1981 CB750 Custom carb syncing problems

[snyper2099]

The bike ran when I bought it but, it ran poor!
I should give some background on the bike...work I've done thus far,
-I took the carbs off, disassembled them and cleaned every part in them with carb cleaner.  Everything seemed to be moving freely and open.  I set the screws 1.5 turns out on each carb.
-I removed the air box and cleaned it, installed a new air filter, installed new spark plugs, changed the oil, cleaned out a bees nest from the stator compartment.
OK, now it runs better but...

When you first start the bike it starts just fine but, it only idle's around 1500-1800 RPM's and I want to get it closer to 1000.  If I attempt this, it stalls.

I have made one of the homemade carb syncing tools found on the forums here and it works very well!  I am having one cylinder on my bike (#4) that is really not firing at the same rate or may not be firing at all.  NOTE: The bike does not backfire.  I'm using ATF as the sync liquid.  Seems like the sync tool is working correctly but the #4 cylinder is not firing correctly.  This is way more than an air screw adjustment.  I know it's firing but the exhaust on that cylinder does not heat up at the same rate as the others.

Anyone have a good direction to look before I tear the carbs off the bike again and start cussing some more?  Should I swap plugs even though they are brand new?

[motorbiketim]

Shoot a little carb cleaner in the #4 carb while it's running and see if the rpm's pick up, if so then it not getting fuel on that cylinder. If it makes no difference then you may have a fouled plug or an ignition issue. Also, have you checked the compression?

[snyper2099]

I haven't checked compression.  I'm uncertain how to check it.  I will try the carb cleaner thing out over thee weekend and post my results.  I'm also going to change plugs 3 and 4 to see if it changes anything and someone else suggested I pull the #4 plug to see if it's getting a solid spark.  Thanks!

[cameronjm]

I have spent many hours doing what it sounds like you’re doing.

This is what I now know and maybe you already know but, here it is.

JC Whitney sells a nice vacuum system for syncing your carbs and last time I checked it was like 25 bucks.

If the carbs are vacuum controlled, I found that the air box needs to be installed for the carbs to work correctly, the air box helps sync the carbs.

Also, make sure the exhaust pipes are about the same temperature, and make sure that you do not overheat the bike while tuning the carbs. FYI: Keep your eyes on the heat coming from the pipes.

I think the number 2 carb is the factory set one, and you should adjust all other carbs to match it.

Good luck.

Cameronjm
 

[snyper2099]

Just thought I'd give an update, I've got all the cylinders firing together.  What I did was remove the coils and plug wires and found one of the plug wire connections to the coils was corroded.  I cleaned it with sandpaper.  I also unscrewed the ends and cleaned the ends on the plug connectors as well.  I reconnected everything and now all cylinders fire together.

Now I've discovered the stator is failing.  I've ordered a new one and will post how the bike is doing after I install it.  I used a multimeter and the wiring fault flowchart to figure out the stator was bad.

[nauticus28]

Do you have to buy a shop manual for the bike or were you able to find a free pdf version online? I've been looking for the free pdf version but without much luck.

[DWightSperber]

what did you find out? check the rubber boots. if they are cracked, they suck in excess air and would cause it to run faster. if boots are cracked, it will make it impossible to synch. spray wd-40 or something around the rubber boots at both ends.

[Yamahafixer]

You may want to swap coils 3 and 4 and see if the problem follows the swap.

[ccRider246]

Have you found the manual yet? You can download it a dansmc.com