Topic: Gas in Air BOX!!! - CB750 K5 -

[Colonel]

All,

I just bought a 1975 CB750 and I am trying to make it my daily rider..That's my first Classic bike ever and I have a lot to learn (had CB Seven-fifty 90, CBR 1000FN 93, GSXR 1100 95 in the past)
I have had some problems -with overheating, loss of power and erratic idling. I have been through technical manuals and forum to diagnose and started working on the issue.

As I removed the air box to check on the air filter I discovered that I had gas in the box (probably 1/8")!!! 

What could be leading to this???

Thanks, Stephen

[heffay]

you probably have a stuck float or two or three or four. 

[Colonel]

Heffay, thanks for the 'quick' reply/hint...
So I am looking at a carb job I guess... That's going to be a first for me   

I guess you need to go through this to deserve such a bike!
As for the posts; where should I post the questions? Apparently I posted in the wring forum...

Cheers, Stephen

[Scrubs]

not difficult. pop the float bowl off of each carb...No need to remove the whole carb, the bowls are held on by little clips
underneath each one. eye ball each float to see one isent drastically out off true with the rest. carefully re attatch the
float bowls so the bowl is not jamming them and see if that helps. If not - you will probably have to set the float(s) height to spec.

PS If you havent already done so, it wouldent hurt to give her a full service either.
valve clearance, cam chain adjust, point gap, timing, carb sync. very simple to do on these bikes, you may need
to invest in a couple of tools if you don't already have to make the job easier such as a carb balance tool but they
are all indispensible and good investments if you don't already own.

[OldSchool_IsCool]

I think you posted in the correct forum.  I suspect Hef saw your list of "other" bikes and didn't notice that you are talking about a CB750 K5.

Something else you can try that doesn't involve any disassembly is to close the fuel valve (petcock) on the gas tank, then one by one, drain each float bowl into a clear jar/container.  There is a set screw on the bottom of each bowl and hopefully a drain tube that leads back to the swingarm.  Put the open end of the tube in the jar and open the set screw/drain valve 2 or 3 turns. 

Is there any water or other crap in the jar?  Open the petcock and let fresh gas flush the bowl.  Tap the bowl with the butt of a screwdriver to knock any loose crap free.  Close the drain valve.  Repeat with next carb.  When finished, see if the air box still floods.  If yes, then on to dropping the float bowls and checking float height.

+1 on a tune up.

[super pasty white guy]

If you complete your profile, you may find that there's a member nearby who might help you...  should you want it.

My thought would be that if you've got carbs with sticking floats, it might be a good idea to pull the carbs and make sure that they're good and clean.

spwg

[heffay]

I think you posted in the correct forum.  I suspect Hef saw your list of "other" bikes and didn't notice that you are talking about a CB750 K5.

It was originally in the bike of the month forum... was moved very quickly following my reply.  (which I have since removed.)   

[Colonel]

All,

The bike ran great for some time... but this morning as I was ready to go to work, the bike did not maintain idle and suddenly pissed gas from the overflow line of carb #3...
Then slightly in the air box (could see gas leaking from the box). Shut it off, switch gas off... drained bowl #3 and removed cap to check... Thought the float or float valve was stuck... Nope.
Pushed the bike in the garage and drove to work...  

Back home, decided to dismantle carb, inspect and clean... (BTW, thanks to the posts of Honda Carburetion Manual and the illustrated carb work at http://www.salocal.com/sohc/tech/carb/carb.htm)...

This is now done... Main jet was loose (completely) on Carb#3 but floats (all) were perfectly free and adjusted... I am ready to reassemble tomorrow but would like to know how to simply adjust the throttle opening... Some recommend using a 1/8 drill bit to calibrate easily the opening... Any other hints?

[jeepster]

You can buy/borrow or make a vacuum carb sync tool, (e.g. morgan carbtune pro ~$75, or search this site for instructions on how to make one) then use the carb adjusting tool(morgan sells one for around $18), or a long socket (8mm I think...) and long flathead screw driver to dial in the setting. Beware that the previous owner may have sealed the locknuts with loctite or epoxy to keep the settings from changing. (mine did) If this is the case, don't fore the locknut, apply a little heat and it should break the bond. Keep in mind, that you will have to have your valve clearance, timing, pilot screws, and idle adjusted properly to sync the carbs.

Valve clearances are relatively easy, but make sure you buy a bike specific set of feeler gauges, (motion pro makes a set, they're bent and a bit narrower than standard gauges.) a Honda valve adjusting tool would also be helpful here...Maybe get a pilot screw adjuster as well, basically a long flathead screwdriver that is bent 90 degrees at the business end.

Of course there are many ways to accomplish the same tasks without these tools, but they will definitely make your life easier. Get them if you have the $$.

