Topic: '72 750K2 drops a cylinder when warm

[smccloud]

Ran great until after this weekend when I pulled the engine to find a lost tappet adjuster locknut.  I dropped a plug on the ground so I put a new set in tonight and I have the same damn issue.  I know I had a hard time getting the socket off of #2, but I just need ideas of what to look at here.  I won't be looking at anymore tonight because I'm to pissed off to, but I don't want to have to keep riding my dad's new fangled DOHC CB750F/K/A/whatever (he put 4 bikes together to make one).

[HondaMan]

If it's not a too-tight tappet (do they all have .002" intake/.003" exhaust clearance?), it could be a bad plug cap or the wire to it. Try watching it on a humid night, to see if any dancing halos appear in the dark near #2. Mine did that, and I couldn't see it inside the Vetter fairing lowers, until a rainstorm made it dance up my leg at a stoplight. Man, that hurt. 

It would run OK, but cold-blooded, then when good and hot, #3 would quit, or at least run weak. The plug cap was old and the resistor was nearly 15,000 ohms (i.e., burned open).

[smccloud]

If it's not a too-tight tappet (do they all have .002" intake/.003" exhaust clearance?), it could be a bad plug cap or the wire to it. Try watching it on a humid night, to see if any dancing halos appear in the dark near #2. Mine did that, and I couldn't see it inside the Vetter fairing lowers, until a rainstorm made it dance up my leg at a stoplight. Man, that hurt. 

It would run OK, but cold-blooded, then when good and hot, #3 would quit, or at least run weak. The plug cap was old and the resistor was nearly 15,000 ohms (i.e., burned open).

tappets are all at .002" intake/.003" exhaust.  i think it took me over half an hour to adjust them since i wanted them all perfect.  i will check the caps tonight.  what pisses me off is the fact that it ran great until i pulled the engine out to find the stupid locknut for the number 1 intake adjuster

now i have to ride my dad's fancy dohc cb750 to work today (i'll take a pic)

i am also going to re-do all the wiring extensions i did for the ignition box i bought from you just to be safe (i.e. in case i managed to get one cold solder joint and not notice it).

[smccloud]

If it's not a too-tight tappet (do they all have .002" intake/.003" exhaust clearance?), it could be a bad plug cap or the wire to it. Try watching it on a humid night, to see if any dancing halos appear in the dark near #2. Mine did that, and I couldn't see it inside the Vetter fairing lowers, until a rainstorm made it dance up my leg at a stoplight. Man, that hurt. 

It would run OK, but cold-blooded, then when good and hot, #3 would quit, or at least run weak. The plug cap was old and the resistor was nearly 15,000 ohms (i.e., burned open).

just out of curiosity, what should the resistance read?  since it runs fine when cold..............

[HondaMan]

A good Denso plug cap reads between 3000 and 5500 ohms. I have seen new ones recently at 6000 ohms, though. When they burn inside, it goes up in resistance. When a burned one gets hot, it leaks the voltage to the sides of the cap, away from the center clip.

A cold solder joint on the YELLOW or BLUE wire, or a loose or damaged green wire, could potentially cause trouble - but, it would be with 2 cylinders at a time, not just one.

[smccloud]

A good Denso plug cap reads between 3000 and 5500 ohms. I have seen new ones recently at 6000 ohms, though. When they burn inside, it goes up in resistance. When a burned one gets hot, it leaks the voltage to the sides of the cap, away from the center clip.

A cold solder joint on the YELLOW or BLUE wire, or a loose or damaged green wire, could potentially cause trouble - but, it would be with 2 cylinders at a time, not just one.

well, i was wrong with the 1 cylinder at a time, its all 4.  just not all at once.  didn't even get on the highway tonight before it started.  i'm wondering if i didn't get a good connection when i hooked the 5 wires for the charging system back up............

i'm gonna check that tomorrow night.  if thats the case, i will be working on it saturday; until then i'm still stuck on the new fangled dohc (i miss my baby).

[HondaMan]

A good Denso plug cap reads between 3000 and 5500 ohms. I have seen new ones recently at 6000 ohms, though. When they burn inside, it goes up in resistance. When a burned one gets hot, it leaks the voltage to the sides of the cap, away from the center clip.

A cold solder joint on the YELLOW or BLUE wire, or a loose or damaged green wire, could potentially cause trouble - but, it would be with 2 cylinders at a time, not just one.

well, i was wrong with the 1 cylinder at a time, its all 4.  just not all at once.  didn't even get on the highway tonight before it started.  i'm wondering if i didn't get a good connection when i hooked the 5 wires for the charging system back up............

i'm gonna check that tomorrow night.  if thats the case, i will be working on it saturday; until then i'm still stuck on the new fangled dohc (i miss my baby).

Can you check the battery volts with a meter?
When all cylinders are acting up, it is usually a single source, like a loose/bad connector in the headlight, a poor KILL switch contact (operate that thingie about 10 times to "wipe it clean" inside the cover), or a marginal connector at the coils. The Ignition, you'll remember, makes you unplug the black/white wire at one of the coils, then plug into the Ignition, then plug that coil back in to the extra Ignition female. One rider reported that he broke off the wire from the male on the black "wye" right there, and he had to use the spares I send along to fix it. This could possibly be similar?

[smccloud]

A good Denso plug cap reads between 3000 and 5500 ohms. I have seen new ones recently at 6000 ohms, though. When they burn inside, it goes up in resistance. When a burned one gets hot, it leaks the voltage to the sides of the cap, away from the center clip.

A cold solder joint on the YELLOW or BLUE wire, or a loose or damaged green wire, could potentially cause trouble - but, it would be with 2 cylinders at a time, not just one.

well, i was wrong with the 1 cylinder at a time, its all 4.  just not all at once.  didn't even get on the highway tonight before it started.  i'm wondering if i didn't get a good connection when i hooked the 5 wires for the charging system back up............

i'm gonna check that tomorrow night.  if thats the case, i will be working on it saturday; until then i'm still stuck on the new fangled dohc (i miss my baby).

Can you check the battery volts with a meter?
When all cylinders are acting up, it is usually a single source, like a loose/bad connector in the headlight, a poor KILL switch contact (operate that thingie about 10 times to "wipe it clean" inside the cover), or a marginal connector at the coils. The Ignition, you'll remember, makes you unplug the black/white wire at one of the coils, then plug into the Ignition, then plug that coil back in to the extra Ignition female. One rider reported that he broke off the wire from the male on the black "wye" right there, and he had to use the spares I send along to fix it. This could possibly be similar?

you were right on your first guess, #3 plug cap is bad.  although i was thinking i messed up the wiring when i extended the wires for the ignition i bought from you.