MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

TR parts and Triumph parts, TR bits, Triumph Car Spares and accessories are available for TR2, TR3, TR3A, TR4, TR4A, TR5, TR6, TR7, TR8, Spitfire and Stag and other TR models are available from British car spares and parts company LBCarCo.

Triumph TR6 - Carburetor: SU HIF 6

What is the best needle and spring combination for the TR6?

Thanks.
Charles Edwards

What is the advantage converting to twin SU from Strombergs? Will I gain more HP or torque?, will I get better gas mileage?

I keep hearing that the SU are great but no one tells me why or what makes them so great. Is this a special secret club that you swear on oath not to tell all?

I do know that converting to Webbers (3) will net you significant HP gains albeit at the cost of driveability and fuel economy.

So please share.
Steven

Charles--I don't know the spring designation, but I have 'BAG' needles in my HS6's (not sure if that would apply to HIF's). I heard 'BAX' has a good profile as well.

Steven--The HS6's are a much simpler design and thus have less to tweak/fail and can be more reliable. Missing is the temp compensator, overrun bypass valve (although they did come with a spring-loaded bypass valve on the throttle butterfly--easily replaced with a solid butterfly), and dashpot rubber diaphram. Adjusting the jet is easier too from the bottom of the carb. From a performance standpoint, I don't think there is much difference over the ZS's. The cold start mechanism is simpler too (just lower the jet!). The only negative I've heard is that the throttle shaft bushings wear faster (they don't have a rubber seal like the ZS's). All in all, if you subscribe to the "simpler is better" school, SU's are the ticket.

Rick O.
Rick Orthen

Steven, I have SU (Skinner Units) on mine and they are very simple and easy to work on them. Looking for a repair kit but Moss and the others only have kits for Strombergs. If any one knows of a company that do a rebuilt, please let me know.
Angel L. Traverso

Correction to SU, its name is Skinner Unions
Angel L. Traverso

I'v sold about 10 pair of SU HIF6 carbs on eBay and all my customers are happy with BAG needles on the TR6. People, including me, seem to like the simplicity of the SU. There is no doubt the Zenith work well, after all hundereds of thousands were installed at the factory and ran for millions of miles. I'v been told the difference is noteable after the carbs go out of tune and need to be rebuilt. That is when the simiplicity of the SU becomes more evident. For example, one little hole in the rubber part inside the Zenith damper and game over. By comparison, no rubber part in the SU; lots of examples like that.

The HIF6 had some mixture control stuff to compensate fuel delivery for temperature. More efficient but more to go wrong. Also, the HIF6 has a valve on the throttle plate that needs to be removed for better air flow etc.

For rebuild parts contact Joe Curto on the internet. I purchased rebuild kits for my HD8's from Joe - good guy. Talked with him on the phone. He's a walking encyclopedia for SU's.

JP
John Parfitt

Looking for the guy who put a curse on my SU's...after many faithfull years of service, decided to overhaull them...ordered a repair kit from Moss, replaced throttle shafts with oversized ones. After ultrasound cleaning and glass beads blasting, etc., found out that the jets were to small for the needles. Remember Murphy's Law?...The kit its an original SU kit, someone put the wrong jets on the bag...calling Moss for the right replacement. One last advise..If it is not broken, do not fix it...!!!
Angel L. Traverso

I read a comment from a racer that after switching from Z-S to SU's, he had more power in turns, but lost a good bit on the straight away. Everything else considered for a street TR6, there is little advantage to the SU's IF the Strombergs are maintained in good shape. And it's really easy to keep them in good shape. Personally, I wouldn't do it.

Brent
Brent B

Guys,
I've just re-built my ZS 175's, and to Brent's comment above, they are ALMOST easy to keep in shape. The only hitch I ran into is one bypass valve that refuses to close. I confirmed the problem by removing it and covering the holes, and the idle drops to where it should be.

I noticed on an earlier thread that some of you have removed the endcap on the valve to allow adjustment. Is it just drilled out or is it more complicated? I'm tempted to put a solid gasket under this valve and run with just one, but thought I should try to do it right first.

By the way, the Buckeye Triumph instructions on this were great. I never did a carb before, and they made it really clear.

Thanks,
Mark

Mark Hauser

Mark--Use a suitably sized punch to knock the cap out on the BPV. Make sure you have a O-ring seated below the threads on the valve stem upon reassembly.

Rick O.
Rick Orthen

Rick,
Do you mean punch from the inside (with the stem protected) or punch the outside and push it thru? Also, what's the story on the o-ring? I don't have one in mine, though the rebuild kit has a quite a few (for other models I assume). Is it under the cap?

