QuestionI have a 1981 Suzuki 250 dirt/street that starts great, but dies after fifteen minutes. It won't restart until I let it sit for about 30 minutes. It will run for about five or ten minutes and then have to wait for a cool down period. I have replaced the plug - but with the same results.
AnswerHi Alan,
Review the following tech files for troubleshooting information. If you cannot find the cause, I recommend having a local tech inspect the problems for you.
I return tech files by email. There was no email with your question as requested in my instructions. I've pasted a couple of tech files below for your convenience.
Respectfully,
Mark Shively
The &!@^%*#! Thing Won't Start
The first thing to do is a process of elimination:
In order for an engine to start and run, it needs 3 things:
1. Spark at the correct time
2. A proper fuel/air mixture and exhaust flow
3. Sufficient engine compression
Cylinder Compression Check: If you just acquired the motorcycle, you should first do a full inspection before you try to get it running. Please see this page on restoration. If this is the case, I'd do a compression check first because it's easy. First check the quantity and condition of the engine oil. Add oil as necessary. I wouldn't change it unless it's terribly dirty. The engine could be bad so you'd waste the oil change. Hook up or purchase a good battery. Leave out the spark plug and crank the engine for ~5 seconds. This helps to circulate the oil. Let it set for ~15 seconds and do it again. It's not a good idea to continuously crank the engine because the starter can overheat and wear prematurely. Most motorcycles need a minimum of 100 psi in compression to run start and properly. Compression should be checked when the engine is warm, but that's kind of hard to do if the motorcycle is not running. So just hook up a compression gauge and open up the throttle all the way. Crank the engine until the needle on the compression gauge stops moving. This should take no more than 5 seconds. Very roughly, 100-120 psi is good, 120-140 psi is very good and 140 + psi is great. If you don't have this level of compression then you have bad valves and/or bad piston rings. If you want, you can add a teaspoon or so of oil to the engine through the spark plug hole. Then try the compression test again. If compression is higher, then it's likely you have bad rings. The oil will seal the rings and give you better compression. If compression is the same, then you have bad valves/seats and/or valve clearances out of service limits. If you get any bad readings, perform a leak-down test to further determine the source of the leak.
Got spark? There are a few ways to check for spark. The easiest way is to remove the spark plug and put it back in the spark plug cap. Then ground it to the engine - you should be able to lay it against a non-painted portion of the engine. Make sure there is good metal-to-metal contact. A darkened area may help to see the arc. You should see a definite spark at the tip of the spark plug. It's hard to see in direct sunlight. If the color of the spark is blue, that means it's a strong spark. A white color is less strong, and a yellow color is weak. Most MC starting systems are a bit weak (compared to cars). Even a yellow spark should start your engine just fine. Next, the spark must occur at the right time. Just about all engines made since around 1980 have electronic ignition. These systems rarely can be set. This is good news. After working on maybe 50 MC's, I've never seen one go out of time. Motorcycles with points are another matter. You'll have to follow the timing procedure in the manual to get the timing correct. In general, the spark occurs just before the piston reaches the top of its stroke. There is a slight delay between the spark plug fire and the fuel-air mixture ignition. That's why the spark occurs a little early. Most ignition systems have an advance unit build in. As the engine speeds up, the time of the spark retards a little bit and helps the engine fire the mixture at the proper time.
Starting Fluid: Assuming sufficient compression, squirt in a bit of starting fluid and see if she'll fire. If so, great! That means that you have sufficient compression and the proper spark. You should get the engine to run for a second or so on starting fluid. If so there's a good chance you can get it running. One caution here - don't run your engine for long periods on starting fluid because it's hard on the engine.
Fuel/Air Mixture: The last thing is the fuel/air mixture. You could be getting too much or too little of either. If the motorcycle has been sitting for a while, it's likely the gas has evaporated and left a bunch of sludgy deposits. This condition is known as “varnishing.” Ask the seller when it last ran or if its been stored for any periods of time. Varnishing begins to occur in as little as 3-4 weeks. If it's been more than this, you should thoroughly clean out the fuel system and the carburetor. Disconnect the fuel line and drain the old gas. I've been able to burn old gas with no problem by using it in my truck. Add fresh fuel and make sure it flows smoothly through the petcock (on the bottom of the fuel tank). If the petcock is controlled by vacuum, you'll have to draw a vacuum on the vacuum line in order for fuel to flow. The vacuum line will be the smaller line on the petcock. Do you get a good, steady flow of gas? If so good, if not, you will have to remove the petcock and clean it. The gas flow should stop when you release the vacuum. Next, you'll have to remove the carburetor and clean it. There are a low of tiny fuel and air passageways in the carb that must be clean it order for your engine to run right. Carefully take the carb apart. Visit: www.bikebandit.com to view parts microfiche on-line. The carb has tiny passageways and jets that get gummed up. The motorcycle just will not run correctly if the carb is not completely clean, so take your time. I use a gallon container of carb dip (you can get a gallon at your local auto supply store for ~$10). First remove all rubber and gaskets or they will be damaged. After a ~20 minute soaking I blow out the carb with compressed air. You have to get all the jets and the passageways clean. Be sure the jets are clear, especially the starter jet. Sometimes carb cleaner won't clear out these tiny jets. Use a guitar/piano wire, a strand of copper electrical wire, or a tiny drill bit pin vise. Be sure not to increase the size of the hole in the starter jet or you'll change the tuning. Carefully reassemble everything, checking for cracks in the rubber carb boots and the gaskets. Replace them if they are old and cracked. Be sure and remove all rubber and gasket material first or they could get ruined. Carb spray cleaner also works if you're diligent. (If you have an electric choke, check it to make sure it works. Check the resistance between the wires - you should see around 10 ohms or less. Measure the length of the choke assembly. Remove the choke from the carb and plug the wires into a 12V source for 15 minutes. The choke body should be warm. Measure the length again - it should increase by around 1/8".)
