Idle Mixture Adjustment *Help*

Jbon · #1 02-13-2016, 05:22 PM

I noticed that my new (to me) RV6A with carburated O-360 is adjusted a little lean on the ground. When pulling the mixture to cutoff, there is no rise in RPM. Other than that, the engine starts and runs fine. I made the normal adjustment until I saw a 35-40 RPM drop, but I was a bit concerned because to get that adjustment, it took about 5 or so turns from full in. It was enough that the needle was slightly wiggly (though it takes about 15 turns to unscrew it).

The references I've looked at indicate that the proper adjustment will normally be found with 1-1/2 to 2 turns. So what's the deal?

For what it's worth, I have an Electroair EIS in place of the right mag.

Thanks for the input.

aerhed · #2 02-13-2016, 05:53 PM

Check for induction leaks first.

Jbon · #3 02-13-2016, 07:58 PM

Quote:

Originally Posted by aerhed

Check for induction leaks first.

Okay. Can you give me the short course on how to best go about this? Also, what might be other symptoms of an induction leak?

Thanks.

aerhed · #4 02-13-2016, 10:14 PM

One symptom is lean mixture at low manifold pressure. The flipside is a mixture that gets richer as manifold pressure increases. At a manifold pressure several inches below ambient while rich of peak, you should see higher than usual egt on a cylinder with a leak. Unless they all leak. Most common leak is the gaskets at the cylinder flanges. Look for yuck around them. If they are very old or have a lot of time, change them anyway. Under a buck apiece. Next leak source is the connector hoses near the sump. Next up is the carb throttle shaft or carb gasket. A carb with a rattly shaft will suck air from the sides, that's air that doesn't go past the idle ports. Less common is a leaky swage in the tubes going into the sump. Any of these will make you have to extend the idle screw. So will junk in the idle circuit of the carb, but you save that for last.

Jbon · #5 02-14-2016, 10:49 PM

Thanks. Good info. By cylinder flanges, do you mean where the intake tube attaches? My engine is low time, only about 220 hrs, but it has sat, and those gaskets are probably 19 years old. Is there any trick to help confirm a leak, such as spraying Marvel Mystery Oil, or WD-40 and looking for smoke?

The engine runs very smooth at 1000 RPM. . . A little less so at 700. Would raising the idle a bit influence the idle mixture?

BTW, I'm only monitoring one cylinder's EGT

Thanks again.

aerhed · #6 02-15-2016, 01:59 AM

Some advocate using starting fluid while idling (tied). You'll definitely know if you spray a leak spot, but it's dangerous. You don't want to raise your idle. In fact you should be able to get closer to six hundred. And yes, I meant where the intake tubes bolt to the cylinder for the gasket leak.

Jbon · #7 02-15-2016, 07:34 AM

Thanks again. Given the age of these gaskets, I'm thinking its time to change them anyway.

brooksrv6 · #8 02-15-2016, 05:25 PM

To test for induction leaks use a shop Vac that the hose can be set to BLOW. Run it a while to clear the hose and stick it into the air box intake and check your Induction tubes by spraying with soapy water.

Jbon · #9 02-15-2016, 07:44 PM

Quote:

Originally Posted by brooksrv6

To test for induction leaks use a shop Vac that the hose can be set to BLOW. Run it a while to clear the hose and stick it into the air box intake and check your Induction tubes by spraying with soapy water.

Cool. Great idea. I'll try it.

lr172 · #10 02-21-2016, 10:38 AM

I had a similar situation. I started at 2 1/2 turns out. I started backing out 1/2 turn each time I had the cowling off. I also had a stumble on acceleration. I stopped adjusting when the engine performed well. I am probably four turns out now. I have no acceleration stumble and no issues on deceleration. My plugs have no deposits and never have issues with mag checks (i.e. deposit build up at idle). I still show no rise in RPM when shutting down.

I may be a tad lean, but I think that is ok. I'll leave it here until I experience noticeable problems. I may back it out another 1/4 turn at next oil change. Apparently it is not uncommon for the adjustment to be 4-5 turns out. Just be sure to stretch your spring or add washers to keep tension on the screw.

Just my experience here. Also, my GAMI spread is decent for a carb setup, so I don't believe that I have an induction leak.

Larry