TR4/4A Zenith Stromberg questions

[Zimmycobra]

Folks:

I have a '65/66 TR4A IRS with the ZS carburetors (175 CD). During the restoration of the past several years, I rebuilt the carbs and used the Moss rebuild kits for all the parts and needles, etc. While the car does run fairly well (it seems) there are a few issues that I am going to try to address. While the choke mechanically works fine, it will stall out the car when cold. Idle seems to be a bit difficult to adjust properly. From many years of playing with Webers on a big block Cobra replica (very different animal), my feel is that the car seems to be running very rich at idle. When operating the choke, it adds to the problem causing the stall. (My guess at this time)

What I learned from my Weber experience is that the old rules of thumb and settings are no longer correct due to the very different formulations of today's fuels. Hence my question - what is the correct needle and baseline adjustments that are correct for today's fuels? I am planning to remove the carbs and check all of the settings, needles and their positions, look for any other problems, and then re-install.

Any info and thoughts would be greatly appreciated.

 

[TR3driver]

So, what happens when you try to adjust the idle mixture?

One of the nice features of the constant depression carbs is that the mixture adjustment affects the entire range instead of just idle. Thus, simply setting idle mixture should compensate reasonably well for changes in fuel density (like our current E10 fuel). I've even run pure ethanol, just by lowering the jets (on SU rather than ZS carbs, but very similar).

Full choke should be necessary only for starting from cold. As soon as the engine fires off, you should reduce the choke setting to the minimum for smooth running.

It's also worth nothing that many other things can cause poor mixture. Bad cam timing, wrong valve lash, faulty ignition advance, weak ignition are just a few of problems that can masquerade as carburetion problems, even with an exhaust gas analyzer or Colortune. As a rule of thumb, if I can't get a clear peak when doing the "lift the piston" test, there is something else wrong. By "clear peak", I mean that there should be a definite point where the idle rpm bumps up and falls back to original, rather than staying up (too rich), or falling below original (too lean). If it takes more than 2 flats (1/3 turn) of the mixture nut to go from rich to lean then something else is wrong.

Aftermarket cams or even badly worn stock cams will also affect the mixture calibration curve.

Oh, yeah, one other thing. E10 has less energy per gallon, hence requires less air and takes the mixture lean rather than rich. If you are running rich, it's not from using E10.

 

[Zimmycobra]

Randall:

Thanks for the response. Just a bit more info - running stock cam and I have recently reset/rechecked timing, valve lash, ignition, etc. to be sure all is as good as I can get. Not sure I am rich but just my feel for the behavior. Actually, I guess what I was looking for is what would be the baseline or starting settings for the carbs from people's experience with the fuel currently. For example, what do you recommend for the initial turns out for the jet to get the right starting point for the mixture today, what needle should be used, what setting for the float (if different than the manuals state). If nothing is really different, I will just go back to starting with the settings that are in the original manuals and guides and then adjust from there. Just hoping to get any guidance that may have changed to save some time and effort. As always - thanks.

Zimmy

 

[poolboy]

Do your carbs have the adjustable jets ?
If so, see how the depth below the bridge compares to the carbs for the TR6 with non-adjustable jets for a baseline setting.
They sit approx. 0.10" below..
Did you make sure the diaphragm had it's index tab's in their homes ?

 

[trrdster2000]

zimmy, when you take the bottom screw all the way up, can you still lift the piston and it falls, if not the jet needs to be centered and that is a pain. When you get it so a half turn lets the piston drop that will be very close and now take it down 1 and 1/2 turns from the top and that is you set up setting. I find that you can go back about a 1/4 turn with a newly built carb and todays gas and be close. Make sure the choke adjustments are correct as they can play a big roll in the idle.

Wayne

 

[mtlman8]

Hi all, I'm having a carb issue. I had a perforated diaphram, so I replaced it with a new one. Plugs, ignition seem great. My problem is when I adjust the mixture screws nothing happens. I rebuilt the carbs about 3 years ago. What could it be? These are 175 cd2's. BTW. They are running rich no matter where the adjusting screw is. Thanks.

 

[trrdster2000]

mtlman8. Float adjustment or you used gross jets and they are stuck. Take the bottom off and remove the needle and seat, flush with lacquer cleaner, check the floats and see what happens.

Wayne

 

[poolboy]

It could be what trrdster2000 said or it could be what you are turning.
Can you identify what you are calling the adjusting screw ?
Use a part number from Moss, if possible.

 

[mtlman8]

I'm turning part # 365-480. Mixture adjusting screw.

 

[poolboy]

OK thanks.
I wanted to be sure.
https://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=29128&SortOrder=230

 

[mtlman8]

Thanks trrdster2000. Now I have a place to start. that is the correct screw?

poolboy

 

[poolboy]

Yes, that's how you adjust the mixture with that version of the ZS carb.
https://www.marineengine.com/parts/technical_information/sierra_18-7085.pdf

 

[glemon]

The early Zenith is a very simple carb and should be easy to adjust, pretty much like a contemporary SU with a rubber diaphragm instead of a machined piston. I am surprised when you adjust nothing seems to happen, sometimes when you to enrichen the mixture nothing happens at first because the jet is stuck and won't move easily on its own (but when you try to lean it out the mechanical force pushes it up so no such problem). You will also want to check and make sure the choke is not stuck open, which is actually lever that lifts the piston slightly on this carb I think.

 

[mtlman8]

Thanks for the help guys. I think I have it figured out. One Diaphram was perforated and the other one was distorted due to the needle not being seated in the jet, which caused the piston to sit high above the bridge. I also found that one of the nut and bolts that hold the clamp in place for the choke linkage was gone. I'll start with these issues then press on. Thanks again for the help.