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How to Diagnose Motorcycle Carburetor Problems

Friday, December 12th, 2014

Lean, Rich or Needing Adjustment?

Before attempting to fix a Motorcycle Carburetor problem, it is very important to come up with the correct diagnosis so a costly rebuild isn’t done un-necessarily.

Motorcycle Carburetors are relatively simple devices. Their primary function is to deliver the correct amount of fuel/air mixture at a given throttle opening (as selected by the rider). However, as with all mechanical devices, Motorcycle Carburetors will wear over time and will also require periodic tuning and service or a complete rebuilding.

Motorcycle Carburetor problems often fall into three areas: rich mixture, lean mixture, and incorrect adjustment. Diagnosing Motorcycle Carburetor problems is relatively easy and follows some telltale symptoms.

Three Motorcycle Carburetor Problems

1) Rich Mixture means the Motorcycle Carburetor is delivering too much gasoline. Typical symptoms of a rich mixture are:

• Poor fuel economy
• Sluggish acceleration
• Choke not needed from cold starts
• Sooty or black spark plugs
• Sooty or black muffler end pipes
• Strong smell of gasoline when machine is at idle
• Uneven running (will often slow from regular idle rpm’s and stop)

2) Lean Mixtures means the Motorcycle Carburetor is delivering too much air. Typical symptoms of a lean mixture are:

• Backfires as the throttle is closed (primarily during coast-downs)
• Lurching acceleration
• White or light grey spark plugs
• Requires excessive amounts of choke to run/start
• White or light grey muffler end pipes
• Bluing (on chrome systems) of the exhaust header down-pipes

3) Incorrect Adjustment applies to Motorcycle Carburetors that have incorrect adjustment of the air/fuel screw and the balance between two or more Motorcycle Carburetors – where fitted. Incorrect adjustment can produce any of the previously noted symptoms. On multi-cylinder machines, with separate Motorcycle Carburetors for each cylinder, the following symptoms are typical of an adjustment problem:

• Poor overall performance
• Rattling sounds from the clutch
• Engine tends to stall easily
• Erratic acceleration
• Poor fuel economy
• Misfires and/or backfires

Correcting Motorcycle Carburetor Problems

Lean Mixtures: This condition is generally caused by the owner fitting after-market accessories such as exhaust systems, air filter systems or replacement Motorcycle Carburetors of a different type or size. In addition, if the fuel level in the float chamber is set too low, insufficient fuel will be drawn through the main jet. Some Motorcycle Carburetors have a slow speed fuel adjusting screw that regulates the fuel/air mixture in the lower rpm range.

Some Motorcycle Carburetors have a low speed air adjusting screw. Turning this screw clockwise will reduce the amount of air entering the Motorcycle Carburetor, and will therefore richen the mixture (refer to a shop manual for correct settings).

If no changes have been made to the bike, and it previously ran well, a lean mixture can be traced to a leaking inlet manifold or leaking exhaust (often at the interface of header pipe and cylinder head).

Rich Mixtures: This condition is primarily caused by dirty air filters, but it could also result from the owner fitting replacement exhausts and/or Motorcycle Carburetor systems. If the fuel level is set too high in the float chamber, a rich mixture will result.

Incorrect Motorcycle Carburetor Adjustment: This situation is mostly caused by poor maintenance. With the inherent vibration of all engines, Motorcycle Carburetor parts (primarily adjusting screws) tend to rotate, and therefore change their positions. Low-speed running jets and multi-cylinder balancing screws are the items most prone to self-adjust during normal operation and often require periodic corrections.

Vacuum Problems

Saturday, August 17th, 2013

Compression is the first test of all tests…

Spark is the 2nd test

Fuel is the 3rd test, and each of the above has a routine in order to follow, so you are not guessing.

Vac Leaks:

A vac leak is unmetered air allowed in error to get by the intake. This effects carbs and injection equally bad….

You do not want a vac leak on any engine ever, but they are very common.

Sometimes you might not know it if the leak is small, and other times a engine won’t start at all. So symptoms run a broad range of what a vac leak can do.

Bikes with CV (constant velocity) carburetors are prone to get cracks in the rubber diaphragm inside the carb, which is what raises the slide as a vacuum is drawn above it. This internal vacuum leak can make the bike idle erratically, and not want to rev up, to rev slowly, or sometimes to bog down and then rev suddenly. They are always “keyed” so that they only fit in one way, and must be installed carefully to avoid damage. The WD-40 trick doesn’t work as well here, as often the bike is already running a bit rich as the slides won’t rise to provide air. (See below)

Some symptoms are a sudden lean condition, which can result in loss of power instantly and a matching increase in fuel consumption.

