Topic: Noob Needs Answers about his 76' CB750

[Travis..]

Quick Intro: My name is Travis, I live in Greensboro NC and recently completed my Safe Rider Course, and purchased a 76' CB750. I really don't know anything about motorcycle mechanics and am dying to learn more; cannot stress that enough. I have several questions and genuinely am looking for help and advice.

First off about my bike: It is very close to stock, the only change has been the Air box has been replaced by Pods and someone bobbed the Front Fender. It starts with no issues whatsoever, and from my limited experience it seems to run well. My biggest question is that the bike seems to want to stall and turn off after it's warmed up when I pull up to a stop. Sometimes it turns off before I even get my foot down. I'm not sure if it's user error or the bike. The only way it seems to stay alive is if I barely roll on the throttle to keep it awake.

The other key information I want to give before requesting advice is that the previous owner said something was wrong with the Petcock. If I understood correctly he said he always leaves it on Reserve or else he noticed a (Leak?); at least i'm positive he said he always had to start the bike on reserve because if it was switched to On the Carbs would flood. So basically I'm not sure the effects of running the bike with it always switched on reserve.

I appreciate all help and replies; also please dumb down your replies for a complete beginner. To give you an example: The only mechanical work I've ever performed on this motorcycle is a battery change and an oil change; past that it might as well be a spaceship to me. 

[lucky]

With pods it will be running very lean and will want to quit when you stop and it gets hot.

Since you are a beginner and it is such a big bike I would encourage you to buy a stock air filter and stock air box. That way you can focus on riding the bike.
A bike that quits running in traffic can get you killed.

You did not tell the forum if the exhaust is stock or aftermarket.

[Travis..]

Lucky, thank you for the reply. First to answer the exhaust question it is stock, I'm not home to post a picture but it has the two stacked mufflers on each side. In regards to the Air box and Filter: what site would you recommend I find the parts from and is the install a serious job or something with enough research and planning I could pull off consulting my manual?

[Dream750]

My biggest question is that the bike seems to want to stall and turn off after it's warmed up when I pull up to a stop. Sometimes it turns off before I even get my foot down. I'm not sure if it's user error or the bike. The only way it seems to stay alive is if I barely roll on the throttle to keep it awake. 

Welcome.
 
This could be a simple idle speed adjustment. If your under-the-seat owners manual is missing, I’d suggest picking up a copy. It contains tons of useful info that will help you correctly understand, operate and maintain your K6. The owners manual and tool kit were originally supplied by Honda when the bike was sold new. Attached pix gives a sample owners manual overview and reprints can be found on eBay. 
 
Anyhow, to adjust your idle speed look for the single spring loaded idle adjustment screw as in this example:

 

[flybox1]

welcome to the forum.
download, or buy a copy of the shop manual  or clymer book for your bike.
start by reading the maintenance sections.  ask questions here, and tackle the items as you become familiar with your bike.
the forum is here to guide you through the difficult areas.

probably difficult to fine an NOS airbox, so look @ ebay. 

carb flooding... you'll have some carb cleaning/rebuild to tackle so this functions properly (no leaks) with the intake (airbox) changes you make.
take the words of the PO (previous owner) with a grain of salt.   

[mono]

Lucky, I'm going to partially disagree with you here --

Travis - i think that while it is easier for us to give tuning advice if you're running a stock setup, you don't need an airbox.  If your bike runs strong when you 1st start it, but then wants to bog down after warming up, you can probably make a few minor adjustments to the carbs to get it running consistently no matter the operating temperature.

however, you will want to educate yourself as to the basics of carbs and how they work before you go doing anything drastic.

My CB550 has different carbs than yours, but was doing the same thing (because as lucky said, it was running lean), and I was able to simply adjust the air screws on the carb bodies to get it running steadily even when warmed up. 

[Old Scrambler]

You can post your needs on this site............look in the parts wanted section. If no luck there, ebay usually has a few..........but be ware of quality.

Second..........you can install it yourself but be sure to get a complete box with the intake rubbers that fit the carbs. Then get a new air-filter. I prefer the K&N. Its pricey but lasts forever. Any CB750 box will do...........but the ones marked with an F on the side or the later K7 and K8 boxes work best.

[Tugboat]

Is your bike a K model or an F?

