Marvel Carburetor issues

[6643]

On the Stromberg the rounded bottom of the bowl faces you when you install it. It's been so long since I've done a Marvel I can't say for sure on that one.

[6597]

Stromberg 1.pdf

(159.66 KiB) Downloaded 101 times

[bill_e]

Thank you all for the excellent suggestions! I pointed my A&P toward the filter screen in the carburetor and while that was completely clean, the brand new fuel line we installed at annual was partially blocked. Looks like a manufacturing issue with the inner lining that significantly reduced fuel flow when we did the suggested flow test. It doesn't appear that the line had been kinked or smashed at any point, but was junk regardless. Got a new, new line and fuel flows the way it should now. Hindsight being 20/20, the one thing changed in the system was the thing that was wrong. Now I just need decent weather again to go test it, but should be all set.

[6643]

Hindsight being 20/20, the one thing changed in the system was the thing that was wrong.

It's usually applied to issues in flight, but,
"When things suddenly start to go down hill, undo the last thing you did."

It's not unknown to have the inner lining of the hose get caught on the barb of the fitting during assembly and end up forming a flap that blocks flow.

[V529]

Thank you all for the excellent suggestions! I pointed my A&P toward the filter screen in the carburetor and while that was completely clean, the brand new fuel line we installed at annual was partially blocked. Looks like a manufacturing issue with the inner lining that significantly reduced fuel flow when we did the suggested flow test. It doesn't appear that the line had been kinked or smashed at any point, but was junk regardless. Got a new, new line and fuel flows the way it should now. Hindsight being 20/20, the one thing changed in the system was the thing that was wrong. Now I just need decent weather again to go test it, but should be all set.

Glad you got it resolved..............loss of RPM's are no picnic. Funny how it's the new parts you'd least expect to fail.........that fail.

[bill_e]

I wanted to follow up with the final-final resolution to my carburetor adventure. As it turned out, while the partially blocked fuel line was one problem, it was not the source of the rough-running in the intermediate RPM band. When I finally got weather warm enough to go test it after the fuel line was fixed, I found that it was still extremely rough between ~1900 and 2100 rpm static on the ground. It would start and idle fine, run at full power just fine, but not in that intermediate band. Playing with it there, I found that applying carb heat still immediately correct the rough running. Also, giving it a shot of prime immediately corrected the rough running for a brief period of time. All things pointed to the engine being too lean in that rpm band. Having ruled out the airplane fuel system (measured fuel flow rate from the disconnected fuel line on either tank at more than 38 gal/hr), I chose to overhaul the carburetor. Sent it off to Bolduc Aviation in the Twin Cities and they did great work. Trouble was, it still didn't run smooth in the intermediate RPM band when I got it back. At this point I did a bunch more research here on the forum and got in touch with Victor Grahn. He pointed me to D&G Supply in Niles, MI. They've got a flow bench that allows them to assess the size of the carburetor main jet and fine tune it within the overall tolerance to get me just slightly richer in the intermediate power range without causing it to flood out at takeoff power. They say they had to open it up by about 4%. I just got back from the test flight and everything runs exactly the way I want it to. So thank you, Victor, thank you D&G Supply, and big thanks to my A&P for bearing with me on all this. This forum and community are absolutely essential to keeping these airplanes up and running.

[8413]

bill_e,
Thanks for the feedback. Could come in handy one day.
Josh

[V529]

Bill,

Glad you got it resolved! Yeesh, too many folks have been down this road. (me included!)