DaveO's Zenith 40 TIN rebuilding thread

[DaveO90S4]

Hi all,

I have decided to start a new thread (rather than hijacking other users' threads) about my Zenith 40 TIN carb rebuilding and tuning and jetting etc.

The short background is that a couple of years I bought and installed a Chev Corvair 2.7L engine - equipped with zenith 40 TIN carbs - into my formerly 1.8L 4-cylinder Porsche 914. The vendor advised me before purchase that the carbs needed to be rebuilt/ retuned etc.

I've spent quite a lot of time on and off playing with jets and venturis etc. As purchased it had 34mm venturis, and had some transition flat spots. The carb rebuilder reckoned I should go to 30mm venturis, which is what he rebuilt the carbs with. Far too rich, but no transition problems.

Using my enforced downtime I've decided to have a go at getting these carbs performing properly.

I have now:-
1. Added the heat protecting phenolic blocks under each throat, along with new gaskets to and bottom;
2. Removed the problematic over-run enrichment plumbing and plugged all holes;
3. Re-installed the 34mm venturis.

I expected the larger venturis to make it run leaner, but not as lean as it does. Horrible transition and AFR as lean as 18:1 to 20:1.

So time to get stuck in properly methinks. More to come.

Cheers
DaveO

I have the zenith manual, plus carb sync tool, float level tool and accelerator pump squirt volume measurer.

[DaveO90S4]

For starters I decided to measure accelerator pump volumes - which should be 0.5cc +/- 0.1cc per event per nozzle.

With the accelerator pump shaft 'tappets' just barely touching the accelerator pump diaphragm pins at rest, when I open the butterflies fully I only get about 0.3cc on LH side and about 0.4cc on RH side. When I rotate the activating shaft directly, I can get 0.6cc. So the diaphragms, glass bead check valves, etc all seem to be up to the job. When I activate the throttle linkage (to get just 0.3cc) I can then manually press the accelerator pump shaft to get quite a lot more movement in the diaphragm, to yield 0.6cc.

So, in a nutshell, the accelerator pump cam at the end of the accelerator pump activating shaft does not seem to have enough 'ramp' to rotate the shaft sufficiently to then push the diaphragm plungers far enough to get the required squirt of fuel.

I can adjust the screw and lock nuts into different effective lengths - but that will not increase the effective travel of the diaphragm so will not yield increased fuel squirt volume.

I am considering modifying the cams on the end of each carb so as to give them more ramp / lift so as to then rotate the activation shaft more and hence increase the fuel squirt volume.

But that seems like quite a radical first step. Has anyone else experienced this problem, and if so, how did you overcome it?

With thanks and regards

DaveO

[juanbenae]

third person thread title? you think you are gordo or something?

[DaveO90S4]

Interesting observation. The latter portion of the accelerator pump diaphragm seems to produce more nozzle volume than the earlier portion of travel (ignoring actuating cam tamp profile). This has allowed me to get very close to 0.5cc per squirt without any cam ramp profile changes.

[DaveO90S4]

[QUOTE=juanbenae;10832920]third person thread title? you think you are gordo or something?[/QUOTE

Thanks for your helpful comment

[LakeCleElum]

Thanks for you being willing to help Dave....I have to do this soon on my '69 with a 2.2 engine.....

[Hi_Fi_Guy]

I'll admit when I rebuilt mine I was more focused on jetting and venturi size than trying to get maximum squirt from the accelerator pumps. Very interested to see your results.

[DaveO90S4]

Carbs back on - with no carb tuning whatsoever I think the results are quite promising.

30mm venturis, 55 idles, 135 mains, standard emulsion tubes.

Vast improvement over the 34mm venturis I was trying.

But of course quite a lot of carb syncing and test driving to do, so this is by no means the final jet / venturi selection.

I've never tried to tune carbs before, so this is going to be a first for me. Fingers crossed.

Cheers
DaveO

[scooperman]

would like to see DaveO's throttle linkage for the Zeniths on his Corvair engine. (First post here so not allowed to message members yet.)

[Nditiz1]

I'm pretty sure I bought DaveOs zeniths a few years back. He had a modified ball cup setup the looked closely like a vw hexbar linkage.

Here are some pics shown on a WTB ad.
https://forums.pelicanparts.com/porsche-911-used-parts-sale-wanted/1118788-wtb-weber-zenith-pmo-carburetors-my-3-0l.html

He may still have other pics. I have his email if you PM me.

[scooperman]

very much appreciated. As soon as I get permission from the you-have-posted-enough-times forum algorithm, I will PM you.