Author Topic: Slow vacuum prob CB550 (Read 4023 times)

twosixtk · « **on:** June 12, 2012, 01:24:13 PM »

I get @ 2-3 miles on my '74 CB550 before vacuum starts to hold gas from carbs. I rebuilt carbs with new needles/jets/seals. Set needle at lower most ring (highest internal setting) as per what was found. Also ruled out gas cap vent blockage. What can cause this slow vacuum problem? TX!

dave500 · « **Reply #1 on:** June 12, 2012, 01:30:48 PM »

fuel tap blocked/fuel strainer blocked/kinked or blocked fuel lines?

iron_worker · « **Reply #2 on:** June 12, 2012, 02:57:25 PM »

Blocked bowl vent?

How did you rule out the gas cap?

Could it be restricted fuel flow? Slowly fills up the bowls but not enough to maintain while riding? Clogged filter? Gunked up petcock?

IW

Elpresidente · « **Reply #3 on:** June 12, 2012, 06:39:15 PM »

This forum rocks... took the 500 for a stroll, started bogging down... had a helluva time getting home. Looked at the forum, saw this post... went out, cut back one of the gas lines (it was a little long, had a bit of a kink in it) and checked out the petcock bowl.
Bike fired up and ran like a scalded dog! Thanks guys!

twosixtk · « **Reply #4 on:** June 12, 2012, 08:12:26 PM »

New fuel filters, no kinks in new lines. Rode with gas cap off, still happened. Gas pours out of petcock. Again, runs awesome for a few miles, slowly starved of fuel. Let it sit, fuel catches up and bike runs fine.

luap · « **Reply #5 on:** June 12, 2012, 09:25:39 PM »

air bubble in the line, if your runnign the 2 fuel filter try pulling them off see if that helps

twosixtk · « **Reply #6 on:** June 13, 2012, 09:23:22 AM »

Thanks for all of your help. Afraid to run without filters. Re-checked petcock and is clear. Could this be a carb synch vacuum prob?

LesterPiglet · « **Reply #7 on:** June 13, 2012, 09:27:46 AM »

See how fast fuel drains from the float bowl drain screws, your float valve/s may be partially blocked.

twosixtk · « **Reply #8 on:** June 13, 2012, 09:29:47 AM »

#s 3 and 4 no flow...

LesterPiglet · « **Reply #9 on:** June 13, 2012, 09:31:09 AM »

Bowls off and clean those valves.

twosixtk · « **Reply #10 on:** June 13, 2012, 09:34:38 AM »

That'll be next step. Thanks!

TwoTired · « **Reply #11 on:** June 13, 2012, 10:22:04 AM »

Does it do the same thing with the fuel selector on reserve?

How full is the gas tank?

twosixtk · « **Reply #12 on:** June 13, 2012, 11:56:06 AM »

Same in reserve. Tank is nearly full. Pulled bowls 3 and 4. #3 had flow from the float valve orifice. Blew out #4 hole with carb cleaner and got it to flow. Back together. Rode 1 miles same problem. WTF?

TwoTired · « **Reply #13 on:** June 13, 2012, 12:39:55 PM »

Sounds like you have something in the fuel line moving about and blocking passage. To confirm, when symptoms occur, shut off the fuel valve and engine at that point, coast to a stop, then either drain each bowl to measure fuel quantity in each, or remove each bowl to confirm which carb(s) have the problem. Only one, a pair, or all of them?

Duanob · « **Reply #14 on:** June 14, 2012, 10:28:01 AM »

Anybody notice his needle position is in the leanest possible? Its amazing how much difference one clip position makes. Not that that has anything to do with vacuum leaks but still. Check your plugs to see how white they look. Also have you ever replaced the intake manifolds O-rings? Vacuum sync?

twosixtk · « **Reply #15 on:** June 14, 2012, 11:08:44 AM »

Thanks, I will check plugs. Never had this engine apart so no new rings, etc.

Motoguy23 · « **Reply #16 on:** June 14, 2012, 11:14:50 AM »

check the float bowl vent. if your vent is blocked then there is fuel able to get to it but there is a vacuum in the bowl and it will do just like you said.

I have it circled in red.

twosixtk · « **Reply #17 on:** June 14, 2012, 05:06:15 PM »

Had carbs off AGAIN. These vents both drained when I turned them over. #3 is is running 100º hotter than others at exhaust...#4 also a bit hotter the #1 and #2.

luap · « **Reply #18 on:** June 14, 2012, 05:43:46 PM »

do you have to roll off the throttle for the bike to catch back up an run good again like you mentioned above, Daunob did point out your in the leanest position on needle clip which sounds like where you are running the bike, could just be starving motor for fuel. correct me if im wrong but 3 position is stock for those carbs,
are you running the stock air box an stock filter?

twosixtk · « **Reply #19 on:** June 14, 2012, 08:10:42 PM »

I am running the stock air box. Only had bike 1 year. Clips were at #1 when I got it. This aft I moved them up to #2. Bike running worse now but with same problem. As for getting off throttle, no, carbs die when throttle is on but only after a couple of miles of smooth runnins. Again. Thanks for all of Your help.

dave500 · « **Reply #20 on:** June 14, 2012, 10:38:50 PM »

upside down floats?

luap · « **Reply #21 on:** June 15, 2012, 12:23:34 PM »

did you pull out the emulsion jet an clean it real good, theres 6 or 8 small holes that need to be open, sounds like your dieing low mid range I would check those, an measuring the floats correctly 22mm is done on with the carbs on their side not upside down

twosixtk · « **Reply #22 on:** June 17, 2012, 09:24:23 AM »

Thanks guys. Will check emulsion jets and floats. Bruised some ribs so I'll be back at it asap.

twosixtk · « **Reply #23 on:** June 18, 2012, 04:00:17 PM »

SO, all floats are right side up BUT the #3 and 4 throttle valve bodies are showing more gap at the bottom than #1 and 2. At least 1/8" more light showing through when at rest. Could this be the cause? How do I adjust these to match?

LesterPiglet · « **Reply #24 on:** June 18, 2012, 04:06:24 PM »

That wouldn't cause your problem but you will need to sync those carbs with a Morgan carbtune or similar.

lucky · « **Reply #25 on:** June 18, 2012, 04:09:26 PM »

Quote from: Duanob on June 14, 2012, 10:28:01 AM

Anybody notice his needle position is in the leanest possible? Its amazing how much difference one clip position makes. Not that that has anything to do with vacuum leaks but still. Check your plugs to see how white they look. Also have you ever replaced the intake manifolds O-rings? Vacuum sync?

Yes I noticed that right away, but he did not say if it is a CB550 K,F

?
The 1974 CB550K would be 4th groove down from the top of the needle. This is the STOCK Position.

1974 CB550F would be 2nd groove down fro the top of the needle.

He said, "Set needle at lower most ring (highest internal setting) as per what was found. "

Who knows what that means either way it is wrong.

We would need to know all the facts first before we could help.
He is not done yet. He did get the kink out of a vent line(he said fuel line??)

lucky · « **Reply #26 on:** June 18, 2012, 04:24:04 PM »

Ok..I will ask what no one did.

Does the owner know the difference between the fuel line into the carbs and the vent hoses?
Does he have a fuel filter on a vent hose???

The owner needs to read about syncing the carbs and what that means.

FACTS so far:

1974 CB550 what?
Intake type ?
Exhaust type?
Slide needle? We know its on the top groove or the bottom groove .Thats it.
Main jet?
idle jet?
mixture screw?

We will keep digging.

News:

Author Topic: Slow vacuum prob CB550 (Read 4023 times)