drilling a Zenith Carb?

[WallyB]

Greetings all. I've been out of the circuit for a while (literally). On a positive note, I finally have ol' 394WB certified & licensed. It may cheer some to know it is in Experimental category. No problems other than much confusion,miss mailings & spelling at FSDO. 'Nuff on that. On the down side,I haven't flown it yet due to lack of continous power. The Zenith 1821S side draft seems to be the culprit. No mixture & just one idle adjust screw on top. After several small adjustments on the idle air screw,Engine(1915)starts easily,runs fine up to say,25-2700 RPM, coaxing of the throttle will get it up to about 33-3400 RPM static. Any further forward throttle movement will then immediately make it sputter on down to 1000 RPM + or - 200. When it is at 33-3400, the slightest small application of the choke seems to keep the engine steady at 3400 RPM until reducing due tocylinder head temps getting a tad hot. Steve Bennet says the main jet must be drilled out (size?) until it maintains full power. Have any of you folks used and or drilled a Zenith carb or had similar problems with yours? My apologies for this lengthy essay, but Zenith infohas been hard to find thus far. Happy 2008 to all. Wallyb

 

[Idaho Spud]

If this is a type of Zenith carb I've seen, it should have a float.
I would first check the float level for proper height. Too low a setting will
not let enough fuel into the bowl and will starve the engine of fuel at higher
RPM. Too high a setting will cause the engine to flood. A lot of carbs like
that had a scew in main jet. you could screw one out an replace it with one
that had a larger or smaller hole. If you do drill out the main jet use a number
drill bit like a 28,32,43,etc. Start by finding out the size of the hole in the one
you have and go from there.
Jim C.
Sonerai-1 Idaho Spud

 

[Gaston]

Hi WallyB

the use of the choke should enrich the mixture and thus lowering temp.CHT is not as responsive to mixt. as EGT,maybe temp rising is due to the longer time running,and normal.
What are your numbers...

if temp really increase with the use of choke,I would look for a leaking intake past the carb, so leaning the mixture...

good luck

Gaston Dorval
C-GPXD S2L

 

[WallyB]

Hi Idaho Spud, thanks for your comments and...drill size info. Yep, this is a float carb and Bennet says it's a Zenith as he sold it to me. It has no visible markings & I haven't found Zenith Mfgr. as yet. On the engine runups: they have been done with no cowl attached using AV100LL gas. Numerous different settings of the (only)external idle air jet screw on carb top have been used in these run-ups. When engine has reached 3300-3400 RPM, it will hold sometimes for few secs & I apply a minimal amount of choke to sustain this rpm. I baack it off as the rear heads get up to about 400 -420 deg after 3-4 minutes or so at this 3400 static rpm. I'm gonna check the carb mount flanage for leaks as well as other points of interest in the intake system. Next, off comes the carb and chek out the main jet & float as you mentioned. Also have to scout the i-net for someone who has the gaskets etc for this carb. As Gaston Dorval says...good luck! I have heard some of the Sonerai folks use & really like the Zenith so, maybe I'll hear more on that. Wallyb

 

[Idaho Spud]

WallyB,
Does this carb have a T type screw on the bottom of the float bowl and
a slotted screw underneath the mounting flange? If so the bottom is the
mixture adjustment screw and the top one is the idle mixture screw. bottom
one kinda like mixture control on regular a/c carb, it works in opposite direction
turn in to lean mixture and turn out to richen mixture. If you go gpas site and
look at there carbs,it shows mounting instructions and has some good info.
Jim C.

 

[Gaston]

Hi wally B
I dont think 400-420 deg. is too hot, and related to a mixt. problem.
in this static running, cooling is limited, and such readings are quite normal I think.


the best way I found, to find a leaking intake is to pressurise the carb/ intake with a blower,and vaporise a light solution of water and soap on all the intake system ,making sure you turn the engine by hand to have both valves closed at the same time on all 4 cylinders during the process...
If no bubbles show up anywhere then you're OK...

