Stalls out under load

[Tartan30guy]

Hi all. I have a A4 that ran great on the hard but is sputtering and dying out when I try to increase RPMs in FWD now that I'm in the water. Out of gear when i increase RPMs it kind of hesitates a bit on the way up but gets there. In FWD I can get the RPMs up a bit but when I apply more throttle RPMs drop, the engine sputters then quits completely. However, with the choke pulled out I can increase RPMs right up but it just doesn't sound right when I do that.

Thre A4 is a late 90s (from what I'm told) Moyer marine A4. Electronic ignition. I've just done a refit of my Tartan 30, and part of that was a new fuel tank, new lines and filter, oil changed, new plugs. I've cleaned the glass bowl out on the carb and I've blasted carb cleaner into the carb after taking off the flame arrestor, but I continue to have the problem. The hour meter (which works) shows 486 total hours. Not sure if that represents actual hours since install.

I'm thinking of trying to adjust the lean/rich mix screw as if the choke is pulled I can rev up ok, which tells me I'm to lean right now (right??) but before I did that I thought I'd join here at Moyer and ask for help on the issue.

I look forward to hearing your opinions. Thanks!

[scratchee]

Have you seen this carb troubleshooting video from Moyer? I've cued it up to a point that may address your problem, but you should watch the whole thing anyway.

https://youtu.be/kHuemGC_PHA?t=166

[ronstory]

... and welcome to the forum.

[tenders]

Potential cause 1: carb issue, as discussed in the video
Potential cause 2: blocked exhaust

[Administrator]

Only under load?

Bill

[Tartan30guy]

Potential cause 1: carb issue, as discussed in the video
Potential cause 2: blocked exhaust

So let's talk about exhaust blockage. Right now, the A4 exhaust is exactly what I'd expect; good water flow and exhaust gasses coming out. When I increase RPM, the water flow and exhaust gasses increase as well. With this in mind, I'm not thinking the exhaust is an issue. Am I missing something? Thank you

[Tartan30guy]

Only under load?

Bill

Well not completely true. In neutral, when I apply throttle, the RPMs increase but not as smoothly as would expect, it struggles a bit with "hiccups" as I increase RPMs.

Under load (in forward) when I apply throttle I get a slight increase in RPMs before the engine sputters and quits.

I'm thinking there is a blockage in the carb and I need to remove clean it.

Thoughts? Thanks for your input, Dave

[Surcouf]

Well not completely true. In neutral, when I apply throttle, the RPMs increase but not as smoothly as would expect, it struggles a bit with "hiccups" as I increase RPMs.

Under load (in forward) when I apply throttle I get a slight increase in RPMs before the engine sputters and quits.

I'm thinking there is a blockage in the carb and I need to remove clean it.

Thoughts? Thanks for your input, Dave

question: your engine should revs up very quickly without load (without "hiccups") "almost explosively" would say Mr Don M. Does it do that when not in gear? If yes, I would recommend to check the carburetor of course like everybody else, but would recommend to check first that the shaft turns "freely" by hand. If you have a piece of line entangled in your prop shaft, that could explain some of your issues...
If it does not rev up quickly when in "neutral", that excludes the shaft line, and would point toward your carburetor (assuming your exhaust path is clear (exhaust manifold + exhaust pipe etc...))

[Peter]

Fuel pressure gauge might be a useful diagnostic tool here.

I had a similar situation where engine would only run with choke. Fuel pressure was dodgy.

If you have good fuel pressure it is a carb issue. If not, look upstream.

Peter

[tenders]

So let's talk about exhaust blockage. Right now, the A4 exhaust is exactly what I'd expect; good water flow and exhaust gasses coming out. When I increase RPM, the water flow and exhaust gasses increase as well. With this in mind, I'm not thinking the exhaust is an issue. Am I missing something? Thank you

It is possible to have a partially blocked exhaust that allows some water and gas out but not enough to permit the engine to exhale properly and deliver proper power. This can happen from sediment/scale in the manifold or from a collapsed ply in the rubber exhaust hose, even though the outside of the hose looks fine.

You detect this by disconnecting the hose and running the engine briefly to see if the power response improves.

I agree, the carb is more likely the source of the problem but isn’t necessarily the only cause of the symptoms you’re describing.

[roadnsky]

To add to Tenders comment...
another quick (and simple) test for exhaust blockage is to remove one spark plug at a time while the engine is running and see if you notice any change.
An improvement while running on only 3 cylinders would suggest a closer look at your exhaust.

EDIT: I found THIS in the Tech Tips if you want to read more...

[edwardc]

...and I've blasted carb cleaner into the carb after taking off the flame arrestor, but I continue to have the problem.
...
I look forward to hearing your opinions. Thanks!

In my experience, modern carb cleaners do not have the crud-cutting capabilities of those in my youth (which would instantly dissolve all forms of crud, and most plastics!). I find it is necessary to disassemble the carb and physically ream out every orifice with a small wire in order to insure they're clear. Just spraying isn't enough. Even an almost microscopic particle or layer of gunk can drastically affect the carb.

The wire from the center of a bread tie is just the right size. I've also had good luck with an acetylene welder's tip cleaner. In addition to the main jet and the idle jet, be sure to get the two small holes that straddle the throttle plate in the closed position.

Your case does sound like a failing transition from the idle jet to the main jet, so be sure to pay special attention to the main jet.

[scratchee]

... I find it is necessary to disassemble the carb and physically ream out every orifice with a small wire in order to insure they're clear. Just spraying isn't enough.

The video shows how to access the main jet directly without disassembling. Might get lucky, if that is indeed the problem.

[JOHN COOKSON]

T 30
Is there a "polishing filter" between the fuel pump and the carburetor? If not this would be a good time to add one.
The polishing filter will keep the little bits of crud out of the carburetor.

ex TRUE GRIT

[Mo]

Take a few minutes and check your compression. I worked on one a number of years back and the guy had bought a new carb etc, then changed prop...his issue was lose of compression and it was early in the year, just after winter layup..... It would run, would move in reverse, but when put in forward stall.

Just a thought in your troubleshooting process.