QuestionHi
Hope you can help, I have a 1985 RM80 that idles well but when the throttle is put on backfires and won't run. I have pulled motor down to ground and replaced all seal including crankcase seals.Piston and rings are A1 and motor has top compression.Have checked magneto also,carby floods slightly but when hand is held over intake still backfires under load.Have practicly changed all except for CDI. Can magneto be in backwards? How can I tune bike and check CDI?
Any help would be appreciated
Thanks
Glenn
AnswerHi Glenn,
Have you measured cylinder/crankcase compression? If so, what was the reading? With engine idling well as described, I think compression is adequate and CDI is OK, but I suspect a problem with air intake or air-fuel mixture (jetting). Magneto is in properly. I dont' know of any way to check that CDI other than checking it against a known good unit.
Was RM stored awhile prior to your repairs? If so, gasoline in carburetor may have varnished and is obstructing jets and passageways. This requires a thorough cleaning of the carby to restore proper performance.
Engine bogging is sign of excessively rich air-fuel mixture. Engine with eratic idle and backfiring is indication of lean air-fuel mix. Backfiring is also related to ignition problems. You may have more than one problem occuring. You'll have to work towards sorting the problems by process of elimination.
Review the information files that follow.
Respectfully,
Mark Shively
~~~~~~~~~~~~~~~~~~~~~~
Poor Running at Low Speed:
Spark weak:
Battery voltage low
Spark plug dirty, broken, or maladjusted
Spark plug cap or high tension wiring trouble
Spark plug cap shorted or not in good contact
Spark plug incorrect
igniter trouble
Pickup coil trouble
Ignition coil trouble
Fuel/air mixture incorrect:
Pilot screw maladjusted
Pilot jet, or air passage clogged
Air bleed pipe, bleed holes clogged
Pilot passage clogged
Air cleaner clogged, poorly sealed or missing
Starter plunger stuck open
Fuel level in carburetor float chamber too high or too low
Fuel tank air vent obstructed
Carburetor holder loose
Surge tank duct loose
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
Engine not sufficiently warmed up after lash adjuster installation
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
Backfiring when decelerating:
Vacuum switch valve broken
Air suction valve trouble
Coasting enricher trouble
Other:
Igniter trouble
Carburetors not synchronizing
Carburetor vacuum piston doesn't slide smoothly
Engine oil viscosity too high
Drive train trouble
Final gear case oil viscosity too high
Brake dragging
Air suction valve trouble
Vacuum switch valve trouble
~~~~~~~~~~~~~~~~~~~~~~~~~~~
An engine needs three elements to run: 1) adequate cylinder compression, 2) a spark at the exact time, and 3) correct air-fuel mixture.
With a running engine the first thing that should be done is a compression test.
With a well warmed engine remove the spark plugs and the air filter, if you have constant velocity (CV) carbs. Hold the sliders (throttle slides) up with a finger once the comp tester is installed, and hold the throttle grip wide open as you push the start button. Record this reading and do each cylinder this same way for as many cylinders as there are.
Then do a wet comp test by adding 6 to 10 drops of what ever engine oil you use to each plug hole. Spin the engine over one time before you install the comp tester to distribute the oil you just added. Repeat the same test procedure. On paper you will then have DRY: cylinder 1, cylinder 2, cylinder 3, cylinder 4 and that test reading. Under that will be WET: cylinder 1, cylinder 2, and so on for as many cylinders as you have. Then you'll have a known set of figures.
Next, test each wire and plug for visual spark. Using a plug plier's tool is recommended. You might not get zapped if you use them. In any case don't support your body with the bike. With the key ON hold the plug tight against the engine and push the button and watch the plug spark. It should be bright fat and blue. Set engine timing if it is possible. These days on many bikes this is not possible with out doing tricks. Older bikes can still be adjusted mechanically
If a plug fails test the plug wire first, as the ends tend to fail first. One way is to remove the wire when possible. Some bikes have built in sets with the coil these days.
