QuestionHi
Honda CBR 929 RRY Fireblade
Recently had a spark plug blown out of the head, pulling the last three/four turns of thread. Took the head off and had it helicoiled professionally and took the valves out whilst I was at it. All the seats looked ok with no excessive carbon build-up, valves were within tolerance and new internal springs went in (outers were within tolerance). Everything coated in moly assembly paste, new head gasket used and torqued correctly. Had a problem in that two camshaft carrier bolts stripped using the torque wrench and tightening pattern. Helicoiled them myself and have sorted the valve clearances too.
Cam chain tensioner is OK, IN on the inlet cam sprocket and the EX on the exhaust cam sprocket aligns with the head whist the crankshaft sprocket is aligned at 'T'. No mechanical noises turning the engine over by hand. Not possible to mix up inlet and exhaust cams and the cam lobes on cylinder 1 (far LHS as you sit on the bike) are aligned with the cam sprockets. The connectors to the direct ignition coils can be mixed up but the manual says the one with the blue tape is for cylinder 2.
Bike will start but will not now sit at idle and with the tank lifted and the airbox off you can see the occasional combustion coming back up through the throttle bodies. Engine hits about 8k rpm and starts to misfire. New plugs have been put in and gaps checked.
Have the same model as a track bike and have switched the ECM and get the same problem although looking at the ignition timing with a strobe on cylinder 1, the F mark is well ahead of the index mark (probably the I mark is aligned) although it moves counter-clockwise at 1500 rpm.
Just to add a bit more complication, when I was rebuilding it the exhaust servo burnt out and had to replace that and re-do the exhaust valve cables. Using the jumper cable the index mark is aligned at 6 o'clock although when you remove the jumper it doesnt quite align with the 9 o'clocl position but moves to 12 and then 6 o'clock at 3k rpm and 8k rpm as stated in the manual.
Feel like I'm clutching at straws, any ideas? Waiting on the right adapter to come so I can use my compression tester to see if the valves were not closing properly for some reason.
AnswerHi Robin,
Very good problem description, thanks.
I suspect camshaft timing may be a tooth or two out of time, and/or one or more valves are not sealing.
As you know, the first thing to do is a compression test. Until you know the sealing properties of the combustion chambers, it's a guessing game.
Follow with a leak-down test if compression readings are not within service limits.
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Troubleshooting
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. Crank engine no longer than 10-15 seconds at a time. This prevents the starter motor from over-heating.
Then do a wet comp test by adding a teaspoon engine oil 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 1-3 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.
