manifold pressure

i have a rv-4 with 0320 and have been noticing the manifold pressure is lower than usually.
in cruise at 2400 rpm manifold pressure is 21" used to be 24"
and full throttle only around 24"

on the ground not running it always reads around 29" no matter what day.

could the gauge be bad?

thanks
dan carley

Doesn't seem likely that it is the gauge if it will accurately read 29" on the ground. Did you notice any reduction in performance with this change in behavior? Slower airspeed, reduced RPM, reduced fuel flow, etc.?

These readings indicate an increase in vacuum and should coincide with reduced power output. If you are seeing performance symptoms, I would check for blockage before the carb as my first step. Also be sure that throttle cable hasn't moved on you, prohibiting a WOT setting.

Larry

MP

Don't know how you have your MP plumbed but I had a 3 with slow MP response and sticking, the hose connected to the engine had swollen inside and restricted the air, gauge was fine.
Food for thought.
Rt

I had the same thing happen, I was using a aero quip hose. I took the hose off, squirted soap and water over the hose and plugged one end while pressurizing the other with the air hose. Hundreds of micro bubbles appeared everywhere on the hose. I replaced the hose and everything is fine again.

These hoses often accumuate oil in them, try cleaning them out.

manifold gauge

i did not notice any difference in performance same rpm's and same air speeds at rpm settings. just the gauge reads low

danny carley

manifold

i think i found problem carb heat flap was locked in a little open position not sealing probably, think that would be it?


dan carley

dan carley said:

i think i found problem carb heat flap was locked in a little open position not sealing probably, think that would be it?


dan carley

Click to expand...

I don't think so. Seems several have had issues with blockage in the hose causing similar symptoms. Cleaning or replacing that hose would be my first troubleshooting step.

Larry

manifold

i did clean out the line,was nothing in it. today's pressure outside was 29.92 while i had the line off the gauge 28,92. it's off 1 in. i don't think you can adjust the gauge.


dan carley

MP won't match altimeter

dan carley said:

i did clean out the line,was nothing in it. today's pressure outside was 29.92 while i had the line off the gauge 28,92. it's off 1 in. i don't think you can adjust the gauge.


dan carley

Click to expand...

They will only match if you are at sea level. If you are about 1000' above sea level it is correct.

manifold

the airport i'm at is 50 ft above sea level

danny

dan carley said:

i did clean out the line,was nothing in it. today's pressure outside was 29.92 while i had the line off the gauge 28,92. it's off 1 in. i don't think you can adjust the gauge.


dan carley

Click to expand...

I don't believe you can adjust these without factory calibration tools. However, you could consider a tear down and cleaning. I have heard of people reviving these types of instruments by cleaning the dust out of and oiling some of the precision bearing surfaces. This assumes it is an older unit. If it is newer, it might be failing for a different reason.

Given your lack of performance symptoms commensurate with the MP changes, I would be looking at the gauge or hose as problematic.

EDIT: You can go to the auto parts store and rent a vacuum tool (type used for bleeding brakes). These hand pumps typically have a vacuum gauge as well. You can use this pump to apply different amounts of vacuum to the MP gauge and observe the results. You will need to do a bit of research to build a table of values that converts inches of Vacuum to inches of Atmospheric pressure (This is what your MP gauge measures).

Larry