Cylinder number 4 is cold!

[crackedpiston]

Ok, I have a small problem that has me stumped.  I cannot get cylinder no. 4 on my cb750 to fire.  Here are the trouble shooting methods I've taken.



Check spark plug.  Swapped No 1 plug with no 4 and problem remains at no. 4

Checked plug wires... No change, even with new wire.

Swapped the outputs of the coil between 1 and 4.  Same problem (as expected).

Grabbed a spare plug and plugged it into the 4 wire (with original in 4's cylinder) and fired the motor.  The plug in the wire sparks, but weakly.  That is with the naked eye.

When I attach timing light to 4's plug wire, I have no light.  Number 1 is strong, 2 and 3 are weak and intermittent.

Cylinder is getting fuel... plug is wet with raw unburned fuel.

Compression:  numbers 1,2,and 3 are slightly higher but numbers are good all around!

Battery voltage at start up is 12.4 and indicates higher voltages as rpm is brought up (as per honda specs).



I'm stuck.  I thought low voltage could result in a weaker spark but voltage is good at battery.  I would also expect to still have ignition even if compression was low.  And its not that low... builds 85 psi cold.  I'm using the Pamco ignition system with Ultimate High Output coils and NGK Iridium spark plugs, Keihin 29 CR Special and engine is fresh rebuild.  I'm just frustrated.  If you have any thoughts, I'd love to hear them!

[TOOLS1]

I am going to say that the plug wire wire, or coil is bad since, you said it would not trigger a timing light. If it was triggering a timing light then I would look at the carb, and compression.
Welcome to the forum.
TOOLS

[cdaiscool]

It's a fresh rebuild but compression is 85?

From what I've read and found out from builders before, an engine needs 100 psi to run. You may be too low of compression.

I would first try grounding out the spark plug and see if you've got spark there. And check if its strong or not.

[crackedpiston]

Thanks guys.  I have the firing problem sorted.  And I did make a mistake on the compression test.  Numbers where much better at 145,144,147,143.   Now to set up this dang carb from scratch!

[LukeM]

So...
What was the fix for the spark problem?

Luke M

[cdaiscool]

crackedpiston wrote

Thanks guys.  I have the firing problem sorted.  And I did make a mistake on the compression test.  Numbers where much better at 145,144,147,143.   Now to set up this dang carb from scratch!

Nice numbers!

Setting up the carb's not difficult. I got the Randakk kit, and followed this guide:

http://www.cb750c.com/publicdocs/SeanG/Honda_Carb_Manual_revD.pdf

I'm at the bottom of page 25, but so far it's been super-easy to follow. No complaints from me here!

[fasteddie55ca]

take your cover off the stator plate and check , i had the same problem in my cb 750 c number 4 on my bike was cold with a weak spark, only to find out oil got into my stator  , i cleaned it out with degreaser and use a hair dryer on them . put a new plug in the number 4 , and away it went.
good luck,

[pamcopete]

All dual output coils produce a positive voltage to one plug and a negative to the other. Timing lights for the most part are designed to work on the negative voltage plug because that is what a conventional distributor type ignition does. Some timing lights will not work at all on the positive voltage plug wire. Some will produce erratic flashes of light.

So, if you are having trouble getting a reliable timing light operation on a particular cylinder, switch the timing light to the other cylinder that is fired by the the same coil. If you want to test the cylinder that is not producing a light, then just reverse the inductive clamp on the plug wire.