+1 on going back to the stock air box. Save yourself a LOT of time and headache and just do it. You'll need the carb-->air box rubber boots as well, which are still available from Honda (#12 in this diagram).
http://forums.sohc4.net/index.php?topic=130440.msg0#new

You also need to download the manual for your bike (available here) and read read read. Winter's coming so you'll likely have some down-time to work on getting your bike in good shape before Spring. Do a basic tune-up (timing, valves) and put the stock box back on and you'll most likely be good to go.

As for your petcock, it either needs a good cleaning or, more likely, needs a rebuild or replacement. Again, parts are still available from Honda. It may be that there is rust in your tank which is causing the issues, and if that's the case you'll need to address that too.

Post lots of pics, ask lots of questions, you'll figure it all out. Welcome to the fun!!

[Travis..]

Thank you everyone for the welcome. I have purchased my Clymer Manual and have been reading and planning to work through the "maintenance" section to become more familiar with the operations of the bike.

A big concern for me at the present is whether running the bike with the Petcock set to Reserve is harming the engine? (As in from start, riding, and park the valve is set to Reserve and this is only because I wasn't clear on what the PO said about it, something to do with leave it on Reserve, and felt nervous to change it) What effects does leaving the bike parked with the Petcock set on Reserve have on various parts of the engine? I just want to make sure on my short rides around the neighborhood that I'm not causing damage that could accumulate over time.

[mono]

A big concern for me at the present is whether running the bike with the Petcock set to Reserve is harming the engine? (As in from start, riding, and park the valve is set to Reserve and this is only because I wasn't clear on what the PO said about it, something to do with leave it on Reserve, and felt nervous to change it) What effects does leaving the bike parked with the Petcock set on Reserve have on various parts of the engine? I just want to make sure on my short rides around the neighborhood that I'm not causing damage that could accumulate over time.

no problem with that.  the only thing that's different is that the R setting opens a lower port in the petcock.  However, you'll want to fix it ASAP because if the petcock totally fails you'll overflow the carbs all the time and gas may get in your oil and all over the floor.  :/

[flybox1]

^+1
'reserve' only changes where the fuel flows from within the tank. 

gas is gas, and when it gets to your carbs, they care not where it came from. 
if your carbs are prone to leaking they need to be serviced.

[martin99]

Hi Travis, and welcome.

Just a thought. The fact that it seems to run well other than tickover suggests to me that it might have been properly tuned for the pods. As has been said - sometimes you need to take the PO's information with a pinch of salt. You need to get to know your own bike. In my experience just changing airbox for pods without tuning = crap running.

Soooo... it follows that if you just revert to stock airbox without making the necessary changes to the carbs you could be changing from a lean condition to a rich one? Maybe you're not even lean at all, you're bogging down, not nipping up.

My advice would be to check what, if anything has been done to the carbs before spending any money - jet sizes, needle position etc. Then come back and tell us what you have.

I know a lot of guys on here don't like pods, but if all you need is a few tweaks and you're good to go that's got to be a good thing right?

[mono]

^ my point exactly.

Sent from my SPH-L710 using Tapatalk

[Travis..]

Here is a video of the bike during a cold idle: Idle Sound Issue

 I don't know exactly how it should sound put it doesn't seem right in my mind. It sounds weak to me instead of what I would assume a "purr" should sound like. Also hopefully you can hear it doing a slight skip in short intervals? I'm not familiar with the jargon but it seems to "skip".

When your experienced ears hear this what is your first thought? Also no matter what this boils down to I don't even have a garage at the moment; so the idea of tackling something like adjusting the carbs or something myself seems out of my wheelhouse. Would any of you recommend me having it professionally tuned up so for the next year I can focus more on riding while I continue my mechanical reading and learning?

Or does it sound normal? Thanks.

[Stev-o]

Travis, welcome. 

First things first. You need to learn the positions of your petcock and what they do.

I suggest dis confection the fuel line from the carb and do a test.  No fuel should come out when in the off position, this is very important. Fuel should flow out when in the on position (given there is adequate supply) and in reserve. Please confirm this.

[goldarrow]

your bike sounds just fine especially cold start and it starts right up, may be just a tad rich if you didn't have to use choke...

these bikes "purrs" when you ride them above 4000 rpms, and only get more fun from there on....

[vrm]

I'm hesitant to comment since I'm certainly no expert but as has been said before the stalling at idle might be fixed by simply adjusting the idle speed. Does it idle better when fully warmed up? The biggest problem that I can see with running with the petcock set to reserve is that you might run out of gas.