Rough running/stumble

[Roverguy]

I've been chasing a stumble under certain conditions. 

 

Cruising at 30-35 not accelerating 2500 rpm or so I get this stumble, almost like it wants to stall, but under constant acceleration it pulls fine.

 

If I hit it hard it balks a bit, but then once I hit the secondaries it kicks in just fine.

 

I'm attributing it to the old gas that was in the car.  Since I drained the tank, replaced all the filters and ran a tank or so of fresh 93 it's gotten better.

 

I've done a minor cleaning of the throats and jets with the carb on the car.

 

Its a 32/36 Weber.  Next is to do a complete rebuild of the carb.

 

New plugs and wires, old plugs were nice and tan.

 

New cap and non rev limiting rotor going on tomorrow.

 

Since car is new to me and I have no "recent" maintenance records I'm going to baseline everything so I know its new.

 

Thought on this being fuel/not ignition?

 

[Dudeland]

Go to your dizzy take the cap off and see if you can lift up on the shaft. If you have play in that it may be time for a rebuild.

Someone here will be able to tell you how much it should move. Mine was shot and I got a 123 dizzy which works great. Make sure you follow the instructions.


Sent from my iPhone using Tapatalk

[percy]

13 hours ago, Roverguy said:

I've been chasing a stumble under certain conditions. 

 

Cruising at 30-35 not accelerating 2500 rpm or so I get this stumble, almost like it wants to stall, but under constant acceleration it pulls fine.

 

 

Thought on this being fuel/not ignition?

 

Could be either or both.  Difficult to say via long distance.

 

Generally that carb works well with many applications.  That said, there are countless variables regarding jetting and progression porting, all of which are compromises.  While they may not be "infinite" the posts on jetting and part throttle stumble may seem that way.  Without knowing more about your engine modifications and condition, I would suspect a lean condition that might be ameliorated by a slightly larger idle jet and/or a slight change in main and air correction jets.  I am avoiding the listing of specific sizes since each setup can vary considerably.  Emulsion tube selection can also impact your symptom.

 

Ignition timing, in particular, can also improve some of your described issue, but maybe not without affecting some other area of engine operation and power output.  Again, I would start with jetting and establish a baseline.

[echappe]

Vacuum leaks can play havoc

[TobyB]

-if you can't find anything plugged/broken/loose,

 

first thought is junk in the carb,

and right after that, too- small primary jet.

 

t

[Hans]

I agree with Toby. Ethanol in fuel does bad things when it sits for months. I'd give carb innards and jets a good cleaning with carb spray. Wear safety glasses.

[percy]

32 minutes ago, Hans said:

I agree with Toby. Ethanol in fuel does bad things when it sits for months. I'd give carb innards and jets a good cleaning with carb spray. Wear safety glasses.

 

 

While it is not always easy to purge a fuel system of contaminants,  seems like Rover already took a few steps in that direction.  Granted, he indicates an improvement, but only he can determine how much.of an improvement

 

Quote

Since I drained the tank, replaced all the filters and ran a tank or so of fresh 93 it's gotten better.

I've done a minor cleaning of the throats and jets with the carb on the car.

 

 

Complaints of a lean stumble condition with use of the 32/36 are common and may have nothing to do with fuel quality.  Since plugs are clean and assuming, for the moment, that everything is mechanically sound, a lean misfire during part throttle, warranting slightly fatter jetting, is an easy conclusion to draw.  Jet selection is typically the predominant treatment, but maybe not a complete cure.  Ignition tweaks can be helpful too.

Edited August 5, 2018 by percy

[TobyB]

Nope, I'd say pop the top off the Weber, and suction out all the goo at the bottom of the float bowl...

Squirt a bunch of cleaner in there, and then mop the mess out with blue towels.

Then light blue towels on fire for juvenile fun...

Maybe change the power valve and accelerator pump diaphragm, too.

 

The goo floats around, plugs the primary jet, plugs the power jet, but when you stand on it,

sloshes out of the way.  It's nasty stuff- failed stablilizer, water, cow hair, whatever was

near the corn when they fermented it. 

I have been using non- water- impregnated gas in all of my carbed junk these days,

and it's amazing how much less carb maintenance I'm doing.  It used to be

that every spring, I'd plan on a teaspoon or so of whatever that crap is they use to

get the water to mix into the gas. In every stinking carb.  2 stroke, 4 stroke, air cooled, whatever.

Now, pull the cord, turn the key, push the button, off it goes

(whatever it is)

The worst was the trencher- that old Zenith carb was corroding from the inside out.

I started giving it Marvel in the fall just to preserve the carb.

 

Failing that, again, try a bigger primary jet.  Lean transition stumble's a hard- to- solve 38/38 problem that

you can usually tune out of a 32.36-  sometimes just by adjusting when the secondary starts to open.

 

blah de blah.,

 

t