Topic: I'm sorry for being such a sorry-assed n00b (carb synch trouble)

[gregimotis]

I swear I've read the FAQ, the hints section, and as many search results as I could.  Please help my pathetic self anyway.

The bike is a 1976 CB750F1.


I acquired a Morgan carb tune recently and went to try it out last night.

I did all the standard tune-up procedures first (timing, valves, cam, new plugs, pixie dust)

I set the idle to 1100 (steady idle per Carb-tune instr.)

On first reading I got (from left to right)  11-13-14-17.

My Clymer indicates 14-16 +/-3 so I adjusted all to about 15. (although in another paragraph seems to indicate 20-22)

I set all the idle screws to 1 1/2 out (three and four were there already, one and two were closer to       2 1/4)
         -I played with these a bit, and this seemed to be as good a setting as any

I tightened it down, rechecked everything (re-adjusted, re-tightend)

Went for a test and here is the probem:

Idle is great, very smooth.  Low throttle is great, around the neighborhood throttles feel good.  Steady medium throttle and it will accelerate all the way to the redline.

Crank into the throttle and the bike bogs down like a boxer what got punched in the liver.  If I stay on the throttle the bike will die, if I release the throttle it will recover shortly.


I came back in and re-checked my synch, it's good.  I pulled my plugs... they look just as new as they did yesterday.
Two other things which might be important:

1:  The bike ran pretty darn well before I started this, this throttle problem is new since the carb-tune and idle mixture adj.
2: I am at 7000 feet elevation.

I don't know where to go from here.  Any suggestions would be appreciated.

[gregimotis]

Sorry, forgot to mention:  I did the synch without airbox, per intructions.  I did the initial test run with it also, then an subsequent run with it on.  No significant change.

[Gordon]

I've never heard of doing the carb synch with the air box removed, was that in the instructions that came with the Morgan?  I always make adjustments to the carbs with everything set up the same way as it will be regularly run. 

Assuming the carbs are now relatively in synch, I'd have to say the adjustments you made to the idle mix screws are more likely to be the culprit of the new problem.  Considering your elevation, with the carbs at stock settings you'll be running very rich, so you need to adjust the idle circuit on the leaner side. 

Have your carbs been re-jetted to compensate for elevation? 

[gregimotis]

It seemed funny to me to remove the filter, but yes the instructions said too - and it seems that they would stay in synch either way reletive to one another.

As for the idle screws... again it makes sense that they would be rich except; the problem doesn't kick in until I am around three-quarter throttle.  My understanding from this board was that the screws only effect mixture under one-quarter.

I haven't had a chance to fiddle with it today.

[medic09]

Hey Greg,

Did you recently rebuild the carbs, or just synching what you already had?  Just trying to rule out internals like stuck/reversed float, etc.

We're over at Dave Liggon's about 10:30 to do my wife's fork seals, but that means I'll be around the area if you need something.  You don't need my CarbTune (which Dave borrowed along with the aux tank), but I might have something else you need.

Give a call to my cell (490-0447) or email if you need something!  We'll probably go for an early ride, and be back around 10:00.

Mordechai

[Gordon]

As for the idle screws... again it makes sense that they would be rich except; the problem doesn't kick in until I am around three-quarter throttle.  My understanding from this board was that the screws only effect mixture under one-quarter.

It's true that idle mixture adjustments have more of an effect at idle speeds, but it still has an effect throughout the entire throttle range.

[medic09]

Greg, I ask if you rebuilt them because mine did something similar after a rebuild when I took the bike out on to I-25.  Start up was good, riding in town was good, out on the interstate it just died.  Turns out I had put a float in upside down, but it took a while to find that.

Just fishing here...

[gregimotis]

I did pull the carbs and clean them out good before I did the synch.  Perhaps I'll fiddle the idle mixture a bit and if I can't sort it that way, then pull them off again and look.

[OldSchool_IsCool]

Curious,

I am just now having similar symptoms after a long spring of never a hint of a stumble.  Figures it would happen just as I met up with Wykydtron on his way back home from his camping trip 

I haven't touched my carbs since I got the bike 18 months ago.  Performance was always good so never felt the need to.  I adjusted the tappets and points about 300 miles ago, and Ol' Yeller has been running flawless since.  The only thing I did different is not adding Seafoam to a recent fill up (pause for eye rolls).  I mention this because, once I added Seafoam to the tank, the stumble went away.  As an experiment, I ran the tank to reserve plus a little further then filled up sans Seafoam and the stumble returned (while riding with Wyk GRRRRR!).

