flickroll

flickroll

Well Known Member
Today I noticed my idle MP was about 13.5" at 700 RPM, although to be honest I'm not sure what it normally is for me. That does seem a little high though. So I pulled the cowl and checked the air tube clamps, and they were tight. Also checked where the induction pipe bolts to the engine, and they were tight as well. This is on an O-360-A1A.

Is this OK, or does anyone have any ideas on what to troubleshoot next?

Thanks
 
Do you have any tubing that could be collapsing because of the relatively high vacuum (low MP)? This would have the effect of pinching off the tube with the MP between the pinch and the gauge "locked in." If at very low power the gauge isn't reacting to changes in power setting, that could be it.

Dan
 
DanBaier said:
Do you have any tubing that could be collapsing because of the relatively high vacuum (low MP)? This would have the effect of pinching off the tube with the MP between the pinch and the gauge "locked in." If at very low power the gauge isn't reacting to changes in power setting, that could be it.

Dan
Click to expand...

Dan, good thought. I'll take a look. The MP tubing aft of the firewall is very small, perhaps 1/16" ID. Perhaps a larger tube would be better.

For a comparison, do you recall what your idle MP is?
 
Idle MP

In all the constant speed airplanes I've flown, it's usually between 10 and 15 "Hg. That includes IO-360, O-360, O-470, IO-520, TSIO-520
 
That being the case, sounds like it might be OK. I did notice when I SLOWLY advanced the throttle, the MP dipped a little (~0.5" - 1.0") before rising.
 
Just another data point - in my O-360A1A (with carb) MP comes alive at 10" around 750 - 800 RPM of idling
 
Sometimes an overly rich idle mixture can cause higher MP at idle. How much RPM rise do you get when you slowly close the mixture at idle with a warm engine?
 
PCHunt said:
Sometimes an overly rich idle mixture can cause higher MP at idle. How much RPM rise do you get when you slowly close the mixture at idle with a warm engine?
Click to expand...

I had it set at 75 rpm, but have not checked it in a while. I'll do that when I get back in town. Thx
 
Hi Jim
My O-360 A1A powered 8A will cruise at 90 KTs all day long at 13.5". At idle my MP is less than 10" I think you may still have a vacuum leak in your indication sensor plumbing.

Paul Danclovic
Jamestown NC
RV-8A N181SB
 
Paul, thanks. That one sounds very plausible. I've been working in the panel and could have messed something up with the sensor plumbing. What I may do as a test is run new tubing from the engine take-off back to the sensor and see what I get. Thanks for the tip, it really makes sense.
 
Paul, thanks. That one sounds very plausible. I've been working in the panel and could have messed something up with the sensor plumbing. What I may do as a test is run new tubing from the engine take-off back to the sensor and see what I get. Thanks for the tip, it really makes sense.

Question: do you have a snubber in your tubing, and if so, what size is the snubber hole?

Thanks
 
Problem Solved Thanks to My Friends at VAF

Thanks everyone for your input to my MP at idle issue. After work today I went to the hangar and sure enough, I had a major leak at a splice in tubing aft of the firewall. Forward of the firewall I am using a hose with AN fittings from the #3 primer port to an AN bulkhead fitting at the firewall. Aft of the firewall, to the MP sensor, I had been using vinyl tubing from the bulkhead fitting. This had been spliced, and at the splice point is where I found the leak. So today I ran rubber tubing that is same as is used in a P-Mag installation from the bulkhead fitting to the sensor.

I started the engine, and now my MP at idle is less than 10". Here are some numbers:

700 RPM 10.3"
800 RPM 9.3"
900 RPM 8.9"
1000 RPM 9.4"
1100 RPM 9.5"

I also checked my RPM rise when I shut down the engine, and I found that to be 70 RPM.

Thanks so much for the input I received for this problem. VAF is fantastic.
 
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