Greetings from darkest France.
Before I start, can I apologise in advance if what follows is something that has been covered a thousand times. Please believe me when I say I've spent hours and hours on this forum trying to find answers, not helped by the fact that my internet speed is glacial.
I have a 1972 88" Series III, that - after I blew up the original 2.25 - now has an early '80's Defender 2.5 petrol installed. As I'm a very basic mechanic, the engine was dropped in and connected to the original 2.25 Zenith carb which has a K&N filter. Exhaust is via the standard 2.5 manifold, connected to a bigger bore stainless exhaust.
This set up has sufficed for some time as I've only used 'Olive' for local trips, tree stump pulling and teaching my son to drive. I now want to make the most of the engine's potential, as part of a rolling restoration.
Currently it takes forever for the engine to tick over cleanly and until the engine is really hot I have to use the choke to pull away at junctions, etc.
Reading up on the internet, I'm inclined to think this sounds like fuel starvation. So, first question: Would the original Zenith benefit from a bigger main jet? If so, can anyone tell me what size I should try first? (And if I'm barking up the wrong tree - please tell me!)
Second question: When I bought the 2.5, it came with an HRC1258 inlet manifold and a 32 34 DMTL carb. I've read lots of comments about how easy installing the Weber is - new choke cable, adapting the rod accelerator with a cable to operate the Weber cam, fuel pump etc. However, what I've failed to find is an idiot's guide to how to go about installing the Weber. Is there such a thing on the forum? if so, can someone point me I the right direction?
Many thanks in advance,
Winko

Hi from West Wales, The zenith carb is likely old and worn and was not fitted to the 2.5. I do not see any point in messing with it.
The 32-34 DMTL is the carb that should be fitted and as it came with the engine it should be set for it.
Is it still on the manifold ? If so just refit with new gasket. If not come back for more info.
Cables and linkage you may have to make up.
32-34 DMTL carbs use a two line system for the fuel so you will need to fit a return line to the tank.
It is usual for these carbs to have a fuel cut off solenoid so a wire that goes live when key is turned on is needed.
Dump the K and N and fit a proper paper element filter or the original oil unit.

Thank you Tottot.
Yes the 32 34 carb is still on the manifold, so bolting it on is easy. After that, what you are telling me is beyond my current skills base. If someone on the Forum has pictures of the required linkage, or even better the complete installation showing cable / fuel pipe / wire routing, that'd be wonderful. From my very limited knowledge I think I can power the fuel solenoid from the wires to the coil. Is is just a matter of running a wire direct, or do I need an in-line fuse? Secondly, what's involved in running a return line to the fuel tank?
Sorry if these are idiot questions, but I really am starting from a very low skills / knowledge level - but the forum is already proving to be a massive help.

It has been a while since I have seen a 2..5 petrol so you may need someone who has one to answer all your questions.
There may well be an empty socket on the bulkhead that is a switched live
The return pipe is just like the feed pipe, you will need a two pipe fitting on the tank, same as on a diesel.

Thank you.
Short term I'm going to continue with the Zenith and try to gather together the information and part numbers to allow the switch to the Weber. Can anyone on the forum advise as to what jet size I should try in the Zenith to help the engine pull better?

I'm rebuilding a 2.5 engine to replace the tired 2.25 in my 109 Series 3, so I will be facing some of these problems as well. The biggest issue seems to be adapting the throttle cable from the DMTL to work with the accelerator rod setup in the Series. Hopefully someone on the forum has done this before and can provide some pictures.
In the meantime, to get the Zenith working, it sounds to me like a weak mixture. It could be a blockage in the idle jet or an air leak at one of the joints (manifold-to-head or carb-to-manifold). I doubt that changing the jet size will help, but it would be good to check that it is not dirty/blocked.
The best approach is to strip the Zenith down and clean/check it, then rebuild with new gaskets etc. But, as tottot said, it's likely to be worn and maybe not worth the effort, especially if you have a good DMTL ready to go.

I used a throttle cable from a early 2.5 petrol 90. I drilled a hole in the footwell above the pedal and joined the cable to the pedal about a quarter of the way down the pedal. I used a little ball joint to the side of the pedal . my foot never gets anywhere near the cable.

Thank you Minecab,
Any chance of a photo or two, so I can get a better idea of how you did it?

may have to wait until the new year

That's no problem - I appreciate your help.