What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Interesting engine issue

N999BT

Well Known Member
Patron
I have an RV-4 with a carburated O-360 180 hp engine (-A1D). One side is a bendix mag, the other is Lightspeed ignition. Recently, I have noticed that when I am decending at a relatively high speed and reduce the throttle to idle so that the plane decelerates, it feels like the engine quits running, or or sometimes quits for 2-3 seconds, then runs, then quits for 2-3 seconds... If I open the throttle when it is doing this it always runs, but it is nerve wracking. I always wonder if this will be (another) dead stick landing.

Do any of you engine guys have an idea what may be causing this? It would be nice to get it fixed.
 
I have an RV-4 with a carburated O-360 180 hp engine (-A1D). One side is a bendix mag, the other is Lightspeed ignition. Recently, I have noticed that when I am decending at a relatively high speed and reduce the throttle to idle so that the plane decelerates, it feels like the engine quits running, or or sometimes quits for 2-3 seconds, then runs, then quits for 2-3 seconds... If I open the throttle when it is doing this it always runs, but it is nerve wracking. I always wonder if this will be (another) dead stick landing.

Do any of you engine guys have an idea what may be causing this? It would be nice to get it fixed.

You know the old saying. If it hurts when you do that, don't do that.

Seriously, not sure why you would pull power all the way to idle from a cruise setting. Shock cooling aside, I would think that dramatic changes in fuel flow can't be good for the engine.

I have the same configuration as you, and if I need to slow down in a hurry, I will slowly reduce the power to something like 50% while using pitch for airspeed until I can get into the white arc.

But I prefer to plan out my descents so I can come down at a rate that slowly cools the engine.

How's your run-up idle check?

-Matt
402BD
 
Shock cooling

Run up is fine. I agree about the shock cooling, generally. But ATC sometimes keeps me at altitude to late to do a gradual descent. Also, I generally do reduce to 50% or less for a while before chopping. The other day I was in a long descent with some power on, and I was in a hurry because it was getting dark at an airport w/o lights. I got down to about 2000 ft above airport elevation and had to chop to get it to slow down (I have a fixed pitch prop) and make the runway in mountainous terrain. Then it did the surging dance, and I put in a little power and had to slideslip to make the runway.

The thing that most bothers me about this is that it didn't used to do it, and I haven't changed my flying habits.
 
Try flipping the mag switch off for the electronic ignition and see if it stops. If it does, get two new slicks. ;)
 
Last edited:
Something is definitely not right. Reducing power to idle in a descent should not cause the engine to intermittently stop producing power. As a side note, it won't cause shock cooling either:

http://www.avweb.com/news/maint/182883-1.html

The previously posted question about idle mixture is a good one. If it were mine, though, I would take it to an experienced mechanic based on this comment alone:

The thing that most bothers me about this is that it didn't used to do it, and I haven't changed my flying habits.
 
Check your induction tube gaskets. They get hard and brittle and will leak, especially under low throttle settings, as that is when the vacuum in the tubes is the greatest and will cause a lean cylinder. I actually change mine every couple of years. It's amazing to me how brittle they are, and sometimes you can even see fuel stains on the intake tubes.
Also, what's the temps and humidity like? Carb ice could be a potential, too.

Vic
 
Thanks

Check your induction tube gaskets. ... Carb ice could be a potential, too.

Vic


Vic,

You are the second person that has said that to me and the more I think about it the more it makes sense. I will definitely check my gaskets. I don't think it is carb ice because I have been flying in clear dry western desert conditions for the most part. If I find something I will post for the record.
 
Brent,

Here's an idea that's worth what you paid for it:

At idle, most if not all fuel is coming from the idle circuit rather than the main jets. The idle circuit is designed to dispense only fuel required to make the engine run well at low speed (idle).

