Zenith idle question

[gezas]

Hi, this is my first time here, I didn't want to bother anybody before but now I need some help!

I have a late model Zenith carburetor in my 1967 Cal36 that I bought last year.Previous owner - for 20 years - took care of it and had the carburetor rebuilt 5 years ago.
The engine is running fine when higher RPM - no smoke, steady - but when the throttle is being pushed back toward idle, it would rather stall at 1500 RPM. I have rebuilt and cleaned the carburetor over and over and once managed to have it idle at 700 RPM. I let it idle for long but after I came back from a sail and started the engine again it would do the same as before. Down to 1500 and stall. Since, I've cleaned the fuel tank, I ordered and installed the Moyer inline fuel filter kit, replaced the screen in the sediment bowl....but still no idle?
I am running out of ideas? I would order the new carburetor but I am afraid it might do the same as the cause could be somewhere else? ...but where? Timing, leak around the manifold...? If I can be assured that it is definitely carburetor problem, I would immediately order the new!
Thank you!

[Marian Claire]

How is your idle jet set? I’m reaching here but try turning it in all the way. I had an air leak in my throttle shaft that only showed up above 2200 RPM. Maybe you have an air leak at low RPM that is leaning out the mix. Dan S/V Marian Claire

[gezas]

Zenith idle speed

Thank you Dan,

I tried to set idle all the way in, all the way out. Usualy makes not much of a difference, except all the way in engine would stall sometimes or would just run too rough.
The problem is that I can't really set idle speed when I can't slow the engine down below 1500 RPM? If I could make it run @ 7-800, than I could?
At 1500 RPM does the idle circuit work at all?

I am thinking that the transition from throttle to idle is the biggest problem.
I don't have a brake since throttling down would stall the engine, I have to restart and shift in reverse. Good thing is, engine starts easy.

I read many messages in different places about transtion problems but they all the opposite, from idle to accelaration.

[sastanley]

stalling on throttle back

Hi Gezas, welcome!

As you noted, I'm one of the guys with the opposite problem...if I throttle up too fast, the engine sputzes and usually catches itself while transitioning from idle to main jet, but sometimes it stalls...I've put this problem low on my personal list, but it is there..my solution is to easily throttle up and it is 95% of the time OK.

However, after reading your comment I did some forum searching and found some interesting posts from a thread about 14 months ago that may apply to you.

Dave Neptune has been fiddling with these engines longer than I've been alive I think. At any rate, he's one of the old masters of these motors...ooops...maybe that should be, masters of these old motors.

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!

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 description.
It's probably not the centrifugal advance sticking although it could be sticking just not causing this type of problem.

Dave

Here is a link to the entire thread...it got a little off topic farther along...above are just the first two posts.

http://www.moyermarine.com/forums/showthread.php?t=2968

Hope this helps!

[Marian Claire]

So some times it runs, even rough, with the idle jet, not the idle stop all the way in. Is that right? I think the main jet kicks in around 1200 RPM. You could also try slowly closing the choke as you throttle down and see if that helps keep it running. Someone here will have the answer. Keep us posted. Dan S/V Marian Claire

[Marian Claire]

Gezas: I want to make sure we have our terms/parts right. The idle jet adjustment is on the carb just below the manifold junction usually facing aft. The idle speed adjustment is in with the linkage on the forward side. At least on the MC. Remember that the idle jet controls air flow. So as you turn it in, clockwise, the mix gets richer, out and it gets leaner. It is just the opposite of the main jet. As mentioned in Dave/Shawn’s post: What do your plugs look like? Dan S/V Marian Claire

[roadnsky]

This from Don in an old thread regarding not being able to idle...

"The only thing I can think of that might explain your shut downs (presumably at or near idle) is that the idle mixture might be on the lean side and doesn't pick up the load as you come back past 1100 RPM or so. If turning the idle mixture screw in a half turn or so doesn't cure the problem, I'd check the two idle ports along the side of the top of the carburetor throat to see if one of them is clogged (especially the lower one).

At approximately 1100 RPM, the main discharge nozzle pretty much drops out and the idle system is supplying most of the fuel to the engine, and if the lower of the two idle ports is clogged, the mixture will become very lean during the transition to idle. If the engine can get to idle, and the upper port is open, it will be able to carry the load, but only at around 800 RPM.

This situation is sort of like a flat spot during acceleration, only in reverse. The lower of the two idle ports is referred to by some (including the writer) as the "off idle" port, because this port has to kick in as you move the throttle forward to get you "off of idle" and on to the main discharge nozzle. If the lower port is clogged, you'll feel a flat spot until you get to the main discharge nozzle - if you can even get to the main discharge nozzle.

