Topic: 73' CB750, washed engine, now misfire.

[duganc1717]

So I have been luking here for alittle over a month,  I bought this 73' CB750 alittle over a month ago that was part of a Ebay auction with my bike, a complete disassembled bike, two other engines, and a pallet full of parts.  I split the auction with a co-worker and kept the complete running bike a 73' CB 750 w/around 25K, 16" rear wheel, dyna ignition, drilled front rotor, K&N filter.














Of course the bike has required more repairs than what I thought I was getting into, but after taking the carbs apart twice, installing a new chain, and curing a sticking front caliper the bike was running pretty well. 


Today we took two steps back, I washed the engine trying to remove some grease.  When I was done I fired I adjusted the air/fuel mixture screws, and went for a wuick ride maybe 3 miles.  The bike ran great, came home took a shower left my house and noticed a cylinder not firing. 

So I assume I have a spark plug wire that is wet from washing the bike, I have had this happen many times before on cars, bikes, etc.  So I figure I will ride it to a friends house and it will clear up.  It did not, I have 2 cylinders (I think) cutting in an out.  When I got home I pulled the spark plug boots and sprayed brake clean and compressed air into the ends. 

The bike still runs the same, does anyone have any suggestions?

[cleveland]

If it's not the plugs, you may have gotten water in the carbs.

[bwaller]

Check the plug wires where they exit the coils. OEM plug wires sometimes get frayed & loose in that area after 35 years! Wire connections at the coils may be loose also. If that checks out pull the points cover and make sure the contacts are clean and everything is dry. Cleveland mentioned carbs, you can remove the air filter, let it dry plus do a quick drain of the offending float bowls in case. 

[duganc1717]

There no points on the bike it has a Dyna ignition, but thanks for the ideas.  Just for the record when I washed the engine it was mainly the front of the cylinder head and the oil pan.  I did not spray or drench the whole engine. 

It felt very similar to when I had a float bowl stick closed and the number 3 cylinder was starved for fuel but, I pulled the bowls and they looked good.  Also if there was water in the gas would the cylinder misfire then fire, over and over?  Plus I think I'm dropping more than one cylinder @ times.

[bwaller]

Sorry, missed the Dyna part. Cylinders 1&4 and 2&3 each share a coil so determine which pair is acting up first.

If they are indeed stock coils and wires, unscrew the plug caps and with side cutters remove a 1/4" of wire to get fresh wire core and reattach the caps.

[MoTo-BunnY]

If they are indeed stock coils and wires, unscrew the plug caps and with side cutters remove a 1/4" of wire to get fresh wire core and reattach the caps.

+1 on that - I had the same problem after washing my bike a bit too thoroughly. 

I drained the float bowls, one after another, into a glass so I could see the content and the #3 had a bunch of water in it (but none of the others did, oddly enough).

Then, I took a spark plug cap off of one of the wires and found the end to be green with verdigris, so I cut off about 2cm, added a bit of dielectric grease, and re-threaded the cap back on. I did that to all the wires, as I found them all a bit green on the ends - ick!. I also put some dielectric grease inside the caps, themselves, to help seal out moisture and haven't had any problems since - even after riding through a few of Oregon's driving rain storms. 

[duganc1717]

Sorry, missed the Dyna part. Cylinders 1&4 and 2&3 each share a coil so determine which pair is acting up first.

If they are indeed stock coils and wires, unscrew the plug caps and with side cutters remove a 1/4" of wire to get fresh wire core and reattach the caps.

I had previously cut the plug wires back and they were indeed green  , I'm a little hesitant about unscrewing the caps again because the wires are very short and cannot be cut again.

[MoTo-BunnY]

Maybe just unscrew the caps then and clean + add dielectric grease (to keep out any future water?)   I know what you mean about them being awful short - I couldn't do that trick anything, on mine either, and have them still reach the plugs!   

[duganc1717]

Just alittle update, I started the bike today and it still sounded like it was down a cylinder.  I looked around and saw the no.4 cylinder arcing off the valve cover.   The wire is so short that it constantly rubs the valve cover 

So I unscrewed the boot and put dielectric grease on the threads and inside the little rubber insulator boot, and put a piece a rubber hose between the spark cap and the valve cover. 

Fired it up and it sounded great  , you can actually see it the offending wire in the fourth pic.  Thanks for everyone's help.