Engine lags Severely

[OlindeASP]

Recently got my BC12d, and it has an a65 in it. it idles just fine at around 700-800 rpm, and it runs fine at cruise. However in the transition at around 1300-1500 rpm the engine runs extremely rough and lags. It doesn't matter if you push in the power slow or quick, the engine always stutters before coming to full power. I suspect this is a carb issue. This leads to some puckering moments as full power is applied to go around and the power doesn't come. Any thoughts as to what is going on? I have the stromberg carb with no mixture control.
Thank you!

[Robert Lees]

Idle rpm too high for a start. Tweak the idle rpm to the correct rpm and then report back! Easy and simple to do.

[3Dreaming]

I agree with Rob, the idle speed is to high. This could be an indication of an intake leak, which could be attributed to the stumbling as the throttle is advanced. I would look there first.

Second I don't know how cold it is where you are, but the A-65 with a Stromberg will stumble some as it transitions through the mid range when it is cold. The cold air makes the already lean transition leaner causing the stumble. Adding carb heat can help. During the transition from idle to full throttle a carburetor naturally runs a little lean. The Stromberg has a weep hole to help the transition, and it could be plugged. You might also be able to help by richening the idle mixture. Marvel uses an accelerator pump to richen the mixture during the transition, and that does a much better job.

[Mark Bowden]

Matt,

I experineced his problem, I have a '45 and it does not have a filter element for the carb, just a screen.

The fault was a dirty carb, if your issue is the same, you will have to dissassemble and clean it. The issue I had was the air flow chamber around the outside main venturi passage and the carb body. This core is interchangable to get to different Stromburg applications. I suspect you will find this passage comtaminated. This passage is a transition passage in use during low to mid RPM operation.

I recommend that if you don't have an airfilter element that you need to dissassemble and clean the carb every 2-4 years, It is really quite simple and then you can check the float height, needle seat wear and fuel flow rate at the same time.

Also don't forget to check and clean the metal thimble screen fuel inlet filter. It's the big 1" nut on the LH side. Don't torque this hard, use Easy Turn or Fuel Lube to help seal.

I'll try and find some photos.

[Mark Bowden]

Here is a photo of the offending passage around the outside core venturi body.


+1 on the high idle speed comments above, it needs to come down.
In trying to solve my issue no amount of messing with the idle speed needle had any effect. Because that circuit was not the issue.

[mikeg]

You will find that if equipped with a Stromberg (Bendix) carb, usually an NAS3A1, the engine will stumble a bit as it transitions from the idle system to the main metering system. That's just the way they are. Usually on final when you want to stretch the glide a little. No accelerator pump, remember? 54 years behind one
Later,
Mike
NC29804