are all metering units the same??

[Guest tobyholland]

are all metering units basically the same,theres a cheap one on ebay thought id buy to tinker with

thanks

toby

[Andy Moltu]

Essentially yes -they differ in terms of spring rates & max fuel settings/cam. The outlet union seals can differ slightly - use Dowty or O-ring seals.

 

There were one or two specials built with atmospheric pressure compensators - I've never seen one but apparently they do exist although they weren't fitted to general production Triumphs.

[k_raven_smith]

Hi Andy,

Whilst physically all metering units are the same, there are subtle differences in parts fitted and more critically the calibration of the units.

K R-S

[JohnC]

Hi Andy,

I'm intrigued - I knew there were at least two spring rates used for each of the large & small springs (maybe more, but I have only seen two), but do you mean there were also different cam profiles used? I thought only one profile was used (a straight one).

 

Toby - I have seen four parts differences between CU/MU assemblies:

1. Springs: There are at least two spring rates used for both the weak & strong springs. The ratio in rate is about 10:1 between the weak & strong spring in each pair (from memory - don't quote me!). Pairs are interchangeable in the same CU & between CUs, but you can't mix a weak & strong spring from sets of different rates, as the fuelling curve will end up all wrong.
   
2. MU diaphragm/cam follower - There are two entirely different designs, and you can't put a cam follower of one design with a diaphragm of the other design. Matched pairs of diaphragm/cam follower can be switched between MUs however. NB: I'm talking about the diaphragm between the wet & dry sides within the MU, not the vacuum diaphragm in the CU.
   
3. Non-return valve: As Andy says. Two different designs, using different seals. Providing you use the seal appropriate to the NRV, you can use either type in any MU.
   
4. PRV fitting - I have a very early TR5 unit with a threaded hole in the top of the MU (there's a flat face here on all other MUs I've seen). A local expert with many years experience of these systems tells me this is an alternative position for a PRV (a return goes to the tank from a PRV fitted here). We believe this was used in a very few of the earliest TR5s (possibly only on prototype PI units). It was also a mod fitted to some racers (easy access to PRV). I'm not certain which I have. In any case, a blanking plug fitted here (or a pressure gauge takeoff) allows this PI to be interchanged with others.
   

I've heard of the atmospheric compensator unit Andy mentioned, but have never seen one. I'd love to get my hands on one!

 

Probably more info than you need. In summary, although there are differences, PI units are interchangeable, as are most of the parts therein, with the provisos above.

 

If you do go fiddling, be prepared to replace all seals. Be careful with the diaphragms - they're available but why damage a perfectly serviceable part? Be very careful if you detach the roller assembly (within the CU) from the vacuum diaphragm - the screw thread at the top is plastic, is very easily broken if you overtighten the (metal) nut, and is not available AFAIK (although I do know of somebody who has made some metal replacements).

 

Calibration is a whole different ball of wax. Either have an expert do it, or be prepared to spend many, many hours of trial & error with either a dyno or a wide-band O2 sensor.

 

Corrections happily received!

 

Cheers,

John

[saffrontr]

Hi Toby,

As mentioned previously by Andy a few TR6 and 2.5PI saloons were built with atmospheric compensation units, see attached image. I have seen a number of build sheets at the BMIHT where these were fitted for TR6 bound for Switzeland. They are extremly rare albeit the odd one turns up and I saw the attached one at TR Bitz. You will certainly not confuse one of these for the standard offering. I did a spreadsheet a while back with the intention of writing something for TRaction however never got round to finishing it, at least not yet and it is on the list of things to do. This lists the various part numbers and commission number changes, types of springs etc. If you want a copy then send me an email.

cheers

Derek

saffrontr@tiscali.co.uk