With an ohm meter hooked to the ends of a wire wiggle it. A good wire will have a reading as book spec. A dead wire will not read. Check the wires to the ignition coils and to the ECU (electronic control unit) aka: CDI or TCI. Obviously if there is a compression problem the engine is in need of repair. If you get no spark or weak yellow spark there is a problem. Weak spark can be a weak battery problem. If you were able to crank the bike well 13 times as stated above, probably the battery needs no testing. If the cranking became weak and/or you get yellow spark, the battery needs a charge and some testing.
Compression, spark, and fuel should be tested in this ORDER. If compression and spark are ok, then fuel can be considered. For fuel related problems please see vacuum leaks.
Also consider a leak-down test in conjunction with the compression test.
~~~~~~~~~~~~~~~~~~~~~~~~~~
Is fuel getting to the carb?
YES. If NO, then check
A. for a blocked, or broken fuel line.
B. for blockage in the fuel valve.
C. for blockage in the needle and seat, or failure needle and seat.
D. improper fuel tank ventilation
E. fuel filter plugged
F. failed impulse line
G. incorrect fuel pump installation
H. defective fuel pump Repair or replace.
Is fuel reaching engine?
YES. If NO, then check
A. plugged main jet
B. plugged low speed jet
C. throttle valve not opening
D. rotary valve gear not turning
E. for blockage in the needle and seat, or failure needle and seat.
A. clean main jet
B. clean low speed jet
C. replace throttle cable free up slide
D. repair rotary valve gear
Is fuel running out overflow tubes on side of carbs?
NO. If YES, then check
A. needle valve for sticking
B. floats for damage or leaking
C. float arms for proper level Repair or replace.
Is engine flooding?
NO. If YES, then check
A. check to see if you have spark
B. check to see if choke is on
C. check to see if primer is working properly
Replace plugs
Turn choke off
Repair or replace primer
Is spark present at spark plug cap?
YES. If NO, then check
A. check coil wire connections for proper fit or damage.
B. check spark plug cap for continuity and damage.
C. check for faulty ignition switches
D. check for faulty ignition coil.
E. check for faulty ignition Pickups
First NEVER check for spark without the spark plugs installed in their caps! Start testing the easiest things first. Like a poor connection, then the spark plug leads, cap and spark plug. When checking plug USE a NEW plug not one you have had laying around!
Is spark present at the Spark Plug?
YES. If NO, then check
A. spark plug fouled
B. spark plug flooded
C. spark plug broken
D. incorrect spark plug gap
Replace spark plugs in all instances.
Is engine difficult to start?
NO. If YES, then check
A. air cleaner blocked
B. engine flooded
C. throttle valve open too far
D. water in the fuel
E. poor ignition timing
F. no compression
G. enricher valve closed
H. intake leak
I. improper fuel mixture
J. blown head gasket
K. faulty spark plugs
L. gear box improperly loaded Clean filter
Drain fuel, clean tank, and carb, replace with new fuel
Repair engine
Repair gear drive
Does engine backfire or kick back?
NO. If YES, then check
A. dampening box faulty or disconnected
B. ignition wires reversed
C. ignition timing out of adjustment
D. flywheel key missing or sheared
E. faulty ignition switch
F. failed crankshaft bearing on PTO end.
G. failed MAG seal oil has contaminated condenser/points
H. gear box needs update/reconditioning Repair or replace.
Does engine run on one cylinder despite good spark in both plugs?
NO. If YES, then check
A. improper plug gap
B. fouled spark plugs
C. spark plug cap defective
D. no crankcase pressure
E. blown or leaking head gasket
F. cracked cylinder
G. poor ignition timing
H. crankcase air leak
I. needle clip has failed on one carb Repair or replace.
Does engine cranks without resistance?
NO. If YES, then check
A. stuck rings
B. broken rings
C. excessive piston to wall clearance
D. burnt, broken or scored piston
E. piston dome penetrated by pre-ignition
F. blown head gasket
G. spark plug is loose
H. head bolts need torqued
Repair or replace.