This might be found to be true if heat cracked a vac line to the petcocks while you were riding, or the line just lifted off

Other types of symptoms cause what is known as “Hunting” Which is idle RMP that will not stay correct. The idle goes up to a given range maybe even to 2,200 rpm and then will drop to 600 RPM and go right back up as if a demon has the grips.. Or the bike might go to 2,200 RPM and stall forcing you to restart.

Lesser leaks might effect idle, and what was correct yesterday suddenly is high today.

Turning down the over all Throttle linkage screw (THE ONE MAIN ONE MADE FOR FINGERS)
will work to lower idle sometimes but is a mis-adjusting when you should not do that..

If the leak becomes worse the idle will do what ever the leak demands..

The leak leans out the correct mix of 14% to 17% fuel to air and makes the mix indeterminable, ALWAYS lean…

On bikes each carb can have leaks, and manifold mount for any carb can have vac leaks.

Any throttle plate shaft can leak on either end. Any vac lines can leak on either end.

And any test port can have a bad cap, and so also leak.

Most bikes don’t have vac operated accessories, with the one exception of vac operated petcocks. A vac operated petcocks WILL say, Pri = prime, On/Run, AND Res = reserve.

There is NO OFF setting….. Also the petcock will have 2 lines each… One line is for fuel and the other is a vacuum line telling the petcock the engine is MAKING vacuum, and to turn on the petcock diaphragm to pull open the on off valve with in the petcock.

The way a internal combustion engine works creates vacuum. I have never seen any bike with a vac pump.

In my experience vac lines in general do not deal with heat and weather well. They crack, split, and become brittle, and should be replaced once a year, as well as gravity feed fuel lines.

To locate a vac leak you need a can of WD-40 which is probably the best thing you can use WD-40 for.

Also you can use WD-40 to test whether or not idle mix is right. This chemical beats ether hands down for use as a engine starter as well, and will not cause engine damage in moderate amounts.

WD-40 makes what you can’t see, and probably what you can’t hear findable.

You need to listen to know….

SO to tell if idle mix is right, Spritz a shot right at the intake with a running engine, and listen…

Does the idle go up? Or does the idle go down?

If things are correct the engine has all the fuel it wants and the idle will drop, as the engine wants no more…

If the idle goes up you are lean…….

If you have 4 carbs and all go down but one, then that one is lean…. Why it is lean remains a question.

Maybe the setting is wrong, and the pilot is in to far (mix screw on a car).

[ Often a book setting will say 2, or 2 1/2 turns out. That is a place where a fresh built engine should run to start, and IS NOT always the best mix for any given cylinder]

Or maybe you have a vac leak…and so adding fuel in the form of WD-40 causes the idle to jump to who knows what, and that depends on the un-metered air.

It is possible for a bike to run on 2 cyls out of 4, and have the two dead cyls fire up above idle speeds as the engine approaches mid range RPM.

So finding leaks becomes a bit of hit and miss, as you spritz about the carbs after a initial shot into the carbs.

Each time you spritz you must listen, so with a air cooled bike you might want a fan on the engine.

Places to spritz are the manifolds looking for loose clamps, throttle shaft ends, and any vac line ends and components vacuum operated. On injected bikes any Throttle body lines, and injector bases, also any vac operated components as you find them.

Often times vac leaks are misdiagnosed as clogged carbs, and bad plugs, wires, pick ups coils and more…

Common Carb Problems

Saturday, August 17th, 2013

Before attempting to fix a carburetor problem, it is very important to come up with the correct diagnosis.

Carburetors are relatively simple devices. Their primary function is to deliver the correct amount of fuel/air mixture at a given throttle opening (as selected by the rider). However, as with all mechanical devices, carburetors will wear over time and will also require periodic tuning and service.

