SU HS4 Mods

[Mark]

Hi all

In the process of getting the Spitfire runing and been having problems with the engine reving itself slowly up after a period of idling perfectly. No air leaks I can find. Currently fitted with HS2, I am about to fit some HS4 carbs asI bought a rebuild kit several years ago and never used it. I has a race spitfire years ago that was fitted with SU's that had been tweaked by TLD. would never pass the MOT with them. reduce Co and the Hydrocarbons went off the scale. I swopped them for standard HS4's to get an MOT then switched back. Car ran terrible with standard carbs even with the richer needles fitted. but on the face of it couldn't tell what had been done to them but they made a massive difference to the performance. So what tweaks make a difference. Thinning the spindles. sharpen the leading edge of the butterflies to try and increase air flow. What are worthwhile mods?

Cheers

 

Mark

[spitNL]

Hi Mark,

 

Have you seen this thread?

http://www.sideways-technologies.co.uk/forum/Blah.pl?m-1299782857/s-all/

Lot's of info what flow mods work best with carbs.

 

Linking to the old forum, cause the new thread is somewhat broken.

[Mark]

Thanks for that, It seems that Stub Stacks, Ram Pipes makes an improvement, I've never used them but dug out some 4" one's I bought years ago and never used. I am going to thin down the spindle and knife edged the butterfly. I've soldered up the valve so I'll see how it goes.

Cheers

 

Mark

[Mark]

Started to rebuild the carbs with an SU kit supplied by one of the main suppliers for the HS4 and converting it to non waxstat. I bought this a couple of years ago but only just opened it. I can see that I am missing some clips, but the main problem is the choke lever that connects to the bottom of the jet appears to be the wrong shape and fowls the head of the bolt that holds the float chamber onto the carb body. I may have put it together wrong but I don't think so. Anybody have an idea if this is the correct link for the HS4? I have a set of HS6 carbs with a similar set up but the link goes into the front of the hole on the cam on the carb, as apposed from behind so clears the bolt by a mile.

 

Cheers

 

Mark

[spitNL]

Are you sure it should go round that way?

I've never rebuild a non-waxstat HS4, but to me it looks like the lever should hook up in the slot.

Like in this pic.

[James]

I seem to remember they go on another way— at least I had the same issue.

[Mark]

Hi

Yes it's the correct hole the other is for the linkage. Here's a pic of another set of HS4's that i have. Somebody replaced the waxstat but kept the old linkage, but didn't connect it. I can make some more clearance by filling the head off the bolt and replace the rubber with something thinner which may work.

[Mark]

I did try it the other way around but then the link steps out by 10mm missing the hole in the jet.

 

Cheers

 

Mark

[spitNL]

Sorry, you're correct, the other hole to which I pointed IS for the linkage.

 

Is that the old bracket you're using?

If I look at the contents of the conversion kit on Rimmers there are new brackets, so they could very well be different from the old ones.

http://www.rimmerbros.co.uk/Item--i-RL1474

 

 

CHeers,

 

Frederick

Edited June 1, 2011 by spitNL

[mark spit]

The little brackets are the crucial bit.

 

I've chopped a bit of Fredericks picture.

Pretty sure the bracket goes onto the choke cam mech and the new metal actuating rod goes through the black circled hole in the picture.

Seeing as you have another set of carbs, you can simply take the drop linkage bracket off them and attach it to your nice new rebuilt carbs and the new rod. This was done on mine when I changed the waxstats which were disasbled anyway to standard jets.

Mark

[Mark]

Hi

 

The linkage on the other set of carbs is for the waxstat jet and has a different fitting which wont work with the non waxstat jet. I will have to work around it. The "full kit" I supposedly bought including spindles butterflies etc. is missing bits and I have probably been supplied the wrong parts. As I bought the kit a couple of years ago now its a bit to late. It is just frustrating the amount of parts I have bought from triumph traders over the last couple of years that are incomplete, wrong, badly made etc. ie. Vitesse front wings, rear edge is wrong (found out after I fitted them) so now have to cut new wings off Vitesse rear wings 10mm to short. wrong fuel pump with a doggy shaped cam, wind screen rubber wrong shape, roto flex rubbers made from bubble gum, could go on all night, (but in the wrong section)

 

Cheers

 

mark

[AlexC]

I modified my HS2,s in stages and I found the largest improvements were when I reduced the spring seat pressure and removed the dampers(no it doesn't flutter) and the other large gain was chopping off half the spindle and thinning the back BUT if I did it again I would leave both sides on the spindle but thin the front on to .050" or less and the rear(threaded) to 0.080" or 0.070" if i was feeling particularly cheeky. Also lower the countersinks in the shaft and solder the screws for safety. I found increased piston travel only made a difference above 6k rpm.

[AlexC]

OH! and of course and mentioned use full radius stub stacks, the longer ones will work better at lower rpms.

[Nick B.]

Bollox, had just written a long(ish) post when the computer crashes (again...)

 

The waxstat conversion kits is of questionable quality compared to the original non waxstat linkages. The chokercontrol can be hard to get right due to tensions in the bracket leading to a sticking choke. I have chosen to put my effort into dellorts rather than sort out my HS4. So I still have to manually push the jets up when I have had the choker engaged.

 

Anyway - here is a few snaps - hope it helps.

 

 

 

 

 

 

 

 

Cheers

Nick

[spitNL]

Nick, did you bend the link like that yourself, or did it come that way in the kit?

 

Sam, if the waxstat jet is in good condition you could also re-use it and convert it to non-waxstat.

http://home.mindspring.com/~purlawson/files/SU_Waxstat_Jet_Tube_Conversion.pdf

 

 

Cheers,

 

Frederick

[Mark]

Nick

 

Great pictures, thanks for making the effort it's really helpful. I will have to do something similar but it's going to be tricky to get it right, as you say any pressure on the jet will make it stick.

 

Cheers

 

Mark

[Nick B.]

Nick, did you bend the link like that yourself, or did it come that way in the kit?

 

I didn't bendthe link myself - it came that way.

 

 

Nick

 

Great pictures, thanks for making the effort it's really helpful. I will have to do something similar but it's going to be tricky to get it right, as you say any pressure on the jet will make it stick.

 

Cheers

 

Mark

 

Your welcome - no great effort.

 

Shouldn't be to hard to replicate the bend. And whether you ebend it or not, you probably have to work it quite a bit to get the jets right. Best of luck with it.

 

 

Cheers

 

Nick

[Mark]

Hi

I put the carbs back together today and decided it was easier to file the head off the bolt that was in the way, than try and put the missing 'kink' in the link supplied. It's working OK and the chokes aren't sticking. I left the spring clip off the end of the link as it stopped the link sliding slightly in the hole at the end of the jet, causing the jet to stick. I narrowed the HS2 linkage as these are going on my MK3 Spit and decided to keep the mechanical linkage for now so couldn't use the HS4 Linkage. I thinned the spindles and knife edged the butterfly and removed the valve. I used the old parts to play with so I still have most of the Kit. I probably could be more radical with the spindle, thinning it more but didn't want to weaken it as the butterflies are under a lot of tension from the return springs. Its probably 40 percent thinner, with no screws or valve obstructing the Venturi so should flow better than before. Can anybody tell me what the clips are called on the end of the link? it's the round disc with the four triangles in the middle that pushes onto the link and bites in.

 

Cheers

 

Mark