Stromberg 97 heisitation

Help. I have a 1938 21 stud motor that has an off idle hesitation
Carb is relatively new, but has been attacked by ethanol. I’ve cleaned it out several times but it still won’t come off idle without backfiring unless I pull the choke out
Runs perfect otherwise

 

Not an answer to your problem but I run no ethanol gas in all my old cars, it damages the diaphragm in the fuel pump and parts of the carb. Quick Trip, Casey's and a local dealer have no ethanol gas here. I would check that the accelerator pump and the power valve if the accelerator pump isn't working the choke makes the carb richer and power valves can fail due to a backfire.

 

Stromberg power valves can fail with a backfire? Didn't know that.

As said, check all the soft parts and stop using ethanol.

The power valve on a Stromberg doesn't engage until the accelerator pump hits the bottom of the well. I'd think the problem is something else gummed up or sticking. Maybe the emulsion tubes have crud around them. Search "emulsion tubes" on the HAMB and you'll find the proper method for removal and reinstall. But it's gonna cost you another gasket kit (buy a good one from Stromberg, not a cheapo).

 

Is your accelerator pump working? With the engine not running, look down the throat of the carb and open the throttle fully. You should see a healthy squirt of gas as you move to throttle. If you don't, you will experience a hesitation and possible backfire on acceleration.

 

Accelerator pump works good. Carb works perfect, other than just off idle. If I stab the the throttle, runs great but I can't drive it like that all the time (I doubt the axles would last)
Again, runs perfect, starts right up, idles smooth at 500rpm , runs great at WFO I can't just crack the throttle a little bit

 

A carb backfire can blow a power valve.
Not an exhaust backfire.
Easy check. Remove the power valve and blow through the diaphragm. You should not have air movement.
The new power valves are known to have a bad seal.
Good ones are out there.
Are you running a stromberg or a Holley?
When screwing in your idle needle valves does it effect the idle?

 

82% of all carburetor problems are the ignition.

After the emulsion tubes, check to make sure the distributor is clean, advance is working, and the vac brake still has its leather tip and is hooked to the manifold.

 

There is no rubber diaphragm in the power valve of a Stromberg. Just a spring and plunger.

 

Tore apart both carbs twice chasing this very problem. Would have bet a paycheck it was something I overlooked in the carbs.
Gave up and installed a condenser.
Problem solved.

 

I've never gotten involved with diagnosing or repairing a Stromberg carb but it sounds like I already don't know as much about them as I thought.

So just for clarification ... set me straight here.

Your statement leads me to believe that the power valve on a Stromberg is strictly actuated by the mechanical movement of the accelerator pump plunger. Does manifold vacuum not come into play at all as far as determining when the power valve operates? I'd never even thought of this as a possibility, considering the power valve or power piston operation on a more modern carb. Though it does seem like an acceptable, if somewhat less accurate means of temporary fuel enrichment during heavier engine load conditions.

 

The power valve is actuated by the bottom of the accelerator pump hitting the small button on top. Or can high pressure from the accelerator pump going down in the well push the valve open too?

Eitherhow, this is probably not the OP's problem.

 

I did go thru the distributor
Starts a little better (I mean the motor doesn’t seem to turn over 1 time before starting)
Idle screws have been jacked around & yes, that makes some difference in the idle, but not the heisitaion. I’ve adjusted the static timing & the vacuum break & seen a difference EXCEPT for the off idle issue
Again, runs fantastic everywhere except just off idle. It does run better in warm weather (a rarity in northwest Ohio)

 

Try advancing the distributor a degree or two and see what that does.

 

I have jacked the timing until it really rattled entered
I just had the carb apart AGAIN & played with those emulsion tubes again (enlarged the holes a very little bit) and that seemed to make a difference. Not perfect, but a big improvement
I do have 1 more question: I noticed the tubes don’t sit............centered where they come booster. They need to be rotated for the “slash” to sit just right. But of course they can’t be
Wonder if anyone else has come across that

 

I do wanta thank everyone who replied
I learned something from everyone
It’s running much much better
Thanx to All !!!!!

 

If you over tighten, the emulsion tubes will actually crush causing the holes in them for fuel to pass through to be very small. Check to see that they are not bulged out from over tightening. Not saying this is your problem but just something else to check.

 

Did you remove the tubes from the carb body? The gunk gets caught in the narrow cavity between the emulsion tube and the body. Can't be seen when all assembled.

 

I have a similar problem with my 97. When cold, off idle stumble unless choked to the point where it misses higher up. Better once warm. I pulled the emulsion tubes yesterday, all clear. Not sure what to do, thought about enlarging the holes. And yes, there must be a trick to starting them back in so they line up straight..

 

Yeah it's on the rich setting. It's not just taking off from a stop, it will sputter down the street at just off idle. Goose it and it's fine, and fully warmed it's OK, maybe still lean but not sputtering. I'll just put it back together and hope I cleaned something out. Thanks.

 

Ha, I was just thinking the same thing. I haven't removed the base from the engine, next step.. That's a neat thing about these carbs, you can remove the venturis and leave the throttle plates. Thanks again M

 

I tried in place but all the squirt goes out the holes under the throttle places, directly across from the screw holes. So I popped it off to put a thumb over those. The transfer holes aren't plugged but it's not shooting out like it does the bottom holes. Might dunk it in the Berryman's overnight.

 

Do you play guitar or have a guitar playing bud?
Guitar strings work to clean out small air passages in carbs following a little juice.

Joe

 

I can see crud in the idle hole and the intermediate speed hole. A torch tip cleaner works fine for these but you have to bend a 90 degree angle on the end to get at those holes. Do you have the correct gasket (carb to manifold?). A "G" string would be about right for a reamer, too. Lots of rust in the bore. I would give it a soak in the acid tank before I did anything. That spray carb cleaner isn't going to fix that. Cleanliness is essential for proper operation. Even lint from a wiping rag can screw things up.

 

Funny you mention that, a neighbor came over yesterday wanting to solder a connection on his guitar, which I did. He has a gig tonight, I'll get whatever strings he breaks.

 

The 94's on my banger would do exactly as you described. If I held the throttle slightly open, it would stutter along until I rolled the throttle open then it was fine. I finally opened up the idle tubes (the little guys hiding under the accel pump discharge nozzles) a little and it fixed it. I don't know if that is something that would help on a Stromberg. The idle tubes are similar...