Question about 1979 Honda CBX

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After engine Why 1979 honda cbx won't idle right? Starts easy, runs great.

After engine is warm and you rev engine, idle will slowly drop and engine dies. Adjust idle up and engine runs at higher rpm but rev engine and slowly idle will drop and engine dies.

Posted by Anonymous on

6 Suggested Answers

6ya6ya
  • 2 Answers

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Posted on Jan 02, 2017

rselvy
  • 83 Answers

SOURCE: engine missing threw full rev range and rpm fluctuating at idle

Sounds like you definitely need to do a major tune up, that being you need to adjust the valves and cam chain. You most likely have very tight valves which will cause the erratic idle and misfiring. I'd also check to make sure you don't have any air leaks in the intake manifold. A cracked manifold will certainly cause a problem like this. One last thing would be to make sure the carbs are synchronized. If they are out of synch you will never get the bike to idle smoothly.
Let me know if these recommendations help or not. I'll advise after you post your findings.

Posted on Dec 08, 2008

  • 1 Answer

SOURCE: 1983 honda GL1100I, New spark plugs, good

When I recently ordered parts from Saber Cycle, they included a piece of information that pertained to my motorcycle, which is also a 1983 Honda Goldwing GL1100 Interstate. Here is what it says, I hope this helps:

Air Cut-Off Valve

The air cut-off valve is an extremely important part of the fuel/air intake system that is frequently missed even my competent mechanics, most of whom are not even aware of its existence or how it functions.

Every GL1000 has one, located in the air intake tube, between the air cleaner box and the manifold. Every GL1100 has four, located in the top air intake passage of each carburetor, and every GL1200 has one located in the plenum box below the air cleaner box similar to the GL1000.

These valves are "air blockers", along with the carburetor o-rings, serving to restrict air flow and increase the fuel/air mixture ratio when the carb slides are in the lower RPM range, up to closed so that the machine will start and idle smoothly with virtually no choking and accelerate rapidly, with no stumble or hesitation.

When typical starting requires heavy and prolonged choking to keep the bike running, when stalling and hesitation occurs even while "warmed up" and when backfiring pops while the throttle is shut off while cruising, most people will start fooling with the air screws to try to overcome the problem, only making it worse along with reducing their gas mileage.

Running with dysfunctional air cut-off valves can causes severe engine damage or failure. When the valves are dysfunctional, the engine pulls nearly total air in the low RPM range through the carbs and on to the combustion chamber. The fuel/air ratio is extremely poor with pre-ignition long before the normal 38 to 40 degree firing point, generating an enormous amount of heat, with no power, which is trapped a longer period of time in the cylinder before the exhaust valve opens.

This heat goes mostly up through the cylinder head, gradually incinerating the head gaskets and allowing engine coolant (anti-freeze) to seep into the engine. You can easily spot when you are in trouble by a puff of bluish-white or white smoke on first starting the engine. Don't leave it alone. Change head gaskets immediately or you could take out your bottom end bearings, ruining the engine.

For those of you installing new exhaust systems, you will discolor your headpipes, either blue (lean state) or gold (rich state). You can destain your pipes using "blueaway" or "blue-off" but be careful not to rub the chrome, just dab gently.k These compounds are usually purchased at a Harley dealer or Harley oriented aftermarket shop.

Honda genuine pipes, like many Yamaha pipes, have a double wall for a portion of the headpipe which masks this discoloration-but also masks a bad state of tune. Even if your pipes are original and still in good shape, don't think that you are OK if the bike has any or all of the dysfunctional air cut-off valve running and performance characteristics.

If your 1975 to 1987 Gold Wing is not getting 50-58 miles per gallon (20-23 KM/Liter), you have one of the most obvious signs of trouble. It is likely your air screws have been backed out to try overcome the effects of the poor running, stumbling, starting and stalling and quitting.

Resynchronize all four carbs to the #3 carb vacuum level using mercury carb sticks or vacuum dial gauges. Stay with strict use of the setting levels in either the Honda, Clymer or Haynes manuals. Do not attempt to tune any other way! Guessing will not work and can cause considerable engine damage!!!

Posted on Sep 26, 2009

DoubleClutch
  • 376 Answers

SOURCE: I have a '94 Shadow 600. Engine starts fine.

BEFORE YOU START
You are haveing carb problems.
Make sure that dirty carbs are actually your problem. Lots of things can make a bike run poorly or not start. Weak battery, corroded electrics, old spark plugs, bad timing, low compression, mis-adjusted valves, dirty air filter, and plugged exhausts can all cause poor running. I'll write an article eventually on how to diagnose poor running conditions shortly, but for now - lets just deal with the carbs.

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, o’rings, 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 **** 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 each Venturi (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 o'ring. 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 by spraying cleaner and wiping clean. Other material type floats may require replacement if cleaning is necessary. Inspect the needle valve (float valve) and seat. Check needle valve’s spring loaded pin. It should depress and return smoothly and without resistance. Check the needle valve’s tip for a worn groove. Replace needle valve and seat if either symptom exists. These parts wear together and must be replaced as a set.

Choke Plungers: It is common for Mikuni slide carburetors to have indented or hardened choke plunger pads. If the pads are worn, indented or hardened with age, then the idle of your bike will vary wildly as the pads no longer seal well.

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. Verify that numbers are the same for all jets on models with in-line cylinders. A few transverse-4 models and V-engines, the inner and outer carbs use some different size jets and it's important to not mix them up. If you have dial or verneer 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).

Posted on Nov 25, 2009

Testimonial: "Highly detailed info. Thorough step by step procedural descriptions. Very helpful."

  • 311 Answers

SOURCE: 83 vt 500 shadow: the carb on the bike is

hi,i might be wrong,but the first place i would start looking is the carbs,there are two of them and it has all the symptoms of imbalanced carbs,if you have a set of carb balancers then the job is relatively easy,if not,then you will have to take it to a shop to get done,the adjustments on the carbs are very very fine,the difference between running perfect and not running well is about 1/50th of a turn on the balance adjuster screw,if the carbs have been tampered with or as is normally the case,as the bike ages and all the parts start to wear the carbs will go out of balance,and it will severely affect engine performance,get the carbs tuned up and i am pretty sure you will have a new feeling bike when you get it back..hope this helps...cheers

Posted on Mar 27, 2010

yz-maniac
  • 127 Answers

SOURCE: My 2007 Honda CRF100 wont idle, backfires when rpm...

sounds like its time to adjust those valves to me 0.003-0.004 on the filler gage or have done at shop.

Posted on Jun 09, 2010

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