Hey all,

i'm working on my 1981 cb750 Custom and have an issue with the carbs (i'm reaally new at hondas and engine stuff in general).  it idles great and starts great but when you whack the throttle it bogs out.  The plugs are black and wet (way rich).  I'm trying to figure out what model this Keihin Carb is but can't find crap off google (been searching for an hour).  I even searched the numbers off the side of the carb.  I'm trying to get as much info on it as possible so I can find where to get the right jets for this thing.

While searching I found this website http://www.dansmc.com/  it's a pretty nice read for beginners like me trying to learn about engine work/repair though some of it may be dated by now.

Hell it got me thinking it could be a combination of issues this bike might have but I'm gonna start with replacing all the gaskets in the carbs since I found a leak on #2 from the fuel line's "T" and am going to try and lean it out a bit while doing so.

I ordered the appropriate manual for this bike from Helm Inc. but until that arrives if anyone knows resources or has FHE with these let me know.  I need someone's brain to pick.

Enjoy your weekends and ride safe 

i don't know.

but i bet these guys do.

thanks man, i just found 2 DOHC Honda forums by searching for that instead of the specific parts/part#'s i've been trying.

With a bike of that age that's likely been sitting a while, there's practically no doubt you'll need to get inside and rebuild the carbs.  Luckily, that's a very easy thing to do and you'll probably find the carb kits still available from your local Honda dealer (though you'll probably have to order them) and are usually pretty cheap.  In short, your pilot jets are certainly clogged, as well as some of the passages in the carb bodies.  The kits should come with new jets, gaskets and o-rings.  Simply pull each carb apart, clean all the passages thoroughly, replace the jets, gaskets, o-rings and any other parts that come in the kits.  Check float height per the spec that'll be in your manual, reassemble, and all should be well.  You will likely need to have the carbs synched, which can be done at home if you have the right tool.  If not, it shouldn't cost an arm and a leg to have the dealership do a simple carb synch.  I know there's four carbs there, but just go slow, keep everything organized, and you'll find that they only go together one way.  You really can't make a mistake.  Just go easy on the soft phillips head screws on the float bowls.  After all this time they're likely to be semi-frozen in place, and they're easily bludgeoned.  It is often easier to give 'em a whack with an impact before you round the heads with a hand screwdriver.  Keep surfing those CB 750 forums, but in the end a really good parts guy at your dealership is your best friend.  Remember to bring him/her your VIN number to help identify the correct parts when you order them.  Sage words from a parts guy since 1979.

Carbs for these bikes are a subject of endless debate on the various DOHC Honda forums. Do you have the correct stock airbox/filter arrangement, or are you using aftermarket pod filters? Apparently pods cause all kinds of problems, something to do with airbox vacuum being necessary to make the slides lift (they are a CV design). Also, is your choke fully open, and does your accelerator pump work? Are you using the factory vacuum controlled petcock, and, if so, is it actually flowing enough gas to keep the float bowls filled as you ride? Any of these faults can cause the sorts of problems you are describing.

I've got one of these bikes, and after rebuilding the carbs a number of times I came to the conclusion that what most most of my fuel related problems were not caused by gunk inside the carbs, but rather by mechanical things like poorly adjusted accelerator pump likkages or airbox leaks.  Your results may vary, however. Good luck!

PS - The best DOHC forum that I know of is at http://www.cb1100f.net. However, it seems to be down just now when I checked it.

you guys rule, another reason to love the DMF 

In order to help the diagnosis I did a big write up of the bike's info which might help getting it running right.

Sorry for the long post but I've dug up a bit of info on this particular bike so far:

Hey all,

Pardon the long post.
 
I'm extremely new to CB750's and engine work.  I'll break this down and give as much info as possible in order for you to get an idea what I need to do.  I've asked the guy i bought it from a ton of questions in order to help me understand what this bike needs/doesn't need.

1981 CB750 Custom 24k miles (how it currently looks):




How it runs:

Starts instantly and idles great.
Shifts smoothly.
Accelerates well up to 7k RPM if done lightly then bogs a little after especially while gettin on it.
Bogs and stutters really bad when i whack the throttle open hard mostly above 6500/7k.
Since it's my first Inline 4 I didn't know what to expect but a friend that rode it said it doesn't pull anywhere near what his fathers cb750 did.


What I've checked/done/noticed:

I've only rode it to work and back (3 miles one way) and it's been great but after a 30 mile trip on it I noticed what appears to be an oil leak that I saw on the 4th and 5th fins of the head on both sides.  A friend behind me said it was smoking a decent amount mostly on deceleration.

Upon pulling the 4th spark plug (far right plug) I see that it's black and wet.

I also noticed that the T joint from the fuel line into the 2nd carburetor is leaking and have already gotten a carb gasket set from Honda.

The gas tank is currently at a radiator repair shop to fix the small leak on the right rear side of it as well.

While running the last bit of fuel out of the carbs I found all four headers to be loose including one nut that nearly worked itself completely off the bolt.


What owner said he did to it:

Pulled the engine painted it and painted the frame, converted it to a cafe racer style (as pictured), installed the CF 2Brothers exhaust 4-1 and put the pod filters on it.  He said he cleaned the carbs and jetted it using Dynojet needles 132.5 (he thinks) main jet, with the 3rd position on the Needle jets.  He didn't do a vacuum test to sync the carbs but did it by eye.  He also said he did the valve timing.  As far as the charging system he said he replaced it with a permanent magnet system to alleviate the stator charging issues altogether.  The guy said he is a honda tech so i trusted that he knew what he was doing.

What previous owner to that guy told me:

He found it in east texas and it was covered with dirt like someone used it for motocross.  His intentions were to restore it to orginal and had it running sweet till he tried a new exhaust in which he decided to adjust the jetting along with the new pipes.  He gave up on it and sold it to the guy i bought it from.

The guy may have been a Honda tech, but the pod filters say otherwise.  If you look at any modern high-performance (or otherwise) motorcycle you'll see the factories making every effort to provide larger and larger still airboxes.  Your original airbox design on that CB 750 would be far better than those pod filters, which were a cafe fad leftover from the '70s that never really did work.  That would be the first thing I'd do is install the stock airbox and filter.  From there, tighten up your exhaust and go through the jetting to make sure what it has inside it instead of relying on the previous guy's feeble memory.  It sounds a bit as though the bike is running lean, despite your black spark plugs.  The dark wet plugs are more likely related to the oil leak you can see at what is probably a leaking head gasket.  Not a very uncommon malady on the inline 4 Honda 750s.  They'll run forever, but leak like a sieve while doing it!  Even if you get the jetting sorted out, you may find that as the leak progresses and the engine continues to ingest more and more oil, it'll start oil-fouling the spark plugs.  This will also hinder your ability to properly jet it.  And as the previous post indicated, the CV carbs don't react well to the pod filters and need the stability of a calm large airbox to react properly to throttle inputs.  You've got your work cut out for you.

Could he retrofit the older non CV carbs to it?