Topic: CB750A Carb Hic-cup

[Fran Fitzpatrick]

Friends,

     Its me again.  Thanks for all the help.  Today's issue is a Hic-cup from the #4 carburetor.  My bike died on the side of the road a couple of weeks ago (blown fuse).  I replaced all fuses but now the bike runs a little rough.  The #4 and #3 carbs both hic-cup a little, mostly the #4 and the exhaust on the #3 / #4 side is a little weaker then the right side exhaust.  I'm thinking its could be a timing issue... but as always that is just a shot in the dark.

Fran

[raymond10078]

1 and 4 share a point set and a coil, as do 2 and 3.  So . . . the symptoms don't point to ignition.

Can you better describe what a "hiccup" is?

Do you have a filter on the fuel line?

[77Pinto]

Sometimes carb. issues act like ignition, sometimes its the other way around.  Start with the basics.  Check the timing, check the valve adjustments, pull the carb.s and check for debris, do a comp. test on all four, check your plug wires/coils, check for vacuum leaks.



Bill

[raymond10078]

Yes, I was also thinking carbs, which is why I asked about a fuel filter.

Follow 77Pinto's suggestions - but I would only include the carbs if you DO NOT have a fuel filter.

P.S.  If you don't have a fuel filter - buy one!

[Fran Fitzpatrick]

Guy's

     Thanks for the advice.  I have a fuel filter installed. By the carburetor hip-cupping I mean a small back fire (no flame, just a puff of air) toward the air filter    I had the valves and timing adjusted several weeks ago by a shop that specializes in older Honda's. While checking the spark on the #3 plug the engine started easier and idled better with the #3 plug pulled out of it's hole. 

     I have a rubber fuel line with the filter installed.  The line may have a small kink or sharp been in it.  I'll see what happens with the line rerouted.

    I don't have the tools for a compression test, but I can trace the vacuum lines.   

Fran

[raymond10078]

Then . . . it does sound like #3 is running lean.  If you have a filter, then the next check is for an air leak.

Make sure that carb boot clamps are tight - both at the head and the carb.  But - your boots are probably rock solid, and getting them to seal correctly may take new (slightly smaller) clamps.  With the engine idling, spray WD-40 around the boot to carb and boot to head connections.  If the idle speed increases, you have a air leak.

[Fran Fitzpatrick]

Sir,

   Yes the carburetor manifolds are old and hard.  Replacements just don't exist.  I was able to buy a used set that were better then my own set and have since installed them.  I rerouted the fuel line to remove the kink and sharp bend, reinstalled the air box and took to bike for a spin.  I think maybe the pressure of the air-box pushing against the carburetors pushing against the manifolds cured the problem because the bike is now running about as good as ever.  But as it is with this old bike a new issue has arisen.  The rear break pedal now will not disengage completely after the break is applied.   I guess tomorrow I'll trace out that system.  As always thanks for all your help. This is about my sixth post in the last year and each time you and others have offered great advise.  If case you were wondering my bike is posted in the galley section (storm trooper).

Fran 

[raymond10078]

Yes, the carbs will run lean without the air box installed.

Glad your bike is back to running good.  I bet though, that the #3 carbs idle mixture screw could use a 1/4 turn richer - since it reacted to the lost airbox before the other carbs . . . .