83 Carb Guru Needed

Started by cvincer, May 15, 2013, 05:35:23 AM

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cvincer

The carb is an  '83 & was running great ......... now, at idle, the  rear cylinder is not firing.   At general road speeds & accelerating it goes ok  (except for when you back off on the throttle approaching traffic lights etc .... again the rear cylinder stops firing).

Have obviously cleaned the carb several times without luck.

So, can anybody point me to an  airflow diagram  for an '83 carb?? .....please, please, please.

While I'm here;  I understand how an old fashion choke works (pulling the choke lever causes a butterfly valve to restrict airflow thus enriching the mixture) ................ but how does the 'enricher' work on our Mikunis?

Thanking you.

QBS

Sounds like the idle/low speed circuit in the rear carb is plugged.

Jimustanguitar

#2
Quote from: cvincer on May 15, 2013, 05:35:23 AM
So, can anybody point me to an  airflow diagram  for an '83 carb?? .....please, please, please.

I would check out the "updated resources" thread. There are links to most of the manuals, and I'm sure you can find a flow diagram in there.

Quote from: cvincer on May 15, 2013, 05:35:23 AM
While I'm here;  I understand how an old fashion choke works (pulling the choke lever causes a butterfly valve to restrict airflow thus enriching the mixture) ................ but how does the 'enricher' work on our Mikunis?

A choke reduces the air entering the intake and makes the fuel/air ratio richer for starting. The enricher circuit achieves the same rich air/fuel mixture, but it does so by increasing the fuel instead of reducing the air.

Rikugun

#3
How did the inside of the carbs look - any unusual debris? Tank condition rusty? Has the fuel filter suddenly taken on a "full of stuff" look!?  :laugh: Just trying to figure why it would suddenly drop a cylinder at idle. You might want to confirm both plugs have spark at idle just to cover all the bases.

The red line looks to be pointing at the transition holes and the single idle outlet hole is just downstream of those. The enrichener outlet is quite a bit larger and may be hiding in the shadow northeast (in the pic) of the afore mentioned holes.

For what it's worth, here is a flow schematic from the Yamaha manual:
(note what they call the "stater" plunger is a typo and should be starter plunger aka the enrichener.)
It is far better to grasp the universe as it really is then to persist in delusion, however satisfying or reassuring.  Carl Sagan

cvincer


Jim                That sounds logical, thank you.     Strangely if I put on full choke, as I'm riding along with a warm engine, it
                                                                           falters momentarily before continuing without a problem  ....whereas the
                                                                           old car with a butterfly valve in the throat, would die.


Rickgun        Inside of the carbs looks spotless (I add carb cleaner at every fill-up)
                 
                    Tank has no rust  (thanks to this Mediterranean climate)

                    Petcock filter & inline filter (clear casing) don't show any contaminants.

                    Despite the good spark, replaced the spark plug & TCI ........ no difference


QBS              Yes I thought that from the first, & tried the ultrasound followed by high pressure air.  No luck hence this post.

                     With Rikguns  explanation that the group of tiny holes below the butterfly are transition holes ....... the slightly

                     larger single hole immediately below, is for the idle/low speed circuit + the flow diagram,  I'll try to work out

                     where the idle/low speed circuit begins, & then blow air down the circuit.  The weekend is coming.

Rikugun

Generally speaking an enrichener circuit is more effective at small throttle plate openings i.e. near or on the idle stop screw. As such, it can be used to help diagnose idle and low speed fueling issues. If employing some amount of the enrichener helps ridability it seems logical you may have a lean condition like a partially plugged jet or passageway. On the other hand, if adding the enrichener made it worse you might assume you were too rich to begin with like float set too high or mixture adjusted too rich.
It is far better to grasp the universe as it really is then to persist in delusion, however satisfying or reassuring.  Carl Sagan

cvincer


So, with the '83 carb again in pieces I used the ultrasound & high pressure air again ...... with no improvement.

In desperation, with the carb in situ, I removed the jet indicated & dumped 2 tankfuls of high pressure air down the hole
(beware of getting sprayed with petrol if you try this).

YES, rear cylinder firing, engine running rough.  10 minutes with the home made carb balancer (warm engine) & I'm in business.

Rikugun

"in situ"  Now I have another Latin phrase to ad to my repertoire  :)
It is far better to grasp the universe as it really is then to persist in delusion, however satisfying or reassuring.  Carl Sagan

supervision

 The pilot jet is under the carb body you must remove a plug to gain access then remove the pilot jet and clean it
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