Question-------------------------
Followup To
Question -
Good morning. I'm having problems trying to fine tune the carburators. It starts fine, but it starts backfiring and rough idle. Spark plugs are new, its timig is OK, and firing order is OK. What else can I do?
HELP ME!
Thank you in advance.
Answer -
Hi Luis,
"...Good morning. I'm having problems trying to fine tune the carburators. It starts fine, but it starts backfiring and rough idle. Spark plugs are new, its timig is OK, and firing order is OK. What else can I do?"
You'll have to provide some history of this motorcycle and repairs. was it stored? What have you've done so far? Otherwise, my reply will be as brief.
Respectfully,
Mark Shively
Was it stored? = Yes, for almost two years.
What have you've done so far? = I installed new piston rings, new gaskets, new spark plugs, new headers, replaced valves oil seals.
What else must I need to do?
AnswerLuis, thanks for the update.
Review the following file on carb cleaning. I suspect the carburetors may need a thorough cleaning.
Perform a cylinder compression test before cleaning carbs. This will tell us the sealing properties of the combustion chambers.
Use my email for reply: mshively1@woh.rr.com
Respectfuly,
Mark Shively
Cylinder Compression & Leak-down Testing 101
You need 3 things to make a gasoline engine work, compression, fuel, and ignition. Without enough compression the engine will not function. Low compression causes you to crank the engine longer when trying to start it. As you crank the engine it slowly builds pressure inside and will eventually fire up. I cannot think of other problems it will lead to because once you have lost compression; you will not have a working engine, so nothing else can break.
What does it indicate as weak points in the engine and how is the leak down test are related? There are various possibilities for low compression and the leak down test pinpoints them fairly well. Your engine is an air pump and the leak down test measures the engine's ability to hold air. The combustion chamber has a theoretical limit of holding 100% of the air/fuel mixture that is drawn into it and compressed prior to being ignited by the spark plug.
The leak down test measures with a flow meter the actual amount of air the combustion chamber will hold. Weak points will be noted by the sound of rushing air. So, if the rings on the pistons are weak, listening at the oil filler cap while doing the leak down test will allow you to hear the sound of escaping air. A bad exhaust valve will allow air to rush out the tail pipe so you stick your ear to the tail pipe and you hear the noise of air passing by a bad exhaust valve.
The compression test uses a gauge to measure the build up of pressure inside each cylinder as you crank the engine. The cylinder leakage test uses compressed air that is forced into the combustion chamber. Both testers are screwed into the sparkplug hole.
To check the condition of the rings, you need a leak down test. This test measures the ability of the cylinders to maintain compression, thus testing the integrity of the rings and valves. The cylinders with low compression MAY have poor rings, but check the valves first. I had the valves done on mine, hoping that would cure the problem, but it was terminal; 3 of the cylinders had more than 50% leakage cold and about 70% hot - bad rings! I had a complete overhaul done. Now it runs like it just came from the factory (better, says my mechanic). If it isn't using oil, the rings are probably good for many more miles.
A compression test tells you the compression provided by the piston on the compression stroke. It is a relatively instantaneous measurement, but if you keep the meter on the port you can see how the compression decreases over time. If it decreases too quickly, something's not right; either leaky valves or rings. The leak down test quantifies the compression fall-off as a percentage over time and (usually) at cold and hot engine temperatures. It is a better indication of engine condition and is sometimes used to predict remaining engine "life." If your rings are bad, you'll usually see two things:
- low compression
- high (or higher than usual) oil consumption
You may also see white smoke from the exhaust in extreme cases. If there is no smoke and the compression tests are good, then the engine should be fine. If you still have doubts, check the valve guides and valves (if you see a puff of white smoke when you leave a stop light - after decelerating) it could be valve guides or seals. A valve job is easier and MUCH cheaper than a ring job. A skilled mechanic can usually determine if it's the valves or rings.
