Engine missing slightly at idle after annual.

How to keep the Cessna 170 flying and airworthy.

Moderators: GAHorn, Karl Towle, Bruce Fenstermacher

Post Reply
akschu
Posts: 8
Joined: Thu Jul 13, 2017 11:39 pm

Engine missing slightly at idle after annual.

Post by akschu »

Airplane:
1949 c-170A
o-300-d swapped in years ago
Not sure model of carb, I'm going to head up to the hangar and look at it later today.

Symptoms:

Engine missing a little bit below 1200ish rpm, runs very strong above 1600 rpm.
Engine idle is pretty poor below 800rpm, before annual it ran fine at 700rpm.
Engine gets 2425 static and hits 2500 on climb out.
Fuel totalizer reads 11gph on climbout.

Changes at annual:
#2 cyl had an exhaust valve that was pitted and leaking, but cyl still got 62/80 compression.
Exhaust valve seat was ground and valve ground so it seals up nicely now, but the cyl was removed and reinstalled.
Fuel flow transducer was installed in line between gascolator and carb.
Fuel line between gascolator and carb replaced with areoquip 303.
Fuel line has an additional s-curve in it to make sweeping radius to make everything line up.

Troubleshooting:
I noticed that I wasn't getting a 25-50 rpm rise at cutoff (1000rpm) so my first theory was that the additional stuff in the fuel system may cause the idle mixture needle to be slightly off and the engine was lean misfiring at idle, but turning the idle mixture counter clockwise 2 whole turns out doesn't seem to help. The transition to full power is poor and I'm still not able to get a rise at cutoff. Normally that adjustment is pretty fine, so that's making me think I got some dirt in the carb, especially since all of that was apart.

Anyone have any thoughts before I pull the carb and look for dirt? I'm thinking I'll need to order a gasket as well. What other gaskets should I get. I was thinking about getting mag gaskets in case I end up there next.

schu
User avatar
GAHorn
Posts: 20967
Joined: Fri Apr 12, 2002 8:45 pm

Re: Engine missing slightly at idle after annual.

Post by GAHorn »

First thing is to try to discern if it’s fuel or ignition.

The cylinder work will have stressed the ignition leads to those spark plugs. Use a tester to make sure the leads are good (no leaks or shorts or open circuits.). The spark plugs should also be tested or replaced.

While the cylinder was off were the valve springs tested/passed? They weaken over time and have specs found in the Ovhl manual.

The fuel line which was replaced (with added “S” turns) ... make certain no low-points exist in the routing of that line. Although the gascolator should trap any water in the fuel passing thru it, a low spot in that supply line can allow water to “pool” and restrict fuel flow.

Gaskets are relatively cheap. Buy any you might think you need to have on-hand.

Was the carburetor worked on during the other engine work? When was it last overhauled? (Operating hours and chronologically also.). Was it’s bowl and screen serviced? Is the accelerator pump correctly working and adjusted? (This would contribute to a halting acceleration during power-application, and air leaks can cause low-RPM missing.)

Check the intake/induction to that cylinder for leaks since those hoses, etc were disturbed.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
akschu
Posts: 8
Joined: Thu Jul 13, 2017 11:39 pm

Re: Engine missing slightly at idle after annual.

Post by akschu »

Thanks for the reply
gahorn wrote:First thing is to try to discern if it’s fuel or ignition.
Agreed, but hard to figure out. Both can cause missing and roughness near idle.
gahorn wrote:The cylinder work will have stressed the ignition leads to those spark plugs. Use a tester to make sure the leads are good (no leaks or shorts or open circuits.). The spark plugs should also be tested or replaced.
The plugs were tested on a tester during annual. I can test the lead easily enough. I'll pull the plugs too. If they look different than the others, that might give it away.
gahorn wrote:While the cylinder was off were the valve springs tested/passed? They weaken over time and have specs found in the Ovhl manual.
Not tested, but I watched my mechanic pull them, and they were pretty stiff. I suppose I could purchase new springs and ask the mechanic to replace them. This seems further down the list.
gahorn wrote:The fuel line which was replaced (with added “S” turns) ... make certain no low-points exist in the routing of that line. Although the gascolator should trap any water in the fuel passing thru it, a low spot in that supply line can allow water to “pool” and restrict fuel flow.
Careful consideration was given to this. The S-turn takes a sweeping uphill to the carb. The fuel flow seems fine.

