Topic: Carb Clean Rough Low Idle

[Maine Ride]

So I have been systematically working my way around the 'matic. Starting with an oil change & filter ect...I recently started to notice some bogging down when taking off from a stop. My first effort was attempting to use Seafoam to see if it would clear up. This resulted in good performance for a couple of days then a rapid decrease in performance from a stop. After extensive reading on pulling the carbs I was confident enough to try my hand at it yesterday. I removed the air box assembly then finally the carbs. The bowls were opened up and cleaned as well as the brass venturi tubes. I confirmed that the accelerator is holding pressure and releasing cleanly. Both the bowls and the venturi's were heavily coated in a rust colored grime. After a good soak in Simply Green, rinse and dry, I bench synced and returned them to the bike. The reinstall was uneventful. Upon start up the bike sounds great with the choke on when it is warming up. When the choke is taken off and the RPMs drop to idle it intermittently runs rough. There is a noticeable sound of something not being right. I observed the carbs without the air box on and noted that when the bike starts to run rough there is a "sneeze" so to speak from the #2 carb. When the carb "sneezes" there is a cough sound followed by a brief mist released from the choke end of the carb then the RPM stabilizes and for a few moments everything is alright. I have not checked the plugs yet so it may just be an issue of a fouled plug but I figured I would ask.

John

[raymond10078]

I have to ask about the basics:

• Are the valves properly adjusted?
• Is the timing correct?
• What is your idle speed?
• Have you sync'd the carbs yet?
• Did you install a fuel filter? - Assuming that some/all of the "grime" came from the tank.
• Any chance your carb boots are leaking air? - You could try the WD-40 (or similar) spray on the boots when idleing test.

[Maine Ride]

I have to ask about the basics:

    Are the valves properly adjusted?
    Is the timing correct?
    What is your idle speed?
    Have you sync'd the carbs yet?
    Did you install a fuel filter? - Assuming that some/all of the "grime" came from the tank.
    Any chance your carb boots are leaking air? - You could try the WD-40 (or similar) spray on the boots when idleing test.

I have not yet touched nor studied valve adjustment.
Same above for timing.
I am unaware of the actual RPM's due to ignorance of how to attach a device to the bike. I did increase the idle with the pilot screw adjustment hoping to see a difference either way.
The carbs have been bench sync'd but not sync'd with a gauge set.
I did not install a fuel filter, that is on the list. I did inspect and flush the tank when I purchased the bike.
The previous owner stored the bike for 2 years without any fuel treatment.
It could be a leak from the boots, I'll give it a spray tonight when I get home.

[raymond10078]

Just for starters:

• The idle is set using the large knurled knob on the right side of the carb set.
• The pilot screws are not for idle, but for idle mixture.  If you're way too lean on a pilot, it will cause idle problems.
• I connect my RPM meter to a point set when setting idle.
• Carb synch can't be done (precisely) until everything else is done (valve adjustment, timing).
• Carb cleaning is a big chore to get done right - every passage must be clean.  Before I assemble the carbs, I make sure every passage is open using a can of carb cleaner and the thin spray thing on the can.  Squirt in one end/hole, and make sure that the spray comes out the corrects holes/ports.
• Another thought - did you set the float levels?

[Maine Ride]

I did set the float levels. To 1/16th. I took my time with the cleaning and ensured water was passing through the needle valves when rinsing.

[Maine Ride]

I made two videos of the symptoms with the air box off

CB750A rough idle

CB750A rough idle 2

[raymond10078]

Listening - the idle is set way too low.  Set your idle speed up first.  Look at the service manual for the idle adjustment.  Then adjust the pilots as described in the manual.  But . . . you really need to have the airbox on first.

Second, I really believe that you'll get a MUCH better response to your questions if you post this question on the main (non-Hondamatic) forum.  As I point out at the top of the Hondamatic page - when dealing with stuff that is "generic" bike-to-bike, it is better to post over there:

http://forums.sohc4.net/index.php?board=2.0

Usually this forum is more active - been pretty dead today . . . .

[Maine Ride]

Kandrtech I really appreciate your advice and suggestions. I will give the idle and adjustment tomorrow and report back. Thanks again!

[LesterPiglet]

50cc 4 pot automatic? Amazing.
Seriously, the idle speed is far too low.

[lucky]

Carb adjustment is the VERY LAST THING in getting the engine in proper running condition.

ALSO: The cam chain slack must be taken up before you adjust the valves.
The workshop manual tells you all of these things in the proper order.

[Maine Ride]

Thanks!

So I adjusted the cam chain as well as checked and flushed the accelerator "connectors", the rubber lines connecting 1-2 and 3-4. Fired the bike up and it sounded better. I used an infrared thermometer on the exhaust approximately 8-10 inches down from the manifold. I registered and double checked what I assume is outside proper operating tolerances of 1 & 4 indicated in the 400-430 degree range with 2 & 3 in the 160 degree range.

[Fran Fitzpatrick]

Had the same problem... My carburetor manifolds were cracked and sucking air. Try spraying WD40 between the carbs and engine and see if the bike idles any better.  If it does the manifolds are sucking air.

[salukispeed]

Something to think about is that 2-3 are on the same points, condensor and coil so a revisit of the points condition and timing using a timing light may help. Good Compression and Clean and properly adjusted/working ignition is a must first check . If you use a timing light and it misses a flash at the same time the engine falters might indicate an ignition issue. If these are perfect you can start to look at and work thru carb issues.

IMHO