[Jbon]

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]

Check for induction leaks first.

 

[Jbon]

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]

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]

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]

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]

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

 

[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.

 

[Jbon]

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]

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

 

[Jbon]

. . . A carb with a rattly shaft will suck air from the sides, that's air that doesn't go past the idle ports.

Okay, I finally got out to the airport today to do some more detective work. I hooked up my (clean) shopvac and pressurized the air inlet. The intake tubes were tight, but I'll be changing the gaskets anyway because of age.

What was more troubling was a pretty good leak around the throttle butterfly shaft where it protrudes from the opposite side of the carb. The shaft is not wobbly, but I can push it in and out slightly when the butterfly is wide open. Doesn't move when shut. The air leak varied a bit, but was quite noticeable. I also saw much smaller bubbles around the mixture screw.

I described this to one of our local A&P's, and he didn't seem to think it was a big deal, but your comment on how a leak could affect the idle circuit makes sense to me. Again, the engine idles great at around 1000 RPM, maybe a bit rougher at 700, but it bothers me that I'm not seeing the RPM rise I should at shutdown, and that when I made the large adjustment, to get a 40 RPM rise, it didn't stick.

Do you think this is the problem?

Anyone know a good carburetor shop in the DFW area?

 

[lr172]

Okay, I finally got out to the airport today to do some more detective work. I hooked up my (clean) shopvac and pressurized the air inlet. The intake tubes were tight, but I'll be changing the gaskets anyway because of age.

What was more troubling was a pretty good leak around the throttle butterfly shaft where it protrudes from the opposite side of the carb. The shaft is not wobbly, but I can push it in and out slightly when the butterfly is wide open. Doesn't move when shut. The air leak varied a bit, but was quite noticeable. I also saw much smaller bubbles around the mixture screw.

I described this to one of our local A&P's, and he didn't seem to think it was a big deal, but your comment on how a leak could affect the idle circuit makes sense to me. Again, the engine idles great at around 1000 RPM, maybe a bit rougher at 700, but it bothers me that I'm not seeing the RPM rise I should at shutdown, and that when I made the large adjustment, to get a 40 RPM rise, it didn't stick.

Do you think this is the problem?

Anyone know a good carburetor shop in the DFW area?

The shaft bushing interface is not designed to be air tight and contains no seal. Air leak there is expect and is not causing a problem. the shaft has no mechanism to stop side to side movement other than the butterfly valve in the throat. You should have side to side play.

Do some more research on this before you get too concerned. Many mechanics tune idle by sound and performance. If your engine is running well don't concern yourself too much. Go research mixture adjustment procedures. I have seen some good articles with more detailed advice. It may just be the testing procedure that you are using.

Larry

 

[bruceh]

Mike Busch has a good article on in-flight diagnostics for finding/diagnosing induction leaks here. It assumes you have full EGT information available from your EFIS.

 

[aerhed]

Yah, they're not airtight. If its worn bad you will see it most at like 3/4 throttle. Not talking slight movement, but actual rattle the shaft with your fingertips worn. The bubbles around the screw are from your air supply blowing back through the idle ports (little tiny holes by the throttle plate edge at idle).

 

[Jbon]

Thanks guys, I'll do some more research on mixture. Good to know that leakage there is normal. The engine seems to run okay. No stumble on acceleration, or fouled plugs.

What started this whole thing was trying to figure out why I'm getting a 150 RPM drop on the Electroair ignition during run up, vs 50 on the mag. The guy at Electroair indicated that because of the higher efficiency of the EIS, I shouldn't see much of any drop. When I mentioned my mixture issue, he said to get that squared away first. Again, plugs look good. The leads look pretty good too, but they are nearly 20 years old. Any thoughts there?

 

[lr172]

What started this whole thing was trying to figure out why I'm getting a 150 RPM drop on the Electroair ignition during run up, vs 50 on the mag.

Idle mixture would be pretty far down the list of likely things to cause that. If taking the mag off-line, I would expect a small to minimal drop in RPM. THe first place that I would be looking at is timing (Mag and EI) and the second would be testing each plug and wire on both sides.

Larry

 

[Jbon]

Makes sense. The bottom plugs (EIS) look good, as do the leads, though they are 19 years old and have spent a lot of that time sitting. Perhaps swapping the upper/lower plugs would be a good place to start, followed by replacing the leads.

 

[lr172]

Makes sense. The bottom plugs (EIS) look good, as do the leads, though they are 19 years old and have spent a lot of that time sitting. Perhaps swapping the upper/lower plugs would be a good place to start, followed by replacing the leads.

Unlike choosing a wife, looks mean nothing here. By test, I mean with an Ohmmeter.

Larry

 

[lr172]

Idle mixture would be pretty far down the list of likely things to cause that. If taking the mag off-line, I would expect a small to minimal drop in RPM. THe first place that I would be looking at is timing (Mag and EI) and the second would be testing each plug and wire on both sides.

