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Author Topic: 750F PD42A carb issues.  (Read 1725 times)

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Offline KilledByGreg

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750F PD42A carb issues.
« on: September 04, 2018, 04:19:11 PM »
Hi, this is my first time posting on here after a long time of lurking. I have a question about my "77 CB750f, and i just cant figure out the issue. I recently completed a top-end rebuild in July, and since I have fired the bike up, I've had a intermittent popping noise coming from my carbs, paired with puffs of white smoke (it looks to be all of them). So, I was wondering if any of you have ever experienced this issue before? The carbs have been disassembled and cleaned, which helped my bike not being able to run with the choke off. But, now after that issue, I'm having this... Any help would be awesome. Thanks.

Here's a quick video. Please note, my hands are completely off the bike when the idle changes.
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Offline flybox1

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Re: 750F PD42A carb issues.
« Reply #1 on: September 04, 2018, 08:31:32 PM »
unless your bike is specifically jetted for velocity stacks or pods, NEVER run them without the airbox.
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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 KilledByGreg

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Re: 750F PD42A carb issues.
« Reply #2 on: September 05, 2018, 04:13:11 AM »
Quote from: flybox1 on September 04, 2018, 08:31:32 PM
unless your bike is specifically jetted for velocity stacks or pods, NEVER run them without the airbox.


I understand. I removed them for videos purposes.
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Offline Skoti

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Re: 750F PD42A carb issues.
« Reply #3 on: September 05, 2018, 05:41:28 AM »
Are you sure it's a carb issue?

Maybe try running the motor in the dark and look for electrical short circuit sparks from around the ignition coil connections, spark plug leads and contact breaker point wires or electronic ignition.
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Motorcycling is life, anything B4 or after is just waiting.

Honda CB750F1 1976
BMW R1150RT 2002
Norton Commando 850 Roadster 1974
Jawa 350 Blue Style 1992

Offline KilledByGreg

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Re: 750F PD42A carb issues.
« Reply #4 on: September 05, 2018, 05:56:55 AM »
Quote from: Skoti on September 05, 2018, 05:41:28 AM
Are you sure it's a carb issue?

Maybe try running the motor in the dark and look for electrical short circuit sparks from around the ignition coil connections, spark plug leads and contact breaker point wires or electronic ignition.

Good idea. I will do that tonight. Does this issue seem symtomatic of the potential issues you stated?
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Offline Skoti

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Re: 750F PD42A carb issues.
« Reply #5 on: September 05, 2018, 06:11:47 AM »
Not especially, but normally the two areas to check first are fuel supply and ignition system, and since you've already attended to the carbs maybe it's time to look at the ignition?
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Motorcycling is life, anything B4 or after is just waiting.

Honda CB750F1 1976
BMW R1150RT 2002
Norton Commando 850 Roadster 1974
Jawa 350 Blue Style 1992

Offline flybox1

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Re: 750F PD42A carb issues.
« Reply #6 on: September 05, 2018, 07:38:30 AM »
Revisit your valve adjustment as well.  If a valve is not closed or sealing, you can get that result....
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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 KilledByGreg

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Re: 750F PD42A carb issues.
« Reply #7 on: September 05, 2018, 12:42:28 PM »
Quote from: flybox1 on September 05, 2018, 07:38:30 AM
Revisit your valve adjustment as well.  If a valve is not closed or sealing, you can get that result....

Thanks, hopefully I will get time to check my valves tonight after I run the bike in the dark. I’m just hoping I didn’t mess anything up during he rebuild.
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Offline KilledByGreg

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Re: 750F PD42A carb issues.
« Reply #8 on: September 09, 2018, 07:42:56 AM »
Adjusted all gaps so a .001" oversize feeler gauge could not pass through the Clymer manual specs. (.003" gap can not fit .004" etc.)- No luck. Ran the bike in a pitch black garage, and saw no electrical issues. Still stumped  >:(
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Offline TwoTired

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Re: 750F PD42A carb issues.
« Reply #9 on: September 09, 2018, 09:37:22 AM »
Does it still have the stock cam?
Has it been degreed properly?

If the intake valves are closing late, the following compression stroke will push air into the intake runners and out the carbs, resulting in white puffs of atomized fuel/ air at the carb entrance.

The other possibility is bent valves that can't seal.
A compression or leak down test on each cylinder should yeild insight about that possibility.

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 KilledByGreg

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Re: 750F PD42A carb issues.
« Reply #10 on: September 11, 2018, 05:15:44 PM »
stock cam, degreed properly per Clymer manual. I will check the engine with both of those tests. thank you!

Quote from: TwoTired on September 09, 2018, 09:37:22 AM
Does it still have the stock cam?
Has it been degreed properly?

If the intake valves are closing late, the following compression stroke will push air into the intake runners and out the carbs, resulting in white puffs of atomized fuel/ air at the carb entrance.

The other possibility is bent valves that can't seal.
A compression or leak down test on each cylinder should yeild insight about that possibility.

Cheers,
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Offline KilledByGreg

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Re: 750F PD42A carb issues.
« Reply #11 on: October 12, 2018, 04:29:41 PM »
Quote from: TwoTired on September 09, 2018, 09:37:22 AM
Does it still have the stock cam?
Has it been degreed properly?

If the intake valves are closing late, the following compression stroke will push air into the intake runners and out the carbs, resulting in white puffs of atomized fuel/ air at the carb entrance.

The other possibility is bent valves that can't seal.
A compression or leak down test on each cylinder should yeild insight about that possibility.

Cheers,
Sorry for the late reply, I was out of town on work. I ran a leak down test, and cylinder 3 intake dropped 25%. I could feel air passing the valve when I stuck my finger in there. Looks like the previous owner changed the pistons at some point, too. I had to reorder the proper pistons via CMSNL. Looks like I'm digging back into this bike over the winter. Thanks a lot for all of your help.
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