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]

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]

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.

Originally Posted by 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!

Originally Posted by Dave Neptune

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]

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]

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]

Originally Posted by gezas

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]

Originally Posted by gezas

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.

[Marian Claire]

Thinking out load again: Seems like if the pump can supply enough fuel for high RPM it should supply enough at low RPM. An electric pump might not if you had very low oil pressure and a low oil pressure switch but even then it would idle for awhile and then shut down. Have you tried choking as you throttle down? Or just putting something over the flame arrester to limit the air flow. Is your carb a four bolt carb? They have a history of air leaks.
I am on the MC so it is a great day. Dan S/V Marian Claire

[gezas]

I noticed that after mounting the carb (two hole),I can not prime the pump by hand. It feels empty. The engine has to turn over to move the fuel through.
The other thing that might be a problem is actually the in-line fuel filter that I recently added. It is more restriction and more work for the pump?...especially for an old one.
I will try to by pass it by connecting it to a spare tank and hold it up higher? That might show a difference?
I will try blocking the air intake.
Thank you, have a nice day on M/C, i can only do above things tomorrow!

[sastanley]

gezas..I learned this last year when I had the same 'problem' with my mechanical fuel pump...if there is already fuel pressure in the line, the 'priming handle' will act as if nothing is happening...this 'nothingness' can also occur if the rod which pokes out of the engine to move the diaphragm is in such a position that you cannot move the diaphragm with the handle.

I completely emptied my fuel line/pump/carb (along with the inline Racor filter) last year when I forgot to open the petcock at the tank, after running the engine for about 10 minutes. It took about 20-25 seconds of cranking to get fuel to the carb so it would sputter..3 more seconds or so of cranking and she fired right up. The only other consideration I can offer here is if you have long cranking times while priming, watch for possible overload with raw water back into the engine from the exhaust, as the water pump also spins during cranking..(this is especially an issue with C-30's like my boat)

It looks like there also may be a little confusion with the 4-hole question made by Dan...the Zenith carbs either have 4 screws or 5 screws holding the two halves together...I think he was implying that the 4-hole versions sometimes get air leaks, since one of the screws was eliminated, which can cause the lean condition. (FYI - Moyer sells the 5-hole gaskets, which work with either style. It should be obvious which one you have once you know what you are looking for.)

I wouldn't go running off and buying carbs & fuel pumps yet..it is hard to be methodical about the troubleshooting and trying one thing at a time. Did you install fresh gaskets when you put the carb back together? When you sprayed carb cleaner, if you detected ANY shift in engine speed, that could indicate a leak. Do you know of anyone else around with a good carb you could test on your motor, or another A-4 that you could install your carb onto? I agree with Dan that if the engine runs at higher RPM's it is unlikely the fuel pump causing your problems..

When I took my carb apart and thoroughly cleaned it last winter, I did not poke the holes with a wire, but I intend to add this to my arsenal of tools for the future.

edit - one other thought...did you use a fresh gasket on the flange where the carb bolts to the manifold? Due to the way our carbs bolt to the manifold we can't see this surface of the manifold to determine if it is clean and smooth without contorting ourselves..make sure that is a good clean surface with a fresh gasket as well. I have the Indigo PCV thingie here, so I have two gaskets & four surfaces to worry about.

[roadnsky]

Some more info along the lines of what Shawn mentioned with the 4-bolt vs 5-bolt carbs...

"Carburetor Internal Pressure Regulation

It's always gratifying to pass along a technical note that can be implemented at practically no cost, and which has the potential for improving the performance of a large number of engines in the Atomic 4 fleet. One such technical issue recently came to our attention, which could well explain why many late model carburetors (through the late 1970's) might produce a richer than normal fuel mixture.

I ask your patience in sorting through this rather lengthy note, and the good news is you'll find that the fix is much easier than the explanation. In fact, if you're on a really tight time schedule, you can skip to the last paragraph and blindly follow the suggested fix without ever understanding the problem.

BACKGROUND: All Zenith carburetors produced since the mid 1980's have a fifth retaining bolt in the front of the upper and lower housings. Prior to this generation of carburetors, there were only four bolts holding the two housings together, and all four bolts were located around the float chamber. This configuration left no fastener to pull the front of the housings together. How this fifth bolt (or the lack of it) relates to a rich running condition can best be understood by a quick review of how the internal pressure within the carburetor is regulated to maintain inlet pressure instead of ambient atmospheric pressure.

INTERNAL PRESSURE DISTRIBUTION WITHIN ZENITH CARBURETORS: If you look in the upper part of the intake throat of a carburetor with the flame arrestor removed, you'll see a small hole just in front of the choke valve. You can see this hole in the schematic of a Zenith carburetor on page 4-3 of our service and overhaul manual or in the attached image.