[OldSchool_IsCool]

I am ready to reassemble tomorrow but would like to know how to simply adjust the throttle opening... Some recommend using a 1/8 drill bit to calibrate easily the opening... Any other hints?

Noted carb expert and all around great guy, TwoTired, mentioned that he uses a flashlight method to bench sync carbs.  I haven't read his procedure, but I think it goes like this:

- pop the tops off each carb to access the slider adjusters
- throttle return spring and idle adjuster screw need to be mounted
- place a flashlight into the intake throat of carb#2 (or what ever carb is the reference carb, i.e. no slider adjustment screw)
- Turn the idle thumb screw until you see light in the output throat
- now adjust the idle screw until the light just barely disappears
- with each remaining carb, put the flashlight in the intake throat
- adjust the slider screw (not idle screw) until you see the light fade to nothing.

I hope I got this right.  If anyone knows different, please post the corrections!

[Colonel]

This is now done... Main jet was loose (completely) on Carb#3 but floats (all) were perfectly free and adjusted... I am ready to reassemble tomorrow but would like to know how to simply adjust the throttle opening... Some recommend using a 1/8 drill bit to calibrate easily the opening... Any other hints?

More to the story:
One bowl had some rust (powder) deposit at the bottom... From very informative posts: "...another cause of stuck jet and carburetor flooding..."
Might be the seafoam working its way and removing some rust from the inside of the tank (light surface rust is present).

Two options for me now:
1- re-assemble the carburetors (upon receiving the replacement jet), roam free and repeat the exercise next time rust particle is stuck in a jet 
2- clean the tank now; re-assemble and roam free... of trouble! 

What do you think???

Performed valve clearance check and everything looked OK from the top. However, from the inside of the beast I noticed deposit on the valves (looking through the intakes holes using a flash light); the bike has low miles but I guess it sat for a long time.
However, I think this will gradually go with the seafoam... Am I right? Does any of you can recommend something to clean the valves? (besides taking everything apart  )

Thanks

[jeepster]

Clean and coat the tank with por15. Unless you like taking carbs apart every few weeks?

[jaknight]

However, from the inside of the beast I noticed deposit on the valves (looking through the intakes holes using a flash light); the bike has low miles but I guess it sat for a long time.
However, I think this will gradually go with the seafoam... Am I right? Does any of you can recommend something to clean the valves? (besides taking everything apart  )

Some guys say Marvel Mystery Oil added to the gas works wonders...... others say it won't ruffle a bird's feathers.....

~ ~ ~ jaknight ~ ~ ~

[Colonel]

Clean and coat the tank with por15. Unless you like taking carbs apart every few weeks?

jeepster,

Did you use Port15? If positive, can you share your process steps?
I assume you used first their 'Metal Ready'; how  do you uniformly apply this inside a tank?
Then, how do you apply their Por15? It's a paint/sealer right? Just pour it inside and shake???

Thanks

[jaknight]

PM (email) TwoTired and talk to him about Por15.

I think you will be introducing a complete bucket of worms.  It can block passageways, disintegrate and cause a myriad of problems.

Like so many other products, you have people who support it and people who cannot stand it.  Just as with the Marvel Mystery Oil, which when added to gas, is supposed to be an upper combustion chamber cleaner.  Some say they have pulled apart the cylinder head after 70,000 miles and the valves looked brand new squeaky clean........ others just flat out say it doesn't do anything.

Research Por15 as much as you can....... putting it in is a big commitment.

~ ~ ~ jaknight ~ ~ ~

[OldSchool_IsCool]

I am ready to reassemble tomorrow but would like to know how to simply adjust the throttle opening... Some recommend using a 1/8 drill bit to calibrate easily the opening... Any other hints?

Noted carb expert and all around great guy, TwoTired, mentioned that he uses a flashlight method to bench sync carbs.  I haven't read his procedure, but I think it goes like this:

- pop the tops off each carb to access the slider adjusters
- throttle return spring and idle adjuster screw need to be mounted
- place a flashlight into the intake throat of carb#2 (or what ever carb is the reference carb, i.e. no slider adjustment screw)
- Turn the idle thumb screw until you see light in the output throat
- now adjust the idle screw until the light just barely disappears
- with each remaining carb, put the flashlight in the intake throat
- adjust the slider screw (not idle screw) until you see the light fade to nothing.

I hope I got this right.  If anyone knows different, please post the corrections!

I just tried TT's flashlight bench sync method for the first time last night.  My #4 slider needed lowered by quite a bit.  I haven't verified the carb balance with my vacuum meters yet, but she sure seems to be running GREAT right now!  Idles smooth at 1K, good response to throttle twist AND release.  No bucking at mid throttle and no popping on decel.  

The process was simple, quick and subject to manometer tests, I dare say accurate!

Bike is a '77 550K with stock air box, foam air filter, PD carbs and 4-4 exhaust.