Larry

Just thought about this more. Are these mag drops done at 1600-1800 RPM? Idle mixture has zero impact after about 1200 RPM. I would consider not taking too much advice from the EI guy who suggested you sort out your idle mixture for behavior at 1800 RPM.

Larry

 

[Jbon]

Agreed. That makes sense. Next step is to break out the ohm meter. These leads are "Autolite". Carbon, I assume. What resistance per foot should I be looking for?

Thanks

 

[lr172]

Agreed. That makes sense. Next step is to break out the ohm meter. These leads are "Autolite". Carbon, I assume. What resistance per foot should I be looking for?

Thanks

I wouldn't worry too much about the EI harness and plugs. Any decent setup should have plenty of power to overcome problems. If I remember, most auto leads werr 500 ohms/foot and the plugs are 5K. Also, you are getting the sizeable RPM drop when you turn off the EI, correct? That points to the Mag.

I would worry about the Mag side. I don't know the spec for the leads, you'll ahve to look that up. You can do all four and any major difference would be a problem. The plugs should be 1K and they often fail and give high resistance. The mags don't pack that much of an electrical punch and high resistance in the plugs will cause a weak spark.

While I don't have an EI installed yet, most seem to report a very small or non-existant drop when the turn off the Mag. This makes sense as the advanced timing of the Mag ends up negating the need for spark from the Mag. The fact that you have a 50 RPM drop makes me think that your mag timing is advanced beyond spec.

 

[mahlon_r]

Try leaning the engine manually at mag check rpm and then doing the check. Or do the check at a much higher rpm say 2200 or so. If it is better it isn't a ignition issue. Are you positive you are running on the mag and not the EI when you are seeing the 50 rpm drop? It is easy to get mag switch confusion especially with toggles and think you are running on the right side and really running on the left.
Good Luck,
Mahlon

 

[Jbon]

Also, you are getting the sizeable RPM drop when you turn off the EI, correct? That points to the Mag.

No, actually I'm getting the 150 RPM drop when turning off the mag, 50 RPM when turning off the EIS.

Thanks

 

[Jbon]

Try leaning the engine manually at mag check rpm and then doing the check. Or do the check at a much higher rpm say 2200 or so. If it is better it isn't a ignition issue. Are you positive you are running on the mag and not the EI when you are seeing the 50 rpm drop? It is easy to get mag switch confusion especially with toggles and think you are running on the right side and really running on the left.
Good Luck,
Mahlon

I have tried the check at a higher RPM. No difference. Yes I'm sure of the switch orientation.

Thanks for your input

 

[lr172]

No, actually I'm getting the 150 RPM drop when turning off the mag, 50 RPM when turning off the EIS.

Thanks

Sorry then, you are correct in focusing on the EI. I would put an ohmmeter on each wire (let us know the results) and just replace the plugs (the auto plugs should be cheap). Have you confirmed the timing on the EI recently? This needs to be done for both the mag and the EI. If those all check out, you should then check the coil as well as proper grounding of all EI components (the coil may not need to be grounded if it is a wasted spark style). You will need to go to your EI to found out the proper resistance levels for that coil. Failing coils can often be identified by checking the resistance levels against the spec.

 

[Jbon]

Okay, I got some readings on the plug wires. Here's what I found.

#1 - 3.75' measured 21k ohms. Should be 18.75
#2 - 3.25' measured 10.3k ohms. Should be 16.25
#3 - 2.7' measured 7.95k ohms. Should be 13.5
#4 - 3.1' measured 10.8k ohms. Should be 15.5

Looks like I need new plug wires!

 

[sirlegin]

I have a dual lightspeed and the mag drop on both sides were 30-40. I reset the timing and now I have 0-10 drop on each side. I'm also going with iridium plugs

 

[Jbon]

I have a dual lightspeed and the mag drop on both sides were 30-40. I reset the timing and now I have 0-10 drop on each side. I'm also going with iridium plugs

Speaking of timing. Electroair has you using an automotive timing light to fine tune the timing. Because of the engine baffling being in the way, you'd have to stand in front of the propeller with the engine running to get this reading. Not my idea of fun!

 

[lr172]

Speaking of timing. Electroair has you using an automotive timing light to fine tune the timing. Because of the engine baffling being in the way, you'd have to stand in front of the propeller with the engine running to get this reading. Not my idea of fun!

I am adding a custom EI and need to do the same. Don't stand in front of the prop. Use the timing mark on the back of the flywheel. Put the engine at TDC and attach a coat hanger to the engine case and bend it so that it points to the TDC mark. That is your new reference and you can shine the light on it from behind the prop. There is also a 20/25* mark on the back if they have you time to that reference.

Larry