Engine will not rotate- lower end frozen?
NO. If YES, then check
A. debris in crankcase
B. crank baring frozen
C. broken connecting rod
D. piston seized in cylinder
E. badly rusted crankshaft
F. foreign material has enter intake on liquid cooled engines locking rotary valve gear
Repair or replace.
Does engine start but will not idle or misses just above idle?
NO. If YES, then check
A. spark plug gap incorrect
B. worn piston and rings causing low compression
C. Carb idle circuit plugged
D. too much oil in gas
E. plugged carb jets
F. air regulating screw out of adjustment
G. stuck rings
H. poor ignition timing Repair or replace.
Does engine die when throttle is open?
NO. If YES, then check
A. jet needle E clip installed above spring cup
B. improper carb slide used, newer models have cutout in slide
C. incorrect spark plug gap
D. excessive prop load
E. fuel pump defective
F. float level too low
G. vacuum line leak
Repair or replace.
Is engine slow to accelerate or does not reach full throttle?
NO. If YES, then check
A. fuel mixture to lean
B. fuel mixture to rich
C. poor compression due to broken, stuck or worn rings
D. float level to high
E. too much prop load
F. too much prop mass weight inertia
Repair or replace.
Does engine run rough, smoke or vibrate excessively?
NO. If YES, then check
A. enricher valve open
B. water in fuel
C. prop out of balance
D. fuel mixture too rich
E. gear box needs update and or service
F. air in fuel lines
G. fuel pump defective
H. intake leak
I. engine mount broken or loose Repair or replace.
Does engine run poorly at higher throttle settings?
NO. If YES, then check
A. Engine misses at higher RPM
1. incorrect spark plug heat range
2. incorrect fuel mixture (too rich or too lean)
3. spark plug fouled or defective
4. poor ignition timing
5. ignition cables arching
6. ignition coils defective
7. dampening box defective
B. Engine "loads up"
1. jet needle position to rich
2. main jet too rich
3. dirty air cleaner
4. poor ignition timing
5. too much oil in fuel
6. worn carb tuning components
C. Engine emits "knocking"
1. timing too far advanced
2. piston to wall clearance excessive
3. crank bearing burned, failed or grinding
4. excessive wrist pin play
5. circlip missing from piston
6. excessive carbon deposits
Repair or replace.
Does engine overheat?
NO. If YES, then check
A. fuel octane too low
B. fuel mixture too lean
C. ignition timing too far advanced
D. prop load excessive
E. cooling duct blocked
F. carb out of adjustment
G. instruments or sender defective
H. low coolant or incorrect anti-freeze to water ratio
I. poor cooling system installation
J. failed water pump impeller Repair or replace.
Engine rpm surges or won't hold steady?
NO. If YES, then check
A. exhaust back pressure tuning incorrect
B. exhaust system improperly modified
C. engine requires modified jets in mid range
D. needle and clip located above white plastic retainer
E. on 532 Rotax ignition coils and or dampening box wired wrong
F. intake socket failure
Repair or replace.
Does engine lose power after standard warm up or will not reach full throttle?
NO. If YES, then check
A. piston seizure
B. stuck rings
C. prop load excessive
D. main jet to rich
Repair or replace.
Does engine show high CHT temps?
NO. If YES, then check
A. poor ignition timing
B. loose or slipping fan belt
C. prop load excessive
D. fuel mixture to lean
E. obstructed cooling ducts
F. poor engine design installation
G. defective gauge or sender
H. lack of oil in fuel
I. poor piston to wall clearance
J. damaged fan
K. failed fan bearing Repair or replace.
Does engine show excessive EGT temps?
NO. If YES, then check
A. air intake leak
B. air/fuel mixture to lean
C. changes in density altitude
D. changes in humidity
E. changes in air temp
F. timing too far advanced
Repair or replace.
~~~~~~~~~~~~~~~~~~~~~
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).
