Topic: CB550 -76 won't stay running

[Torkel]

It's almost done now! The bike is a barn find and I've had it down to the bare frame and rebuilt it up again. I now have an issue: The bike starts, runs/revs for perhaps 5seconds and then dies. It sounds as if it won't get fuel and it stops responding to the throttle.

-Sparkplugs are new.
-Carbs are cleaned and rebuilt.
-Ignition is spot on.
-Valve tappets are set to spec.
-Battery is charged.
-Exhaust is not clogged (been there - done that - took a while to realize).

She fires on all four and sounds like a horny angel when she revs. Next on my todo-list is to balance the carbs, but I can't keep it running/idling to get that done. I have the Petcock hooked up to one of the fuel lines and the other plugged, since the tank sits next to the bike on a work cart. Fuel flows freely from the tank - petcock and tank are clean.

Do I need both fuel lines hooked up to the carbs? From what I can tell, fuel should flow thru all four carbs anyway - am I missing something? I must be missing something...

[przjohn]

You need both lines hooked up.

[TwoTired]

You need both lines hooked up.

This^

One line feeds the right two carbs, the other line feeds the left two carbs.

Cheers,

[Torkel]

Got it.

For educational purpose: Since my bike is firing on all 4, there is obviously fuel traveling to all four carbs. Is it only reaching 2 carbs thru the vent/balance tubes? And when the fuel level drops in the two hooked up carbs, the other 2 starve?

[TwoTired]

PD carbs have a single fuel rail.  In the US these appeared on 77 and 78 K models.  In the US your, carbs should be stamped 087A on the carb bodies for a 76 K Model, or 069A if it is a 76 F model.  These both receive fuel from T fittings between pairs of carbs.

These earlier carbs have feeds between 1&2 and 3&4.  There is no other way for the carbs to receive fuel, or share between pairs.

They can be fed fuel from a single line if there is an additional T in the fuel path to feed both pair of carbs.

Cylinders cannot ignite without fuel.

Either you don't have stock carbs, you actually have fuel flowing to all stock carbs, or only half your cylinders are firing.  These are your only options which meet the laws of physics in our current universe.

Check your head pipes for even heating.

Cheers,

[Torkel]

Thanks for the info. My carbs have two separet fuel lines feeding 2 carbs each, no doubt, just as the -76s should have. I was thinking about the balancer between each set of carbs, but I forgot that they don't feed between 2 and 3.

I hooked up the second fuel line and that did the trick. She is spitting a little and running a bit rought, but she idles steady. I'll move forward with balancing and adjusting the carbs this weekend. The bike was clearly running on 4 cylinders, not on 2 (I would notice the difference  ). I did fill up the carbs thru both lines at first, since I started trying to run it on only the bowls (no tank connected). I guess it's possible I had a little fuel left in the bowls on the carbs not hooked up. It's the only explanation I can think of at least.

Thanks for the help, everyone.

[TwoTired]

As long as there is fuel in the bowls it can run.  The slow/idle jet draws from physically higher in the bowl reservoir, so it starves well before the main jet at the bottom of the bowl as the engine runs, and stops idling.    On the highway, the engine will sound odd (going lean) first with fuel starvation, before it quits completely.  But, that's when the reserve setting is selected.

Cheers,

[Torkel]

TwoTired: Thanks, that makes a lot of sense.

One step forward and 2 steps back: I've hooked up my 4 vacuum clocks and connected my fuel lines to the tank, placed on a cart next to the bike. All cylinders are showing very low vacuum. I know that the balance between the carbs are more important than the actual value, but it's still lower then I expected. I tried to adjust the carbs, but I see very little change happening, even if I blip the throttle a bit and let it come down to idle again.

The bike is loud and sluggish when I rev it. So I sprayed some starting fluid at the seal between the head and the intake manifold and the bike reacted. So, I have an intake leak. I've also ordered new exhaust gaskets, because I think I have one or several exhaust leaks too.

My goal of test driving this weekend failed! But it could be worse. I'll fix the leaks and get back on it. To be continued...

[bryanj]

The O rings between manifolds and head are the same as rocker cap ones but if the rubber manifolds are hard its worth changing them.

[Torkel]

The carb rubbers are new. But the O-rings in the manifold that seals to the head are from -76. Same as rocker caps?! Perfect, thanks for that!

[Torkel]

I have improved things, yet not reached all the way.

New orings in intake manifold + new copper O-rings in exhaust fixed the leaks. She sounds better.

I'm very much a vintage bike-noob, so I've added a couple of pics so you all can double check if anything looks wrong.

- The idle screw on the carbs is all the way in (as in: very high idle), but the bike still stumbles and dies on idle, as if it was set too low.
- The bikes starts easily in 1/3 throttle. It fires, stumbles and dies if started on 0 throttle. It won't even fire if cranked with any amount of choke, even when cold.
- When I rebuilt the carbs, I went to a 115 main jet, to compensate for the Pod filters. I also ordered size 40 "Slow jets" (se picture) but once I took the carbs apart, I couldn't figure out where they would go. The fuel screw on the side (idle fuel?) is much larger. I assumed I ordered the wrong parts.
- Fuel screws /Idle fuel are out 2 full rotations.
- The bike has a 4-1 exhaust of unknown type/brand/flavour.
- Timing is correct, tappets are set, fuel flow in good, spark plugs and wires are new.
- The carbs have 2 fuel rails, but I can't find the markings you, TwoTired, mentioned. See attached pic for reference.

