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michaeljf

2" SU's on TR6.

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Hello,I am just wondering, and this will probably cause a lot of spilt cups of tea ,if anyone has any experience with tuning 2" SU's on a TR6 (fitted with a Mk2 2.5 PI motor,extractors into a 2.5 single exhaust,123+ distributor,K&N front mounted air filter with 3"tube running back to fabricated air box) Carbys are AUD411 (Rover 2000 TC USA spec.?) with AAB Biased needle which I plan to modify to a Fixed type needle + the neck of the jet. Regards,Michael.

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Hello again, Sorry, there are two of the above carby's attached.Regards,Michael.

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Just my opinion of course, but the engine would need some serious other tuning mods before needing 2 " SU carbs. A pair of 1.75" HS6 carbs will work for the spec you have.

Next steps could be some work on the cylinder head, a longer duration cam and triple Weber 40DCOE carbs. Or Lucas PI of course.

Nigel

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My concern about the larger carb is would the engine provide enough vacuum to make them responsive enough, especially off the line and lower RPM.  Bigger hole, same draw of aie = less flow velocity across the needle = bad,  Also, the pistons will be slightly heavier, meaning more flow is needed to move them.    The 2.0L GT6 used 1.50's so the 1.75's are already a step up.   The 1.75's are good to up to 150 bhp (so I've read), so unless you're going beyond that, I guess the big question is why?   If its more performance you're looking for there are far more proven ways to get it.

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You will struggle to get them to work any better than 1 3/4" and will probably find they are worse, Jaguar ran 1 3/4" on 3.8 Mk2 engines with twin OHC cross flow heads rated to about 210 HP  as standard so stick with them.

Stuart.

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Hello all,Thanks for the replies. At the moment it is running lean,so we have changed the needles from floating to fixed .Tomorrow we will see if we can get the air/fuel ratio sorted a bit better with a heap of needles a friend who gave me the carbies  and races a TR3A with 147 helpers AT the wheels and is running a pair of the same carbies. (mind you his is a whole lot more modified than I plan to have mine).I also have the fuel injection that was on this motor when I put it in the car. I was just wondering if there might have been someone else out there who had tried something similar.Regards Michael.   

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Hello,I'm back. I have decided to rebuild (finally) the original motor.Started getting a bit suspicious when I took the tappet/rocker cover off. Alloy valve spring retainers,next out with the pushrods ,they are black "pressed together"and a lot lighter than standard ones,then off with the head,the valves didn't leave a lot of room in the nicely shaped chambers, then to the pistons ACL,a fair bit of "grinding "around the big ends and crank,then out with the cam which is mounted in bearings and it has kastner a letter and number on the end,the oil pump pickup is also different to the one in the PI motor. It also had/has two fuel pumps,a named/numbered roll bar,alloy wheels plus the exhaust I mentioned earlier.   So far.....regards,Michael. (maybe when it all gets back with some new bits it might have nearly enough vacuum to make the carby's work.)

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What cam is in the Mk2PI engine - is it the same as the late (CR) TR6 or the TR5/CP TR6.

If it'd the former it should work with carbs if set up right, if it's the latter then you might benefit from a cam change.

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3 hours ago, michaeljf said:

Hello,I'm back. I have decided to rebuild (finally) the original motor.Started getting a bit suspicious when I took the tappet/rocker cover off. Alloy valve spring retainers,next out with the pushrods ,they are black "pressed together"and a lot lighter than standard ones,then off with the head,the valves didn't leave a lot of room in the nicely shaped chambers, then to the pistons ACL,a fair bit of "grinding "around the big ends and crank,then out with the cam which is mounted in bearings and it has kastner a letter and number on the end,the oil pump pickup is also different to the one in the PI motor. It also had/has two fuel pumps,a named/numbered roll bar,alloy wheels plus the exhaust I mentioned earlier.   So far.....regards,Michael. (maybe when it all gets back with some new bits it might have nearly enough vacuum to make the carby's work.)

It sounds like your engine has had some serious work in the past. It's unusual to go to the expense of lightweight pushrods and valve caps unless without also fitting an exotic (probably competition spec) camshaft.

If it was mine, I would want to find out the cam spec, either by looking up the Kastner reference or by measuring valve timing and lift directly. It could be the cam you have is too wild (too much duration/overlap) to give power at reasonable rpm for road use.

3 hours ago, Andy Moltu said:

What cam is in the Mk2PI engine - is it the same as the late (CR) TR6 or the TR5/CP TR6.

If it'd the former it should work with carbs if set up right, if it's the latter then you might benefit from a cam change.

The Mk2 PI uses the 311399 cam, same as the CR series TR6. It's quite tame with timing 18/58 58/18. It will work well enough with the carbs but the 308788, fitted to Mk1 PI and Mk2 Vitesse/GT6 is even better for carbs, with timing 25/65 65/25. I've been researching the subject of Triumph cams to use with carbs for a 2500 engine I'm building for my GT6. More info available on Chris Witor's website by downloading his camshaft pdf:

https://www.chriswitor.com/technical.php

 

Nigel

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Hello all,Thanks for the input.As the rebuild progresses all the advice given will be considered.Thinking of 77mm pistons and the Kastner cam is an S4. Regards,Michael.

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