As you decelerate, you're depending on the lower idle port once again to kick in to carry you smoothly as you transition between the main discharge nozzle and the upper idle port."

[Baltimore Sailor]

Could there possibly be damage to the needle and seat of the idle mixture adjustment itself? How would one check for that beyond noting that moving the screw in and out made little to no difference?

If you took the carb off and took out the adjustment screw, do you think it might be visible to the naked eye?

[gezas]

lean mix?

Thank you for everyone!

I will be away for a couple of the days, please don't give up yet! I don't want to be defeated by a carburator? ...but if I have to accept that at the end, would a new carb be a solutiuon?

Dan: I haven't checked the plugs.
The idle jet screw, I know but where is the idle speed adjustment, is it the screw in the linkeage that stops the throttle from closing too far? The one at the carb end of the throttle cable? If yes, I tried to adjust but engine stalls if I go too far. (below 1500rpm). I read once that set timing (rotate distributor)while engine is under load (in gear during idle). I just can't getthe engine run low RPM?

Baltimore Sailor:
I replaced the idle jet screw from the rebuilt kit but haven't checked the threaded part of it?

roadnsky:
I checked and cleaned and checked both of the two idle ports along the side of the top of the carburetor throat. They are clear but i am not sure that when the throttle plate is closed, it should cover the lower but not the upper port? In my case it covers the lower but there is a small gap between the plate and the throat. Is that ok? Could that make it lean as it sucks air through the venturi eventhough the throttle plate is closed? Basically the plate doesn't seal the throat completely since it is in a slight angle.

sastanley:
I appreciate your research, it sounds like lean mixture might be the cause but I don't know how to make it richer, due to above problems?

[Marian Claire]

Gezas: Yes that is what I refer to as the idle speed adjustment. Try slowly choking as you throttle down. That should make the mix richer. More than one person on the forum has had a similar problem and it turned out to be an air leak in the carb. Good Luck. Dan S/V Marian Claire

[tenders]

When you've cleaned the carb, have you reamed out the very small holes (idle ports) with a fine wire? I have a piece of wire expressly for this purpose, unkinked, and carefully saved from the tag the alternator shop used to attach my name to my alternator several years ago after a rebuild.

[gezas]

idle ports

thank you "tenders", I have blown compressed air through and squirted carb cleaner as well. After all this cleaning, even if it would work today, I would always wonder when is itg going to clogg again or "lean out"?
Would a new carb really be more reliable? That is my dilemma. The cost is not much but first I wanted to fix this one. I learned a lot in the process, that for sure. Thank you for your advice.

[roadnsky]

thank you "tenders", I have blown compressed air through and squirted carb cleaner as well. After all this cleaning, even if it would work today, I would always wonder when is itg going to clogg again or "lean out"?
Would a new carb really be more reliable? That is my dilemma. The cost is not much but first I wanted to fix this one. I learned a lot in the process, that for sure. Thank you for your advice.

There are quite a few of us on this forum who have gone that route.

In my case, completely starting from scratch on the fuel system (carb/fuel pump/lines/racor water separator filter/polish filter) solved all of the hard starting, stalling, rough running issues. Not to mention the worrying!
Just an opinion, to help you make that decision...

[tenders]

thank you "tenders", I have blown compressed air through and squirted carb cleaner as well. After all this cleaning, even if it would work today, I would always wonder when is itg going to clogg again or "lean out"?
Would a new carb really be more reliable? That is my dilemma. The cost is not much but first I wanted to fix this one. I learned a lot in the process, that for sure. Thank you for your advice.

I'm not a carb expert but it seems to me that your symptoms could be readily explained by a blockage of those ports, and I am not sure that compressed air and carb cleaner are failsafe ways of making sure that those tiny passages are clear. Reaming them out with the wire is pretty much a guarantee.

I agree with your philosophy of wanting to repair this but when all is said and done there are certainly worse things than having an extra carb lying around. I have one myself, and an extra water pump. The new carb will certainly help you isolate the problem, but not necessarily fix it; just isolating the problem might be well worth the price of the carb. But consider that the risk of clogging is probably not going to go away with a new carb. If there is an upstream contributor to the clogging problem like inadequate filtering or a fuel pump that is spalling junk into the fuel, a new carb is going to clog too.

[gezas]

Hi, I started the engine and sprayed carb cleaner around the throttle shaft and mounting seal. I think the rpm increased but wasn't very dramatic change. I checked the plugs and they were dry. I really think at this point that first I should replace the fuel pump since I don't think I could clean the carb any better? I will ream those small passages in the throat one more time and i will get back when that's done.
Thanks to all for the support and advice and hope one day I can help out from my experience.