Engine stalls when throttle increased

[Gabe]

A couple of weeks ago I was taking the boat out and found that I had no power once I exited the marina. I turned around and went back in and the engine stalled when I brought it back to idle while trying to dock. I am pretty sure the lack of power was due to a fouled prop which I cured with a good bottom cleaning. I followed advice found here and cleaned and rebuilt the carb with the moyer kit. I also replaced all the fuel lines and added a fuel/water separator. Now the engine starts and will idle (after adjusting the idle screw about 1/2 turn in from the factory setting), but as soon as I give it some throttle it stalls out. I am assuming this means it is running lean? Since i replaced most of the fuel system I am also assuming I did something wrong and there is probably an air leak somewhere. Does the fact that it idles ok and doesn't die until the throttle is increased suggest a location for the airleak?

I am assuming an air leak resulting from my (attempted) repairs because before I took the carb off I never had a hint of a problem with the engine at higher rpms until after it sat for a month and the carb got gunked up.

[ArtJ]

Try bleeding the main passage plug again. You may have stirred up some
crude which became lodged in the main jet.

Regards

Art

[Mo]

some ideas

Does engine have the PCV kit installed. If so the mixture screw can be brought in some ... have seen that necessary on a couple of engines.

Also check the seal on the separator...ensure it's screwed on tight. Double check all clamps to ensure tight.

Ensure the gasket tightening the carb onto the manifold is good and tight as well.

Lastly, check timing.

[Harris Huberman]

Stalls when put in gear

Hi, Gabe -

Hope you've solved your issue.

I'm adding to this thread as I have a similar sort of issue - and I assume the differential diagnosis of stalling under load may be the same.

My A4 currently starts and (to me) sounds fine in neutral but when I go to put it into gear - forward or reverse - it promptly stalls.

I thought it might be a reversing gear issue - But the reversing gear seems to be working okay: I can use my hand crank to manually turn over the motor without too much difficulty (with sparkplugs in, btw) - with the gearshift in neutral, the prop shaft doesn't turn - while when I push the gearshift into forward it goes into detente, and the propshaft does rotate. Sounded right to me. But again when I actually start the engine and put it in gear, it promptly quits.

Today I rebuilt the carb, thinking it might be a clogged main jet, and in fact it now idles more smoothly. However, when I go to put it into gear, it still stalls.

Earlier I checked compression with finger over the spark plug hole test - all 4 cylinders seemed fine. The engine (a '61 early model) was rebuilt by Don's shop in 2007, and I've used it lightly since.

Key bit of recent history - I was having a problem with no spark (and followed and much appreciated input from this forum guiding me to eventually replace my coil, ignitor and fuel pump ... but that's another thread) and finally got spark and the engine starting. But in the process I both removed the distributor to change the ignitor, also had all the spark plug cables off. As I said, the engine idled fine afterwards, but that's when I started having the problem when engaging the reversing gear. So I'm thinking high on my list must be dumb things I did when reinstalling the above. I thought I'd timed the distributor to fire #1 at TDC, but ... And again I think I've correctly placed my spark plug cables (1,2,4,3 yes?) but ... I'm open to suggestions to systematically test these and/or other possibilities.

Ideas?

Thanks,

Harris
"Sahalja"
City Island, NY

[hoya]

Stall fix

Harris,
We experienced the same problem last year. Our problem and fix were simple. While changing plugs and wires, I mixed up wires 3 and 4. She idled like a kitten, but any load stalled her out.
Hope that works for you.
Lee

[Harris Huberman]

Thanks, Lee - definitely will check it.

Harris

[Mo]

Learned from mistake.

For you guys having no power in gear here's something I learned this year. Thumb compression method to rule out compression issue may be misleading....

While working on a fella's boat I relied on it to dx that we had adequate compression. Compression tester used later to tell us the problem was low compression. In the meantime he changed prop, carb, fooled around with gearbox, timing, ....allot of time wasted.

Turned out the problem was low compression; Head gasket leak as well as valves not seating properly...combined they caused a problem. My thumb did pop off each cylinder with the thumb test.

