Baffled by my Zenith carburetors - Help...!

[Luis A.]

After diligently reading just about every post on the subject, I tackled my carbs. I only got to the carbs after adjusting timing, new plugs, new plug wires and valve lash adjustment. The car was running rough when not under load. On my first pass, I readjusted all the linkages per the book, adjusted the choke, and the minimum opening of the throttle plate. After all the basic adjustments it was running much better, almost perfect, but noooo, I couldn't leave well enough alone. I wanted to sync the carbs.

Before getting to the sync part, I noticed the rear carb was dripping fuel at idle but not the front one. Easy enough, go from the 1 mm washer to the 2 mm one for the needle valve. It still dribbled. I bent the the float arm in the direction that would have the arm reach the needle valve with less fuel in the bowl. Did this three times but each time the fuel still dribbled out at idle.

When I cover the little idle air hole on the top, the car runs much smoother, which indicates the mix is lean. But how can that be when there is still dribbling of fuel?

I decided to try and measure the fuel level in each carb and see how different they are. I took carbs off the engine and placed them on the workbench. No fuel spilled out when doing this. Took the top off and after blowing very low pressure air verified the needle valves were closed. Though a small opening on the top I used a a small rod as a fuel dipstick and verified that the level is about 4-5 mm lower in the carb that is dribbling fuel.

So I'm baffled.. How can the level be lower on this carb but still drip fuel when the front one has a higher level and doesn't drip?

Why does the carb that drips at idle, also runs better when the small idle air hole is covered?

I have both carbs on the workbench and want to finish this off today but I'm stuck... help!

 

[Stevehose]

You should sync the carbs, if one is drawing too much air it will cause dribble at idle. Start with the idle mixture screws out 1.5 turns and the throttle plate screws just off closed.

And you can test the float needle valves by swapping them between carbs, if the other carb now drips, there's your answer. Are these new valves or did you re-use the old ones? I ended up using my old ones because I didn't like the quality of the new ones (they didn't have the little ball bearing in the tip) - I cleaned them very well, verified they worked, and put them back in.

The combination of the above solved my dribbling issue.

Go back to stock washer size too. Keep us posted.

 

[Luis A.]

Thanks Steve. Laptop on workbench. I have a new valve on non-dribbling one and old on dribbling --HOWEVER, that is because like you I went back to the old to see if the new was the problem. Agree on quality. Will go back to old ones after I verify they don't leak by blowing air while closed.

Good point on dribble caused by too much air being sucked in... I'll straighten out the float to correspond to the one on the other carb. While I have them all apart (again) I'm double checking the insulating plate between the butterflies and the tops and ensuring all small holes are open. I'm keeping the original one instead of using the paper/mush one in the kit. I'll apply a light coat of blue hylomar to prevent any leaks being careful not to blob over the holes.

Will report back later!

 

[Stevehose]

Before reinstalling check the 4 large screws that hold the lower body to the mid body (they are accessed from under the carb pointing up - and retighten these - they will settle in a little a day or 2 after rebuilding and could be a future source of vacuum leak.

 

[Stevehose]

and also make sure the floats arent hung on a piece of ill-fiting gasket - I think deQuincey had a post on this.

 

[deQuincey]

and also make sure the floats arent hung on a piece of ill-fiting gasket - I think deQuincey had a post on this.

+1

here is the issue that steve mentioned:

http://www.e9coupe.com/forum/showthread.php?t=9535


by the way, the dripping fuel, as steve said might be due to air passing in excess, if you close the main butterfly using the screw located near the choke, you will find that the drop ceases

be careful if you install a double washer to reach a 2mm distance of float valve, it is almost impossible to assure fuel tightness, i was told that we should have a 1mm distancein any case !

 

[61porsche]

Stick a vacuum guage on the rear manifold. Loosen the butterfly screw/ position for the the best, highest, etc. Too much air is passing through the carb. Wen you move the guage to the front, you want them both as close to being equal a you can get.

Find a peice of vacuum hose- go over every part of the carb listening for air leaks. Idle screw o-ring, choke diaphram, the small gaskets that hold the choke housing on, etc.

