1980 CB750 Rev issue

[boostaholic]

So my 80' cb750 has been running great lately! BUT not today. So a few days ago I packed my Carpy 4-1 exhaust and reinstalled it. Ran fine and felt good. I didn't however rev it out and take it into higher RPMS. Today however, I did take it up in the higher revvs and noticed the engine starting to sputter and lose power. At idle the bike wont revv past what I think is 6k (my tach is sticky). None of these issues were present before packing the exhaust. Obviously packing the exhaust created more back pressure, which would most likely cause it to run rich... Right? Pulled plugs they look the same as before, maybe a little lean? Ill post a picture below. I checked the voltage while revving and it jumped up to 16 volts at higher RPMs. Other than packing the exhaust it just sat for a few days.

With all of that being said, I have noticed the valve train sounds kind of loud. I'm not sure what its supposed to sound like but it just sounds loud to me. Also, the timing chain sounds kind of loud.

Any help on both issues would be great!

[boostaholic]

Also I should mention I rejetted with 125 and 72s. I currently have mixture screw maxed out. ~4 turns.

[shinyribs]

Could be a fueling issue. You can start with the basics of ensuring good fuel flow ( clean petcock, clean filter,etc).

Whatcha got for an air filter? Pods are notorious for not wanting to rev out.

Also, these CB's ( SOHC and DOHC) rely heavily on a strong battery for good ignition. A weak battery, rregardless if the bike is charging fully or not, can cause one not to rev out.

The black, sooty areas of the plug are most likely from idling, putting around. Impossible to get away from as we need to idle from time to time. It' possible your idle mixture is a little rich, but I wouldn't seat that yet.  But that ashy electrode strap is textbook lean. I mean, like, without a doubt...lean. I would bet a dollar to your doughnut you're running out of fuel at higher revs. Whether that is due to jetting, intake/exhaust changes, or lack of fuel delivery will have to be figured out with good old fashioned headscratching, testing and tuning. Good luck!

[boostaholic]

I will have to check the petcock and fuel filter as I haven't since I've had the bike. Air filter is a stock paper? element.

I haven't load tested the battery but it seems really strong. Never had a problem with it starting up. The lights do dim when idling though..

The bike has stock airbox and a carpy 4-1 with stock baffle in it. I have upped the size of the jets already, seems strange that a free flowing exhaust would need that much more fuel over stock.

[Truck]

there is a filter on the top of the petcock that sits inside the tank. You may have an aftermarket filter inline between the tank and carbs. There may be a small square box mounted to your carbs with fuel lines running to and from it. This is a vacuum on/off, I've heard this can cause fuel problems.

[shinyribs]

Truck wrote

There may be a small square box mounted to your carbs with fuel lines running to and from it. This is a vacuum on/off, I've heard this can cause fuel problems.

Great point ^^

Vacuum operated petcock can be a dog. Search for SOS (Spawn of Satan) and you may find more on it.

Running the stock airbox is a GREAT thing. Keep it and your life will be much simpler  

The FSM should a breakdown on diagnosing and repairing your vac petcock if that is the issue. Or you can go to a manual petcock. Whichever suits your budget or tastes better.

[boostaholic]

Okay, so I replaced the fuel filter and checked for a screen in the petcock (wasn't there). Fiddled around with the mixture screws and its running DECENT.

First off, I have the idle mixture screws all the way out. I mean literally about to fall out. With the mixture screws at normal adjustments (like 1.5-3 turns), the bike sputters and doesn't run on all 4. Blip the throttle and it bogs and pops.

I have the factory airbox on the thing with basically a straight pipe. I have 120 mains and 72 slow jets. Also, occasionally it'll sputter up in the top end like its running out of fuel, not all the time though.

I have cleaned the carbs thoroughly, accelerator pump was filthy and plugged. Once the bike is sufficiently warm and it has its 17 turns of the mixture screw (more like 5.5 but whatever), it runs like a top. When its cold, pulling the choke in and bliping the throttle results in fast responsive revving, but cold revs without choke are non existent.  

I guess my questions is, should I go bigger on the jets? Or should I just let the bike be finicky and old and just ride it and stop worrying about it.

[TOOLS1]

If I were you. I would fix the vacuum leak and sync the carbs. That will fix your problem.
TOOLS

[boostaholic]

So you can say without a doubt in your mind that its a vacuum leak? I'm not disagreeing you, your certainty just astounds me.

[TOOLS1]

Everything you said in that last post points directly to a vacuum leak. The running better when cold with the choke on, and trying to compensate for a lean condition by adding more fuel, and the popping while accelerating all point to a vacuum leak. And the engine noise at idle is caused by the carbs not being in sync, which can also be caused by a vacuum leak.
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[boostaholic]

Alright! I'll check for leaks then. Thanks!

[shinyribs]

Refusing to idle with the idle mixture screws where they belong is a text book sign of vac leak and/or plugged pilot jets.

[boostaholic]

After much spraying and diagnosing, I have found a significant vacuum leak at the throttle shaft. I assume its those pesky old fabric style o rings.

So then I guess the question is, where do I find those things? And, should I put in my 110 and 68 jets while i'm at it?