Topic: Bad Misfire when warm?

[Porscheguy912]

Hello all,

first off: 1978 CB750F, 120 mains, pods, 4-1 aftermarket wrapped header with a cherry bomb, new points and condensers (static timed then checked with strobe), rebuilt carbs 3 times with last time using a ultrasonic tank. (bench synced) will be vacuum synced soon. idle circuit is 3 1/2 turns out and it still takes some effort to keep it running while cold and on choke.

Just as the title says, the beast starts, warms up, but then misfires really bad while hot. So bad that it will often die.
It pulls good but when you open her up she fumbles and coughs; but if you stay on the throttle it clears up and revs well. The problem is that once i've been riding for about 5 minutes or so it starts to misfire so badly that it sounds like its only running on 3 cylinders or less.

It left me on the road at night a few weeks back and the only thing lucky about that was that after it cooled down it started right back up. Once it did, being dark outside i noticed little blue sparks arcing from the plug boots to the head. So i got new boots thinking that was the problem and it still misfires.

I then thought it could be the wires so i cut and drilled out the coils, epoxyed in copper wire "spikes" and replaced the wires. Still no dice. I then thought maybe a coil is bad, so i pulled the plugs to hope that 1-4 or 2-3 looked sooty. Plug number one looks great and 2,3 & 4 all are half nice and tan and half sooty.
Just for kicks i pulled off the coils from a friends 76 cb750 to see if they would make a difference.
Nope, same symptoms.

Also, while i pulled the coils, i noticed the wiring harness looked a little melted. So i took the whole thing off, unwrapped it and noticed one of the ground wired looked kinda melted. (the section of green ground that spans the top of the engine where 3 wires connect into one) I wasent sure if it was melted from heat from the engine being so close or because it has so many wires connecting into one. I replaced it with a slightly larger gauge wire checked the resistance and re-wrapped the entire harness.
Still misfiring.
I am next going to looking to fuel flow. All of the floats have been set and checked. I do have a inline fuel filter and no screen in tank. (i'm ordering one today) I especially dont like the petcock on the 78 tank. the fact that there is no tube for the reserve line in the tank (reserve draws fuel directly off of the bottom of the tank) kinda bothers me. Does anyone know if there is a different petcock that will fit that has two tubes in the tank like my old 74' Cb550 ?

So, with all of that being said, is there anything that i am missing Electrically that could lead to a misfire when warm? Could it be carb boots? they seem tight but not hard. I sprayed starting fluid on them during the misfire and there was no change in engine rpm or tone.

I was going to wait until i vacuum synced the carbs to post but i figured any input would be helpful.

Cheers!

[SHORTROUND]

40 low speed jets
130 high speed jets
drop one on the needle clip/ move clip down one to the pointy end.
bench sync
throw them on the the bike an sync with gauges
ride the snot out of it

where to get jets? z1 enterprises $1.08 ea. get some 130's possibly 125's and 135's. that way your're prepared to fiddle with it a little bit. total cost about $20.

none of the honda petcocks have two tubes, well none that i have worked on.

[Hush]

Does your bike have points or electronic ignition? sorry gotta ask as I know zip about the 750 and people on here are always changing ignition parts.
If all that carb syncing and fiddling doesn't help, look at the condensers at your points, cheap little things that will drive you mad if they have gone bad.

[Dunstall_74]

Does your bike have points or electronic ignition? sorry gotta ask as I know zip about the 750 and people on here are always changing ignition parts.
If all that carb syncing and fiddling doesn't help, look at the condensers at your points, cheap little things that will drive you mad if they have gone bad.

PG Mention straight away that he set it up w/ new points and condensers.  Sounds to me like it's running lean, possible float level issue like ShortRound mentioned.  I think 130 mains would swing it just a tad the wrong way though.  I've heard 120's are good.  I used 125's for quite awhile, and they worked well.  I ran that size with stock coils and a Dyna-S.  Also, keep an eye on the fuel that your running.  The idle jets are almost always the culprit for the poppity-pop-pops...  anyway all this I'm sure you already know.   BTW Honda did have petcocks with 2 spigots, they were stock on the early models, I think K0-K2.  I am running one on my bike (Dunstall tank).  They are nice because they have a sediment bowl that has a screen that you can take off on the fly and clean out the crap that you're getting at the pump. 

[Hush]

We used to run a product called REDEX through our bikes in the 70's, one squirt per tank full and it cleaned the carbs right out!
Anyone else used that stuff?

[Dunstall_74]

We used to run a product called REDEX through our bikes in the 70's, one squirt per tank full and it cleaned the carbs right out!
Anyone else used that stuff?

Never heard of it but it sounds great!  Where do I get some?  I have to be careful of what I use in my tank as I've sealed it with Hirsch tank sealer.  I can't use anything with MEK's.  Seems like the idle jets on our bikes are a constant issue.