Mark
Mark Hauser

Here's an excerpt from TR6wolrd.com:

"IDLE BYPASS VALVE

More Voo-Doo and magic. If it has been a while since these things were overhauled the diaphragms are probably hard as a rock. They are the funny looking assembly on the forward side of the carb right next the brass colored throttle shaft. Remove the assembly by extracting the slotted screws. Once the assembly is removed, extract the phillips head screws.

Just to let you know there is a spring inside so don't be surprised if the thing pops apart. Please note that the diaphragm has a gasket on both sides. When you re-assemble this unit put it back the same way or the diaphragm will not be properly supported. Moving the adjustment screw should be avoided if possible. Getting these things back into adjustment can be a chore.

This diaphragm is not part of a standard kit, so if you want it put it on your parts list. The gaskets usually are part of the kit. Go figure."

The first time I rebuilt these buggers years ago the kit came with 2 gaskets. One was designed to let air through with the valve closed - the middle "damn" had an opening. Naturally, the idle was too high. Don't use that one even if that's what came out.

Also - did you renew the spindles & seals? If you've never done it, it probably needs to be done. That was the biggest air leak I had.
Brent B

Brent,
I went thru the kit a couple of times, and the only gasket it comes with is the one with the gap between the two holes. The front carb works fine with it anyway, but I'll check around tomorrow to see if someone has the other version.

I did replace the spindle seals. Overall, if I drop down the idle by covering the bypass ports with tape, it idles nicely around 800-900 rpms, so I think this is my last hurdle.

Thanks for the info,
Mark
Mark H

I had to end up replacing the spindles as well to get a good idle - but after that (and new needles) the carbs have been perfect!

You got a good idle by taping the by-pass ports. It's not recommended to permanently by-pass them. Try
1) check that the valve seat is not gouged or worn. You may want to try a little polishing compound to be sure it's seating well.
2) If adjusting the spring doesn't help, you can use a piece of material from another gasket to blank off the passage. That may work for a while until the spindle leak causes the idle to go up again.

Brent
Brent B

Umm - I got hurried so #2 above isn't very clear. What I meant was that you could modify the by-pass valve gasket with another piece of material (a small piece of another gasket, for instance) to close the leakage path.
Brent B

Mark--You have to disassemble the BPV to remove the end plug (I guess you could try drilling without disassembly, but you'd risk buggering up the valve stem). The O-ring fits over the slotted head of the valve stem, slide past the threads to the landing. I think the ring is 1/16 in cross-section. It keeps air from being drawn through the BPV cover where the adjustment screw is accessed.

Rick O.
Rick Orthen

Rick/Brent,
After digesting the info here, and talking to TRF, I decided to 'bypass the bypass'. As noted above, one of the attractions of the SU is the absence of the bypass, so I guess this is part-way there. When I mentioned it to TRF, they said it was done by many others. I cut a blank gasket to seal off the bypass on the faulty carb and voila, problem solved. Its idling nicely at 850 for the first time since I've had it.

Still need to stand the test of some more miles, but this may be a viable alternative to fighting with the BPV. And the gasket should slip by any judge (I've read the debate on concours vs. fun)

Thanks as always for all the input,
Mark
Mark H

Mark--Great to hear the blanking worked. It would be nice, however, to get the darn thing to work right to begin with. But I understand your frustration on that front. Maybe now you'll be able to successfully do the piston lift maneuver to tweak mixture. I'd like to hear if the fix has a noticeable effect on performance (engine braking, exhaust pops, etc.).

Rick O.
Rick Orthen

Rick,
It took a while to get you feedback on the results of the bypass-bypass, because we are only getting ocassional driving days here in NC. The bottom line is that after about 100 miles on the rebuild, the elimination of 1 bypass has been only beneficial. Remember there's still one left, and some of this improvement may be due to the carb rebuild overall.

I never had any exhaust popping on this car, so I can't report on that. However, the engine braking and idling are MUCH better. One of the reasons I started this was to get down to low rpms quicker when shifting to give my aging clutch a break. Changing gears at 1500 was no fun. Now shifting is much smoother, and idle is comfortably in the 900 RPM range at idle.

I certainly don't suggest getting rid of a working bv, but I wouldn't hesitate bypassing a funky one rather than lose any more hair over it. Its easy enough to reverse if the need arises.

Mark
Mark Hauser

I would have been interested in whether the "early" gasket would have fixed it, too.
Brent B

Angel

I think I saw repair kits for the SU's in the last TRF catalog.
Curt Anderson

This thread was discussed between 14/12/2002 and 21/02/2003

Triumph TR6 index

This thread is from the archive. The Live Triumph TR6 BBS is active now.