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Wisconsin Engines Single cylinder up to V4 engines.

Wisconsin Engines

Zenith 68-7 - too rich mixture?


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  #1  
Old 02-13-2012, 11:05:53 PM
Kane Kane is offline
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Default Zenith 68-7 - too rich mixture?

Hello all,

I'm trying to get this carburetor to function properly on my Wisconsin THD, and I have a few questions. Firstly, I have cleaned the carb carefully and all gaskets are brand new. On throttle-up to high idle, I'm getting a significant amount of black smoke out of the exhaust and then it clears up. I'm assuming this is a too rich idle mixture and fuel is pooling in the manifold or cylinders and not combusting. The carb is a fixed main jet model, and the idle mixture is adjusted with a screw. I have the idle mixture screw backed out which should be a very lean mixture. Why am I getting all the black smoke when the throttle is opened? Is there something I'm missing?

Additionally, what position should the throttle plate be at in a closed (ie: idle) position? All the way closed so as only the idle holes are open, or should it be open a little bit?

Thank you very much for any help. It is much appreciated! Cheers!
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Old 02-14-2012, 12:02:54 AM
Andrew Mackey Andrew Mackey is offline
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Exclamation Re: Zenith 68-7 - too rich mixture?

Check your float and the float level. It sounds like the float level is set too high. Some of these carbs tend to puddle, no matter what you do. When at idle, not all the fuel drawn into the intake vaporizes, nor is all drawn into the cylinder. The gas then puddles in the bore, where, when the throttle opens, the excess fuel is then drawn into the engine. Nature of the beast I guess. Setting the float level a little lower might help aleviate the condition, it probably will take trial and error to get it right.
Andrew
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Old 02-14-2012, 12:48:38 PM
mmcdonald mmcdonald is offline
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Default Re: Zenith 68-7 - too rich mixture?

You can also replace the main jet plug with an adjustment if necessary. Also seen many discharge nozzles, jets, etc. that have been opened up with a drill. Other post about float level is also correct. One other thought is a plugged up air cleaner will cause the same.
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Last edited by mmcdonald; 02-14-2012 at 12:49:52 PM. Reason: additional info
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Old 02-14-2012, 11:05:59 PM
Kane Kane is offline
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Default Re: Zenith 68-7 - too rich mixture?

Alright, so the carb float level also affects the idle circuit as well as the high-speed circuit? I did have the carb running a lot leaner the last time it was together, with much less smoke upon opening the throttle. But since then, I have replaced the head, carb, and intake/outtake mainfold gaskets, as well as cleaning the carb thoroughly. I've affected something that's for sure! I will also try removing the air cleaner from the system to see if it makes a difference, but I think its pretty clean.

I will pull it apart and get after it again this weekend and report back on my findings. Thanks for the thoughts guys. Cheers!
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Old 02-15-2012, 01:39:24 AM
coyote62ny coyote62ny is offline
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Default Re: Zenith 68-7 - too rich mixture?

look the carb over there is a float bowl vent to outside air if it is pluged it will also cause this. i had the same problem once and found that vent pluged cleaned that out with a small wire and compressed air cured the problem
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Old 02-15-2012, 04:15:18 AM
Andrew Mackey Andrew Mackey is offline
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Exclamation Re: Zenith 68-7 - too rich mixture?

To answer your question, yes, float level will affect both the idle and high speed. Too much fuel in the bowl, richer mixture. Too low a level, lean out, and transition problems will occur. Make sure the needle and seat on the inlet are in good shape as well. A worn assembly will not function as well, allowing more fuel into the bowl. As others have noted, check the air filter for restrictions. if it is or has a foam element, (some filters use both paper with a foam cover) if it is old, it will not pass air as well as a fresh filter. Also, if a paper element, if it has gotten wet, it will not flow air properly. Water changes the ability of the paper to pass air, even after it has dried out! If you dis-assemble the carb again, have a close look at the gasket between the upper and lower half of the carb. Make sure that the gasket is comperssed evenly across the entire contact area. I have seen carbs, where the castings have warped over time, due to internal stresses built in when the carbs were cast. This warping led to air leaks between the fuel bowl and the carb air passage, leading to gas getting thur into the intake passage. This is not common, but it does happen. Over tightenning of the screws between carb halves can also warp the castings.
Andrew
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Old 02-15-2012, 09:59:31 AM
Kane Kane is offline
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Default Re: Zenith 68-7 - too rich mixture?

coyote62ny, any thoughts on where the bowl vent is?
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Old 02-15-2012, 10:32:25 AM
Andrew Mackey Andrew Mackey is offline
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Exclamation Re: Zenith 68-7 - too rich mixture?

There may be a small hole drilled in the top, but more often than not, a tube or pasage run from the upper section of the carb into the inlet of the carb. If a brass tube, it should be about 1/8 to 3/16" in diameter, and will be visible in front (outside) the venturi, probably near the front of the carb. If it is a drilled passage, you usually see plugged passages, and there will be a small hole near the outside of the venturi, about 11/6" in diameter. Look at the interior of the upper section of the carb, for a hole in the float chamber roof. You can blow carb cleaner into it and see where it vents. WEAR GLASSES or a face sheild when you try this, in case the vent is blocked! You don't want an eyeful of carb cleaner - it can be very painful and may damage your eyes! If you see the vent in the carb throat, you can reverse flow blow cleaner thru it to see where it enters the float chamber. I forgot to mention this in my earlier post: While you have the carb apart, check the gasket and the carb mating surfaces for any blocked ports or passages.
Andrew
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Old 02-15-2012, 12:06:51 PM
RSCurtis RSCurtis is offline
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Default Re: Zenith 68-7 - too rich mixture?

