Engine stalls if throttled down quickly

[Baltimore Sailor]

I'm already planning on a complete tune-up for the spring: new distributor cap, rotor, wires, plugs -- I already have the electronic ignition. I have a brand-new carburetor as of last spring. So this question is more general interest than a pressing need, at the moment.

Basically, my engine behaves like this: starts right up and runs smoothly. However, if I throttle down quickly the engine will go right down to almost stalling, then "catch" again and come up to where the idle is set. Sometimes it does stall out, but starts right back up again. If I slowly throttle down it will settle into its idle quite nicely.

Is this the sort of behavior that changing the above parts will cure? Or is it indicative of something else, maybe the weights in the distributor (which looked fine the last time I looked at them in the summer)?

I'm also not quite sure about the idle mixture setting, as the screw is difficult to get to, but I have at least tried to set it as best as I can. Could it be that I need the adjustable main jet add-on for the carb, or should the brand-new carb be OK as it is?

I'm interested in seeing everyone's opinion, and thanks!

[Dave Neptune]

Idle screw

Sounds like the idle is a little to lean, try backing the idle screw out (counter clockwise) 1/4 turn and check again. Usuallyif the engine dies when rapidly returning to idle indicates a lean mixture, if it is to rich it will chug stumble then die and your plugs should be a little black or sooty. If the plugs are sooty try turning the Idle screw in a 1/4 turn although this sounds unlikely in your case discription.
It's probably not the centrifugal advance sticking although it could be sticking just not causing this type of problem.

Dave

[Baltimore Sailor]

Not being facetious, but what's the difference between "dies" and "chug stumble and dies"?

My plugs are usually always a bit sooty, but they're the colder plugs -- I haven't found the good hot ones yet (can't remember what they are exactly). That's another thing I'll be taking care of in the spring, getting those hot plugs.

[Dave Neptune]

Stalls

If the engine dies from returning the throttle to the stop in a lean mode it will usually feel like the engine was "shut off", if it dies from being rich it will usually get your attention somehow befor it actually dies---sputter stumble done. If yoiur plugs are sooty you are to rich, try leaning it until you start to loose rpm then 1/8 - 1/4 turn out and see what happens. be sure the engine is warmed up good before adjusting the idle, a cold engine wants more fuel than a warm one, thus the choke.

Do you mean the J-8 as the colder plugs or the J-12's?

Hope this helps

[Baltimore Sailor]

I see. Mine stalls as if it were shut down more than with any kind of drama, but my plugs are a bit sooty as well. Hard to figure which scenario I've got with those symptoms, but I'd say the plugs are more revealing -- I'm probably running a bit rich.

Ah well -- I'll be replacing all that stuff in a month or so anyway, so I'll figure it out then.

I've already got the tune-up parts I mentioned earlier plus the crankcase recirculation PCV valve and the upgraded idle mixture screw on my list. Lots to do once the weather starts to warm up.

Can't wait to get to it all.

Almost forgot about the plugs: I think I've got the champions, but I forget what the number is.

[roadnsky]

AUTOLITE 437's

I'm pretty sure that the hotter/cleaner burning spark plugs are the AUTOLITE 437's. I haven't tried them yet, but saw that claim in an earlier thread...

[CalebD]

Sooty plugs

Apparently sooty plugs can be the result of an obstruction in the exhaust system.
See the thread linked here and scroll down to the picture of the collapsed exhaust hose: http://www.moyermarine.com/forums/showthread.php?t=1350

[roadnsky]

Apparently sooty plugs can be the result of an obstruction in the exhaust system.

I had this issue just last week. I had suspected it for a while, especially after reading various threads on this site.

It turned out to be the rubber wet exhaust hose from the hot section to the water lift muffler. It had collapsed internally right at the point where the water was intro'd to the hot exhaust just as Don had described in a few posts regarding exhaust problems. (I suggest reading those past posts)
Anyway I decided to bite the bullet and also replace the hot section too. I'm glad I did. Found crud and rust in there also. She runs great now.

I'm posting this because it bears considering as a possible cause of Baltimore Sailor's problem in his original posting. I did have this symptom before replacing the exhaust, although mine was the "chug stumble and dies" variety...
-jerry

[Baltimore Sailor]

Thanks for reminding me!

Replacing that rubber exhaust hose was also on my list of things to do, but I'd forgotten it. (It's a mental list.)

Is that stuff just standard auto radiator hose? What's the dimension?

[CalebD]

Roadnsky,
I read the entire thread but think the photo of the collapsed hose is worth 1000 words. I get sooty plugs and the 'chug, stumble, die' syndrome if my plugs get fouled. Changing the plugs seems to take care of it but the real culprit is probably in my exhaust system.
Guess I'll be measuring those hoses myself come spring.
Balti,
I'm not sure that there is any guarantee that your exhaust setup/hose dia. will match that of others. Probably best to measure the fittings upon removal of old hose.

I'm not really looking forward to this one.

[roadnsky]

Is that stuff just standard auto radiator hose? What's the dimension?

Definitely not auto radiator hose...

It will depend on your HOT EXHAUST SECTION and your WATER LIFT MUFFLER.
I used the MMI Inline cooling water exhaust entry fitting (MM Part #EXHT_01.1_324) at the end of my hot section and both that and my water lift muffler take a 15/8" Wet Exhaust Hose that I bought at West Marine.

Here's a pic of my old hose. I'll take a better one of my new hose and post tomorrow...
-jb

[roadnsky]

Exhaust System Sketch

Here is a sketch of the Exhaust System that Don posted some time ago...

[tenders]

Search defender.com for "exhaust hose." You'll see lots of it, much cheaper than West Marine.

I replaced mine last year -- I used the "with flex wire" type and I think it was $10-$11 a foot so it was probably 2" or 2.25" inner diameter. An easier job than I expected. The inside turned out to be OK but the outside had taken a beating.