Carburetor problems generally fall into three areas: rich mixture, lean mixture, and incorrect adjustment. Diagnosing carburetor problems is relatively easy and follows some telltale symptoms.
Three Carburetor Problems

1) Rich Mixture means the carburetor is delivering too much gasoline. Typical symptoms of a rich mixture are:

* Poor fuel economy

* Sluggish acceleration

* Choke not needed from cold starts

* Sooty or black spark plugs

* Sooty or black muffler end pipes

* Strong smell of gasoline when machine is at idle

* Uneven running (will often slow from regular idle rpm’s and stop)

2) Lean Mixtures means the carburetor is delivering too much air. Typical symptoms of a lean mixture are:

* Backfires as the throttle is closed (primarily during coast-downs)

* Lurching acceleration

* White or light gray spark plugs

* Requires excessive amounts of choke to run/start

* White or light gray muffler end pipes

* Bluing (on chrome systems) of the exhaust header down-pipes

3) Incorrect Adjustment applies to carburetors that have incorrect adjustment of the air/fuel screw and the balance between two or more carburetors – where fitted. Incorrect adjustment can produce any of the previously noted symptoms. On multi-cylinder machines, with separate carburetors for each cylinder, the following symptoms are typical of an adjustment problem:

* Poor overall performance

* Rattling sounds from the clutch

* Engine tends to stall easily

* Erratic acceleration

* Poor fuel economy

* Misfires and/or backfires

Correcting Carburetor Problems

Lean Mixtures: This condition is generally caused by the owner fitting after-market accessories such as exhaust systems, air filter systems or replacement carburetors of a different type or size. In addition, if the fuel level in the float chamber is set too low, insufficient fuel will be drawn through the main jet. Some carburetors have a slow speed fuel adjusting screw that regulates the fuel/air mixture in the lower rpm range.

The carburetor shown in the accompanying photograph has a low speed air adjusting screw. Turning this screw clockwise will reduce the amount of air entering the carburetor, and will therefore richen the mixture (refer to a shop manual for correct settings).

If no changes have been made to the bike, and it previously ran well, a lean mixture can be traced to a leaking inlet manifold or leaking exhaust (often at the interface of header pipe and cylinder head).

Rich Mixtures: This condition is primarily caused by dirty air filters, but it could also result from the owner fitting replacement exhausts and/or carburetor systems. If the fuel level is set too high in the float chamber, a rich mixture will result.

Incorrect Carburetor Adjustment: This situation is mostly caused by poor maintenance. With the inherent vibration of all engines, carburetor parts (primarily adjusting screws) tend to rotate, and therefore change their positions. Low-speed running jets and multi-cylinder balancing screws are the items most prone to self-adjust during normal operation and often require periodic corrections.

Dealing with Ethanol

Wednesday, July 25th, 2012
Ethanol is a blend of gasoline and ethyl alcohol. Ethyl alcohol is an excellent solvent and is hydroscopic, that is it adsorbs water. Acting as a solvent, ethanol can damage the o-rings, accelerator pumps, air-cut valves and virtually any rubber that it comes in contact with, over time. 
 
The dissolved sealants can be ingested by the engine which can cause damage and/or fuel leaks. Fuel tanks built of certain materials are not immune to having a problem. Ethanol has a cleaning effect on tanks that releases fine metallic particles which will pass through most fuel filters. READ: Change your filters more frequently.  The dissolved metals will clog fuel injector nozzles and carburetors. Ethanol added to a fuel tank contaminated with water will cause expensive repairs. The water in the tank will combine with the ethyl alcohol to produce a noncombustible layer of liquid in the tanks that will stop most engines cold.
 
We have made the decision to upgrade to Viton when customer is willing to pay the additional cost and all rebuilds now include NEW OEM jets (no more cleaning), NEW float valve needles and where called for NEW accelerator pump and/or air-cut valves. It is a good idea when the carbs are being rebuild to replace fuel lines with “Multi-Fuel” rated fuel hoses whenever possible.
 
Multi-Fuel hose has this printed on the outer sheath of the hose. Currently Aeroquip brand AN braided and socketless hose is Multi-Fuel rated, other brands may not be. If you have older braided hoses, even stainless braided hoses, the rubber inside these hose can begin to break down over time with exposure to ethanol rich fuels. Watch fuel filters for chunks of broken down rubber, an indicator that the hoses are being compromised. Use Sta-Bil fuel additive to reduce the effects of Ethanol on your fuel system. 
 
Higher concentration (for example E85 or fuel with 85% ethanol) will have corrosive effect on rubber, plastic and/or aluminum parts that are not designed for it. Aluminium can be protected by anodizing it which will add protective layer to it.  Smaller concentration (E5 or E10) will not affect unprotected aluminum parts at all. They will still have negative impact on unprotected rubber parts (fuel lines, injector seals etc). Fuel tanks (or the lining within) might have problems withstanding the ethanol and should be evaluated for this problem. 
 