Judging from your location, don't rule out carburetor icing. Heat the intake air with a hair dryer and see if it makes any difference.
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Old 02-16-2012, 12:35:15 AM
Kane Kane is offline
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Default Re: Zenith 68-7 - too rich mixture?

Thanks all so much for your input. I will check it out again this weekend, when I'm back at my shop, and report back!
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Old 02-16-2012, 01:09:51 AM
coyote62ny coyote62ny is offline
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Default Re: Zenith 68-7 - too rich mixture?

if i remember right the bowl vent hole is in the top half of the carb just above the seam where the top meets the bottom. if its pluged it may be easyer to find from the inside of the top. this vent hole is real small. it may also be a tube that goes from the inside of the top into the throttle bore they did use both it depends on the carb and how old it is. the one i had trouble with did what you are experincing and also would flood over with the tank above the carb . [gravity feed] if you got a carb kit it should show the vent in the instructions. hope this helps you. i would also put a main fuel mixture screw assembly in the carb if you can get one. that way you can have a means of adjustment would make it run better and more fuel efficent.
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Old 02-18-2012, 01:04:23 AM
Kane Kane is offline
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Default Re: Zenith 68-7 - too rich mixture?

Well, I pulled the carb again and gave it a thorough once-over! The float was set WAY too high which would have given a higher fuel level in the carb then normal... I reset the float as per the Wisconsin THD manual. Additionally, I was unable to locate the small bowl vent hole.

I fired the engine up, and it's getting a MUCH smoother idle, with a nice smooth throttle-up and no stumbling. I held a napkin to the exhaust and I'm getting only a little bit of black soot after 20 seconds on both high speed and idle. However, I'm still getting a significant amount of smoke as I throttle up. The engine doesn't stumble or spit or anything, but it definitely belches a lot of smoke... more than I remember it doing a year ago when it was last running. I want to say the smoke is more blue and white in color (rather than solid black), but it was getting dark out. Is the white smoke unburnt fuel?

I did a little feathering with the throttle: there was very little smoke upon throttle-up if I quickly cycled between idle and high speed. This leads me to believe there's still raw fuel puddling in the cylinders. I'm wondering if the oil is getting washed from the cylinder walls and that's why there's so much blue smoke. By the way, I'm getting compression readings of 80-100psi between the two cylinders.

So what do you guys think about the amount of smoke? One guy said the excess fuel is nothing to be concerned about... does that sound right?? Should I take a video and see if I can post it on Youtube for you all to have a peak and make a diagnosis? Any thoughts? Thanks again for the invaluable help all!
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Old 02-19-2012, 09:54:57 PM
sprkplug sprkplug is offline
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Default Re: Zenith 68-7 - too rich mixture?

If it looks white, then it's actually light blue, meaning oil is being burned. If it were me I wouldn't worry too much about any BLACK smoke when you transition off idle, especially since you say the engine doesn't stumble on throttle up. It goes with the territory somewhat on an engine with no accelerator pump in the carb, very few cylinders, usually a slow idle, and not the smoothest passages from carb to combustion chamber.
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Old 02-20-2012, 05:25:56 AM
Andrew Mackey Andrew Mackey is offline
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Exclamation Re: Zenith 68-7 - too rich mixture?

A little smoke on transition isn't a problem. Burning oil will soot the exhaust. Is the engine broken in? if not, it needs to be loaded, in order to get the rings to seat properly. just idling the engine will not force the rings to seat into the cylinder. if the cylinder walls are glazed (mirror shiny), then the engine will probably smoke a bit. If just for show, oh well. For use, check oil level often. Your compression is acceptable. You might try setting the float just a bit lower, if you think it is still burning too much fuel, but it sounds like you just about have the mixture dialed in. I think that oil consumption will be your issue, with the 'white smoke being noted. NOTE: oil can get into the combustion chamber in a few ways: 1) past the rings, Worn or not broken in cylinders and rings will pass oil. And 2) past the valve stems. Excessive valve guide wear will allow the engine to pass oil past the stems. 3) oil dilution. Make sure the oil has no gas in it, and that it is the proper weight for the engine.
Andrew
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Old 02-23-2012, 12:49:56 PM
Kane Kane is offline
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Default Re: Zenith 68-7 - too rich mixture?

The engine specs SAE30, but I'm using 10w30 oil... is that fine? When I bought the engine it had NASTY oil in it, so I change it immediately. I think the new oil is the detergent kind, so I changed it again. I will keep on changing the oil until I get the oil running clean and then hopefully the inside of the crank will be free of mass amounts of oil sludge.

As for the carb, I pulled it once more again, adjusted the float for a little leaner mixture. Additionally, I sanded the lower carb's half to better fit the upper half as it was warped... Bob's your uncle, I did something right! I get a tiny little puff of black smoke upon opening the throttle and everything else is perfect. So I'm assuming the carb was sucking air or something. I will keep an eye on the oil and the oil level though, but I'm definitely not getting any white/blue smoke anymore, so I'm happy about that. The engine lopes a little for a few minutes while it is warming up, and then it is all good.

Thanks again for you guys' help... it was very valuable! Cheers!
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Old 02-24-2012, 10:29:49 AM
jamest jamest is offline
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Default Re: Zenith 68-7 - too rich mixture?

Have run a THD for years and mine throws smoke from idle to full speed then runs just fine with no smoke. Reringed it years ago so I know its not oil.
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Old 02-24-2012, 12:57:27 PM
Andrew Mackey Andrew Mackey is offline
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Exclamation Re: Zenith 68-7 - too rich mixture?

Probably worn valve guides. Won't hurt anything, just a little smoke on start up. Once the engine warms up, it burns off any residue from the oil. Nothing to worry about.

Kane, glad you got it sorted out. Good luck with it!

Andrew
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