I've already woken up my son by trying to keep the bike running, apologized to the Mrs, served her a glas of wine and pushed the bike back into the garage for today. Some guidance would be much appreciated.

Could my balancing screws be so far off that they cause the low idle?

[Scott S]

 [- The idle screw on the carbs is all the way in (as in: very high idle), but the bike still stumbles and dies on idle, as if it was set too low. ]

 I bet that when you rebuilt the carbs you have the slides and idle screw messed up. Did you bench sync the carbs? What does the gap on the NON cutaway side of the carbs look like? Slides aren't in backwards are they?

 I would back out on the idle screw to midway and use the sync screws to open the slides. You should only see a ting gap on the non-cut away side.

[Torkel]

I bet that when you rebuilt the carbs you have the slides and idle screw messed up. Did you bench sync the carbs?

I did not. Not sure how that is done: Just set all the carbs to the same setting? I didn't touch the sync screws when I rebuilt the carbs.

What does the gap on the NON cutaway side of the carbs look like? Slides aren't in backwards are they?

Slides are in the right way (pretty sure they don't fit backwards). I assume the "cut aways" are the sloped shape on the intake side of the slides? I see where I went wrong here: I didn't realize the sync screws moved only the sliders. I thought (duh!) they moved both slider and needle.

I stuck my phone in #4 carb and snapped 2 pics (see attached). The gap does look larger than "tiny".  I'll remove the cars and "bench sync" the carbs tomorrow.

Thanks!

[Scott S]

 Well, first of all, if your idle screw is a the way in, back it out and try that before removing the carbs.

[Torkel]

Well, first of all, if your idle screw is a the way in, back it out and try that before removing the carbs.

I played with several different settings. Max in on idle screw has the least bad symptoms. Something is clearly off.

[Scott S]

 I just saw the tags on the pics. It doesn't look like there's any difference between all the way in and 50% on the idle screw.

 You are calling the big, single screw on the main rack of carbs the idle screw, correct? Not the 4 individual idle mixture screws on the carb bodies, right?

 Are you sure you're operating the choke lever correctly? You wouldn't be the first person to get it backwards.

 If you are moving the large, single idle screw and the slides are open that far, then something was assembled wrong when the carbs were rebuilt. No way the slides should be open that much at idle.

[Scott S]

 It just hit me that those pics are from the air box side and show the cutaway.

 I just so happen to have a set of rebuilt carbs sitting here. Here's what my baseline starting point looks like:

 Idle mixture screw.
 Slide height on engine side.
 Slide height on air box side.

[Torkel]

Awesome! Your pictures are really helpful - now I have something to compare to. I’ll pop the carbs off and adjust them on my workbench.

No, I know how the choke works. It both has markings and I see what is open/close with the air filters off.

I think the lack of visual movement is mostly down to poor camera technique. I clearly see the whole throttle rack and the sliders inside the carbs move when I adjust the idle screw. I think it’s just the sync screws that were a mile off when the bike was parked 20 years ago and I didn’t reset them when I rebuilt. Now the very low vacuum levels and the lack of reaction when I try to sync makes sense too.

To be continued...

[TwoTired]

When you bench sync the carbs, make sure that with the idle knob backed out, that your "Master" carb (which you designate), can reach the "floor" and cut off all air supply.   Then you lock that one down and never adjust it's travel ever again, even during vacuum sync.  What you will do is adjust all the other carbs to the setting of the master, regardless of where the big idle knob moves it to.

This will allow the idle knob to control the lowest slide position and not run out of adjustment range.  Never chase vacuum level on the master carb.  Adjust other carbs to match what it has.

Cheers,

[Torkel]

That makes a lot of sense - thank you!

[Torkel]

Confused... 

So I yanked of the carbs and saw that the sliders where a bit all over the place. Good news, since that meant I had found the root cause! I was certain! I was confident! I was... wrong?

So I followed the procedure you guys described (see pics) and... the bike behaves exacly the same.

It only starts if I give it about 1/3 throttle. It won't run on idle, no matter how far in I turn the idle screw. It won't start or run on choke.

EDIT: Just realized, I havn't been able to find any markings or numbers on the carbs. Can someone confirm that those are in fact the stock carbs for a CB550 -76? The bike was a barn find and I don't even know if it was running before it was parked.

[Scott S]

 You said you ordered slow jets but couldn't figure out where they go.
 You DO have pilot jets in the carbs, don't you? They should have been right next to the main jets in the carb bodies. It should have been pretty obvious.

 And while we're at it, did you check float heights or fuel level in the bowls?

 What do the plugs look like?

[Torkel]

Oh wow! Yes, that is it of course. (facepalm icon?) I just checked some pictures on the web and saw right away where the pilot jets go. My carbs didn't have any installed when I took them apart to rebuild them! I just assumed the carbs were complete, ordered a rebuild kit and replaced each parts I removed. It never struck me that something like one of the jets would be missing.

Geez... I may be new to bikes, but I've done enough wrenching on old cars to remember: Never, ever assume that the previous owner did anything right.

OK, off to the garage...

Edit: Yes, I did set the fuel level in the bowl. I did that right after I completely missed that the pilot jet was missing...

[Scott S]

 Yep..... that'll make it run like crap!

[Torkel]

OK, carbs have been off - got some slow jets installed - back on the bike. I'm getting pretty fast at removing/installing the carbs at least. 

Start up will have to wait until tomorrow. My son has been exceptionally hard to get to sleep and I'm not risking waking him up... again...