Unable to lean for max RPM, why?

[riobison]

Carb leaning

It's like my engine is always running lean?

In leaning my RV4 w an 0360 and FP prop I'm not able to lean to peak power. I fly out of an air port at 3700 asl and when leaning to max power I'm not able to see on my digital tach the increase in rpm. Nor am I able to hear it. I will keep leaning to until it starts to miss and that part is normal . I took it up to 12,500 asl and pulling 2600 rpm (maximum) and tried to lean again for max power but again I could not see nor hear an increase in RPM to tell me I'm at max power. Instead I would lean until the engine would start to miss. I have left the fuel pump on, checked and cleaned the air filter and inspected everything that I can see externally and everything seems normal. I have 10 hrs on the plane and it has done this since I have had it.

Has anyone any ideas or suggestions on this?

I have adjusted my carb heat and now I can see about a 30 to 40 rpm drop with it on. Not like a Cessna but from what I have read this is normal for the RV's

When apply full power from an idle or go around it will always hesitate before going. This I think is because I don't have an accelerator pump so this is probably a different issue.

Otherwise the engine seems to run good and seems to get off the ground quick at 3700 asl, climbs over 1600 fpm at 2250 rpm and top end is 200 mph. Oil temp is normal. As far as EGT and Cyl head temp the gauges always seem to be on the low side so I don't put a lot of faith in them.
This leaning thing seems a little weird to me.

Thanks

Tim

[mahlon_r]

The hesitation is normal on an O-360. Never saw a MA4-5 carb that didn't have a accelerator pump. But there is no external indication they have one all the parts and linkage are internal.
You say you have an egt gauge.... use it to determine the amount you are rich of peak egt rather than looking for a rise in RPM. If you see a rise of 75 degrees or more from full rich to peak egt, in cruise, at altitudes below 4500 feet the carb isn't all that lean. If you are seeing less than 50 degrees then it could be richer but still Ok in my book. Less than 25 degrees of leaning capability at 4500 asl is too lean and you should pursue it in my opinion.
EGT will give you an excellent indication of how rich or lean you are, by analyzing the spread from full rich to peak at any power setting.
Good Luck,
Mahlon

[riobison]

I put on 500 hrs in my Cardinal 177A with an 0360 and it never hesitated not even once but it had an accelerator pump. My RV4 has an 0360 A1A and there is no accelerator pump, at least no externally. I'm not sure of the carb model but I will pull that data next time the cowl is off.

This is my 1st experimental so some things are new to me and are specific to these aircraft due to their design differences. Most of them are good and some questionable but are acceptable.

I'm not a high time pilot but I do have over 700 hrs in a dozen different models from the big three and I could always hear and see an increase of 50 to 75 rpm when leaning for Max Power on the ground or in the air. This is the 1st plane that I have not been able to do that with. And at 12,500 ft I would really consider that abnormal.

On a side note it is hard to believe the performance out of this thing. Doing circuits and only burning 6 gap is unbelievable (my 7ECA burnt more than this) I was triple checking my numbers as it seemed so low. But being throttled back to stay with the flow of the traffic explains it.

I just completed my 1st 2 hr XC between 6500 and 8500 ft @ 2600 rpm while burning 8.75 gph and 200 mph TAS.

Thinking about it still excites me.

[miyu1975]

Can someone explain to me in simplistic step by step terms how to determine ROP, Peak and LOP using EGT i guess.

[BobTurner]

Ryan: slowly lean (slow so the gauge can catch up) and watch the EGT. It will go up, up, up, then go down, down, until the mixture gets so lean the engine stumbles and/or stops. Put the mixture control in the position where the EGT was a maximum, you are now at peak EGT. Go richer, you are rich of peak. Go leaner, you are lean of peak. Note some carb'ed engines may run rough before even getting to peak.

Tim, it sounds to me like your carb is too lean. There is an idle mixture adjustment screw. Usual ground test is to idle about 800 RPM, full rich. Suddenly pull the mixture to cut off. Just before the engine dies there should be a small, like 50, RPM increase. If none, you need to enrichen. If it's 100 it's too rich.

[riobison]

Hello Bob,

When shutting it off, it does not pickup any rpm what so ever. So I will en richen the idle circuit with that screw.

But with it being on the idle circuit, will that have much if any effect on the mid to top rpm where I spend most of my time and could be the most costly if it's not set up properly?

I'm competent in tuning Hi Perf 2 strokes like in snowmobiles but I'm not overly familiar with these carbs so I'm making some assumptions here that's all.

Thanks

Tim

[BobTurner]

The answer is yes. Although it is set at idle, it will affect the mixture thru the whole range of power.
Let us know how this works out.

[riobison]

thanks

Tim

[PCHunt]

Seems to me that a carb needs to be set up so that it is rich enough at the condition that requires the most fuel flow.

Wouldn't that be at sea level on a cold day?

I have an O-320-E2D, and the carb was too lean on the ground. I ended up drilling the jet, and: ...Much Better! Search for extensive threads on the pros and cons of drilling the jets out.

You can always lean the mixture, but you can't make it any richer if you are already at full rich. And if full rich is not enough, you are running too lean at high power settings at low altitude, such as during takeoff.

You might try leaning at fairly high power on the ground, and see if you can lean to a peak EGT. If not, then you are likely too lean at full rich on the mixture.

Hope this makes sense, and other more expert folks please chime in.

[PaulR]

Originally Posted by BobTurner

The answer is yes. Although it is set at idle, it will affect the mixture thru the whole range of power.
Let us know how this works out.

Is this correct? I have always been under the assumption that just over idle, the idle mixture went out of the circuit and it ran on only the main jet. If that assumption is true, then adjusting the idle mixture would have no effect on higher power settings.

I'd just like to know which is correct.