Gaskets are relatively cheap. Buy any you might think you need to have on-hand.

Was the carburetor worked on during the other engine work? When was it last overhauled? (Operating hours and chronologically also.). Was it’s bowl and screen serviced? Is the accelerator pump correctly working and adjusted? (This would contribute to a halting acceleration during power-application, and air leaks can cause low-RPM missing.)
gahorn wrote:Check the intake/induction to that cylinder for leaks since those hoses, etc were disturbed.
The induction hose was replaced and the screws were tight. I could chalk the airplane, start it, and introduce something that will change the RPM if it leaks. Is there another way to do this that's better?

I'm leaning towards dirt in the carb, that part of the system was disturbed and it seems really odd that I can back the idle mixture screw out 3.5 turns and still not have a good 50rpm rise on shutdown at 1000rpm.

I think I'll go in this order:

Pull plugs to see if they are working and darker
Run the engine for a while and look for a cold cyl with a temp probe
Test harness
Triple check intake leaks.
Remove mixture screw and look for dirt
Remove carb screen and look for dirt
Remove and clean carb

Sound reasonable?
User avatar
eskflyer
Posts: 84
Joined: Sat Feb 02, 2019 1:48 pm

Re: Engine missing slightly at idle after annual.

Post by eskflyer »

You say the plugs were tested on a tester. Was a ohm test done on the plugs?
AA16, SHORTWING and SPAMCAN FLYER, JP
akschu
Posts: 8
Joined: Thu Jul 13, 2017 11:39 pm

Re: Engine missing slightly at idle after annual.

Post by akschu »

Yea, my mechanic has a full on sparking plug tester.
User avatar
c170b53
Posts: 2527
Joined: Mon Apr 22, 2002 8:01 pm

Re: Engine missing slightly at idle after annual.

Post by c170b53 »

You could pull the float bowl plug and look for debris from your new fuel lines but it might just be an induction leak. I’d look carefully at the new cylinder’s intake elbow and for anything associated with the intake spider. Using a hand held temperature probe might also help.
Jim McIntosh..
1953 C170B S/N 25656
02 K1200RS
akschu
Posts: 8
Joined: Thu Jul 13, 2017 11:39 pm

Re: Engine missing slightly at idle after annual.

Post by akschu »

I found one intake elbow was leaking so I fixed that and it runs okay now. It's as good as it was before annual. I still can't get a 25-50 rpm rise on shutdown, but now that I'm thinking about it, my airplane never really did that that well.

What could be wrong with the carb that causes that? Plugged mixture tube?

schu
hilltop170
Posts: 3481
Joined: Sat May 06, 2006 6:05 pm

Re: Engine missing slightly at idle after annual.

Post by hilltop170 »

Sounds like an induction leak to me.

With throttle closed, there is max pressure differential between intake manifold and ambient air pressure which will suck in raw air leaning the mixture causing at least one cylinder to misfire. When throttle is opened, differential drops and raw air entry/lean mixture trails off and cylinder wakes up.

Before you spend any money, a no-cost test will let you know if it is an intake leak.

Remove the cowl and the carb heat hose from the muffler side. Turn on the carb heat. Duct tape the discharge from a leaf blower or shop vac to the loose carb heat hose. Turn on the blower and spray a soapy water mixture from a pump spray bottle onto all of the intake connections. Don’t worry about leakage thru the carb heat valve, the blower will overwhelm it and pressure up the intake. Immediate soap bubble foam will indicate any leak at its source.

Even tight hose clamps and flanges can leak, especially ones that have just been worked on.
Richard Pulley
2014-2016 TIC170A Past President
1951 170A, N1715D, s/n 20158, O-300D
Owned from 1973 to 1984.
Bought again in 2006 after 22 years.
It's not for sale!
User avatar
GAHorn
Posts: 20967
Joined: Fri Apr 12, 2002 8:45 pm

Re: Engine missing slightly at idle after annual.