Use the two mm gasket.

Use a socket on the fuel valve. Snug and a little more. ( My best guess is that this is it- dribbling.)

Don't bend anything. It doesn't work. No shims,etc.

Check and trim the gasket- it hangs up on the seam of the float. Even just a little can do it.

The original spacer- it had paper gaskets on both sides.

 

[Luis A.]

Hey guys,

Thanks for your help.

I'm confused now by the 1mm vs 2mm suggestions for the needle valve washer since the manual indicates it should be 2 mm but some very knowledgeable posters here suggest the 1mm. I have the 1 mm installed now and also verified the gasket is not getting in the way of the float. I had to interrupt my work this afternoon to resume the rest of my life and will be back to this early tomorrow. I'll report back then.


Steve, the 4 large screws are very tight and I don't see how they could relax since I'm not using the kit's cardboard insulator but the original bakelite one which is very rigid and should not give.

Jerry, my idle screws have no o-ring nor do I see it in exploded parts drawings. Should it have one?>

Stay tuned!

 

[61porsche]

13111257847 O-RING (04/71 to 07/73) Yes, there an O ring on the mixture needle valve. Use your original one if that's what you have. It's hollow at the tip.

Good luck!

 

[Stevehose]

Good deal on the bakelite - the replacements suck. If you have no o-rings you may be able to find a workable solution from ace hardware or HD/Lowes. Something to provide a snug fit on the mixture screw shaft and seal with the carb body but still allow turinng of the screw. The throttle screws have no o-ring.

 

[deQuincey]

my two cents

Hey guys,

Thanks for your help.

I'm confused now by the 1mm vs 2mm suggestions for the needle valve washer since the manual indicates it should be 2 mm but some very knowledgeable posters here suggest the 1mm. I have the 1 mm installed now and also verified the gasket is not getting in the way of the float. I had to interrupt my work this afternoon to resume the rest of my life and will be back to this early tomorrow. I'll report back then.
probably i am not absolutely correct, but i have checked this in person with proud expert and carburetor parts salesman ruddies in berlin, my car at least must have the 1mm washer, might there be euro specifications?, i dont know

Steve, the 4 large screws are very tight and I don't see how they could relax since I'm not using the kit's cardboard insulator but the original bakelite one which is very rigid and should not give.
the original bakelite is fine, but it should be together with an o-ring in one of its holes (best) , or with paper gasket in its two sides

Jerry, my idle screws have no o-ring nor do I see it in exploded parts drawings. Should it have one?>
my iddle screws hadnt an o-ring neither, jerry is referring to a differenttype of iddle screws that came on later models, mines only have the screw and a spring, also the screw is not hollow at the tip

Stay tuned!

 

[Luis A.]

Som photos to clarify the idle screw situation. Evidently there are two types of screws. My 73 Euro has no o-ring and has a spring, like DQ's. Looks like this:

The one Jerry and Steve are describing, with o-ring, must be like the one below. This one is from a spares set of carbs I have of unknown origin. Notice the sleeve around the opening where the crew goes which provides a sealing surface for the o-ring. My carbs have nothing like that.

 

[Stevehose]

One other thing since you have your carbs apart - check that the top sections aren't warped. Put them on a flat surface and see if you can see light through the sides. If so, I wet sanded mine flat by soaking a sheet of fine sandpaper and laying it on a granite counter - the water keeps it in place while you slowly sand the top flat holding it flush to the sandpaper with both hands, finish with 600 grit once true.

 

[Luis A.]

OK, so here's the latest. The suspicion that the gas dribbling in the rear carb is due to too much air being sucked is likely correct. In order to achieve balance between both carbs and stay around 900-1000 RPM's, the rear carb's throttle screw needs to be all the way out, that is, not opening the throttle plate any more than it is normally set to open at minimum.

So... I took my bright light and shone it in down the main barrel trying to compare both carb's butterfly opening at minimum (warm idle) and the rear carb's is definitely slightly more open than the front. I adjusted the butterfly to be open .7 mm with the fast idle screw when the screw is resting on the highest point of the cam. I'll have to take it off again and see what happens to that setting when the engine is warm and off choke...

