Manifold Pressure Jumping

[aiu0462]

Just flew 3 hours in my "new" to me 260. The manifold gage was jumping back and forth 1". Between 20 and 21". Where do I start to look? Intake gasket on #2, seem to have some black around it. How does an orifice go bad when it's just a restricter hole. Is it located in the line to the MP gage?? Also had a thumping from the front. Felt like a rabbit was hitting it's feet against the firewall. Don't know any other way to explain it. Caused the instrument panel to shake some. Annoying, Need it fixed. 300 hours on a 3 blade Hartzell Simitar. Having it balanced this week-end. Hope that cures it. Comments welcome.
Steve

[Zach Grant L1011jock]

Steve,
Take the MP hose off the back of the instrument. Use a small screwdriver to turn the brass metering screw in a bit. Too far and it will respond too slowly or not at all, too loose, and it will jump on every intake stroke. That is the simple issue.

Thumping in the front end...when were your Lord mounts replaced last? Where you in turbulance? Is your nose gear door secure? Are the lower cowling support rods in place? Is your nose gear system tight it might be jumping up and down in turbulence (no play in drag links, and clevis in good shape. Also pivot points not loose. Need to jack the plane and pressure the nose gear in all directions to look for wear as well as shake). Have you had a good dynamic balance on the prop recently (since the last prop dressing episode)? I know it is not much help, but I hope this gives you a few places to start.

-Zach

[aiu0462]

Zach,
You are awesome. Great places to start. I found a Comanche mechanic today and the nose gear is what he suggested. Also the set screw on the back of the MP gage. Ordered new mains and James is going to put them on next week and while the airplane is in the air, he is going to look over the other items. Prop is getting balanced this week-end. Thanks so much for answering a newbie's questions.

Steve

[Zach Grant L1011jock]

Thats what this site and ICS is all about! All of us were new to the plane at one time. Trouble shooting at a distance is a bit of a shot in the dark, but at least maybe we can get you pointed in the right direction once in a while .
-zach

[aiu0462]

pulled the MP gage out and tried to adjust screw tighter. I say tried because it was frozen. Won't move either way. Put it back and it still jumps a little. New gage in my future. Also, could not find a hole in the MP line coming off #4. It goes in the shop next week for new main tires and a nose gear door check. Balancing the prop this weekend. One other question, is it normal for these engines to "PoP" a lot when you pull the throttle to idle during landing and while practicing stalls?? Always thought that back firing could damage engine.
Thanks,
Steve

[Zach Grant L1011jock]

Steve,
Poping at closed throttle is indicative of an overly rich idle mixture. No it will not hurt the engine, but a big enough pop will hurt the muffler. What is happening is there is too much gas to be burned in the cyl with the available low volume of air, and when it reaches the hot muffler and some additional O2, it ignites and you get the backfire, essentially the muffler can is the new combustion chamber. When it goes in the shop, ask them to lean the idle mixture a bit. They should know how to do that. It is pretty simple!
Good luck, and bring on the questions. None are too big or too small!
-Zach

[Matt Kurke]

Here's a section in the Service Manual covering the topic. I've illustrated the ease of which one can search for anything in the manuals on CD.

Matt - Comanche Gear

EDIT: I see the attachment didn't make it. I'll email Dave.