Thinking I was onto something with Seafoam, I added some.  The stumble is better, but is not gone.  I don't think Seafoam is the solution, but I do think it adds evidence to carb problems being the culprit.  Wyk thought maybe varnish in the main jets since acceleration was pretty solid up until about half throttle.  While  in nutral, we blip-ed the throttle up to redline with no sign of the stumble.

I'm going to watch this thread closely for a solution and will post results of my fix attempts too.

[OldSchool_IsCool]

I figured out my stumble problem.  You're going to LAUGH!!

I installed an in-line fuel filter some months back.  My tank isn't rusty or anything and I did replace the in-tank filter just last fall.  Guess its a "couldn't hurt" extra precaution thing.

Well anyway, the in-line filter was the high-point in the line as it snaked its way down to the carb fuel gallery.  Well, somehow, probably due to running out of fuel in the main tank or taking the tank off recently, the in-line filter captured a large air bubble, reducing the fuel flow to a trickle.  Once the float bowls emptied out, there just wasn't enough flow to feed the beast above about 6k RPM!

I have since remounted the in-line filter so that it is stanting upright on top of the fuel gallery.  No way it can trap an air bubble now.  And the beast growels like she used too!!

[gregimotis]

Not solved yet.

I took the carbs back off and pulled them apart; Disassembled everything, checking to be sure it's all put together correctly.  I dropped the needle clip one notch to compensate for a rich condition I've always.  Put it back together and re-synched.

The bike runs great in normal conditions - idle sounds nice, accel is smooth and strong... unless I crank the throttle.

I'm going to read as much as I can about carburetor online and if I get no ideas, I may take it to a mechanic I know.  Not in the spirit of the forum, I know.

[Gordon]

I dropped the needle clip one notch to compensate for a rich condition I've always.  Put it back together and re-synched.

Lowering the needle clip raises the needle, which richens the mixture.

[medic09]

... if I get no ideas, I may take it to a mechanic I know.  Not in the spirit of the forum, I know.

   A scandal!

Let me know if you need something from my garage.

Well, if it is carbs is youir throttle linkage okay?  Floats okay?  Main jet clear?  (I know you checked all that, just reiterating.)  Can there be an ignition/electrical component to this?  Sometimes carb and electrical problems imitate each other.  Is your timing and advance okay?

Where's TT when we need him to analyze and do a differential diagnosis? 

[RRRToolSolutions]

I don't have any experience running at 7,000 feet, but I hear what I believe to be a lean condition at mid-range. The mid-range mixture is most effected by the needles as they allows the main jets fuel to flow faster/sooner. The pilots can and will over-lap during accleration and part throttle settings. Since you had some screws opened to 2 1/2 turns - I'd guess you had them compensating for a lean mid-range mixture. If it's rich, it will blubber and run heavy. If it's surging and has a tendancy to vary rpms at a constant throttle setting - then it's lean. Lean mid-range can be best adjusted by raising the needles (lowering the clips). If 1-2 clip notches doesn't solve it, then a slightly bigger main may be needed. You don't want a lean condition. All carbs should be set the same with regards to vacume (carb sync), needle height, and idle mixture settings.

Are you sure that airbox is back on and properly fitted? Additional airflow could cause mixture changes.

Regards,
Gordon

[gregimotis]

Replies:

Gordon, yes, I actually raised the clip from the stock middle to one up from such -sorry to cause confusion.

My screws are currently set at one turn out (which is in the normal range, although maybe a bit rich for the altitude)

The the throttle response within normal use (no throttle up to three-quarters open) is good, neither bogging down nor hunting.  I could ride for days without ever noticing a problem.  The problem only presents at ~85% throttle: really crank the throttle, and the bike bogs and dies within a second.

It could be electrical except that the problem only began after I pulled and disassembled the carbs.  The only electrical work I did was to inspect the points and eyeball the timing, both were in good shape, no tools were applied.
Other than fouled plugs (current ones are almost new) from the rich condition, the bike has not had any electrical problems.

I'm confident the floats are right and the innards are spotless.



