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Good day 750 fans. This is brand new for me, the blogging thing. I have a 1972 CB 750 that has not run for over 30 years. The top end came off for for cleaning and gasket replacement; it was all in pretty good shape. The carbs have been rebuilt with kits and a very good manual by Mike Nixon. The points ignition was ditched and a Pamco electronic ignition installed with new coils, wires caps and ends. This setup is real sweet. My bike starts up almost instantly. Replaced the clutch friction plates, the entire front brake assembly, the battery and tires just to mention the big items. The tank and side covers have new paint with a 3 color scheme. Now for the issue.
At idle, 1000 rpm the exhaust pops and still pops at 3000 rpm and a easy cruise. Bike seems to have good acceleration, no popping and does not pop on deacceleration. The exhaust system is a 2 into 1, each side made by Hooker Headers. The aircleaner is aftermarket box over all 4 carbs and a 1/2 inch thick foam filter strip by K & N. That's it for now. Any help is greatly appreciated.
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Administrator
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Welcome to the forum.  Popping out of the tailpipe at idle could be a sign of retarded ignition timing. The ignition is firing with the exhaust valve open,basically. Either that or you could exhaust valves that are not sealing properly. Either worn or rusty seats,valves(common if it's been sitting) or the valves could be set too tight causing them not to seal.
Your new carb kits,did you verify that it came with the same size main and pilot jets as the originals? This could cause problems also.
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Interesting, thanks for the reply. I timed the ignition with a light and the valves were freshly adjusted at .002 intakes and .003 for the exhaust. The popping is also at easy cruise say 3000rpm. The valves were lapped into the head and the lap pattern looked good. Could it be valve timing. i spun the engine over 3 times when I checked and rechecked the lineup marks. .Engine has 30,000 and no visible evidence of cylinder wear, like the ring land lip. The carb jets I am not sure about.
A rider friend said it is an air leak at the exhaust head flange fitting...............does this make sense?..................DJ
-----Original Message-----
From: "shinyribs [via Honda CB750'S]"
Sent: Jul 24, 2013 11:55 PM
To: rattle can dave
Subject: Re: Exhaust popping
Welcome to the forum.
Popping out of the tailpipe at idle could be a sign of retarded ignition timing. The ignition is firing with the exhaust valve open,basically. Either that or you could exhaust valves that are not sealing properly. Either worn or rusty seats,valves(common if it's been sitting) or the valves could be set too tight causing them not to seal.
Your new carb kits,did you verify that it came with the same size main and pilot jets as the originals? This could cause problems also.
I included a shot of the bike for the heck of it. Rattle can laquer from Auto Zone..........is this paint any good?................Thanks again...............DJ
Montvale,Virginia
Experience is something you get just after you need it.
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Administrator
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If the motor was out of time it wouldn't rev good at all. If your bike revs good then I wouldn't think that was it.
It's possible that it's an exhaust leak,but I'd still take a look at the jets. Especially the pilots.
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Good advice, thanks for all your help.
What do you think of the POD type air filters for these carbs?..........................Blessings...........DJ
-----Original Message-----
From: "shinyribs [via Honda CB750'S]"
Sent: Aug 7, 2013 12:41 AM
To: rattle can dave
Subject: Re: Exhaust popping
If the motor was out of time it wouldn't rev good at all. If your bike revs good then I wouldn't think that was it.
It's possible that it's an exhaust leak,but I'd still take a look at the jets. Especially the pilots.
Montvale,Virginia
Experience is something you get just after you need it.
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Administrator
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Dave-
Go to the Search window on the web page, plug in "pod filter" and read on the many opinions of pods for these engines. The SOHC models usually run well with pods, the DOHC models (with the constant vacuum carbs) not well at all. It _can_ be done on the DOHC models, but be prepared for lots of jet swaps and tuning to make it go.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
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To fix this problem start with a compression check first.
Make sure the throttle is wide open during the test, and all of the spark plugs are out.
Then you need to tell us WHEN it is popping. Confusing so far.
On deceleration, or while the bike is stationary and you rev it up?
Does it pop while trying to accelerate?
If the bike has not been re jetted with that intake and exhaust,
it will always keep popping.
It is running too lean.
On a Roadstar Adventure.
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Thanks for your help with this one....................very good info................Ride safe & Blessings on your day.................DJ
-----Original Message-----
From: "LukeM [via Honda CB750'S]"
Sent: Aug 9, 2013 9:49 AM
To: rattle can dave
Subject: Re: Exhaust popping
Dave-
Go to the Search window on the web page, plug in "pod filter" and read on the many opinions of pods for these engines. The SOHC models usually run well with pods, the DOHC models (with the constant vacuum carbs) not well at all. It _can_ be done on the DOHC models, but be prepared for lots of jet swaps and tuning to make it go.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
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Thanks for your input, the timing was set with a light so I am pretty sure it is ok,. Head was cleaned and decarbonized, valves lapped and new oil seals installed. The valve adjustment is a good idea and my next easy place to check....................thanks again.........ride safe................DJ
-----Original Message-----
From: "shinyribs [via Honda CB750'S]"
Sent: Jul 24, 2013 11:55 PM
To: rattle can dave
Subject: Re: Exhaust popping
Welcome to the forum.
Popping out of the tailpipe at idle could be a sign of retarded ignition timing. The ignition is firing with the exhaust valve open,basically. Either that or you could exhaust valves that are not sealing properly. Either worn or rusty seats,valves(common if it's been sitting) or the valves could be set too tight causing them not to seal.
Your new carb kits,did you verify that it came with the same size main and pilot jets as the originals? This could cause problems also.
Montvale,Virginia
Experience is something you get just after you need it.
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Statements like this always are a red flag to a mechanic.
Quote:
"Thanks for your input, the timing was set with a light so I am pretty sure it is ok""pretty sure" just means you do not if it is or not.
The static timing MUST BE CORRECT.
Timing light check
just tells you if the advance unit is working correctly.
On a Roadstar Adventure.
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I got down on my knees and stuck my face in the timing hole. It was as close as the thickness of the timing mark on the casting. My pretty sure is that close..........Blessings on you.......DJ
BTW...................I think I will recheck it. I have a light with a inductivfe pickup and it is attached to wire #4.........DJ
-----Original Message-----
From: "Lucky 1 [via Honda CB750'S]"
Sent: Aug 12, 2013 1:16 PM
To: rattle can dave
Subject: Re: Exhaust popping
Statements like this always are a red flag to a mechanic.
Quote:
"Thanks for your input, the timing was set with a light so I am pretty sure it is ok"
"pretty sure" just means you do not if it is or not.
The static timing MUST BE CORRECT.
On a new adventure now.
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CB 750 NO PACK L FRONT.jpg (207K) 