 

[61porsche]

Luis,

Check the amount of air that the second barrel is contributing by holding the palm of your hand over the opening. ( it does the same as holding the air bleed closed.) Compare it to the front carb. You're looking to see if one is appreciably more that can upset the main barrel. ( lean)

Another way is to reach down to that lever and briefly touch it to see what effect it has.( rich)

 

[Luis A.]

Luis,

Another way is to reach down to that lever and briefly touch it to see what effect it has.( rich)

Which lever? Secondary?

 

[Stevehose]

Yes the secondary - now that Jerry mentions this, I also had this syndrom - the secondary barrel of one carb was just slightly open - you can also test by squirting some carb cleaner into each barrel and compare how long it stays in there before draining. Make sure your secondary butterflies are flush - this may require turning the adjustment screw and/or ensuring it is not binding when coming back to idle - you can slightly loosen the screw that holds the butterfly to the shaft and make sure the metal seats correctly in the barrel.

Which lever? Secondary?

 

[61porsche]

Down by the choke diaphram is the lever with the arrow. You can actually reach down and "blip it". If done softly, it introduces a little air. If the speed increases, it's rich. If the engine stumbles, the mixture is weak.

I know this is sometimes confusing to the best of us. The closer you get to ideal, the smaller the adjustments get. Sometimes it's the opposite of what you think. That's why the manual says to turn the needle for instance one way then the other. ( FI works exactly the opposite)

I'm used to using the palm of my hand for the secondary only. It tells me by the amount of suction, which carb needs a little less. I guess I could make up an adapter for an instrument to get even more precise?

 

[Luis A.]

So here's what my problem was... I forgot to disconnect the rear carb's linkage...! It's a good thing I'm not running a nuclear reactor...

OK so with that out of the way, I was able to synchronize both carbs at around 1000 - 1040 RPM's expecting them to come down to around 900 with the filter on (more later).

I also was able to get a tight fit of the syncrometer on the secondary barrels and they were very, very close as seen below. When I had the carbs out the car, I had checked them carefully and observed the secondary plates were closed what visibly appears to be all the way, but not binding at all.

One carb:

The other:

After I had put the air intake/filters on and drove the car a bit I realized the RPM's really didn't come down but stayed around 1000 - 1050. When I once again tried to sync them, by bringing out the throttle screw and then the idle screw, to try and keep the air/gas relationship somewhat in sync, I just can't get that low without the car running a bit rough. I was turning both in 1/4 turn increments.

All the while, this car's exhaust smells a bit rich to me. Since I bought it, running rough, and now, running a lot better, it still kind of smells rich/sooty to me. When I stop at a light, and the exhaust stream catches up with me, I can smell it. It's not smokey, just a bit smelly. So it may be running rich but it's proving hard for me to adjust it by ear/eye. Is there a way to measure tailpipe CO levels without spending a fortune on a fancy analyzer?

 

[Stevehose]

I gave up on the manual's "make final adjustments with the mixture screw" advice and turned the mixture screws rich until the engine idle started to drop then turned them towards lean to smooth out the idle - you may want to try going a little leaner still - it will backfire when too lean but these carbs like being on the rich side. Did final adjustments with the throttle screws. May not be correct procedure but it has worked for a year. I keep my idle at 1000 for when the a/c comes on so if you have such it's ok to leave it there.

Are you using a separate rpm meter or the one in the dash? the dash ones are not reliable - mine indicates 100-200rpm too high so if yours reads 900 it could actually be 700 and explain the rough idle. My dash gauge reads 1150 and my real meter reads 950.

With old cars and modern crappy gas, we have to improvise to get it to run as best possible.

I recently bought an old Heathkit CI-1080 exhaust gas analyzer for $60 on eBay - they come up often although usually around $150 - I got lucky. Haven't used it yet but will at some point, but again the spec number at this point may be irrelevant as long as it runs well.