Medic, do you have an 'carb analyze and fix automatically' tool in your garage?  I'd really like to figure it out rather than give it to someone.

[medic09]

Greg,

Hang in there.  You already know that the folks here will be as tenacious as you choose to be.  If you want to work through this till the bitter end, uh I mean to conclusion, then someone here will help you do it!  Are you home working on this today?

[RRRToolSolutions]

Well, raising the clip on the needle (effectively dropping the needle) and turning those idle mixture screws in gave it a double shot of "lean". I'd still contend that you're lean and opening the throttle more is pulling a gulp of air that isn't matched with enough fuel.

My 02.

Regards,
Gordon

[gregimotis]

I was rich before and raised the needle clip - thus lowering the needle (leaning the mixture) one notch.  Hopefully, that gets me close to correct for the altitude.  The idle screws are at one out, the range is 1-1.5 per TT's carb FAQ and my manual - further out=leaner; so they should be a little rich if anything.
Still, I will try turning them half or three-quarters in each direction and re-test for change.   I may plug chop it and make sure the mixture is where I think it is.

All that said, further testing has revealed new information:

-The problem only presents at RPMs below ~3000 in any gear.
-Above 4000 the bike pulls as it normally would at high throttle.
-Right around 3000 the engine reaches a break-over point where it bogs down a bit but is still able to wind up until eventually it 'gets it's legs'.


I rode the bike about 15 miles yesterday and really, I never put the throttle down near hard enough to encounter the problem.  Even if I were dragging the lights, I'm not sure I would twist it that far before hitting higher RPMs (if at all).  My plan is to futz with the screws a little, and check the plugs for hints as to the mixture;  Bump this thread once or twice with new information, and keep an eye on it.

BTW Medic, I usually browse here from work (I'm a 9-5er) and tinker with the bike here and there in the eves and weekends.  No 48/72 shifts for this kid

[TwoTired]

I haven't seen a clear description of what problem you are trying to solve.  I get the impression it is throttle response.

When the throttle is snapped open, the carb throat pressure approaches outside atmospheric.  This means their is no or little vacuum to draw fuel through the metering jets, until the venturi effect caused by increasing air velocity creates some fuel draw.

A 76 750 F1 does not have an accelerator pump.  It relies on having a rich idle to get the RPMs high enough for the venturi action to take over.

The other place an extra boost of fuel can be available is the emulsion tube area, and this volume is effected by float height settings.

From Idle RPM, you will not be able to snap open the throttle slides without a wheeze engine response, unless you set the carbs up for slobbery rich settings.  From low RPM you should be able to make 1/2 the total throttle twist range with good acceleration in any gear.

Air filter restriction effects low speed throttle response greatly.  Low restriction means the carb throats are already close to atmospheric pressure and drawing minimum fuel from the metering jets.

To make the idle richer and improve sudden throttle opening changes, turn the air screws in to richen the idle mixture, and/or raise the fuel level in the float bowls.  If your air screws are hollow tipped, this will limit their adjustment ranges.  To go beyond this range requires larger idle jets.

Cheers,

[gregimotis]

...It relies on having a rich idle to get the RPMs high enough for the venturi action to take over...From Idle RPM, you will not be able to snap open the throttle slides without a wheeze engine response, unless you set the carbs up for slobbery rich settings...

Now that makes some sense.  The bike has always been rich, and the very first thing I did was lean out the idle screws - they were set about 2.5 turns out.  Currently they are at factory setting (1.25) -which I understand to be sea level appropriate
I will play with the idle screws later and see what develops.

[kine8282]

TT so turning the air screws in = richer and air screws out = leaner?

[TwoTired]

TT so turning the air screws in = richer and air screws out = leaner?

Yes, air screws restrict the air coming from the air jet to be premixed with the idle jet before delivery to the carb throat.

Do not confuse these with Idle Mixture Screws (IMS).  Some carbs have these instead.  These screws restrict flow of the mixed air and fuel from the idle circuit just as it is delivered to the carb throat.  The lean/rich change operates in reverse for such carbs.

cheers,

[roadkill savior]

After you removed the sync gages did you get the 4 little screws back in and tight with the little washers?

[gregimotis]

Good question, and yes.  The #2 was a bear, but I got it finally.

Probably won't have much oppurtunity to work this weekend