Topic: motor running on 3

[aglick87]

Here's a good one for all the motorheads out there.  I have been wrenching on my 550 all winter to bring it back to its original brilliance.  I rebuilt the motor, and thoroughly cleaned the carbs by dissassembly and dipping in a 2 different solution buckets.  I have been working the past few weeks on getting everything into the frame and running.  I fired it up over the weekend and to my surprise, I am only running on 3 cylinders.  #2 will not take off.  Here's what troubleshooting I did.  I checked for spark against the motor and it was good.  I swapped #2 and #3's plug wires to make sure it wasn't a bad plug wire.  I swapped 2 of the spark plugs and all the plugs are good.  So I ruled out ignition as a  possible cause.  I know it has good compression because it was just freshened up.  I also pushed the kick lever by hand and could feel compression at equal intervals.  So I assume it has good compression.  My next step was fuel delivery.  I backed out the drain screw on carb #2 and gas came pouring out.  So I know the bowl is full.  Is their anything I am missing?  The intake clamps are tight and not leaking air.  My first guess was that maybe the idle jet was clogged somehow.  But to see if that was a problem, I opened the throttle a little bit to the point where the carbs would be drawing off of the needle.  And still it would not take off.  So lastly, I took the filter out and sprayed starting fluid into the #2 carb and fired the motor.  And wouldn't you know it, the #2 pipe became slightly warm.  But once the fluid burned off, it was back to 3 cylinders again.  What do you guys think of that?

[kghost]

Assuming that the cam lobes for that cylinder weren't round.

It just about has to be the carbs. When you dipped them did you remove the jets and check all the orifice's?

Did you spray cleaner or air thru all the passages in the body of the carb?

[bryanj]

If thats a 77/78 i will just about guarantee the pilot jet is crudded up. You can get them out to clean if you are carefull.

[Raul CB750K1]

You gave us the answer. If it fires with starting fluid, everything is fine but the gas supply. Check the carburetor.

Raul

[aglick87]

I had every removable piece off and out of that carb.  Everything was soaked in the buckets and I also sprayed carb cleaner through the two air circuits that start from the rear of the carb. And I had the pilot jets out and ran a small piece of copper wire through all of the holes.  Then gave each of them a blast of cleaner and each one misted cleaner out very well.  But even if it was the pilot jets, it still should've taken off when I turned the throttle back to about 1/3 of the way.  At this point, the idle circuit has very little to do with anything.  That is the confusing part.  It's almost acting like the bowl doesn't have gas in it.  But it does.  This is a 78 by the way.  Any thoughts are welcome.

[Raul CB750K1]

Maybe a wrong float height? If it's too low gas will come into the bowl but will never reach enough level. If you attach a piece of transparent tube to the drain plug, hold it pointing upwards and open the drain screw, you will see how high the gas reach without having to dismantle the carburetor.

[MRieck]

Maybe a wrong float height? If it's too low gas will come into the bowl but will never reach enough level. If you attach a piece of transparent tube to the drain plug, hold it pointing upwards and open the drain screw, you will see how high the gas reach without having to dismantle the carburetor.

I'd check that float height too. Is the plug wet after the enging is running (fuel soaked) or dry?

[don_m]

Go back to square 1 & check the compression with a guage first before pursueing unknown problems.  Before you diagnose carb problems with your lever of logic, first stand on the solid ground of mechanical soundness, & place the fulcrum on solid ignition.
Cheers, Don Madden.

[migopod]

I'm having the same problem on a 76 cb550 only with #4.  Ignition, valves and compression are all good,
and I just put new jets and the like in the carbs.  When I run the engine for a while the #4 plug comes out
slightly damp, although not wet.  If it was getting the appropriate fuel mix and just not igniting, how wet
should the plug be?

[migopod]

Oh boy was I off about my #4 cyl.  What i discovered might be something worth checking into on yours.

I hooked up the sync gagues to the manifold, and #4 was reading a lot higher than the other three.
when i tried to adjust the vac down the needle didn't move at all.

It turned out that one of the pegs that's involved in lifting the carb piston when the cam rotates was
broken.  When i replaced that, the carbs synched well and the engine suddenly worked quite well.

Why is it so easy to overlook the simple things?

I'm having the same problem on a 76 cb550 only with #4.  Ignition, valves and compression are all good,
and I just put new jets and the like in the carbs.  When I run the engine for a while the #4 plug comes out
slightly damp, although not wet.  If it was getting the appropriate fuel mix and just not igniting, how wet
should the plug be?