<b>Notes from The Great Ethanol Scam written by Ed Wallace</b>
 
Not only is ethanol proving to be a dud as a fuel substitute but there is increasing evidence that it is destroying engines in large numbers 
 
“Does the average citizen understand what this means? In from 10 to 20 years this country will be dependent entirely upon outside sources for a supply of liquid fuels … paying out vast sums yearly in order to obtain supplies of crude oil from Mexico, Russia, and Persia.”—Yale Professor Harold Hibbert, ethanol promoter, 1925 
 
More than one major transportation-based industry in America besides Detroit is on the ropes. For the fourth time in our history the ethanol industry has come undone and is quickly failing nationally. Of course it’s one thing when Detroit collapsed with the economy; after all, that is a truly free-market enterprise and the economy hasn’t been good. But the fact that the ethanol industry is going bankrupt, when the only reason we use this additive is a massive government mandate, is outrageous at best. 
 
Then again, the ethanol lobby and refiners have a solution to ethanol’s failure in America: Hire retired General Wesley Clark as your point man and lobby the government to increase the amount of ethanol in our fuel to 15%. The problems with that proposition are real—unlike ethanol’s benefits. 
 
Where’s the Logic?
First, the primary job of the Environmental Protection Agency is, dare it be said, to protect our environment. Yet using ethanol actually creates more smog than using regular gas, and the EPA’s own attorneys had to admit that fact in front of the justices presiding over the Third Circuit Court of Appeals in 1995 (API v. EPA). 
 
Second, truly independent studies on ethanol, such as those written by Tad Patzek of Berkeley and David Pimentel of Cornell, show that ethanol is a net energy loser. Other studies suggest there is a small net energy gain from it. 
 
Third, all fuels laced with ethanol reduce the vehicle’s fuel efficiency, and the E85 blend drops gas mileage between 30% and 40%, depending on whether you use the EPA’s fuel mileage standards (fueleconomy.gov) or those of the Dept. of Energy. 
 
Fourth, forget what biofuels have done to the price of foodstuffs worldwide over the past three years; the science seems to suggest that using ethanol increases global warming emissions over the use of straight gasoline. Just these issues should have kept ethanol from being brought back for its fourth run in American history. 
 
Don’t let anybody mislead you: The new push to get a 15% ethanol mandate out of Washington is simply to restore profitability to a failed industry. Only this time around those promoting more ethanol in our gas say there’s no scientific proof that adding more ethanol will damage vehicles or small gas-powered engines. With that statement they’ve gone from shilling the public to outright falsehoods, because ethanol-laced gasoline is already destroying engines across the country in ever larger numbers. 
 
Got a Spare $1,000?
Last July was bad enough for motorists on a budget—gasoline prices had shot up to more than $4 a gallon. But for some the pain in the pocketbook was about to get worse. At City Garage in Euless, Tex., for example, the first of numerous future customers brought in an automobile whose fuel pump was shot. A quick diagnosis determined that that particular car had close to 18% ethanol in the fuel. For that unlucky owner, the repairs came to nearly $900. The ethanol fun was just beginning. 
 
City Garage manager Eric Greathouse has found that adding ethanol to the nation’s gasoline supply may be a foolish government mandate, but it has an upside he’d rather not deal with. It’s supplying his shop with a slow but steady stream of customers whose plastic fuel intakes have been dissolved by the blending of ethanol into our gasoline, or their fuel pumps destroyed. The average cost of repairs is just shy of $1,000. 
 
It gets better. 
 
Scott Morrison is the owner of the City Garage chain in North Texas and he related the story of his technical director’s run-in with ethanol; in December he filled up his E85 Flex Fuel Chevy Suburban at the Exxon station in Ovilla, just south of Dallas. His Suburban died on the spot, because even an E85-equipped vehicle will not run on the 100% pure ethanol that Exxon station was pumping that day. In that case it was not Exxon’s fault but a mistake at the distribution center, and Exxon (XOM) quickly made good for the cost of repairs. 
 
On Jan. 16 of this year, Lexus ordered a massive recall of certain 2006 to 2008 models, including the GS Series, IS and LS sedans. According to the recall notice, the problem is that “Ethanol fuels with low moisture content will corrode the internal surface of the fuel rails.” In layman’s terms, ethanol causes pinpoint leaks in the fuel system; when leaking fuel catches your engine on fire, that’s an exciting way to have your insurance company buy your Lexus. Using ethanol will cost Toyota (TM) untold millions. 
 