You should not get any more than 10% variance between cylinders. I'd start by keeping things simple. First, check all valve clearances; you can lose compression through the valves. Once this is done, check the compression again, and do it a few times to be sure it's not your compression tester that is out of calibration. There is a procedure for testing valve problems, which is also a possibility here. Pour some oil down the spark plug holes just before you take the reading. If the reading changes, then you've sealed the problem with the oil (i.e. your rings are gone), but if it doesn't then it's more likely to be your valves - which means a rebuilt head (much cheaper than the alternative).
As the cylinders wear they get a taper -bigger at the top and smaller at the bottom. You can go up to about .015'' of wear overall or .007 per side. The rings have to expand and contract every time the piston moves. So, they tend to break at high RPM but can handle 3,500 for years. Engine wear is about .001'' per 10,000 miles on average - some better and some worse. If you have a vehicle with lower compression and over 100,000 miles it's a safe bet that you have cylinder wear, the same wear that will break rings at high RPM.
You can sleeve them with no problems. If its standard bore you can go up several sizes over. This will clean out even the worst cylinder wear in most cases, but not deep gouges in the wall. Excessively deep gouges will need a new sleeve.
Re-torquing the head bolts after head re-assembly should be done after a few hours of running on a warm engine. Undo each bolt one at a time only. Start in the middle just like a regular sequence and undo a bit and then tighten up to the final amount which is usually a bit more than the initial setting. You should get another 1/8 turn out of some of them. Also use a bit of oil on the threads and washers so they screw down evenly.
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Carb Cleaning 101
By M. Shively
The elements of internal combustion engines are: correct fuel/air ratio, spark at right time, adequate cylinder compression.
There are many passageways and openings to check and clean. All are important in function and when obstructed or not working properly, have subtle to radical effects on engine performance. Vacuum leaks and carburetor synchronization also have effects on performance and should be inspected and adjusted following the below procedures.
Carb Cleaning 101
Warning: Remove all rubber parts before you begin. These parts usually include vacuum diaphragms, needle valves, orings, hoses, and other parts. Spray cleaners will damage these parts. Do not disassemble individual carbs from the carb bracket.
Air & Fuel Passageways: Trace and learn individual fuel and air circuits from beginning to end. Machines can only drill straight through the cast passageways. To change direction, another angled passageway must be drilled. The union is plugged with a brass or bronze bead. Inspect and clean each passageway with spray cleaner, brushes/pipe cleaners/etc, and compressed air. Remove any discoloration and debris. Look for spray cleaner to exit from one or more passageways.
Jet Cleaning: Inspect jets by holding to light and look through them. You should see an unobstructed round hole. Clean the jets with one or more of the following: jet cleaning wires, soak solutions, carb spray cleaners and compressed air. Re-inspect jets after cleaning and install when clear of obstructions. Some main jets have paper-like gaskets. Most have metal spacers between the jet and the emulsion tube. Some screw directly into a brass emulsion tube which is machined for a 7mm wrench at its float chamber exposed base.
Inlet Fuel Valve: Inspect the needle valve & spring. Press down the tiny metal rod that protrudes from the butt or float end of the needle valve. The spring should move freely and return the rod to its location. Check the needle valve's seat area for a groove or other wear. It should appear highly polished. Some needle valve seats are rubber and wear may not be visible. Inspect the needle valve jet seat. You can clean the jet seat with Q-tips and semi-chrome polish if necessary.
Carb Body Castings: Blow air through the atmospheric vent holes located on the dome of each float bowl chamber. Air should exit via hoses or brass nipples. Inspect the emulsion tubes and passageways (cast towers that jets thread into) for discoloration and debris. Clean interior emulsion towers with a soft bristle gun cleaning brush. Clean the Venturi's (main carb bore).
