Is this a carb issue?

[BobB]

My A4 has developed an annoying symptom: after normal starting and running at idle, if I throttle-up under load, as I increase the throttle the engine speed will reach a max and go no further (and below what I believe to be max rpm), despite increasing throttle. To give you a better idea of what I mean: max rpm is reach with my throttle lever at straight-up, even though it can be rotated ca. 90 degrees more. Occasionally, after the engine runs under load at its max rpm, it will lose power, cough, and the power will decrease. If I don't let off on the throttle, take it out of gear, and pump the throttle a few times, the engine will die. Restart requires pumping the throttle and starting at a throttle setting beyond normal idle. The engine will then run normally at idle (no load) and under load, until it starts losing power again (maybe 10-15 min intervals). It sounds to me like the engine dies of lack of fuel. I can completely eliminate the symptom by simply running under load at a lower rpm (cruising at ca. 3.5-4 knots is fine; cruising at 5 knots or greater causes the symptom).

My A4 has the original zenith carb (no mods) and the original fuel pump (no mods). In the 10 years I've owned my E27 I have not worked on the carb, so I don't know if it ever has been rebuilt. My thought is that a jet is partially blocked and not delivering sufficient fuel. This is a guess. I have the MMI carb rebuild kit and plan to rebuild asap, but I'd like to know what I should be looking for as I take apart the carb. (I've seen the excellent pictures of the rebuild steps.)

I hope I've diagnosed this problem correctly, but if you think I'm in error, please let me know your opinion. Thanks for the time and trouble,

Bob

[Don Moyer]

Bob,

I don't think there is anything wrong with your carburetor. The fact that your engine runs OK at lower power settings is strong evidence that you have low fuel pressure for some reason. This is a familiar problem with Catalina 30's which have fuel tanks lower than the engine and quite some distance away, and don't have a fuel pump with sufficient pressure. The engine will run OK at low to medium power settings, but if the throttle is moved to near full power, the engine will slowly run out of fuel as the carburetor float chamber slowly lowers.

Don

[BobB]

Bob,

I don't think there is anything wrong with your carburetor. The fact that your engine runs OK at lower power settings is strong evidence that you have low fuel pressure for some reason. This is a familiar problem with Catalina 30's which have fuel tanks lower than the engine and quite some distance away, and don't have a fuel pump with sufficient pressure. The engine will run OK at low to medium power settings, but if the throttle is moved to near full power, the engine will slowly run out of fuel as the carburetor float chamber slowly lowers.

Don

Thanks Don. I always appreciate your advice. So, do I live with it, replace/rebuilt the fuel pump, or what?

Bob

[jhwelch]

Before you go to the expense of replacing the fuel pump take your carb.
off and examine the needle valve area. It's possible that it's sticking
just before reaching the fully-open position.

I assume that you've checked your fuel filter to make sure it's not full
of "gunk", which could also be causing a flow restriction.

-jonathan

[Don Moyer]

The real professional thing to do would be to buy a vacuum tester at your local parts store and use the pressure side to plumb into your fuel line just ahead of the carburetor to measure your pressure. Anything under 2 psi at your favorite cruising power setting would be a real smoking gun.

Don