An Unpublicized Trend
Though the media is ignoring it, one can easily find many stories on BMW (BMWG.DE) blogs relating similar problems with fuel systems damaged by the use of ethanol. Certainly that was the case with Christi Jordan and her 2007 Mini. For weeks it was difficult to start; Moritz BMW in Arlington, Tex., inspected it and found severe carbon buildup inside the engine. On her second trip to the mechanics they decided to test the ethanol content of Christi’s fuel and found it was much higher than the federally mandated limit of 10%. This time the fuel pump had been destroyed by the ethanol. The repair bill came to $1,200: As in all cases where vehicles are damaged by ethanol, legally the factory warranty no longer applied. 
 
Jim Keppler, Moritz’s fixed operations director, said he’s had at least 10 other cases of ethanol poisoning in Minis over the past six months. Christi was one of the lucky ones; Moritz covered her repairs. But there’s no telling how many motorists across the nation have had to pay for fuel pumps, or fuel systems, that ethanol damaged. Most were probably unaware of the real culprit behind the breakdown, because virtually no repair shop tests the level of ethanol in the gasoline when these fuel system problems occur. 
 
And there are active lawsuits from boat owners; ethanol broke down the resins in their fiberglass gas tanks, destroying their marine engines. Additionally, those who deal in small gas engines for lawnmowers, edgers, and weedeaters have quickly learned that, as Briggs & Stratton’s (BGG) Web site warns, “Ethanol-blended gasoline can attract moisture, which leads to separation and formation of acids during storage. Acidic gasoline can damage the fuel system of an engine while in storage. B&S strongly recommends removing ethanol-blended fuels from engine during storage.” 
 
Like motorists, if landscaping tool owners put gasoline with more than 10% ethanol in their small engines, that immediately voids any factory warranties. In the case of the Lexus recall, using just a 10% ethanol blend was found to be destroying many of these engines also. 
 
Another Government-Mandated Mistake
It now appears that in just a few years since the government forced ethanol use on the country, engine and fuel system failures caused by ethanol are causing major damage to more and more new and used vehicles. This means the hapless owners are not only paying for snake oil in lower fuel efficiency and more smog, but pay again when it damages their vehicles and lawn mowers
 
We seem to have forgotten, but the promise of turning over farmland for fuel production was to reduce our nation’s demand for imported crude. 
 
But until this massive economic slowdown, as Gusher of Lies (Public Affairs, 2008) author Robert Bryce pointed out, even while the ethanol mandate was being ramped up we were increasing our imports of foreign oil. 
 
Translation: The entire politically stated purpose of using ethanol had already been proven to be a false one before the program even got fully under way.
 
No surprise there. The premise that ethanol could give America the freedom to one day stop importing oil has always been fraudulent. Another fun fact: If we outlawed gasoline and diesel, thereby removing every last car, truck and SUV from our highways—no vehicles anywhere on any road in the country—America would still have to import oil because we would still use more crude than domestic production can supply. 
 
Why is that? Crude oil is also used to make fertilizers, aviation fuel, home heating oil, and many other products. Not to mention polyester suits for car salesmen. 
 
Comment Now, Public!
Pushed into it by the corn growers’ and ethanol refiners’ lobbying organizations, today the EPA is starting to go through the public comment phase on increasing the level of ethanol in our gasoline from 10% to 15%. Time and time again we have heard from these groups, who now claim that there is zero scientific evidence that a 15% blend of ethanol would do any damage whatsoever if the mandate for ethanol were raised. As with all statements made by vested interests, few outsiders have actually taken the time to look and find out whether this statement was true. 
 
In fact, it’s false. 
 
Not one mechanic I’ve spoken with said they would be comfortable with a 15% blend of ethanol in their personal car. However, most suggest that if the government moves the ethanol mandate to 15%, it will be the dawn of a new golden age for auto mechanics’ income. 
 
One last thought: Most individuals who have had to repair their fuel systems in recent years never had the gasoline tested to see if the ethanol percentage might be the problem. Today most repair shops and new-car dealers are still not testing for ethanol blends. They’re simply repairing the vehicles and sending their unhappy and less wealthy customers on their way. But, where dealer and repair shops are testing the gasoline, ethanol is becoming one of the leading culprits for the damage. 
 
Sadly, when a truly bad idea is exposed today, Washington’s answer is to double-down on the bet, mandate more of the same, and make the problem worse. Only this time around motorists will be able to gauge the real cost of ethanol when it comes time to fix their personal cars. 
 
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