Spark Problem with '81 CB900c

[scratchdesk]

Hi all, been lurking quite a while and decided to finally make a post as this has me scratching my head.

I have a 1981 Honda CB900c that's backfiring a lot under load so I was trying to get that sorted.  Sound appears to be coming from Cylinder 2. The bike is new to me and I've gotten it to the point where it idles great but I was noticing a lot of popping while accelerating.  I hooked up a timing light to each spark plug wire and results were:
Cylinder 1 - Spot on and great through revving
Cylinder 2 - Idle good but intermittent misses under load
Cylinder 3 - Idle good but light goes black over 3-4k
Cylinder 4 - Idle good but light goes black over 3-4k

Should I look to the pulser for the problem?  Plug wires/ caps?

I haven't driven the bike hard yet (just around the block) so I'm not sure how the power is but I didn't feel like 3&4 were cutting out which throws me for a loop. #2 sounded like it was missing a lot under load so I didn't want to push it.

Pulling the plugs showed 2 was a bit sooty but 1, 3 & 4 looks good. Have new NGK D8EAs in it now but had a set of Champions in there first, same result with either plug.
Engine and all exhaust pipes are hot after being run for a while.
I've trimmed and re-seated each plug wire... I was trying to keep expenses down so didn't want to just keep throwing money at the problem.

Any clues gurus of the CB?  Easy checks I can do to identify the problem?  Thanks in advance.

[TOOLS1]

Sounds like a problem with the "aircut" valves on the carbs. However with the timing light test you did, I would get an Ohm meter, and check the resistance on the plug wires, and through the coils. The factory service manual shows how to do this. You can download the one for the CB 750 from the home page here. I am pretty sure that the info will be pretty much the same for the 900.
TOOLS

[scratchdesk]

Thank you sir.  I spent a few hours working on it today and put a few of the ideas into play.  Here is where I'm at now:

Basically the bike idles great and rides pretty well.  I am still getting backfires from Cylinder #2 pretty steadily in the 2-5k range.  After that I don't notice any backfiring.

I trimmed all the wires (coil side) and that helped Cylinder #3 and #4.

If I hook the timing light up I get misses out of Cyl #2 and #3 that correspond to the popping I only notice out of #2 in the 2-5k range (both on coil #2).

I checked the resistance on the coils and even tried switching them to see if the problem moved with them, it did not and Cyl #2 continued to pop/ miss.  Both read in the 545-550 range.

I checked the spark unit and that read a little over 11v and then with grounding the blue wire it was at 1.2v which is in spec (0-2v).  I also switched the spark units as they are the same, this had no effect.

Next checked the spark advance with the timing light and it was spot on.


So after basically running through the whole setup everything appeared to be in order.  I went back and did the spark plug pull test by removing the spark plug connections one by one to see what happens.
1,3,4 all either instantly killed the bike or dropped it to 400-500rpm where it almost died. When I pull 2 the bike doesn't seem affected until 3-5 seconds later when it looses about 300rpm (idle around 800).  Could this mean something?

Next on the block is a compression test if I can track down a tool to borrow and perhaps pull the carbs and bypass the air cutoff valve. Any other suggestions?  Could valves have any effect like that?  Or perhaps synch?  The timing light results are what throw me off every time...

[TOOLS1]

Compression check would be next on the list, but you said that the spark test with the timing light kept showing a miss on #2, #3. You really need to figure that out before going on.
TOOLS

[seestheday]

You can also get the CB900C manuals from the cb750c.com site.

[scratchdesk]

Well compression test showed me the light!

I found out cylinder 2 was super low so I pulled the top off and found one of the exhaust valves was too tight.  I need to order the special tool to adjust the valves along with some spacers.

Any clue the best way to go about getting both the tool and spacers?  Trying to get her running before the season is over, lol.

I think while I'm at it I may just pull the cams and replace the valve seals in hopes that takes care of the smoke out #1.

And carbs will be coming off again to get those passages blocked off.