Thorp Air Command -

78WG Engine Woes
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Author:  Andy475 [ Fri Jun 22, 2018 11:18 pm ]
Post subject:  78WG Engine Woes

Hey Everyone,

Had an interesting engine issue develop recently in 78WG. When attempting a touch and go, immediately after pouring on the coals, the engine would momentarily drop 2-300 rpm then spin back up. There was noticeable roughness in the pattern with the mixture full rich and the problem repeated itself when run up on the ground. I had the carb pulled and overhauled it, but everything checked out.

I took it for another spin today, and prior to flight could not repeat the issue. Flew it for 30 minutes and returned to the pattern, only to have the same issue. I did a mag check post flight on the ground and it checked out at 1800, but at max, mixture full rich it would drop from 2100 to 1900,then slowly continue to fall. If I leaned it out and the mags both check out with about a 75-125 rpm drop. Definitely seems to be running too rich across the board, but only when it’s warmed up. I’m stumped. Here’s some other data:

DA: less than 2500’
Engine: Lycoming O-320
Mag timing check out good
Some surging full rich, idle power on final
Only happens when warmed up

Any thoughts? Trying to get this sorted for Oshkosh....


Author:  1albee [ Sat Jun 23, 2018 6:17 am ]
Post subject:  Re: 78WG Engine Woes

Have you checked the condition of your carb heat box / valve and rigging ? Ensure that it is not leaking hot air into the intake system when its in the normal operating position. If it is, you could get an enriched mixture due to the less dense hot air being directed into the intake. Just a thought.


Author:  Jeff J [ Sat Jun 23, 2018 8:15 am ]
Post subject:  Re: 78WG Engine Woes

Just drop rpm or did the engine shake too? I had cracked jug on an O-320 once that would pass a compression check and idle great but when it got warmed up good the engine lost power and shook. Reduce power and it ran great again.

Author:  Andy475 [ Sat Jun 23, 2018 8:38 am ]
Post subject:  Re: 78WG Engine Woes

It’s makes good power when leaned with minimal shaking

Author:  Unclerap [ Sat Jun 23, 2018 2:59 pm ]
Post subject:  Re: 78WG Engine Woes

"Makes good power when leaned" is a clue. Lycomings are set for 20% overrich at sea level. I think what happened is you loaded up the cyls with an over rich mixture in the pattern and it cleared shortly after you applied full power. You need to pull the red knob more aggressively IMHO. Only time full rich is start up, takeoff-climbout, and short final in prep for the go around. +1 on the carb heat box, known weakness on the lycs unless you built your own.


Author:  fytrplt [ Sat Jun 23, 2018 6:20 pm ]
Post subject:  Re: 78WG Engine Woes

Concur with leaning regimen. Long time Lyc rep, Paul McBride, told me that many years ago.

Author:  james peran [ Sat Jun 23, 2018 7:30 pm ]
Post subject:  Re: 78WG Engine Woes

Hmm i have find the mixture adjuster shaft seizing in the tube on the carburetor. took me a while to find that problem. Its spring loaded. From the outside the arm looks like operating normal for the
mixture adjuster but inside its sticking .BTW there was no witness marks .. J.P.

Author:  d.grimm [ Sun Jun 24, 2018 9:37 am ]
Post subject:  Re: 78WG Engine Woes

Carb overhaul?
1. Single piece Venturi?
2. Flow bench checked?
3. High fuel consumption?
4. Mixture has no effect until nearly the end of the travel?
Experienced this in my Pacer, a proper overhaul with a flow bench check fixed everything.

