Marvel schebler carberator

[Tnathan]

Wanted to get some advice about my carburetor. Around 1500 when doing a mag check the rpm would drop a couple hundred rpm. If I keep advancing the throttle slowly all of a sudden the engine would rev up and zoom to 1800 rpm.

my A&P said it was that I need to let the engine warm up more but it does it warm or after flying. They also said maybe a bad mag but it seems to not show any abnormal mag drop during the mag check.

i adjusted the idle mixture and it seems better. Now it’s more of an rpm flat spot at 1500 where as I advance the throttle the rpm stays constant and then then suddenly zooms to 1800. I read some troubleshooter guides. One seemed to focus on the accelerator pump and idle system. The other one said at above 1000 rpm the idler doesn’t impact performance.

any idea what I can check? Is this an overhaul? Or am I being crazy?

[Gilbert Pierce]

When did this start?
Float level doing low can cause this. Hesitation as it transition from the idle circuit to Venturi sucking fuel thru the power circuit.

[Tnathan]

When did this start?
Float level doing low can cause this. Hesitation as it transition from the idle circuit to Venturi sucking fuel thru the power circuit.

Hi Gilbert, I did ultimately find a thread where you made that same hypothesis. You still got it my friend.

It just recently started. I fidgeted around with the idle mixture and idle which made it a little better, but suspect you may be right. As an owner operator not sure there is much more I can do. Looking like I am in for an overhaul. Did some searching at the links on here and it seems all the recommended rebuild shops are out of business. ��. Could buy a new float but from what I read there is some magic to getting the float hight set right. May need a $100 part but it’s going to cost me 2k to get it in there. ��

[walt.buskey]

It was over ten years ago, but I had these guys do my Marvel-Schebler: https://dgsupply.com/

Speaking of setting the float level: As I understand it, mine was messed up because the previous overhauler had set the float level by bending the float tang (too much, as it turned out) instead of using the thick & thin washers supplied with a M-S rebuild kit to lower the needle seat. My engine would absolutely not shut off on full lean. Carb has been working fine ever since, and I thought their price was quite reasonable, which included a full flow test.

[PACERGUY]

What mags do you have and how long ago was the last service?
DENNY

[leifur]

Now it’s more of an rpm flat spot at 1500 where as I advance the throttle the rpm stays constant and then then suddenly zooms to 1800.

This does not seem to be a Mag problem. It has to be the amount of fuel that is somehow restricted at a certain throttle opening. Do you get full RPM on takeoff and climb? Then your float seems to open alright.

[Vagabondblues]

Wanted to get some advice about my carburetor. Around 1500 when doing a mag check the rpm would drop a couple hundred rpm. If I keep advancing the throttle slowly all of a sudden the engine would rev up and zoom to 1800 rpm.

my A&P said it was that I need to let the engine warm up more but it does it warm or after flying. They also said maybe a bad mag but it seems to not show any abnormal mag drop during the mag check.

i adjusted the idle mixture and it seems better. Now it’s more of an rpm flat spot at 1500 where as I advance the throttle the rpm stays constant and then then suddenly zooms to 1800. I read some troubleshooter guides. One seemed to focus on the accelerator pump and idle system. The other one said at above 1000 rpm the idler doesn’t impact performance.

any idea what I can check? Is this an overhaul? Or am I being crazy?

Application? When I worked the phones at one time in my life I did triage on peoples aircraft problems. Always got ridiculed for not getting make and model, part number. For example the MSA-3SPA on the 290 D has a well documented stumble at 1500 rpm. Also if needed on a taildragger you can level the aircraft, plumb bob in Rh door frame and measure the float level to rule out the low float theory. NPT 90 degree fitting with clear plastic tube.

As a note the three fly holes control idle up to 1500 rpm. See attached Lycoming engine school slide. From 1500 up to full throttle it is all the throttle valve and main discharge nozzle. However inn some instances it may be the accelerator pump. Some things to check would be for worn bushings, loose screws or a worn out shaft on the throttle valve. A worn throttle shaft or bushings leaking off venturi air in the venturi could cause the problems . Throttle valve bushings and shaft are overhaul items, as well as the accelerator pump.

Todd

[PACERGUY]

A leaking primer can also cause some strange symptoms.
DENNY

[Gilbert Pierce]

I have had Bobby at Aircraft carburetor and injection services of Texas just do a repair when an overhaul or rebuild was not necessary.
(940) 648-2277

[Tnathan]

Wow, so awesome. Thank you. I will go test that and see what I get. You all are the best. Thank you.��