Now imagine this...
As you start your descent with throttle open, the engine is at reasonably high rpm and thus pumping a relatively large amount of air through it. The throttle plates are open and a good amount of fuel is coming out of the main jets. Now, you close the throttle to idle. The rpm and air flow are still high, however the carb is only supplying the amount of fuel for idle (through the idle circuit). Lots of air, little fuel = very lean mixture and the engine cuts in and out.

To test this, enter your idle descent as above. When the engine starts to cut out, level out to slow the airspeed and engine RPM down. If I'm right, as the RPM decreases the mixture should get richer and the engine should begin to run normally.

A fix might then be to richen the idle mixture a bit.

Just a guess,
David Carr
 
Brent,

Here's an idea that's worth what you paid for it:

At idle, most if not all fuel is coming from the idle circuit rather than the main jets. The idle circuit is designed to dispense only fuel required to make the engine run well at low speed (idle).

Now imagine this...
As you start your descent with throttle open, the engine is at reasonably high rpm and thus pumping a relatively large amount of air through it. The throttle plates are open and a good amount of fuel is coming out of the main jets. Now, you close the throttle to idle. The rpm and air flow are still high, however the carb is only supplying the amount of fuel for idle (through the idle circuit). Lots of air, little fuel = very lean mixture and the engine cuts in and out.

To test this, enter your idle descent as above. When the engine starts to cut out, level out to slow the airspeed and engine RPM down. If I'm right, as the RPM decreases the mixture should get richer and the engine should begin to run normally.

A fix might then be to richen the idle mixture a bit.

Just a guess,
David Carr

Another good suggestion. Thanks!
 
Check your induction tube gaskets. They get hard and brittle and will leak, especially under low throttle settings, as that is when the vacuum in the tubes is the greatest and will cause a lean cylinder. Vic

Good thought, Vic! What we don't always realize is that the MAP is down to around 5" when we pull the throttle and the prop is windmilling. We're used to seeing 10"-12" at idle, and don't realize how much it can drop. If you ever had a vacuum gauge on your car, your idle vacuum was 18"-21", but when you pulled your foot off the gas pedal when going down hill or slowing to a stop, it jumped up to 24"-26"!
 
Now, you close the throttle to idle. The rpm and air flow are still high, however the carb is only supplying the amount of fuel for idle (through the idle circuit). Lots of air, little fuel = very lean mixture and the engine cuts in and out.

Why would you assume that airflow is still high if the throttle plate is closed. The throttle plate controls airflow.
 
If your carb is a Marvel Shebler (sp??) carb, you can get a carb ice detector from Aircraft Spruce:

http://www.aircraftspruce.com/catalog/inpages/icedetect2.php

I've got one on my non-RV airplane and it works great. Recommended.

(No connection other than as a very satisfied user.)


I have the AFS EFIS, and the carb temps have been in the non ice range when this happens. Since my last post I have richened up the idle mixture, 700 RPM with a 50 rpm rise while leaning. It did not fix the problem. At Arlington I talked to Lycoming and Bart at Aerosport. The lycoming guy thought the idle mixture may be too rich, Bart thought the flanges at the inlet to the cylinders may be warped. We shall see.
 
I found the solution(s) I think!

Since my last post I disassembled the intake tubes that go from the sump to the cylinder heads. At least one of them was leaking for sure at the interface between the tube and the head. My guess is that two or three of them were leaking. Then I discovered that three of the the steel tubes that are expanded into the sump were loose; they could be wiggled. One was really loose. They will not fall out due to the design, but there is definitely significant leakage here as well. I sealed the gap with gasket maker (it is fuel proof), installed new gaskets at the tube to head interface, and new rubber tubes for the bottom end.

I test flew it yesterday, I could not duplicate the problem, so for now I consider it solved. It seems to idle better, and it is sure nice to completely chop the throttle in flight and not have the engine cutting in and out. Thanks for all the suggestions!
 
Thanks for posting the solution! I was interested in that issue as I'm planning the same set-up: O-360 with one conventional mag and one Lightspeed.
 
