Topic: Diagnosis, anyone?

[medic09]

I recently bought my '78 CB750K.  It's all there, runs 'okay', but has lacked real TLC for a long time.

I commute about 120+ miles round trip each day.  The bike gets up to, and runs at highway speed (75+ mph), *but* it lags when I open the throttle (like to pass), it sometimes loses power on even moderate hills (other times climbs fine), and even while running at speed it may 'buck' for a while.

I ran Gumout/STP gas treatment through it.  I set the gaps and timing (timing was way off).  I've ordered new points, because the old ones were getting burnt.  I ran it at high speed to 'blow out' the carbs.  New sprockets and chain (which it really needed).  Air cleaner was dirty, but not intolerable; and I've ordered a new one.

Now what?  All the stuff I described persists.  I'm thinking fuel filter, or carbs, or...?

Any input appreciated.

Thanks!

[Rocking-M]

Sounds like the carbs need synchronizing,
you can clean those points up a bit with a emory file (unless your lucky enough to find
a points file) be careful to file them flat. Stick the file in with the points closed and pull
it out straight. Reset them with feeler gauge and they should last till your new ones arrive.

[ElCheapo]

Without knowing when the last time was that the carbs were tore down, I would assume the carbs need to be done. I have seen carbs that run ok and were supposedly just done, and gone through them completely and the bike runs with incredible improvements. Fact is no amount of feeding super cleaner to any carb that is messed up will make it run any better without removing it.  

[jsaab2748]

Old sparkplug caps that are breaking down under load can cause drivability issues. A cheap fix
and available at the dealer. They "screw" onto the wires when you install them. A good idea to replace them especially if they're the originals.

[medic09]

And I forgot to mention, the gas mileage stinks.  Before I tried any remedies, it was around 25/26 mpg.  Now it's around 32/34 mpg.

Keep those insights coming!

Thanks all!

[Sam Green Racing]

I would leave it untill the new air filters arrive, if the old one are over dirty, they will make the bike run rich enough to seem down on power.

Sam.

[medic09]

Interesting thought, Sam; thanks!

Do you figure that could be the simple solution to the whole 'syndrome'?

[jgary]

I had some very similar symptoms, followed some very similar steps.  This past winter I tore down the fuel petcock to see why my reserve wasn't working.  On the 76 CB750, there's a mesh bag that covers the tubes that go into the tank from the petcock.  Kind of like a filter, I guess.  Mine was filty, mayne 1/3 blocked.  Cleaned it up with WD-40, and all is well.

John.

[eldar]

Well it is hard to say what is going on.
You will want to set aside about 5 hours.

Set your valves and timing and adjust cam chain. you will probably want to pull the housing off for the cam chain and clean it first.
pull off the carbs and clean the crud out. You will want to pull the main and slow jets out to clean. Trust me on this one.
clean things VERY well. make sure you can see light thru all the jets. Check and see what your accel pump looks like.
Bench sync your carbs and remount them.
Set your sync using sticks or guages then set your idle mix, preferably with a good tach.
You mileage should be around 40 -45 mpg. You should also have decent power. I can still accel against a 30+ headwind in 5th gear.

[medic09]

Thanks, eldar.  I rather expected that's what I have to do.  It won't happen 'till winter, though.  The bike's my daily ride; and full-time+ school, work, and other obligations keep me from dedicating more than an hour or so at a time.

This evening I drained the tank, pulled the petcock and cleaned the screen/filter (wasn't all that dirty), and put it back together for tomorrow AM's commute.  I'm supposed to be studying just now, but procrastination won out.

Next, I'm going to look for some Seafoam.