Needle Jets & Jet Needles: Clean the needle jets, jet needles, and passageway or tower that needle jet screws into. Clean the emulsion tube (pipe between needle jet and main jet) (Main Jet may screw into emulsion tube). Jet needles are part of the throttle slides. See below…
Throttle Slides: There are several types of throttle slides: Mechanical linkage, vacuum, diaphragm, and cable. Disassembling the jet needle from the slide is not always required for cleaning. If you have vacuum piston type throttle slides (large diameter solid metal slide), avoid cleaning the lubrication from sides and caps. If piston type check cap vents and passageways with air. Clean if necessary and re-lube. If you have rubber vacuum throttle diaphragms, inspect for dry-rot, defects, and tears by gently stretching rubber away from center. Do this until all areas around diaphragm have been inspected. Replace any defective part as described above. Clean carb body areas around diaphragm including air passageways and air jets. Diaphragms have a locator loop or tab fabricated into their sealing edge. Observe this locator upon reassembly. Avoid pinching the diaphragm when reinstalling caps.
Fuel Screws: Fuel screws have sharp tapered ends. Carefully turn one fuel screw in while counting the turns until it seats lightly. Warning: These screws are very easily damaged if over tightened into their seats. Record amount of "turns-in" and remove the fuel screw, spring, washer, and oring. The fuel screw is part of the enrichment (choke) circuit...clean passageways as described above. When carbs are assembled, spray low PSI compressed air into diaphragm air vents located at intake side of carbs. Throttle slides should rise, then fall when air is removed. Lightly lube external moving linkages. Reinstall carbs and follow through with carburetor synchronization.
Throttle Cables: Lubricate cables periodically. If cables are disconnected from carbs or removed for replacement, etc . . . remember cable routing and ensure proper reinstallation routing. Avoid bread-tying, sharp bends, and pinching cables. Adjust cables so throttle grip has about 5mm of play or throttle slides or butterfly valves may not open completely (full throttle)(wide full open).
Float Bowls: Inspect float bowls for sediment, gum or varnish, crystallization, and defects. Clean all pipes, tubes, passageways, and embedded jets with cleaners and compressed air. Remove and clean the drain screw and area. Inspect bowl gasket and replace if necessary. Clean and inspect overflow pipes and tubes, look for vertical cracks.
Floats: There are several types of float materials: plastic, brass, black composite, tin, and others. Handle floats carefully. Avoid bending, twisting, denting, or other means of mishandling. Most floats are adjustable by bending a small metal tab near the float axle end. Do not change the float adjuster tab unless tuning fuel service levels. Clean metal floats by soaking or spray cleaners, if necessary. Replace other type floats if cleaning is necessary. Clean the float axle or pin.
Synchronization: This is a fine adjustment performed usually and preferably with the carbs installed and the engine running. The unusual part is performed with gauged wire with the carbs on the work bench. Carburetor synchronizing balances Venturi vacuum at the exhaust side of each carburetor, resulting with smooth idling and optimized performance at all throttle openings. Synchronization is checked using a set of gauges which are either air vacuum type or liquid mercury type. The gauges are connected to vacuum ports on the intake manifolds via nipple tubes or if sealed with screws, sync gauge adapters will be needed. With the engine running at temperature, and with a fan or means of forced convection aimed onto the engine, the carbs fuel screws and idle are adjusted, then the synchronization is adjusted via adjustment screws on the carbs. A reserve fuel tank is recommended for convenience of accessing carbs during this procedure. See gauge instructions and repair manuals for detailed use of synchronization gauges.
Notes: While carbs are apart, record the jet sizes. Look for a very small number imprinted on the body of the jets. It should be the same number for all. If you have dial or veneer calipers, measure and record float heights. Perform measurements with floats just touching needle valves, though not depressing the needle valve rods. Replace fuel and vacuum hoses. Be sure to use fuel rated hose for fuel. Install or replace in-line fuel filters. It's a good time to remove and clean interior petcock fuel filters. Inspect carb manifolds for dry-rotting, inspect all clamps and air ducts. Inspect, clean, lube, and/or replace air filter(s).
