Trouble shooting carburetors
One of the biggest challenges with troubleshooting carburetor problems is the number of different
possibilities for each symptom, and some that are specific for an individual type of carburetor.
This is general troubleshooting information that can be applied to most carburetors.
Strongly suggest that if you are working to fix a problem after trying one cure, should it
not work, you reset the carburetor to its original settings before you begin the next
step. We see carburetors all the time where one thing after another has been tried, but when the
first change didn't work, it was left anyway, until in the end, the carburetors are so out of spec that
it is only by starting from the beginning can it be sorted out.
An excellent example of this is the matter of hesitation. There are a lot of reasons for an engine to
be hesitating, usually, but not always, having to do with fuel availability. Most assume that it is a
lack of fuel and begin making changes to deliver more. The pump/injection is increased, the float
level raised, jets enlarged, and still the problem persists. At this point, there is so much fuel going
through the carbs that the engine is being flooded, sometimes to the point of gasoline going into
the crankcase. When people call us about this, we start asking questions about the condition of
the spark plugs, the exhaust and when and how the engine is backfiring, because we usually
encounter a RICH condition with hesitation, rather than a lean one. BUT a rich running engine runs
cooler than a lean engine, so work for that careful balance.
Rich, dark spark plugs, sooty exhaust, backfiring through exhaust due unburned fuel
Lean: light spark plugs, clean exhaust, backfiring through carbs
either situation can lead to engine fires!!!
Carburetors work by mixing fuel with air in a precise ratio, delivering it to the cylinder at the right
moment in the cycle for the spark plug to ignite the mixture and push the piston down starting the
next cycle. If anything within that cycle is off, including when the spark is delivered, a poor running
engine is the result. So don't forget to check your spark plug condition, spark plug wires, and
distributor timing as part of your troubleshooting procedure. Do you even have the correct spark
plugs?! REMEMBER engines running at idle tend to be rich, loading up your plugs, causing weak
spark. Keep your RPM's over 3500 when driving to help burn off that idle load up. If you are
doing a lot of idling before setting out - checking distributor, carb settings, etc, clean up the plugs
before your ride.
After carburetor installation, be sure that the throttle arms (from throttle levers to linkage) are
properly adjusted. Please do not assume that because it was okay before carburetor work, it is
okay now. You must also make sure that the air flow carburetor to carburetor is equal using your
idle adjustment screw, then set your idle using the volume control screws.
|1. Egine does not start despite proper ignition and adequate fuel
is this just a hard start, meaning having to pump and pump to get fuel moving to the carburetors? Adding an
electric fuel pump to prime the carburetors may be needed. Modern fuels evaporate quickly leaving carburetor
a ) no fuel in the system: Check fuel supply lines, with ignition off detach fuel line connecting pump with
carburetors and be sure that there is fuel flowing from pump to carbs. If not, pump is the problem.
if there is fuel in the system check the float needle valve, the main jets and the injectors for fuel delivery.
Make sure the carburetor pump system is working.
Is distributor and/or spark plugs working to ignite fuel mixture
b) too much fuel in the system: check float needle valve, float levels, fuel pump pressure, injection
2. Uneven idling: a) improperly adjusted idling - adjust idle mixture screws, idle adjustment screw.
b) idle jets or idle air jets or passageways plugged or uneven sizing - clean jets, check sizing
c) air leaks - worn throttle plates, worn out gaskets allowing air leaks, overly tightened carb bolts
causing warping in the carb body, worn throttle shafts,
d) defective idle mixture screws - points damaged with over tightening, threads stripped from over
e) check distributor for proper advance and holding idle
3.Poor Power transition (flat spot): a) worn throttle plates and/or throttle shaft
b) idle adjustment too lean or too rich
c) improper float level (too high or too low)
d) improper injection quantity (too high or too low)
e) shifting at too low a speed causing rpm's to drop too low
f) carbs not synched
4. engine stalls when throttle lever is quickly closed improper idle adjustment, check throttle arm
5. engine runs unevenly, misses, backfires a) mixture too rich - check fuel pump pressure, float level,
injection, float needle valve, distributor
b) mixture too lean - check fuel lines, float level, clean jets, check distributor
c) check for air leaks
6. High fuel consumption a) carbs running rich or ineffectively - check all the above
b) check distributor, points and plugs
c) check driving habits
And when all else fails, call 356 Carburetor Rescue!