Carbs!

[Oregon_Chris]

Hi everyone!
I'm riding a 1980 CB750C that's been chopped up.  
Im running w/o the airbox, and have the pods on.  I've got a set 4-into-2's for the pipes.  Dont know what kind, but they are kinda loud.  
My problem is that I'm running way too rich.  My plugs have to be changed every week or so.   And that's just from riding back and forth from home to work (4 miles round trip).  I took it to a "mechanic" who pretty much wasted two and a half months of my time and hundreds of dollars without a solution.   Every plug is fouled to the same degree, each time.  I'm thinking my jets may be too big.  
Any ideas?   Thanks in advance.  
I haven't a lot of experience with a wrench, but I'm not hopeless.  A little intimidated by trying to pull the rack myself and try to fix em.  
Chris.  

[LukeM]

Do the spark plugs look black and sooty, or black and shiny? Black and sooty usually means it's running rich; black and shiny means there's some oil getting in there.

Pods will allow more air through compared to the stock airbox/filter.  This means leaner operation.  Same with opened up exhaust vs. stock.  Less back pressure means more airflow = leaner operation.
It's possible that someone put in bigger jets to compensate for that.  In order to find out, you will need to remove the carb bank and check the jets.  Also, check the accelerator pump. Maybe it's leaking or pushing fuel all the time.

My Haynes manual says the primary main jet is a size 68, and the secondary main jet is as follows:
CB750F pre-1980       size 98
CB750F post-1980     size 100
CB750L and CB750K  size 102

Pilot screw factory setting varies between 1.5 turns out from all the way in, to 1.875 turns out. Consult the FSM to find your settings, and start from there.  I think the mixture gets richer as the screw is backed out.  So if you're running very rich, I'd turn one of the pilot screws in, counting the number of turns.  Hopefully the previous owner set them all the same way.  I'd start with 1.5 turns out, and adjust from there.

Good luck, and keep us in the loop.
Luke M

[Oregon_Chris]

Thanks, Luke!   I will check the Pilot.    I've never done carb work before.   I've pretty much heard that the key is to follow directions carefully and keep everything organized, labeled and accounted for.   Any advice for a rookie, or should I just try another shop?  
Thanks again!  

[Oregon_Chris]

Oh, and the plugs are a very flat black.  No sheen to them.  

[TOOLS1]

Lay out a large white sheet over, and around your work bench/area. That way, you will be able to find any small parts that go flying
Just take your time, and take pictures as you go.
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[LukeM]

I'm quoting Re-Run when I say the carbs can't be too clean. Clean, clean, and clean again. I can almost bet that you'll have the carb assembly on and off at least twice. If you can find a shop with an ultrasonic cleaner (one big enough for the whole assembly), that might be a good way to go.  Rice Paddy here in Columbus charges $65 to clean them this way.

Luke M

[sgtslag]

Another option is to set it back to factory:  put in a stock air box, and filter (K&N are nice, give plenty more air to the engine, no re-tuning required, but they're expensive -- around $50-$60), replace the jets to standard.  I've read that our bike's engines are too low of a pressure to need re-jetting for different exhaust systems.  Don't know for sure, but experimentation is king, unless you have an experienced mechanic whom you trust to give you sound advice (which could save you a lot of parts and labor fees).

I would also recommend you get NGK Iridium plugs, after you get the OEM airbox, and filter installed, along with OEM jets, if you choose to return it to stock.  The Iridium plugs are fantastic:  require less voltage to jump the spark gap; fire almost every single time without fail -- almost never misses, period; engine runs smoother, and it starts ridiculously fast; they claim it will increase mileage, and HP, by 3% (not sure if you could actually measure such a small improvement, but the other things are very noticeable...).  The Iridium plugs run $8-$9 each, so save your money until you get the carb issue resolved.  Cheers!

[Oregon_Chris]

Thanks!  

[Oregon_Chris]

That's a thought, too!   I will switch to the NGK's after I get my mixture dialed in.  
Thank you!  

[Oregon_Chris]

Well, I've been doing a bit of reading and research and have decided I'm gonna rebuild the carbs myself.   Does anyone have a favorite or "high quality" recommendations?   I've also heard that for my situation, starting out with jets sized 75&110 are a good way to go.  Any thoughts on that?  
Cheers!

[jordan h]

just curious, i have a 1976 and i'd like to put a 4 into 1 system on it (pretty much for looks and sound) do i have to change the jets? will i do damage if i dont?

[TOOLS1]

It depends on which exhaust system, and several other factors. The only way to tell is to try it. Sometimes they will work. Sometimes they will not. If the bike does not run right, you will need to re-jet. As, for damaging the bike. That also depends on several factors. If it is too lean it could make the chrome pipes turn blue, or even burn holes in the pistons. However this is a worst case scenario.
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