It's Baaaaaack

I flew down to Pullman yesterday to pick up my daughter, and my engine quit about 5 feet off the ground. There was a crosswind so I was working the bank into the wind, opposite rudder thing, and I noticed that I could see the prop blades individually go by. When I touched I added a little throttle, and it just stopped turning. I restarted it on the runway while still rolling, and everything seemed OK. Weird. I had flown through quite a bit of rain on the way down, so I am wondering whether that did it, or what. I did a run up, everything I could think of that would get it to a low RPM on the ground but it ran fine on the way home and landing at COE.

It has been doing the surging thing again lately lately when I chop the power in the pattern. It quits, and then runs and then quits and then runs, but normally as soon as the RPMs get back down to idle, it just idles. I think it is time to get really deep and find out if it is the carb or another intake leak. I'm going to take the carb off and have a look at it. II don't think it is ignition since I have both a standard mag and EI. What else it could be? Puzzled, and a little concerned.
 
I can't help you with your exact problem, but I can encourage you with a capital "E" to not spare any effort in finding and correcting your engine sputtering.

About 10 years ago, a friend who had built a beautiful composite airplane, kept having engine sputtering problems. Many fellow pilots urged him to get it fixed. It was obviously running rich. This engine had come out of a similar homebuilt that crashed with similar engine problems. Bottom line is our friend lost his life due to a rough running engine in the vicinity of his destination airport.

The NTSB found the problem. Someone had installed the wrong part in his fuel injection diaphragm assembly. I realize you don't have fuel injection, but the message is the same. Get it fixed.
 
Last edited:
Having read through all the posts I see others were onto what I was about to write, so here is a changed version of CHECK FOR INDUCTION LEAKS!!!

The list should start with
a. Induction leaks
b. Spark plugs - Gapped correctly!!! Especially at low power settings.
c. leads mags etc...usually is not.
d. Carb settings rarely change so they are last on my list.

As for shock cooling, you can get that when you fly full power into a lake, harbour, ocean or river. Otherwise its an Old Wives Tale. STOP SPREADING THEM.
 
Not sure if this is applicable to aircraft engines but when we used to look for vacuum or intake leaks on our snowmobiles we would rev the engine up and then chop the throttle (to get maximum vacuum) and then spray ether around the outside of the intake/vacuum system. If the engine revs back up again on its own you've got a leak.
 
Check your induction tube gaskets. They get hard and brittle and will leak, especially under low throttle settings, as that is when the vacuum in the tubes is the greatest and will cause a lean cylinder. I actually change mine every couple of years. It's amazing to me how brittle they are, and sometimes you can even see fuel stains on the intake tubes.
Also, what's the temps and humidity like? Carb ice could be a potential, too.

Vic

Same advice if you have the o-ring style intake pipes. I have been battling an intermittent rough running engine for a year. I found 2 plug wires that had worn to the conductor and replaced them. The problem went away for a while but started again this spring. I finally noticed that fuel flow was a little erratic and that as I throttled back the EGT on cylinder 3 would go up instead of down, telling me it was going lean. I pulled the induction pipe and found the o-ring had a couple of black lines around the circumference. Sorry the picture is fuzzy but I could not get the camera to focus. You can see the black lines though.

1256822485_K5xjzJZ-S.jpg


The lines looked harmless. It just looked like a parting line but I worked the line a little with my finger nail and here's what popped up.

1256822421_4pDzLpj-S.jpg


I replaced that o-ring and test flew and the engine did much much better but still was a little rough when I throttled back. This time the EGT was rising a little on #1 cylinder. I'll replace that o-ring tonight and see what happens.

BTW you have to readjust idle speed and probably mixture when you fix these leaks.
 
Have you tried increasing the idle speed slightly? Maybe it needs 750 rpms when hot to stay running? Just a thought, I know it helps with deceleration when it is set low. This is easy and at least worth a try.
 
Back
Top