Author Topic: Engine Hesitation and/or Stumble after idle during 1/8 to 1/4 throttle.  (Read 488 times)

Offline Mrbill

I have Hummel 94/86 (45) horse motor.  I am having issues with getting my Mikuni 28mm carbs tuned and the engine smoothed out.  I am running a fuel pump and Holley Regulator.  (Gas tank is behind the seat). The regulator is set as low pressure as it will go, hopefully around 1lb.  I'm also running 170 main jets in both carbs. My floats are set at 17mm, the top end for 28mm carbs.  I was still getting overflow when the floats were reset so I changed out the main needle valve to a 1.5 and went down to a O-6 Needle Jet.  So, I'm not getting overflow now but, when I start to throttle up there's a hesitation and/or stumble (actually it seems to have always done this) I also have the needles set to the 2nd notch from the top. Once I'm past it, it smooths out and seems okay in runup. And last but not least, the number cyl. 1 spark plug looks rich and the number 2 cyl. spark plug looks lean.  Can anyone provide me some assistance on what to do next?  Thanks, Bill

Offline CHARLES DEBOER

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Re: Engine Hesitation and/or Stumble after idle during 1/8 to 1/4 throttle.
« Reply #1 on: August 06, 2019, 10:28:22 PM »
Call Scott Casler at Hummel Engines. Cell 520-723-5283.  He is willing to help any time I call him.

Offline Mrbill

Re: Engine Hesitation and/or Stumble after idle during 1/8 to 1/4 throttle.
« Reply #2 on: August 22, 2019, 07:58:38 PM »
FYI Scott has always been great to respond with my questions.  However, regarding carbs he said he wasn’t that knowledgeable.  He did recommend both slides were opening at the same time.

Offline Dan_

Re: Engine Hesitation and/or Stumble after idle during 1/8 to 1/4 throttle.
« Reply #3 on: August 23, 2019, 12:46:52 PM »
Here's how they do it down unda.



“A superior pilot uses his superior judgment to avoid situations which require the use of his superior skills.”

– Frank Borman, Apollo 8 Commander

Offline Mrbill