More Carb Sync Questions

[markmartin]

I recently did my first carb sync.  The bike was running well overall and when I hooked up the Morgan Carbtune,  the carbs weren't very far off;  I adjusted them to read better, and after the sync, I do believe I can tell that difference is for the better, and the bike is (still) running very well now. I think it could be a little sharper on the 'blip' test, but aside from that, I have no complaints.  I didn't touch the mixture screws, mostly because a couple of strange occurrences happened while I was doing this procedure that left me thinking 'what's up with that'?  I figured it was time to ask some questions before stumbling on blindly.

1.)   Before I even started the sync process, I decided to adjust my idle up a bit (using the single idle adjustment screw) as it had been idling at about 900-1000rpm.  . When doing this I got some terrible surging,(well over 2000 rpm) even when backing the idle screw back all the way back .  I was able to get the idle to behave after some frigging with the screw and went for a ride, only to get surging when coming to a stop after riding , and then being able to get it to settle to idle by slipping the clutch a bit while in gear.  I finally was able to get the idle back to normal by playing with the idle screw.  I then synced my carbs (while again running into the same surging problems)  and was again able to get the idle back to normal by much frigging with the idle screw after the carb sync. Like I said, the bike is now running fine again, idle is about 1100 when well warmed up, and no surging problem.  That said, I almost don't / wouldn't dare touch the idle adjustment screw (or mixture screws)  as I think the surge problem would come right back.

Anyone have any idea why this surging would happen?  Dirty carbs?  Dirty linkages? Demonic possession?   Air leak somewhere or sucking air in but why would it start doing it, and then go away—and it wasn't happening before I touched it??  I've put about 800 miles on since with no problems.  

*The bike was very warm (10 -15 minute ride) before - when working on it.
*There is ample play in the throttle cables even when it was surging and the carb action didn't seem to be sticking,
*I tried pushing down on the idle adjustment bar/ with a screw driver when it was surging – the idle screw all the way backed off and seemingly bottomed out, but no avail (this was with the tank off)

2.)    The Morgan Carb-tune rods (4 rod model) would barely register when synchronizing at idle, not much better at 1700rpm, and only what I would consider up in the middle of the 'screen' when at about 2800 rpm.  Is this normal?  Should I use the tool upside down as suggested in the users manual?  Anyone else run into this problem while sync-ing an FJ?

3.)   Also, I was getting a small occasional (small) backfire out of carb 3 ? while syncing.  Normal? Not normal?


I've tried staring at the carburators and scratching my chin, but this hasn't seemed answer any questions.  Any help would be appreciated.

Mark

[Harvy]

Mark.....that surging thing...... yeh, it happened to mine just one time about a year ago when I did a synch - only the once - and then it went back to normal and has not happened again....... its got me stumped!

Harvy

[fb747]

Could it be possible that whilst syncing, fluctuating air pressures are causing the carbs to suck through the choke circuit?

Total guess shot in the dark.

[andyb]

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Could it be possible that whilst syncing, fluctuating air pressures are causing the carbs to suck through the choke circuit?

Total guess shot in the dark.

Would indicate leaking orings in the choke circuit, yes?



Adjust the mix screws then.  Something seems wrongish, may as well do what you can to get it right, or live with it.

[SlowOldGuy]

The high idle speed and low intake vacuum are indicitive of an intake vacuum leak somewhere.  Either the intake manifolds on the cylinder side, or it could be a leak past the choke plungers (plungers aren't sealing well).

An increase in speed means air is getting in somewhere.  If you add extra fuel to the intake, the motor will blubber and die.  If you add extra air, the engine will rev up.  The throttle plate controls the amount of air allowed into the engine.  The carb curcuits make sure that air has enough fuel to combust.  Turning the throttle allows more air/fuel into the intake which makes the RPM increase. 

Your low vacuum readings on the Morgan also indicate a vacuum leak.  Look at what the gauge reads when the choke is on (practically zero).

Also, vacuum should be "fairly" constant at a steady state throttle position.  It varies A LOT when you're blipping the throttle, but if you hold the throttle at a constant position, the vacuum "should" build up to somewhere around 20 to 24 on your gauge.  As a note, the vacuum plunges to near when you open the throttle.  When you close the throttle it spikes high. 

When you get popping out of one of the carbs, that "typically" means you've got a plugged (or partially plugged) idle jet.

Continue diagnosing and report back.

DavidR.

[SlowOldGuy]

Oops, my edit time expired.

That should have read "plunges to near ZERO when you open the throttle."

People suck the mercury out of their old style sync gauges when they blip the throttle then allow it to slam shut.

DavidR.

[markmartin]

Continue diagnosing and report back.

DavidR.

Thanks for the feedback.  I pulled the tank and inspected the manifolds and was able to tighten up all the hose clamps on both sides of the carbs (thumb and two fingers tight, not full fist tight).  Boots look fine from the outside....  

I checked all the spark plugs and all are consistant color and look good--if anything on the lean side  -- which would be consistant with the air leakage prognosis. ?   Plugs look most like # 20 on this chart http://www.4secondsflat.com/plug_chart.html

I'll try the carbtune again soon -hopefully tomorrow night,- and see if the vacuum is better.  Hookin' up the brand new Morgan Carbtune was anti-climatic if not downright disappointing with the engine idling and all the rods just sitting there bouncing on the bottom.

Looks more and more like a carburetor rebuild kit and a stainless steel screw and O-ring kit are going to be a winter project for me.  

Mark

[markmartin]

The high idle speed and low intake vacuum are indicitive of an intake vacuum leak somewhere.  Either the intake manifolds on the cylinder side, or it could be a leak past the choke plungers (plungers aren't sealing well).

...............An increase in speed means air is getting in somewhere.   ..............

Your low vacuum readings on the Morgan also indicate a vacuum leak. 

Right on Brother!  I hooked up the carbtune tonight and was able to get a reading of about 10 - 12 at idle (1100rpm) and 18-20 when I set the idle up to about 2200rpm. The only adjustment from the last time I synced was tightening of the manifolds.  As a matter of fact, I could tell the difference for the better on my ride to work and back today as there was not as much of a slight stumble when accelerating (in gear) from idle--before I even hooked up the gauges tonight. 

The surging problem was nowhere as bad, but still had a small issue when bringing the idle up to 2000 and then back down.  It seems that if the idle is set to say 1400 rpm(a bit high I know) it's as though the extra vacuum from the higher idle is sucking more air in somewhere and causing a erratic idle, (o-rings in the choke curcuit-as mentioned in other posts?) but when I bring the idle down to 1000 - 1100 it finally settles down and behaves.  I've no problem with my idle set at 1000-1100, but I should be able to set it to 1400 and not have it surge.

Also, vacuum should be "fairly" constant at a steady state throttle position.  It varies A LOT when you're blipping the throttle, but if you hold the throttle at a constant position, the vacuum "should" build up to somewhere around 20 to 24 on your gauge. 

Yup, I was able to do that tonight.

I still have not touched the mixture screws.  I'm going to run it a bit and inspect the plugs, I'm hoping to get a slightly richer looking plug than the slightly lean that I have after finding and fixing the slight air leak at the manifolds.  Again, the bike is running well, not sure if the mixture screws are even in this equation.?
That said, thanks for the diagnosis.  I'm learnin'

Mark