Topic: Carb synch question

[csendker]

Am I getting this right: during a typical carb sync, you adjust each carb until they all pull an 'equal' vacuum through the cylinders.  Equalizing the vacuums that are pulled really equalizes the amount of air being drawn through each cylinder so they all pull the same.  This is good because; presumably, you have matching jets which gives you an equal fuel flow.  Synchronized air flow and equal fuel flow = happy engine.

But isn't this really just a way to equalize the thermal performance of each cylinder?  Couldn't this also be accomplished by adjusting the carbs until you have matching temperatures at the exhaust pipes (say, near the connection to the engine block)?  I would think that equal thermal performance should also = happy engine.  Or are the adjustment parameters skewed?  Say you adjust to within a half an inch of mercury, which is reasonable to accomplish vs. to within a degree or two which may be an unreasonable adjustment?  Or maybe equal temperatures really don't mean squat?

I haven't put a lot of thought into this, rather I'm just tossing the question out for discussion.  And I've yet to sync a carb, which is also important to note.  But with two more sets of 022A's having been delivered in the past week or two (my first spare set having permanently frozen slides, ugh), I'll be digging into them as soon as my job stops sending me all across the eastern seaboard.

[RRRToolSolutions]

Syncronizing the carbs is so darn easy and makes for smooth idle and insures each carb is opening the same. Next to getting the timing right, carb sync is a good bang for the effort.

Buy the Morgan CarbTune - $100 and it works so easy.....

Gordon

[chung]

Intake manifold vacuum is primarily a mechanical function. Exhaust temp is very important but you must have all the carbs opening together (synchronized) before you can move to the next step.

[Jerry Rxman Griffin aka MuthaF'er]

Chris,

Keep in mind the carb area is the entry point to performance. Equalize everything here and continue on through the exhaust. IE no leaks at the manifolds, head work if your billfold can afford it, proper valve seal, proper valve clearance, timing, exhaust restriction, etc. Carbs are just a beginning and can be a hassle but a necessary hassle. Make sure your valve clearances are spot on. Cheap & easy. Flow and ignition. Thats it.

[csendker]

Buy the Morgan CarbTune

Yes, I know that if I'm ever to get serious about maintaining my bike I'll need one of these.  

But it occurred to me on one of my long drives home that this is all an attempt to equalize each cylinder output so they don't fight each other (they are all tied to the same crankshaft...).  I thought that it's really a thermal equalization in the end (air/fuel + spark = fire) and maybe a thermal measurement might just cut to the chase.

That said, are the little screws that I circled in the intake manifold where you hook up the CarbTune?   

[TwoTired]

Are you going to bung and insert EGT probes into each exhaust pipe?  Or, just monitor the outside header temp?  While exhaust header temp will change with engine RPM, this change is not instantaneous.  There is a time lag for stabilisation, both with mixture changes and RPM changes.  Also, as this is an air cooled engine, the inner cylinders will have a slightly higher exhaust temp due to the added heat of the outer cylinders.  How much of an offset between these will you add to determine proper mixture delivery?  You're also going to have a fan blowing on the engine during synchronization.  You gonna instrument the airflow to each header, as well, to calculate cooling effects?

At idle, the temps will be effected by the Idle Air Screws as well as the slide positions. Better have them optimized beforehand.

Theoretically, if each of your cylinders have exactly the same volumetric efficiency, they will each outlet the same exhaust temp if:
1) The cam lobe lift and duration are identical for each cylinder
2) The rocker lash is identical for each cylinder.
3) Each carb is producing the same mixture for equivalent slide openings.

# 1 is very much unlikely to be true unless the cam has been blueprinted.  Every used Honda 550 cam I've measured has had unequal height lobes.  Close but unequal, nonetheless.  I don't think close tolerances for this part were a priority for Honda.  I wouldn't expect the lobe ramps controlled valve timing to be precision, either.

I think the carb sync term is, in reality, a misnomer.  The true goal is to get all the cylinders to fire with equal power pulses.  If slide one has to be a fraction larger than slide two for them fire evenly then so be it.  Having equal vacuum indicates that each cylinder is drawing equal gulps of air and therefore should produce even power pulses, assuming good general cylinder mechanical health and carburetors that behave identical to each other.

Just some thoughts...

Cheers,

[csendker]

Are you going to bung and insert EGT probes into each exhaust pipe?

  Nope. (I don't even have a CarbTune let alone any exhaust gas monitoring equipment )

Or, just monitor the outside header temp?

  Yup, that was my thought.

While exhaust header temp will change with engine RPM, this change is not instantaneous.  There is a time lag for stabilisation, both with mixture changes and RPM changes.  Also, as this is an air cooled engine, the inner cylinders will have a slightly higher exhaust temp due to the added heat of the outer cylinders.  How much of an offset between these will you add to determine proper mixture delivery?

  Very good points, my theory is falling apart...

You're also going to have a fan blowing on the engine during synchronization.  You gonna instrument the airflow to each header, as well, to calculate cooling effects?

  Hmm, never done this before, didn't think of that.

Obviously, there's a whole lot of other issues that affect the thermal efficiency beyond air/fuel + spark that I hadn't considered.  I see where measuring the vacuum directly gets right to the point of "equal gulps of air" whereas measuring temperature, even assuming it could be done accurately, would contain way too many variables for it to be an accurate balancing method.  I think the concept was right ~ equal thermal efficiency/equal pulses of power ~ but I guess its way too simplistic of a measurement of thermal efficiency.  Find a variable, measure it directly and adjust appropriately.  Then you move on to the next variable and isolate it...

[mrbreeze]

Syncronizing the carbs is so darn easy and makes for smooth idle and insures each carb is opening the same. Next to getting the timing right, carb sync is a good bang for the effort.

Buy the Morgan CarbTune - $100 and it works so easy.....

Gordon

Everything Gordon says here is a totally true story.....Carb synch= smooth......Morgan Carb Tune= easy synch.

[Ibsen]

In addition to what's been said above, another problem when the carbs are out of sync, at least one of the cylinders will try to run faster than the others, putting more stress on the crankshaft. Another problem is that it can cause the engine to overheat if the carbs are way out of synch.

And I agree, the Morgan Carbtune works like a charm.

http://www.carbtune.com/inst.html