Author:  Derek Fritschle [ Sun Jun 24, 2018 10:54 am ]
Post subject:  Re: 78WG Engine Woes

Andy, sent you a PM


Author:  Doug Frantz [ Sun Jun 24, 2018 9:22 pm ]
Post subject:  Re: 78WG Engine Woes

I have a T-18 with an 0-320 E2D that I have owned since 1983, and have flown it about 1200 hours. I have always had to lean the engine about 1/2 " on the mixture control during takeoff and climb to get max rpm. After much searching I found out that different model Marvel carbs have the float chamber vent hole in different locations. The one on my engine (which came off of a '69 Cardinal, and 10- number that ends in -32) is inside the carb throat at the rear. I believe that is the cause of the over-rich condition. At full throttle, the high velocity air coming down the induction box and turning the corner to enter the carb is slightly pressurizing the float chamber, causing the fuel flow to be higher. Add some airspeed ram air to this and it gets worse. This was discussed in the old newsletters, and one solution was to get a 10-5009 carb which I believe was used on Mooneys. I bought one, and the float vent hole is indeed in a different location, on the base of the carb completely outside the throat. This would eliminate the possibility of ram air effect pressurizing the float chamber. I never had the carb overhauled and installed it, as I got used to leaning the engine, and it has run fine that way the whole time. So, I can't really say if that would fix my problem, but it is food for thought. Also, are you gravity feed, or does the engine have a mechanical fuel pump? Excessive fuel pressure could cause higher fuel level in the float bowl and over rich condition. Main question is did anything change on the airplane about the time the problem appeared? Good luck!

Author:  Andy475 [ Sun Jun 24, 2018 11:11 pm ]
Post subject:  Re: 78WG Engine Woes

Alright Chaps,

Here is what we've tried, in order:

1. Rebuilt Carb, check good on flow bench, no issues noted. Its a MS MA-4SPA 10-3678
2. Primer Isolated, no change
3. Airbox cleaned, filter changed, no issues noted
4. Plugs clean, checked resistance and bomb tested, other than being gummed up, no issues noted
5. Visually inspected engine mount, induction system, everything firewall foreword
6. Aggressively leaned in pattern, problem continued

Fuel system is gravity fed. I've flown it 50 hours since November (when I acquired it), and this is the first time I've had issues with it. I've reached out the previous owner to see if he's had any history with these issues.

Test flew it again today after tackling the plugs, issue seems to be getting worst (or my imagination is getting the better of me) with occasional roughness when leaned too. The intent was to eliminate the plug and mixture theories, and I think I'm satisfied that neither of those were the issue. Back on the ground, the mag checks are dropping off more than before. Everything is pointing towards mags now. It's just strange that both are going at the rate, at the same time....

Moving forward: the magnetos (slicks) are high time and have been untouched since 2003. Beyond overhauling/replacing those, I'm at a loss. Hopefully we'll test flight new mags next weekend. Oshkosh is rapidly approaching...


Author:  Jeff J [ Mon Jun 25, 2018 8:03 am ]
Post subject:  Re: 78WG Engine Woes

Some of the slicks were having issues for a while but it would be odd for both mags to go at the same time. Have you done a mag check while the problem is occurring? There is a hazard in doing a check that way. If the engine quits, it will pump the exhaust full of an explosive fuel mixture that could cause damage when lit by turning the mags back on (I liked to make my motorcycle backfire that way when I was a kid).

Author:  Jeff J [ Mon Jun 25, 2018 5:31 pm ]
Post subject:  Re: 78WG Engine Woes

A bad impulse coupler might be able to create this problem but I would expect the problem to be a lot worse. Slicks had a coil AD but, I would expect a different problem if it was a coil failing (hard to start hot). I haven’t been able to find any silver bullets in any of my troubleshooting info for you.

Author:  Fraser MacPhee [ Mon Jun 25, 2018 8:56 pm ]
Post subject:  Re: 78WG Engine Woes

Induction leak? I had one on my 0-320 A2B - changed the induction tube gaskets all round - it ran better - having said that, I have replace two of the cylinders in the past 5 years due to valve wear (rough running)- they were old chrome cylinders that needed a boneyard to feel welcome.

Author:  Doug Frantz [ Tue Jun 26, 2018 11:02 pm ]
Post subject:  Re: 78WG Engine Woes

As Colombo once said, "One more thing." Have you checked the compression to see if there is a jug going down? Didn't see that in the posts, but I could have missed it. BTW, you do have the same carb that I have. Also look to see that the fuel tank vent is unobstructed. Should be a forward facing 3/8' tube on the bottom of the airplane center fuselage between the gear.

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