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Mystery of the Coughing XP-400 Finally Solved!

Chris Engler

Active Member
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For those of you that have been following my "XP-400 Still Coughing" thread, I'm delighted to report I had to start a new thread with a different name since the problem has been identified.

After spending 2 months and changing out fuel servos, mags, exhaust, checking plugs, spider, injectors, confirming fuel flow and pressure, etc. etc., and not finding anything, I spent today at the airport, pulled the engine off the airplane, removed the accessory case and there it was....the cam to crank gear train was "one tooth off".

The engine would start and run fine till 2500 RPM then stumble and sputter rich. Leaning would clean up the coughing and allow 2600 RPM static. Many (including the mechanic on our filed) felt the engine would run rough throughout the entire RPM range with the cam timing off but Birddog from this forum insisted I check as he experienced similar symptoms with an engine with the same issue. BIG high five to Birddog for his persistence in having me check!

Now at least we've got the problem identified and can work on the solution....will be chatting with Superior first thing Monday morning.

Thanks to all those that provided experence, insight, and encouragement as I worked through a frustrating couple of months!
 
The "Mystery of the Coughing XP-400"? I'm glad you solved it, but I was just a little disappointed that you didn't have any help from Jupiter, Pete and Bob.
 
The "Mystery of the Coughing XP-400"? I'm glad you solved it, but I was just a little disappointed that you didn't have any help from Jupiter, Pete and Bob.

I love the reference, but I bet only 1/100 RV'ers will get it.

And now I'm off to the airport in my chauffeur driven Rolls...
 
Had to google...

The Three Investigators is an American juvenile detective book series first ... as the " three investigators" are three boys named Jupiter Jones, Peter Crenshaw and Bob Andrews.
 
how in the heck did that get past the inspector?

Steve - that's the million dollar question. As I noted in one of the previous threads on the topic, my engine was one of 5 "test cell" engines used during the development of the XP-400. The log book that came with the engine shows the following entries (paraphrasing):

1. New engine, test run and in a condition for safe flight

2. 150 hours in the test cell followed by a complete disassembly for inspection and replacement of wear items (bearings and rings). Reassembled in accordance with Superior procedures and work instructions and is in condition for operation.

3. 5 hours in test cell, certification that the engine was manufactured, inspected, and tested in accordance with Superiors Procedures and Work Instructions, and is determined to be in a condition for safe flight.

Each entry of the logbook was signed and certified by Superior.

The one that really has me scratching my head is the last one. My guess is the misalignment of the cam happened during the reassembly following the 150 hours in the test cell...but 5 hours of run time with the cam misaligned and this problem went undetected....I think not!:eek:

For those not familiar with the cam to crank alignment, the photo in my first post on this thread shows the cam gear on top aligned with the idler gear (note the "0" with a "dot" on either side). The crankshaft gear is the one on the bottom in the photo. Note that the "0" on both the idler gear and crank gear should align. On my engine, it's clear that the crank is misaligned relative to the cam by one tooth. As a result, the valves are not in the correct position relative to crank (and piston) positions.
 
I'm glad you've found the smoking gun Chris. When I read your initial post on your engine problems, it really took me back to all the frustration I went through when I was in the same position.

When you get some time I think it would benefit all the VAF future readers if you put the pic of the cam timing into your other post in case someone has the same symptoms in the future.

BTW my engine with the same problem was a carbed 0-320 on a Cherokee 140.

Travis
 
Good point Travis....I added a post to the old thread showing the misaligned cam.

Thanks again....would not have found this without your advice! I hope we can catch up in person some day....dinner will be on me!
 
usermedia.php


For those of you that have been following my "XP-400 Still Coughing" thread, I'm delighted to report I had to start a new thread with a different name since the problem has been identified.

After spending 2 months and changing out fuel servos, mags, exhaust, checking plugs, spider, injectors, confirming fuel flow and pressure, etc. etc., and not finding anything, I spent today at the airport, pulled the engine off the airplane, removed the accessory case and there it was....the cam to crank gear train was "one tooth off".

The engine would start and run fine till 2500 RPM then stumble and sputter rich. Leaning would clean up the coughing and allow 2600 RPM static. Many (including the mechanic on our filed) felt the engine would run rough throughout the entire RPM range with the cam timing off but Birddog from this forum insisted I check as he experienced similar symptoms with an engine with the same issue. BIG high five to Birddog for his persistence in having me check!

Now at least we've got the problem identified and can work on the solution....will be chatting with Superior first thing Monday morning.

Thanks to all those that provided experence, insight, and encouragement as I worked through a frustrating couple of months!
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An easier way to check internal timing.....

Rotate the engine so the piston in number one cylinder is positioned at top dead center on the compression stroke. Remove the rocker box cover from number two cylinder.

Engine timing is checked by first observing the number two cylinder valve rocker arms. Both valves should be closed or nearly closed. The next step is to move the propeller slightly in one direction. Rocker arm motion should be seen as one valve starts to open.
STOP. Now rotate the engine back to the original position with the #1 TDC mark again aligned with the split in the crankshaft halves. Both valves should again be closed or nearly closed. Now move the propeller slightly in the direction opposite from the first movement. Rocker arm motion should again be seen as the other valve starts to open. If the two valves started to open as described with only a small amount of engine movement in each direction, the engine timing is correct.

It may be simpler to rock the propeller slightly with a back and forth motion while observing that first one valve and then the other will start too pen. If movement in either direction exceeds twenty degrees of engine rotation before motion of the rocker arm occurs, the crankshaft to camshaft timing is not correct.
 
Had to google...

The Three Investigators is an American juvenile detective book series first ... as the " three investigators" are three boys named Jupiter Jones, Peter Crenshaw and Bob Andrews.


I didn't catch the original reference, but these were huge books of my youth. My Mom still has a box full of all my books in her attic.
 
It ain't over 'till it's over

Best you check the piston/valve clearance - the slightest lil' touch of a valve to the piston is cause for parts replacement....

Sorry to say, but it might be best to let Superior do a tear-down inspection on their dime.
 
Looks to be retarded a good amount. It's a common practice in race motors to advance the cam a few degrees to find the peak power. You'll no doubt see a noticeable HP increase now that the cam timing is advanced to normal.
 
Best you check the piston/valve clearance - the slightest lil' touch of a valve to the piston is cause for parts replacement....

Sorry to say, but it might be best to let Superior do a tear-down inspection on their dime.

im with mark on this one, nothing against superior, but my confidence in the engine build would be suspect at this time. that should have never gotten by the assembler. did he miss anything else?

bob burns
 
I think I read all of the books that were written prior to 1970 or so ...

I love the reference, but I bet only 1/100 RV'ers will get it.

And now I'm off to the airport in my chauffeur driven Rolls...

Probably my favorite series of books when i was young, except maybe the "We were there" series of historical fiction novels ...
 
Looks to be retarded a good amount. It's a common practice in race motors to advance the cam a few degrees to find the peak power. You'll no doubt see a noticeable HP increase now that the cam timing is advanced to normal.

You usually retard the cam for more top end power but it depends on many factors.
 
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