Post by GAHorn »

gahorn wrote:....Was the carburetor worked on during the other engine work? When was it last overhauled? (Operating hours and chronologically also.). Was it’s bowl and screen serviced? ......
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
akschu
Posts: 8
Joined: Thu Jul 13, 2017 11:39 pm

Re: Engine missing slightly at idle after annual.

Post by akschu »

Carb was not worked on, I don't have the logbooks here, still need to grab those from the hangar. I did remove the screen and cleaned it, but not the bowl.

I ended up getting sick which hosed me from working on it very much, but after thinking about it, I don't recall ever getting a good rise out of it, and the engine ran as well as it did pre annual after I fixed the induction leak, so I decided to mail the carb to ly-con and have it flow tested. If they don't find anything wrong they will mail it back to me, if they do find something, they will fix it, and if they think it needs major work, they can overhaul it.

When I get the carb back, I'll quadruple check the induction. I suspect everything will be good then.

Moral of the story, if you look at those intake runners funny, they leak.

schu
User avatar
GAHorn
Posts: 20967
Joined: Fri Apr 12, 2002 8:45 pm

Re: Engine missing slightly at idle after annual.

Post by GAHorn »

EACH ANNUAL inspection should include removing the drain plug from the carb and catching the drainage and checking it for debris,... and inspecting the inlet screen. The gascolator should be disassembled, and the screen cleaned and new gaskets during reassembly. These actions can detect dirty fuel system issues.

Very few shops that claim to work on carbs can actually “flow check” them on a bench, so it’s a good idea to ask small shops, or to send it to one of the larger facilities such as Consolidated Fuel Systems (now believed to have been acquired by Tempest.). Hatfield Aviation in Houston also did overhauls/flow checks carbs and is a prime contractor for NASA.(dated info)
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
akschu
Posts: 8
Joined: Thu Jul 13, 2017 11:39 pm

Re: Engine missing slightly at idle after annual.

Post by akschu »

Ly-con claims to have a flow bench and can test the carb in all of it's various running states to confirm operation.

I know the gascolator was gone through, but not sure about the carb. I double checked it on my own.

Thanks for the input.

schu
User avatar
GAHorn
Posts: 20967
Joined: Fri Apr 12, 2002 8:45 pm

Re: Engine missing slightly at idle after annual.

Post by GAHorn »

Be VERY CAREFULL when re-installing the carb bowl drain plug. It is NPT and if you tighten it severely it will CRACK the carb bowl. Remember that this is a GRAVITY fuel system and we only need to prevent a leak.... not hold pressure. (Just a very small “dab” of Permatex non-hardening gasket compound on the 2nd row of threads will suffice. NEVER use Teflon Tape in a fuel system.

Hope this helps.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
User avatar
Stankasica
Posts: 16
Joined: Tue Oct 18, 2016 9:08 am

Re: Engine missing slightly at idle after annual.

Post by Stankasica »

akschu wrote:I found one intake elbow was leaking so I fixed that and it runs okay now. It's as good as it was before annual. I still can't get a 25-50 rpm rise on shutdown, but now that I'm thinking about it, my airplane never really did that that well.

What could be wrong with the carb that causes that? Plugged mixture tube?

schu
To get the rpm rise at idle cutoff, you need to adjust the idle mixture.Too much of rpm rise is too rich of a mixture and not enough rpm rise or no rise is too lean.

Stan
Stan Kasica N2458D
KSNC. Chester,Ct
akschu
Posts: 8
Joined: Thu Jul 13, 2017 11:39 pm

Re: Engine missing slightly at idle after annual.

Post by akschu »

Stankasica wrote:
To get the rpm rise at idle cutoff, you need to adjust the idle mixture.Too much of rpm rise is too rich of a mixture and not enough rpm rise or no rise is too lean.

Stan
Yes, I know, but on my airplane screwing it out to 3 turns still doesn't give a rise, but certainly hurts transition. I suspect the idle mixture tube is clogged. I just mailed it to ly-con to have it checked out.
Post Reply