[edwardc]

Harris,

With the engine in neutral, can you twist the propshaft by hand? If not, you may have something fouled on the prop, or your stuffing box may be too tight.

[Harris Huberman]

Yes, I can turn over the engine with the hand crank - in neutral, the prop shaft doesn't turn (of course), but also, in forward gear / detente, I can also turn it over with reasonable effort and the prop shaft does turn, so I'm assuming its not either a fouled prop (I also dove to check) or I'm thinking anything wrong with the reversing gear (although would appreciate input on this last point - is there an adjustment in the reversing gear that could be off and account for stalling? After reading the relevant chapter in Moyer's manual, it didn't seem likely.

thanks,
Harris

"Sahalja"
City Island

[Gabe]

update

Thanks for the responses. I finally got back down to the boat yesterday. I double checked all the hose clamps, they were tight. I think found the problem. I was able to tighten the scavenger tube at the manifold fairly significantly. When I started up the engine ran ok at idle and at full throttle (pulling against the dock lines). It stumbles for a brief second while throttling up. As I increase the throttle rpms increase as expected until the throttle is about 1/3 open, and then the engine stumbles like it is going to stall out but then kicks right back in runs fine. Any idea what would cause this?

[Dave Neptune]

Possibilities

Gabe, the "stumble" when advancing the throttle can be many things of which 3 are most likely. One of the likely ones is "you" advancing the throttle to fast and the A-4 carb has no acceleration pump to give that liil extra squirt of fuel to avoid stumbling~not real likely but possible. The other two are both fuel related first to rich (also rare) and this should be indicated by dark sooty plugs and the other is lean (far more likely) and the plugs will look whitish. The above is assumed that the timing is still OK as the timing being off can do the same. However if the timing was good and you have not moved the distributor the above should hold true.

Dave Neptune

[sastanley]

Gabe..an air leak can definitely cause an issue. Hopefully you've solved the initial problem!

I get a stumble on throttle up too..another suggestion I've received from the experts here is that sometimes a dirty idle jet can contribute, & it stumbles as the carb is moving off the idle jet to the main jet...I guess the theory is that as the idle jet is nearing its maximum threshold, the dirt starts affecting it just before the main jet takes over, but it will still idle satisfactorily. There are two tiny holes in the carb on either side of the main butterfly valve that are part of the idle jet system that get cruddy..carb cleaner isn't always enough & they require a little patience with a small wire & compressed air if you have it.

As carb guru Dave Neptune has already mentioned, my most successful solution until I finally remove the carb for a proper cleaning is to try not to throttle up too fast..I've also sometimes pulled the choke out just a wee bit if I am worried about it stumbling all the way off at idle with moderate success as well.

[Gabe]

hmmm....it does seem like it is happening right when the main jet should start to take over. I just cleaned and rebuilt the carb, but maybe i missed something in the idle ports, or maybe I have a bunch of crap in my fuel tank and just gunked it up again. Timing was fine before and I haven't touched anything other than the fuel system. I'll check the plugs next and see if they suggest anything.

thanks

[edwardc]

hmmm.... Timing was fine before and I haven't touched anything other than the fuel system. ...

How was the timing set? To spec, or "power timed" for max RPM?

I found I had this stumble when my idle jet was set too rich AND the timing wasn't advanced enough. Advancing the timing (but still short ofmax RPM) allowed me to set the idle jet to the proper mix (confirmed by oxygen sensor) and the stumble while throttling up went away.

[Gabe]

How was the timing set? To spec, or "power timed" for max RPM?

I found I had this stumble when my idle jet was set too rich AND the timing wasn't advanced enough. Advancing the timing (but still short ofmax RPM) allowed me to set the idle jet to the proper mix (confirmed by oxygen sensor) and the stumble while throttling up went away.

I'm not sure about the timing. The boat is new to me, so that was set by the previous owner. I just know that it has never been a problem for me, but it is possible that what you are describing is happening. I know my idle jet is set to the rich side of the recommended setting of 1-1/2 turns out. Thanks for the suggestion, I will add that to list of things to try!