Stromberg stumble, is this normal?

[8475]

Hello everyone. I'm getting close to flying my 140. It's been down for about 2 years. Couple weekends ago, I started the engine for the first time in 18 months. It had a pretty bad misfire at idle, which I diagnosed to a clogged idle jet in the Stromberg carb. I cleaned out the jet and as a result, it now idles well, without a misfire. However, I've been unable to entirely eliminate a stumble if I open the throttle rapidly from idle. Is this normal? Or should I continue to diagnose other problems?

[2066]

Dave, you'll likely get much more expert answers on this...but, that said: As best I can understand, when the Stromberg "transitions" from idle to run, there's a "normal" tendency for hesitation, especially if advancing the throttle rapidly. The fact that there's no accelerator pump may play a role, too. FWIW, mine has had a bit of hesitation similar for almost 38 years, even having had it overhauled a few times. (...good idea to refrain from quick throttle advance on final ) It's vital to keep it clean, of course, and good for you in finding and correcting the jet obstruction. It may not be related, but does yours have the mixture control set up for functioning? And, just a final thought (opinion) -- the Stromberg is probably the perfect carb for the C85/140. I like anything that reduces "unnecessary" complexities. Mac

[6643]

I've been unable to entirely eliminate a stumble if I open the throttle rapidly from idle.

Simple solution: don't do that.

As stated above, the carb doesn't have an accelerator pump. It uses what is called an acceleration well, which temporarily enriches the mixture, but its function is highly dependent on a number of different things, including the restriction of the air filter, the size of the air bleed jets, and the size of the holes in the emulsion tube.

Essentially, it was intended more for constant speed operation, not street racing.

When all is said and done, though, as long as it doesn't stall, it's not worth worrying about.

[6183]

Dave, you'll likely get much more expert answers on this...but, that said: As best I can understand, when the Stromberg "transitions" from idle to run, there's a "normal" tendency for hesitation, especially if advancing the throttle rapidly. The fact that there's no accelerator pump may play a role, too. FWIW, mine has had a bit of hesitation similar for almost 38 years, even having had it overhauled a few times. (...good idea to refrain from quick throttle advance on final ) It's vital to keep it clean, of course, and good for you in finding and correcting the jet obstruction. It may not be related, but does yours have the mixture control set up for functioning? And, just a final thought (opinion) -- the Stromberg is probably the perfect carb for the C85/140. I like anything that reduces "unnecessary" complexities. Mac

Mac,
You hit the answer square on the head. Fifty years ago, a gent in the business told me the same thing, because the unit has no accelerator pump it's common to get a stumble if the throttle is quickly advanced (kind of reminds me of friends operating some early jet engines when the fuel control was your hand or when operating a 5 cylinder Kinner radial, open the throttle too quick and the engine will stumble and hesitate).

[8475]

Thanks, everyone, for your responses. I'm glad to know this is normal and that I can move on to another item on the Barriers to Flight list. Getting closer!

[tonycondon]

slight adjustments to the idle air mixture can help.

variations in fuel specific gravity can also effect this depending on how the float level is set. I had trouble with that last fall when I filled up with about 3/4 auto fuel.

[2066]

slight adjustments to the idle air mixture can help.

variations in fuel specific gravity can also effect this depending on how the float level is set. I had trouble with that last fall when I filled up with about 3/4 auto fuel.

Good added point, Tony. I believe the STC may mention this, also (I'm too lazy to look up ). I have had to adjust the idle a bit on those occasions when using mostly auto fuel. I was fortunate for 10 years to hangar/fly out of a small field that had an auto fuel tank/pump -- that made it an easy choice. These days, with added aches and pains of lifting fuel cans + the "safety factor", I find that I use much more 100LL than "car gas"...and, must (reluctantly) admit that the C85 does consistently run/perform better with it.Mac