Next, consider whether your exhaust pipe is clogged. This is especially a problem with two stroke motorcycles. The unburned fuel/oil mixture builds up in the exhaust pipe along with carbon over time. If the exhaust pipe is clogged, your motorcycle will run terribly or not at all. It can be difficult to tell if the exhaust pipe is clogged. The easiest way to tell is to remove it and try to start the engine. Do not allow the engine to run but a few seconds after starting. It will be a bit loud - two strokes will sound like a chain saw. If it runs, you know were the problem lies.
How Was The Old Gas? Did the gasoline smell old? If the motorcycle has been sitting over a year or so, the gasoline will turn into a varnish-like stuff. Then you'll have some serious cleaning to do. The gasoline will have evaporated for the most part, leaving a sludgy, molasses like substance in the gas tank, petcock, fuel lines, and the carb. All of these parts will have to be cleaned. This is especially a problem in the carburetor. Thoroughly flush out the gas tank and gas lines with new gas. I've had good luck using the old gas (in small quantities at a time) in my truck. If the gas tank is not too rusted or dirty, you might be able to get away with just flushing it out. Pour in some fresh gas. Remove the lines from the petcock to the carb. Place a suitable container under the fuel line and draw air on the vacuum line to the petcock (if so equipped) and see how well the gas flows. The flow has to be more than a trickle. It should be steady and fairly substantial. If you get this flow, then you're ok. Run at least a pint of gas through to help flush the system. If you don't get a flow then it's likely your petcock is clogged. You have to remove it to disassemble and clean it. Carefully reinstall it on the gas tank or you could get a gas leak. Reassemble everything and try to start the engine. It will take several seconds of cranking to get the gas to work its way from the gas tank to the carb. Hopefully it will start now! :>) I recommend “Sea Foam” as a fuel stabilizer.
The Big Test: Install a new spark plug, especially if the engine is a two stoke. Clean or replace the air filter as necessary. The carb, air cleaner, and exhaust pipe are all carefully tuned to work together. Most motorcycles absolutely WILL NOT run correctly if any of this stuff is changed in a haphazard manner!
Double check all connections, then crank for 5 seconds. Did it start? Do you hear any popping, like the engine is trying to run? Most engines will not start if you apply throttle while cranking. Honda engines tend to start easier if you apply throttle just as the engine catches. Some motorcycles will not accept any throttle at all until they warm up a bit. If the starter catches, but doesn't start, let it rest ~15 seconds before trying again.
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Engine Doesn't Start, Starting Difficulty
Starter motor not rotating:
Starter lockout or neutral switch trouble
Starter motor trouble
Battery voltage low
Relays not contacting or operating
Starter button not contacting
Wiring open or shorted
Ignition switch trouble
Engine stop switch trouble
Fuse blown
Starter motor rotating but engine doesn't turn over:
Starter clutch trouble
Engine won't turn over:
Valve seizure
Cylinder, piston seizure
Connecting rod small end seizure
Connecting rod big end seizure
Transmission gear or bearing seizure
Camshaft seizure
Balancer bearing seizure
No fuel flow:
Fuel tap vacuum hose clogged
Fuel tank air vent obstructed
Fuel tap clogged
Fuel line clogged
Float valve clogged
Engine flooded:
Fuel level in carburetor float bowl too high
Float valve worn or stuck open
Starting technique faulty
(When flooded, crank the engine with the throttle fully open to allow more air to reach the engine)
No spark; spark weak:
Battery voltage low
Spark plug dirty, broken, or maladjusted
Spark plug cap or high tension wiring trouble
Spark plug cap not in good contact
Spark plug incorrect
IC ignitor trouble
Neutral, starter lockout, or side stand switch trouble
Pickup coil trouble
Ignition coil trouble
Ignition or engine stop switch shorted
Wiring shorted or open
Fuse blown
Compression low:
Spark plug loose
Cylinder head not sufficiently tightened down
Cylinder, piston worn
Piston ring bad (worn, weak, broken, or sticking)
Piston ring/land clearance excessive
Cylinder head gasket damaged
Cylinder head warped
Valve spring broken or weak
Valve not seating properly (valve bent, worn, or carbon accumulation on the seating surface)
Hydraulic lash adjuster damaged (worn, seizure, or spring broken)
Hydraulic lash adjuster oil passage clogged
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Vacuum Leak Testing
An engine needs the elements to run: Adequate compression, spark at exact time, and correct air-fuel mixture.
Compression is the first test. Spark is the 2nd test. Fuel is the 3rd test. Each test has a routine order to follow, so you are not guessing.
Vacuum Leaks:
A vac leak is un-metered air allowed in error to get by the intake. This affects carbs and fuel injection equally. You do not want a vacuum leak on any engine ever, but they are very common. Sometimes you might not know it if the leak is small, and other times an engine won't start at all. So symptoms run a broad range of what a vac leak can do.
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 RPM 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. 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 will work to lower idle sometimes but is a misadjusting when you should not do that. If the leak becomes worse, the idle will do whatever the leak demands. Another symptom is engine starts, idles and runs well when cold, but stalls when warmed.
The leak leans out the correct mix of 14% to 17% fuel to air and makes the mix undeterminable, 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 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 an 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. Same goes for 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 an 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 fuel screw is out too far. [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 unmetered air. It is possible for a bike to run on 2 cylinderss out of 4, and have the two dead cylinders 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 an initial shot into the carbs. Each time you spritz you must listen, so with an 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 vacuum leaks are misdiagnosed as clogged carbs, and bad plugs, wires, pick ups coils and more.
