the engine of air.
  • Badly receeded valves will result in erratic carburetion.
  • Low cylinder compression results in poor engine performance.
  • Carboned-up valves reduce cylinder compression and exhibit similar symptoms as receeded valves.
  • Inadequate cylinder compression will result in an engine that is not able to use the fuel it is getting, which looks like carburetor richness but is not.

Ignition system

  • A dirty spark plug weakens the ignition system, causing a misfire.
  • A spark plug with a crack in its porcelain weakens the ignition system, causing a misfire.
  • A spark plug wire with a crack in its insulation results in a misfire.
  • Ignition coils that have wax evident on their outsides have been overheated and may result in ignition misfires or simply weak spark.
  • Ignition coils that have cracks in the plastic cases have been overheated and can result in misfires.
  • Sticking or worn mechanical advance mechanisms cause incorrect ignition timing.
  • Failing pulsers result in erratic timing that upsets combustion (and on Hondas at least feels just like carburetion -- look for a 4,000~5,000 rpm stumble).
  • Failing spark unts (igniters) weaken spark due to slower switching.
  • A faulty battery results in ignition misfires.
  • A failing electrical fuel pump pulls voltage from ignition system, resuling in ignition misfires.
  • Poor battery terminal connections, corrosion, etc. result in ignition misfires.
  • Loose or dirty spark plug wire connections at ignition coil or at plug caps result in misfires.
  • Excessive spark plug cap resistance results in ignition misfires.
  • Improper ignition timing, burned contact points, or loose or shorting points wires result in ignition misfires.
  • Excessive voltage drop to the ignition coils due to connector and switch corrosion or looseness result in weak spark -- you should have no more than 1 volt drop.
  • Loose or corroded or painted over engine mountings can affect ignition performance by reducing system voltage.

Exhaust system

  • An exhaust system that is too restrictive, whether from improper design, oil accumulation, or rust, can choke an engine and result in poor performance, particularly at high rpm.
  • An exhaust that is much less restrictive than stock can demand of carburetion more fuel, i.e. rejetting.


  • Fuel in the crankcase oil richens carburetion.

Evap system

  • Fuel in the charcoal cannister richens carburetion.

Using the sync gauges for troubleshooting

Here are some more things to think about. The vacuum gauges used to synchronize carburetors can also tell the troubleshooter a few things besides throttle sync. Here are some examples of the kinds of things a vacuum gauge can communcate about your engine.

  • A vacuum reading that progressively weakens, i.e. climbs toward a positive pressure, points toward a tight or burned valve.
  • If this same gradually weakening reading actually reaches zero, i.e. becomes a positive pressure, at a steady idle, this points to an overly-restrictive exhaust system.
  • A reading that spikes upward toward a positive pressure occasionally at a steady idle points toward tight valve(s), sticking valve(s), or a fuel or ignition misfire.
  • Readings that fluctuate widely when the rpm is held at mid-point for a period indicate weak valve springs.

Exhaust gas analyzer use in troubleshooting

Most of us of course don't have access to an exhaust gas analyzer, but for those that do, an EGA can be an invaluable diagnostic tool. Go to this link for a summary of the kinds of problems an EGA can help ferrett out.

The more air/less air test

The more air/less air test is so important you absolutely must consider it part of your troubleshootig arsenal. By giving the engine more air than normal and noting how it responds, then giving it less, and again observing, we can quickly distinguish between the three common areas of trouble: mechanical, electrical, and fuel. How you do the two steps is important however. Let's take a vintage four-cylinder Gold Wing for example. For the more air test, we don't do something so crude as eliminating the air filter element -- that would be so severe a change that the results would completely overcome our objective. Instead, prop open the airbox lid with a pencil eraser or something similar, so that just a little extra air is introduced. Note the results. If the engine responds with improved performance, richness is indicated. Repeat the test going the other direction. This would mean removing the air filter element, duct taping up half its surface area and reinstalling it. If engine performance improves, leanness is indicated.

Two things. First, though some recommend it, doing the less air test by applying the choke is not a reliable method. There are too many variables there, especially on older carburetor designs. Also keep in mind that the results of the more air/less air test is not 100 percent conclusive. That is, just because taking away air improved combustion doesn't mean the problem might not still be ignition. This is because fuel and ignition have an inverse relationship. That is, a reaction when altering one can easily indicate the other. For example, you take air away and the result is better performance. Must be leanness, right? Not necessarily. 80/20 maybe, but not 100 percent. That is, 80 percent likely a leanness problem, but there is still a 20 percent chance the problem is ignition. It's because of that fuel/ignition inverse thing. An ignition system that is failing can be compensated for by adding fuel, and the reverse is also true, a very lean fuel mixture burns better when more ignition voltage is applied. So think of fuel and ignition in pairs.

Also remember that when richness or leanness is indicated, the area of investigation covers from the gas cap to the exhaust pipe, and everything in between. See those parts listed above. A gas cap that does not vent properly will lean the engine, and an exhaust pipe that is clogged for any of several possible reasons will make the engine run rich. Gas cap to exhaust pipe. I once had customer do the less air test by wadding up a t-shirt and throwing it into the aibox. He reported back that the engine responded admirably and led him to locate and correct loose intake manifold clamps. Just a turn on the clamp screws and the engine went from not being able to be revved up, to a happy normal high revver again.

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