1982 CB750sc wont start! help

[82-750sc]

I bought a 1982 CB750sc a few months ago and finally got it running on all four cylinders, one carburetor had a clogged jet and after I cleaned it, it worked great. But today I started it up to show someone the bike and I revved it once to ~5000 rpm and then shut it off. Later that day I tried to start it again but it would not start, only extremely loud backfiring once in a while. I have a fresh battery and the charging system works, petcock flows freely, getting spark on all four with new plugs. I am very frustrated with this, how can it go from working perfectly fine one minute then the next not work at all Ideas?

 

[amc49]

Plugs wet? Fouled plugs will do that.

You must not have much experience with engines as what you decry happens all the time on older ones with wear. Just because it is finally hitting on all four means nothing, you can still be on the edge of a no-start................

 

[82-750sc]

Plugs are wet, I just took them out again and am going to leave them out over night to air out the cylinders. Do you think it's just flooded?

 

[amc49]

Talking 35 year old engine now, compression check, if low the plugs will continue to wet foul as engine heat is not enough to keep them burned clean. The compression spec is around 170 psi.

 

[pantera_dragthewater]

Let the plugs dry. If that doesn't fix it dry the plugs again, and use a little starting fluid to help the initial starting. Hopefully that will give it that extra kick it needs to roar into action.

 

[amc49]

How does one 'dry' them? I ask because plugs wet with fuel generally leave residue that does not simply dry, being oil based. The residue then fouls almost instantly again and a no start, or crappy start that again fouls out to not start again. If by some means the plug DOES fire, if fouled it then carbons the excess fuel on the porcelain to then short down the sides to leak spark just like a wet plug does.

Like said earlier, no compression will foul plugs to make you think too much fuel all day long, just like weak ignition. The DOHC is utterly ridiculous about not sealing from people never running the valves and MUCH trouble comes from that.