I remember that on the last pair of new head I installed ,I had to machine the intake flange on the head because it did interfere with the intake elbow causing a major leak but easily seen upon assembly.

Hope this may help.
keep trying and investigating .You sure will succeed...

Gaston Dorval
C-GPXD S2L

 

[WallyB]

Hi Gaston. and IdahoSpud. I think I'll try your suggestion on pressurizing the intakes and check for leaks .the intake castings from GPAS came with gaskets near 3/16 or 1/4 " thick so I did have to use a much thinner gasket to prevent the castings hitting the cowl. There are plenty of hoses etc to possibly leak for sure. After removing some cables etc, I did finally see a amall print of "Zenith" but no model numbers, but this carb does match the GPAS pix of the 1821S in catalog. this carb has no T-screw type fitting anywhere including the float bowl. On the bottom centerof the float bowl, there is a thin brass nut(no slot) of about 5/8" or so.which I hope might be the main jet or perhaps just a drain instead. I didn't find a set screw at the mounting flange area. In any case, I will check for leaks as Gaston suggested & if ok, pull the carb & attempt to drill out the jet(slowly). I'm still hoping to contact Zenith & also locate a gasket source or make some up. I thank you gents for your interest & help in my problemo. I wanna fly it before I get much older as my oil temp sender(electric) also doesn't work so little things do add up if one waits too long. Wallyb

 

[Gaston]

Hi Wally B...
I also had to discard the thick gasket for the same reason you mention,but I used it as a template to trace and cut heavy paper gasket.using only one gasket is better (I think) than using the thick one which adds onbe more face to seal thus increasing the risk of leak.

You should make sure you have no leak at all before you blame the carb,which is most probably not the culprit...

Gaston Dorval
C-GPXD S2L

 

[phzabriskie]

Hi WallyB, Did you ever resolve your issues with your Zenith Carb. I have been interested in Zenith for a number of reasons. But I get very little discussion on them as most Sonerais are using Aerovee or Revflow. If GPAC is endorsing them there should be some advantage. I got a catalogue from Zenith a while back and the choices looked very interesting to me. Below are the upflow and sidedraft carbs. The Electronic Actuator option is very curious. I know most pilots would be shy of giving up manual control of throttle to an electronic devise, but what advantages might it offer?
As Main Jet Adjustment is optional it may explain why some builders are getting carbs that are not operating as expected.

Hi Gaston. and IdahoSpud. I think I'll try your suggestion on pressurizing the intakes and check for leaks .the intake castings from GPAS came with gaskets near 3/16 or 1/4 " thick so I did have to use a much thinner gasket to prevent the castings hitting the cowl. There are plenty of hoses etc to possibly leak for sure. After removing some cables etc, I did finally see a amall print of "Zenith" but no model numbers, but this carb does match the GPAS pix of the 1821S in catalog. this carb has no T-screw type fitting anywhere including the float bowl. On the bottom centerof the float bowl, there is a thin brass nut(no slot) of about 5/8" or so.which I hope might be the main jet or perhaps just a drain instead. I didn't find a set screw at the mounting flange area. In any case, I will check for leaks as Gaston suggested & if ok, pull the carb & attempt to drill out the jet(slowly). I'm still hoping to contact Zenith & also locate a gasket source or make some up. I thank you gents for your interest & help in my problemo. I wanna fly it before I get much older as my oil temp sender(electric) also doesn't work so little things do add up if one waits too long. Wallyb

I also found this link to carb parts supplier via Gaston.
http://www.su-carbs.co.uk/phdi/p1.nsf/supppages/turner?opendocument&part=2

http://www.teglerizer.com/sucarbs/index.html

 

[Gaston]

Hi Wally B

when you do your testing, is your cowling completely closed or if you proceed with open engine.
when I was working out my setting,I was not able to have the engine performing well because of prop wash leaning out the mixture.
with the complete cowling on, all became much mo easy...

maybe this could help

Gaston Dorval
C-GPXD S2L