This opening in the top of the intake throat connects with a passageway between the upper and lower housings which continues around the venturi tube, into the area of the top of the main well vent jet, and eventually entering through the top of the float chamber. This passageway has the effect of delivering inlet pressure to the main well vent jet and the inside of the float chamber.

NOTE: It's very important to note that while inlet pressure varies with RPM (and how dirty the flame arrester element might be); inlet pressure is always somewhat lower than atmospheric pressure.

EFFECT OF INTERNAL PRESSURE IN THE HIGH SPEED SYSTEM: During normal operation, suction created by the venturi draws fuel from the bottom of the main well just below the float chamber, and as the fuel level within the main well lowers, it uncovers one or more of the small holes in the bottom of the main discharge nozzle. At this point, internal air is drawn into the discharge nozzle through the main well jet to mix with the fuel moving up through the nozzle.

A sort of "tug-of-war" develops between the venturi (low pressure) on one end of the discharge nozzle and inlet pressure (low, but not quite as low) within the float chamber and well vent jet on the other end of the nozzle. If atmospheric pressure (somewhat higher than inlet pressure) is allowed to enter the float chamber and the area of the main well vent jet, the venturi will have an easier time on its end of the "tug-of-war", and a slightly richer fuel mixture will usually result. As long as inlet pressure is maintained within the float chamber and the top of the main well vent jet, a proper fuel mixture will result at any power setting within the normal RPM range.

INSPECTING THE INLET PRESSURE PASSAGEWAY: The simplest way to inspect the inlet pressure passageway in a four-bolt carburetor is to attempt to insert a thin feeler gauge (.002" or so) between the front of the upper and lower housings. If you discover a space between the fronts of the housings, atmospheric pressure will be allowed to enter the float chamber and the area on top of the main well vent jet.

REMEDIATING CARBURETOR INTERNAL PRESSURE REGULATION: The good news (finally) in all of this is that all you have to do to bring the front of the housings together is to disassemble the carburetor and file the top and bottom of the two housings until the surfaces of the housings are flat and meet all around their edges. I like to use a broad (rather coarse) file, but some folks prefer to use a piece of sand paper on a flat surface.

CAUTION: The tendency for the front of the two housings to pull apart is apparently so great that a few folks have reported that they were fearful of removing so much material that the venturi might no longer fit between the two housings. Our advice in these cases is to stop removing metal from the housings and fill in the remaining gap (usually small at this point) with a small amount of sealer on both sides of the gasket in the area of the inlet passageway."

[roadnsky]

Another interesting thought to consider for gezas' original issue of not idling below 1500 rpms. This from a TECH TIP:

"Late Model Carburetor Tidbit

There is a small brass plug (approximately 3/16 inch in diameter) in the underside of the upper housing of Late model carburetors, just forward of the small brass fixed idle jet. The basic purpose of these plugs is to close off the bottom of this particular idle passageway in the upper housing after it was drilled, similar to the way 1/4 inch brass plugs are used to close off the ends of the oil passageways in the block.

At some point, Zenith began drilling small bleed holes in these plugs so as to allow a small amount of air to be drawn in to mix with fuel passing through the idle passageway. Since these bleed holes varied so greatly (from "0" to .057"), I originally had the idea that they can't be too important.

More recently, I drilled a small hole (about .040") in one of these brass plugs in a carburetor which had no hole previously, in an effort to improve poor idle performance. After drilling the hole, I couldn't even get the engine to idle at all - not even poorly.

From this experience, I conclude that whether or not a carburetor has a bleed hole in this plug, as well as the size the size of the hole if it is present, is somehow linked to the internal configuration of the idle passageways of that particular vintage of carburetor, and should not be altered in the field."

Worth checking?

[smosher]

gezas, I had carb issues last summer and Dave Neptune taught me alot. The 1st thing he recommended was a fuel pressure gauge to isolate the flow from the carb. The carb is very easy to take apart and clean I'm no mechanic and I can do it, the people here on this site are very helpfull with good suggestions. Try Searching for his name and read his input.

Sounds to me on your problem, it could be either a clogged idle jet, or too lean. I would 1st reset the idle mixture to 2 turns out. If that doesn't fix it when it happens try closing the choke. Make sure your choke opens and closes.

The carb is very easy to take apart and I saw where someone recommends running a wire through the idle jet. The wire ties on bread work when you peel off the label.

Here's a very good web site on rebuilding the zenith carb http://home.earthlink.net/~ultratom/...rb_rebuild.htm

Let us know what happens

Steve

[baileyem]

A great suggestion Steve; using wire from bread-ties to poke crud out of small jets and passages. I have used them with good success, and would like to offer another source of good 'poking wire'.....guitar, banjo and mandolin strings are really great for this application. They come in a variety of diameters, are stiff enough to resist bending, and one string goes a long way.

Mike