Zenith 40TIN carbs on 3.0SC

[chris_seven]

Just re-built and fitted a pair of Zenith 40TIN carbs on a 3.0SC.

The carbs had the throttle shaft bushes reamed and new shafts fitted along with new throttle plates.

We fitted 36mm venturies that we made and modified the main jets and emulsion tubes.

The manifolds were re-finished and we used some thermal insulators to match to the ports and seal the injector holes.

After a few initial teething problems and some spitting and banging the engine has now settled down well and idles smoothly at 900rpm.

It pulls strongly from 1800rpm to 7200rpm without any flatspots or hesitation.

I think it has lost a little at the bottom end compared to the CIS possibly due to the 36mm chokes and we plan to fit some 34mm parts in the next couple of weeks.

The Air Filters are ITG Foam units that we had custom made to fit a standard sized rain hat.

We had a slight distributor problem initially as we had the vacuum advance connected to a single port and this caused some advance instability at idle.

In the short term we have fitted a 'damping valve' into the vacuum line which has helped significantly but we plan to modify the dizzy further and remove the vaccum advance over the next few weeks.

We have fitted headers and a custom re-packable prototype 2 into 2 Silencer that we manufactured in house.

I would like to thank Paul Abbot for the advice he offered while we were first setting up the carbs.

[Schmierung]

Modified emulsion tubes? Please elaborate. Doing same swap.

[wachuko]

I just love the looks of an engine bay with carbs... looks so clean and neat in there.

[James Brown]

Bravo, i did the same thing and loved the quick throttle response and seat of the pants pull. to bad there is almost no support. But i did find a place that machines new venturies in any size

[chris_seven]

Originally Posted by Schmierung

Modified emulsion tubes? Please elaborate. Doing same swap.

We changed the size of the holes in the existing emulsion tubes and added a few extra holes where there were only two in the Zenith tubes.

We have made throttle shafts in 8mm dia and 8.5mm dia and have turned down the ends so we can use the same size beam coupling as the Weber and I have just ordered a batch of beam couplings (50 off) to get the price of replacements down to a more reasonable level.

We made 4 sets of 36mm Venturies and have 3 sets of 34's being made now.



They have been anodised gloss black since this photo was taken.

We also made Weber IDA shafts in 8mm and 8.5mm dia along with Solex 40Pl and MFI shafts, but we only made these in 8mm as it quite easy to re-bush the Solex and the MFI.

[POR71]

Hello Chris,
We're about to do refurbrish a 3.0 SC engine and just found a set of Zenith carbs....and what beter thing to do then change it to carbs.....I 've send you an E-mail with pics of the carbs....and are desperately in need of some advice....
Many thanks...
Ivan

[911hutch]

I am putting a 930/10 CIS engine into a flared 914 as a track toy and have a perfect set of Zeniths sitting on the shelf.... but am still reluctant to remove the simple cis from the perfectly running 3.0 and switch to an application that is considered some what unorthodox and lacks a technical following. But Mike from Windsor Ct. has provided development information (re: jet size, etc) with an excellent presentation of hard data regarding this application that has helped me to seriously consider this plan. Check out Mike's information or PM him at VFR750as he has been very helpful regarding specific technical questions.