"Non Linear" Fuel Flow

[cwalter]

I have a 1979 M20k with intercooler and Merlyn Wategate. I recently had the fuel pump and mixture controller replaced/overhauled. I've had 2 shops, including a Mooney service center, attempt to set up the fuel system. I get full rich at idle, 24gph full rich at full power, but as I reduce manifold pressure my TIT/EGTs rise. I usually get about 18gph at 2600/30in TIT 1350 in climb and now get about 15-16gph with TIT approaching 1500. In cruise I have to use full mixture to get 75 ROP and at low altitudes it is more like 50 ROP.

This M20K has always been set rich - firewall mixture was over rich on take-off (more than 24gph) and I had to set fuel flow in run-up so I would be at about 24gph on take-off roll. The current shops are setting to the book. Initially one would say it is one of the replaced components, but I'm thinking this may be an issue that had previously been compensated for by this over rich setting.

Has anyone seen this type on non-linearity in the fuel system such that you get full rich at idle and take-off but run best power or less flows in between (all cases firewall mixture)? One of the mechanics suggested that the Merlyn may be driving a higher MP than the mechanical fuel control is "expecting" below full power.

Any thoughts?

[carusoam]

I think the title of your thread might have been better for you, if it were something like...

How to set-up the FF for a K with Engine/Turbo/Controller/Inter-cooler/STCs...

List those details clearly.

Even a C has non-linear FF...

This would probably get closer to the point of where you are trying to go...

Best regards,

-a-

[cwalter]

I (and mechanics) have all the manuals, STCs, service bulletins, etc. from Mooney, Continental, and STC holders to set up the system. I'm asking if anyone has seen problems with M20Ks that "by the book" set ups would result in unacceptable operating characteristics, specifically non linear fuel flows resulting in lean operations between low and high power settings. Non linear fuel flows resulting in normal operating parameters is not what I am referencing.

[jackn]

Sounds like they did set up the FF to 'Book'(SID97-3F) but missed one sentence in the document. If you want to do partial power climbs, set the max FF 5% higher. This would give you around 25.5 at max power, but as youv'e said, you can always lean out a little on TO. I've done this and it works well.

[jackn]

Actually, the LB engine max FF is 24.7 therefore 5% would be 26 gph

[cwalter]

Thanks- I will call that out to the mechanic. The issue exists in cruise as well. I struggle to get 13gph best power flow in various cruise configurations firewall mixture. This happened even after the first mechanic set full rich to about 26gph full power. About 10,000 feet I begin to get some room to dial back mixture to 75 ROP. Given the fuel system was designed with a fixed waste gate I wondered if anyone has seen linearity issues like this due to a Merlyn install.

Thanks for the input!

[jackn]

The fuel system is a closed system and therefore should not vary with altitude. I assume all the UD lines going to the fuel pump are tight. Th Merlyn wastegate has two lines going to it, the UD  and MP pressure lines. It uses the differential to set the boost. I have seen one of those lines sliced by the wastegate's heat shield. May be worth a look.

[N231BN]

I have only had my 231 for five months now but I have worked on twin Cessna's for years. When we had issues with mid range FF it was usually a problem with the idle mixture setting. The TSIO-360 has a different throttle body design from the 520's but the concept should be similar. If the idle pump pressure is set too high you then have to turn down the idle mixture adjustment to get the proper RPM rise. Since the idle mixture adjustment is connected to the throttle it actually regulates fuel flow through the whole range of movement. At take off power it is wide open so the FF is regulated solely by the metered adjustment but once you start pulling the throttle back it starts to restrict the FF again.

My guess would be the gauge they are using for the unmetered pump setting isn't accurate at low pressure. This has fixed the same issue on a couple of Cessna 414's.

[pkellercfii]

I had this problem with my '89 K model (252, TSIO-360-MB engine) when I purchased it. Fuel flow was fine for takeoff, & at idle. At cruise power settings, full rich was just 20-30F ROP at all altitudes I specifically observed it at. The problem turned out to be that the idle fuel pressure was set too low. Be aware that it seemed like the takeoff fuel presssure  looked like it was the sum of the idle and full power fuel settings, and so changing the idle fuel pressure will affect the takeoff fuel pressure & thus TO fuel flow.

 

--Paul Keller

'89K @ RLD

 

[carusoam]

Welcome aboard Gents!

Best regards,

-a-

[N231BN]

Welcome aboard Gents!

Best regards,

-a-

 

Thanks! I have been reading since I picked up the 231 this summer.

 

I had this problem with my '89 K model (252, TSIO-360-MB engine) when I purchased it. Fuel flow was fine for takeoff, & at idle. At cruise power settings, full rich was just 20-30F ROP at all altitudes I specifically observed it at. The problem turned out to be that the idle fuel pressure was set too low. Be aware that it seemed like the takeoff fuel presssure  looked like it was the sum of the idle and full power fuel settings, and so changing the idle fuel pressure will affect the takeoff fuel pressure & thus TO fuel flow.

 

--Paul Keller

'89K @ RLD

 

The idle fuel pressure or "unmetered" fuel pressure is just an adjustable orifice. Unless it is set extremely low it "should" not have an effect on TO fuel flow as the metered aneroid is regulating fuel pressure at TO RPM. Continental SID 97-3 is the procedure for adjusting the pressures. On page 6 it states: "NOTE: Maximum part throttle full rich fuel flow will be achieved by setting the idle rpm (low) unmetered fuel pump pressure to the minimum value specified." This way you need to enrich the idle mixture setting which gives you more fuel through the whole range of throttle movement.

 

If your unmetered pressure was set that low I'm surprised it has enough fuel to idle. Has the engine ever tried to quit on you during landing roll out?

[cwalter]

Great input - thanks eveyone!

[cwalter]

No issues with idle. The engine is full rich at idle. Full idle is very good with rise at cut-off. Idle at 1100 rpm I can dial back a good bit of mixture to get peak rpm. I get 7.8gph for my 1900rpm run-up(firewall mixture) which is what I always set to to get 24gph at T/O. So the issue is between run up and full power. Run up may be low enough power that the variance is not that noticeable.

Thanks again.

[N231BN]

No issues with idle. The engine is full rich at idle. Full idle is very good with rise at cut-off. Idle at 1100 rpm I can dial back a good bit of mixture to get peak rpm. I get 7.8gph for my 1900rpm run-up(firewall mixture) which is what I always set to to get 24gph at T/O. So the issue is between run up and full power. Run up may be low enough power that the variance is not that noticeable.

Thanks again.

 

I would be willing to bet if you have the unmetered fuel pressure readjusted to the low end of the range (6.25psi) and then readjust the idle mixture your problem will go away.