Topic: need advice on 71 CB750 cylinders 2-3 not firing

[westondc]

Still trying to figure out what the issue is with my friends 71 CB750. After we returned from a week long motorcycle trip this bike then sat for about a week (covered) outside. When he went to fire it up only cylinders 1 & 4 would fire. We’ve replaced the points, condensers, spark plugs, checked the timing & tappets and looked over some of the wiring connections.  When correcting the timing we set the point gap to shop spec and points just opening at the 'F' mark. Also set the spark plug gap to spec measurement. We pulled out the plugs and each was getting a nice blue spark. Checked the compression (cold engine) and got around 115 psi on all cylinders. After all this we started the bike up and still no combustion from 2 & 3 cylinders. When we pulled the plugs on 2 & 3 they seemed dry with no signs of firing other than the spark. Unscrewed the float bowl drain on 2 & 3 carbs and they’ve got gas. So… any ideas? Seems odd that 2 & 3 are not firing at all. Next guess might be the carbs…

[KingCustomCycles.com]

99% of the time 2-3 is a coil failure or power wire issue to that coil.  They could spark in free air and not under combustion pressure.  Notice the 1% chance it is elsewhere.

[Spanner 1]

99% of the time if 2-3 are not firing and the plugs are dry then it's low fuel level in the carb bowls due to slow fuel delivery from the petcock which can effect any carb(s) randomly...... IMHO.

[westondc]

Thanks for the info. I've bypassed the yellow wire in the wiring harness and ran a new wire straight from the 2&3 points to the coil, still no fireing. Need to check the black wire to the coil next. Can i test the voltage at the spark plug cap with a volt meter when starting? If so what should the voltage be?

[westondc]

also, if 2&3 are getting gas and air mixture but not fireing then they should be wet and smell like gas when i pull them out right?

[bollingball]

If plugs are dry and you did see spark on all four I would (lean) towards fuel delivery issue. put it on the center stand petcock open wait a couple of minutes start bike,turn off bike within a few seconds before floats have a chance to drop pull plugs and smell & look for fuel.

[bollingball]

I think not on the HIGH voltage test see if you have 12v. going to coil.

[Lars]

Had the same problem once, it was the black and white wire to the coil that was loose.

[KingCustomCycles.com]

The coil power wire (black) can break right at the coil.  Most people test the wire at the connector 3 inches away and miss the break.  It should show over 11 volts with the headlight on or the starter engaged (under load) Only plugs exposed to a very rich mixture will look wet in a short bit of cranking.  It is a fuel vapor in the cylinder and it takes some cranking to get the plugs wet. Connect the black power wire straight to the battery and that will bypass all the other potential problems with wires and components. I would thoroughly investigate the electrical avenue before pulling the carbs.

[mycb750k6]

All I can offer is that when that happened to me a few weeks ago, it just turned out to be a loose black/white wire connection to the 2-3 coil under the tank.

[Head]

I would check the slow jets,first, You stated spark, but dry plugs, Sounds like they are not getting fuel to the cylinders.   Head

[bollingball]

If you have not yanked the carbs yet start it up and give it a shot of starting fluid you should hear them kick in if it's getting fire.

[Duke McDukiedook]

Check your pipe temps and check your jets, then clean them and the orifices with cleaner and compressed air.

[westondc]

thanks for the advice, i think we're going to take a closer look at the wires to the coil (especially the black/white wire) and check the voltage. I'll bypass the wiring harness completely with new wires from the batt and points to the 2-3 coil. the bike had pod filters on it which i pulled off and checked to see of the choke and throttle were operating as they should on each carb, they are. might shoot some starting fluid in 2&3 and see what happens. i think we have a set of used coils in a box that i might be able to try. so from what I've gathered from the forum and our tinkering leads me to believe it is one of the following issues that's causing the problem:

1. bad wiring connection to the coil
2. bad coil
3. clogged carb jets

and of course i will post our findings..

[Head]

You said that 2&3 were dry so that is an indication that they are not getting any fuel! Also you said that they have a blue spark, Which means the electrics are doing their part!     Head

[Head]

You can pull the slow jets with out pulling the carbs off. Drop the fuel bowels, use a stubby screw driver,a small mirror, and plenty of light, I leave the floats in place and am gentle so not to mess the floats up or settings, If you have some one to hold a flash light to shine on the mirror as it it tillted to reflect up in the carb body that makes it easier. I have done this many time over the years.     Head

[westondc]

You said that 2&3 were dry so that is an indication that they are not getting any fuel! Also you said that they have a blue spark, Which means the electrics are doing their part!     Head

Yes they are dry but we were only running the bike for a very short time with the 1-4 cylinders going. 1-4 exhaust pipes get hot but 2-3 stay ice cold. Need to run it a bit longer next time and check the plugs again. I've been told by a few people that a spark with the plugs out might not indicate much since they might not spark under compression.

You can pull the slow jets with out pulling the carbs off. Drop the fuel bowels, use a stubby screw driver,a small mirror, and plenty of light, I leave the floats in place and am gentle so not to mess the floats up or settings, If you have some one to hold a flash light to shine on the mirror as it it tillted to reflect up in the carb body that makes it easier. I have done this many time over the years.     Head

Not a bad idea. I'm trying to eliminate the electrical as the problem since its going to be a pain to pull the carbs. Even if I'm not getting gas through the slow jets, wouldn't it at least fire if i twisted the throttle and got gas mixture through the main jets?

[Duke McDukiedook]

A spark plug will spark under compression if you have you electrics sorted out regardless of anything else unless they are thouroughly wet with gas or carbon fouled, then they might not act like properly.
 

[westondc]

A spark plug will spark under compression if you have you electrics sorted out regardless of anything else unless they are thouroughly wet with gas or carbon fouled, then they might not act like properly.
 

What if the issue is with the ignition coil? like weak power to the spark plug letting it fire with the plug out but not in the cylinder on the compression stroke? wish there was a way to test the output from the coil but the voltage is way to much for a volt meter.

[Duke McDukiedook]

Did you run an ohmeter test on it to test for proper resistance between primary and secondaries?

[westondc]

Did you run an ohmeter test on it to test for proper resistance between primary and secondaries?

I did compare the resistance between both coils (since 1-4 was working). they read they same if i recall. not sure what your mean by primary and secondaries?? bare with me as this deep work with bike electrics is new to me.

[Duke McDukiedook]

You test with an ohmeter across the primaries, then across the secondaries, you did look in the manual on this test, correct?

[cookindaddy]

I am suspecting that the plug cap for #2 or #3 is not making good connection with the lead from the coil. Especially if they are in bad condition and you are twisting them around when doing all these plug tests. The plug cap screws into the end of the high voltage wire. Others have written here about checking the connection between the plug cap and the wire and trimming a bit of the wire, if necessary, to ensure a clean end and then re-threading the plug cap. The high voltage is supposed to jump the gap at the tip of the plug, not additional gaps where the plug joins the wire.

Good luck with this