I noticed some EGT fluctuations last weekend on the front cylinder. Friday I did a quick lap around the pattern and the EGT for the front cylinder was about 100F higher than normal at full power. When I tried to the bring the power back it started climbing towards the limits. I kept full power in until bringing it back to land. At no time did the limits get exceeded.

Yesterday I checked a few things that seemed the most obvious. I found a couple slightly loose buttons (never knew what they were called) on a couple plugs. They were tightened. The plugs themselves looked fine and the gap was correct. I found one spot on the exhaust manifold that looked like it might be a small leak. I cleaned up the smudge and re-torqued the manifold bolts.

Another flight around the pattern showed that nothing had changed. Still about 100F over normal at full power. RPM's were right and didn't drop off. During the runup, both sides of the ignition ran smoothly and dropped the same RPM. CHTs were normal for both cylinders. The suspect spot on the exhaust manifold was still clean. If it was a small leak, it was fixed by re-torquing the bolts and wasn't the root problem.

My next step is to pull the carb and intake manifold to check the seals there. I'll also open up the carb and make sure the jet is not partially clogged, though I think that would have more general ramifications.

I've had an inline spade connector for the EGT probe get loose before. When that happens, the EIS shows the temp dropping, not increasing. Once the connector was tracked down and snugged up, it returned to normal. I don't think a faulty wire would make it read high.

Is there anything I've missed that could cause just one cylinder to lean out? The head looks good and is clean around the seal. I didn't notice any cracks or bad seals around the spark plugs either. The exhaust manifold is in good shape and not showing any cracks. Same for the pipe and muffler.

I really think it must be an intake manifold leak, but let me know of anything else that could do this.

Thanks,
Ian

Ian, we need more info.  What engine, how many hours on it, age, etc.

Dave

Hirth 2702, free aircooled. Single Mikuni carb. 115 hours over about 4 years. Rings decarboned about 10 hours ago, though there wasn't much there. Fuel pump, filters, plugs and exhaust gaskets all changed then as well.

The engine has been totally consistent. I can tell when the plugs need cleaning by changes in the EGTs. I had a minor exhaust leak at around 20 hours, that showed up immediately in the EGTs and was corrected with the correct torque. The torque had let loose a little since it was a new engine.

Limits for this engine are 535F for CHTs and 1256F for EGTs. WOT climb (5390-5400 RPM) runs about 1075F  EGT and 425-475CHT, depending on the OAT. On the rare 100+ day, it runs just under 500F, but cools off with a shallower climb and/or reduced power.

Cruise (5150 -5200 RPM) temps run about 1100-1125 EGT and 390-425 CHT. Mid-range EGT temps peak at about 4500 RPM around 1175 or so. Less if you keep the prop loaded.

Differential between cylinders is about 10-20F for CHTs, with the back cylinder being slightly warmer. Diff on EGTs is usually less than 25F and often within a few degrees.

I'll see a slight shift up in the EGTs in cooler weather and the mid-range peak will get a little more sensitive, but nothing bad.

Plugs stay pretty clean with auto gas and BlueMax mixed at 100:1. The rear plug in the rear cylinder will foul first, but only slightly faster than the others. Running 100LL will load up the plugs faster because of the lead, but it has no discernable effect on temps or power.

I use the tapered copper (or brass?) mesh type of fuel filters. They are clear and get checked every preflight for contaminants and water. There are two run in parallel after a Mikuni 1-in / 2-out fuel pump. They catch very fine particles, but I really don't see much junk loading them up. They get changed every 50 hours no matter what they look like. All fuel lines were replaced this last winter when the plane got recovered and painted. The old lines were starting to get pretty stiff. The crankcase pulse line was also replaced this year.

Is that enough info?

Ian

This reminds me of a similar problem I had with a 447. I replaced the seals and cured the it.

Yup, that's what I was gonna suggest, too....maybe you've got a leaking crankshaft seal.

Dave

When you check your carb don't forget to check the idle jet and airbleed.

Ian: Try swapping the sensors, or the gages first. We shake-um pretty hard in these things. If you have electronic engine monitor, just try the sensors before going two deep. Charlie

Ian: another subject. Do You run a Powerfin, and did You drill Your prop bolts?  Charlie

I'm heading out there tonight to take a look at the intake and carb side of things.

I'm using a Powerfin. I bought prop bolts that were already drilled for safety wire. I'm real curious as to how this could effect things.

I'm pretty confident in the EGT sensors. I just re-did the wiring and all this winter. I'll swap the leads and see if the problem switches cylinders on the EIS.

A front seal leak would make sense with the problems I'm seeing. I won't have time tonight, but I can pull the prop and gear box to take a better look later this week.

Ian

Ian:  Sorry. Did'nt mean it had anything to do with heat. Just wanted to know for My own info. I drilled my cap head bolts. Have'nt quite figured out how to change topics yet.  Just put up with us old guys awhile. Hope You find the problem.  Charlie

Instead of pulling the prop. since you will have the carb off any way, why not just presure test? If leak down is unacceptable then you can pull the gearbox.

I had a similar problem and it was the rubber vibration isolators getting worn on the panel mounts. A slight vibration was causing the gage to read high. I unbolted the gage and held it in my hand and the problem went away. It may not have anything to do with your problem but is very easy to check. I switched to a digital setup and the problem was solved.

Jim Wright,  Building an RV-9A and wishing I still I had my Hi-Max

(Posted to wrong thread the first time)

I still haven't been able to get out to the hangar this week. Work keeps interferring (I need to get my priorities straightened out).

I'm guessing that it was an analog guage that had the vibration problem. I'm pretty sure my EIS isn't susceptible to that.

I talked to Matt at RecPower yesterday. He pretty much confirmed what I was thinking and what most of you guys have suggested. He did eliminate the carb and rubber carb mount. All the problems he has seen with those resulted in fluctuations in both cylinders. That's what I would expect, but I have seen other types of engines get asymetrical flow from carb issues.

The most telling thing is that the rear cylinder hasn't been effected at all. This really points to seals specific to the front cylinder. If the front seal is the problem, I bet the leak is pretty close to the cylinder base. If it was opposite the cylinder, I bet I'd see the rear cylinder starting to act up too.

I'll let you guys know what I find out. Until I get out there and pick up some wrenches, it's just speculation.

Thanks,
Ian