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Author Topic: Carb trouble?? 77' 750K  (Read 1247 times)

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Offline mikejorg

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Carb trouble?? 77' 750K
« on: March 03, 2013, 06:36:24 PM »
Recently cleaned and sycned the carbs. The bike ran beautiful for about 4 days and then died on me. Was in 3rd gear and could feel it wanting to die out. Turned to a side street and as soon as let off the throttle it died on me.

The bike starts and idles fine. At idle I'll give it a quick turn on the throttle and it won't rev, but if I give it another quick turn it will rev as normal. Some times on the first turn of the throttle it will die. With the choke out this is not an issue.

Is this a carb issue? Floats levels off?
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Offline TwoTired

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Re: Carb trouble?? 77' 750K
« Reply #1 on: March 03, 2013, 06:59:28 PM »
Check the simple, first.  Gas tank fuel level.

Pull the spark plugs and look at the deposits.

Recheck all the tuneup check list items.
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Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline mikejorg

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Re: Carb trouble?? 77' 750K
« Reply #2 on: March 03, 2013, 07:32:29 PM »
Fuel level was at half and just put new plugs in. I should pull the plugs out again and check them.

Any other ideas? Anything I should check for?
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Offline toytuff

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Re: Carb trouble?? 77' 750K
« Reply #3 on: March 03, 2013, 07:48:54 PM »
Check your fuel cap. It needs to vent just like humans.

tt
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Offline mikejorg

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Re: Carb trouble?? 77' 750K
« Reply #4 on: March 03, 2013, 08:01:16 PM »
What am I checking for on the fuel cap?
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Offline flybox1

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Re: Carb trouble?? 77' 750K
« Reply #5 on: March 03, 2013, 09:09:44 PM »
after you run it does it create a suction sound when you open the tank cap? that would indicate a vapor locked tank and compromised fuel flow...
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline mikejorg

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Re: Carb trouble?? 77' 750K
« Reply #6 on: March 03, 2013, 10:07:29 PM »
I'll check for that and let you know the result.
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Offline mikejorg

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Re: Carb trouble?? 77' 750K
« Reply #7 on: March 04, 2013, 11:43:54 AM »
I don't believe its vented through the fuel cap, I have a hose coming out of the bottom of the tank. Didn't find any pinches or binds.
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Offline iron_worker

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Re: Carb trouble?? 77' 750K
« Reply #8 on: March 04, 2013, 11:59:00 AM »
As the fuel level goes does the space created must be replaced with air. So yes, there must be a vent to atmosphere on the tank somewhere.

However, seeing as you cleaned and sync'ed the carbs recently I would suspect your own work first.

IW
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Offline TwoTired

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Re: Carb trouble?? 77' 750K
« Reply #9 on: March 04, 2013, 12:05:51 PM »
Quote from: mikejorg on March 04, 2013, 11:43:54 AM
I don't believe its vented through the fuel cap, I have a hose coming out of the bottom of the tank. Didn't find any pinches or binds.
You're wrong.
 The large hose you see is a well drain for the space next to the gas cap, under the door.  The cap itself MUST be vented.  Better find a hole.


...And recheck the points setting.

Cheers,

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Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline flybox1

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Re: Carb trouble?? 77' 750K
« Reply #10 on: March 04, 2013, 12:27:34 PM »
Quote from: TwoTired on March 04, 2013, 12:05:51 PM
Quote from: mikejorg on March 04, 2013, 11:43:54 AM
I don't believe its vented through the fuel cap, I have a hose coming out of the bottom of the tank. Didn't find any pinches or binds.
You're wrong.
 The large hose you see is a well drain for the space next to the gas cap, under the door.  The cap itself MUST be vented.  Better find a hole.


...And recheck the points setting.

Cheers,

+1, lift your tank lid and see the hole in one of the corners closest to the hinge.  if you overfill, this drains directly to the ground (if routed properly  ::) )
might be time for you to go through the 3000 mi tuneup steps. 
get out your shop manual, or download it from this forum.
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline mikejorg

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Re: Carb trouble?? 77' 750K
« Reply #11 on: March 04, 2013, 01:21:35 PM »
Thanks for all the info guys, really appreciate it.

I'll check for the vent near the fuel cap. I'll recheck the points again, I set them to spec before I cleaned the carbs. I have the manual so I'll go through the tuneup steps as well.

Gonna get some clear hose and measure the float height this weekend as well.
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Offline flybox1

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Re: Carb trouble?? 77' 750K
« Reply #12 on: March 04, 2013, 01:25:37 PM »
save the trouble of the clear tube test if you have not taken the carbs off to service them?
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline mikejorg

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Re: Carb trouble?? 77' 750K
« Reply #13 on: March 04, 2013, 02:27:22 PM »
The carbs are in great shape, all brass components look brand new. Everything was clean when I took them off before except for a small clog in the pilot jet on carb #1, but that was cleaned out using fishing line and carb cleaner. Gonna get some guitar string and get in there again. I dunno if the fishing line did enough.
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Offline flybox1

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Re: Carb trouble?? 77' 750K
« Reply #14 on: March 04, 2013, 02:38:34 PM »
visually look clean? or verified?
emulsion tubes pulled out and all holes run through with cleaner/wire?
not trying to belabor....cleanliness of these carbs is really important.
verify your float heights are correct (same level as top of the pilot jet).
« Last Edit: March 04, 2013, 02:42:03 PM by flybox1 »
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline flybox1

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Re: Carb trouble?? 77' 750K
« Reply #15 on: March 04, 2013, 02:40:43 PM »
...also, if you have PD style carbs, you need to ensure the accel pump circuit is squirting properly.
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline TwoTired

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Re: Carb trouble?? 77' 750K
« Reply #16 on: March 04, 2013, 02:50:47 PM »
Quote from: mikejorg on March 04, 2013, 02:27:22 PM
The carbs are in great shape, all brass components look brand new. Everything was clean when I took them off before except for a small clog in the pilot jet on carb #1, but that was cleaned out using fishing line and carb cleaner. Gonna get some guitar string and get in there again. I dunno if the fishing line did enough.

You can't ensure the pilot jets are clean without pulling them out.  You'll see why, once you remove them.
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Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline mikejorg

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Re: Carb trouble?? 77' 750K
« Reply #17 on: March 04, 2013, 03:13:36 PM »
All pilot jets were pulled out and inspected. All were good except the one and that was cleaned.

Flybox I appreciate the belaboring. One of the emulsion tubes had a small clog but was cleaned out with cleaner. I'll go back in with a wire this weekend, maybe something got back in there. Also will make sure float heights are in spec. I'm almost positive I do have the PD style ... To test the accel pump would you turn the throttle to make sure you are getting gas squirting? If so, I did this the first time around and #1 was weak, but then was fixed and squirting ok.

I realize something could have easily snuck back into the carbs somewhere, but appreciate all the help/advice.
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