scratching my head (Magneto/EGT problem)

[krines]

I just completed a major on my engine. First flight revealed EGTs of 1225 on all cylinders except #3 which ran about 975. CHT for number 3 and 4 cylinders match at about 300 with OAT 25 F on climb. Switching from L to R mag had no effect on the EGT. Prior to major EGT on #3 ran 1225. Seems to run smooth but I dont trust my ears. Power seems normal. Ideas? Steve

[docfoley]

Try calibrating the sensors or swapping them to see if the problems moves. Someone else my know the answer to this but perhaps a valve out of adjustment is to blame.

[Bruce Fenstermacher]

I would swap sensors and eliminate a bad sensor first.

[GAHorn]

Yep. Depending upon which model transducer you have, it could be a failed unit, or a bad ground on that particular unit, or other indicating problem. Swap leads with another position for comparison.

[FredMa]

If they still show a difference on the ground also, you can use a handheld ir thermometer to see if there is an actual difference or not before wasting time with swapping parts around.

[krines]

EGT probe checks out OK. I am suspecting intake valve not opening far enough leading to rich mixture and cooler temp. Valve lifting rods where all in spec but some where noticeably shorter than others. I guess you could couple that with a relatively weak spring on the hydraulic lifter and come up short. Think I will put in a new rod unless there are other ideas. I am thinking there is no other way to adjust the valve.
Steve

[GAHorn]

Don't arbitrarily change the pushrod.
Use a dial indicator and measure lift.
(I doubt your theory.)
Check exhaust riser, muffler obstruction. Check intakes for collapsed hoses, (I once saw an induction hose with a collapsed liner which behaved like a dead cylinder), and blockage. Check for induction leaks, etc.

[krines]

Thanks George now I have something to do tonight. Steve

[GAHorn]

You know, perhaps a hyd lifter which doesn't pump up could create a problem. So could a cam with worn lobe.

Run the engine, getting it hot, then quickly measure the valve-lift of the pumped-up lifter compared to others.
(If you don't mind the mess, you can even run the engine briefly with the valve cover removed to observe the rocker action.)

[FredMa]

What do you mean by the rods were all within spec but some were noticibly shorter? How much difference are you talking about? Do you mean that the rods length was in spec or the valve clearance that is set by using different length rods was in spec? The limits are listed in the overhaul manual but I don't think there is much difference in the rods. Maybe .030 or .060 difference. George is right you definitely don't just change the rods arbitrarily. You have to determine the valve clearance by collapsing the lifter first as described in the O.H. manual then measure the clearance. If it is more than the limit then a rod can be changed with a longer rod to acheive the proper clearance. Do not do any of this though. You need to determine exactly what the problem is first before shotgunning parts at the airplane in the hope that you will get lucky. That usually just results in wasted money, time and only produces additional frustration.

[Bruce Fenstermacher]

When the overhaul was done the zero lash measure should have been checked for each valve. And of course it should have been within spec. If not a .030 longer rod is available. The spec is quite wide and all the slop is taken up by the hydraulic lifter. If you were within tolerance with zero lash (flat lifter) which is between.030 and .110 between the valve stem and rocker you are good to go and if the valve is not opening enough you have a bad hydraulic unit or cam.

[krines]

Thanks guys, When the engine was overhauled the rods where sent out for yellow tags and where all approved. It may be that the previous overhaul resulted in different length rods being installed to make up for valve lash. I installed factory new continental cylinders. The hydraulic units and cam were yellow tagged so I have to assume they are good. I recently checked intake and exhaust systems for leaks, changed spark plugs, and compression tested - all OK. My mechanic suggests flying the plane as the rings may need to seat so I now have 5 hours on it. I am not totally convinced the EGT probe is OK. Occasionally when I throttle back to idle I see the EGT pop up to the expected 1400. I tested the probe by hoding a blow torch to it and the #1 cylinder probe, both read 1325 but that is only a static test and there could be a ground problem with vibration, going to order a new probe today. I was present during the installation of the lifting rods and there is no doubt in my mind that there were visual length differences albeit quite small. I was assured the hydraulic units would make up the difference. The plan now is to fly the plane ( It flys like it always has and runs smooth) and install a new probe. If that does not solve it I think it would be easy to pop out those rods and check them. That however does not seem like it is the issue as I believe 4 of the rods looked shorter yet I have no problems with the other cylinders. Steve

[Bruce Fenstermacher]

The hydraulic units and cam were yellow tagged so I have to assume they are good.

They met a specification the each unit by themselves but that doesn't mean that any combination of them working together would meet the 0 lash spec. Resurfacing and line boring case halves changes the cylinder deck height. Surfacing the hydraulic lifter body as well as the rocker arms, fresh new valves and fresh new valve seats all change the required length of the pushrod. That is why unless EVERY thing is Factory new, and even then, the zero lash is checked and the appropriate length rod is used as there is no other way to adjust it. You can not assume since they had yellow tags or some of the parts are new that assembled together they will meet spec.

[GAHorn]

Thanks guys, When the engine was overhauled the rods where sent out for yellow tags and where all approved. It may be that the previous overhaul resulted in different length rods being installed to make up for valve lash. ... I was present during the installation of the lifting rods and there is no doubt in my mind that there were visual length differences albeit quite small. ...I believe 4 of the rods looked shorter yet I have no problems with the other cylinders. Steve

OK, Steve... Let's be certain to use the correct terminology.

RODS....being checked and "yellow tagged"....(if someone makes such a statement) probably refers to CONNECTING RODS ...such as connect your pistons to your crankshaft....NOT likely your PUSHRODS...which operate your valve rocker-arms.

Pushrods are not usually "sent out" for recertification...they are usually visually inspected only, unless some specific reason (such as a failed engine due to stuck or dropped valve) has occured.

[krines]

Thanks for helping me with this everyone and I am a novice so I appreciate all the knowledge you can pass along. I do have yellow tags for 12 lifters which I was equating with the push rods but I now understand are actually the cam followers. So George you are right. I will be replacing the EGT probe this weekend and will have more info to follow. Steve