SU HD8 Throttle Plate Seal and tolerance

[Blue Streak]

Hi I’m hoping some experts can help me with the maximum tolerance I can live with on the throttle plate seal of the rear HD8 carb on my series 2 e type. The carbs were a refurbed set and generally have been done very nicely and the engine runs fine but I can’t get idle below 1050 rpm. I’ve eliminated vacuum leaks (To vac tank or the manifold itself) so took a look at the carbs. The front and centre carbs have a good throttle plate seal minimal light and they trap a 0.04mm feeler gauge all round against the body. However the rear carb throttle plate has a gap in the bottom left and top right quadrants of 0.04mm rising to 0.08mm as I get to the spindle the top, bottom, bottom right and top left will trap a 0.04mm feeler gauge and let no light through. I’ve spent a few hours adjusting the plate but that’s the best it gets, it’s not bad and better that when I took it off, question is whether the tolerance is too much and I need to get a replacement, what do you think?

In the picture you can see the light coming past. Any help greatly appreciated.

[Gfhug]

Hi Blue Streak, can you add your name and car type to your signature, please. It’s much nicer to address you by your name, unless you’re really shy

You say the carbs were overhauled, by whom? If a professional company I’d suggest sending them back. Have you spoken to Burlen? https://burlen.co.uk/ They will sell spare plates, etc. if you can’t get sorted through the refurbisher.

Is everything properly set up on the carbs as per the manual, choke settings, etc.? There could be a source of problems.

Geoff

[mgcjag]

They should seal completely with no light showing.....try turning the plate 180 deg....Steve

[Blue Streak]

Hi, I’ve reached out to the refurbisher but no joy yet and tried reversing the plate and the gap I got was the best I could achieve. Have followed the full carb set up process per the manual with throttle linkages disengaged, ensuring fast idle screw was clear and the springs are in good condition and aligned properly.

I think as you say will probably need a good throttle body/plate to resolve.

Thanks for replies so far.

Rob.
1969 Series 2 OTS

[RICHOT]

I had the same problem a while back - couldn't lower the idling speed and when I did manage to, it soon returned to 1250+rpm.

The butterflies didn't seal perfectly and the spindles had a little play in them.
I sent the carb bodies to Burlens who replaced the butterflies and put in new spindles as they said that they and their bushes were well worn and could have been letting air in.

On reassembly, the idle speed is spot on.

Have you considered that it may also be worn throttle spindles which are adding to your problem?

Good luck!

Richard

[Blue Streak]

Thanks Richard, the spindles have minimal movement in them but you are right this might be my main option as I expect if I put it back on whilst it will be a little better than before it probably will continue to be an issue. Out of interest how much did they charge per carb for the work?

[RICHOT]

Can't find the info for my E Type but:

I had 2 done (for my XK120) in Sept 2021 and the cost for the two carb bodies being refurbed (rebushing the bodies, fitting spindles, discs etc.,) was £202.02 which included VAT and postage back to me.

Worth getting in touch with them, I've always found them very knowledgeable and helpful.

Cheers

[Blue Streak]

Just out of interest here’s what one looks like against the same light which will clamp a 0.04mm feeler gauge all round, just the tiniest bit of light near the spindle.

Rob
1969 Series 2 OTS

[Blue Streak]

That is the picture of the one above is one I presume is good.

[bitsobrits]

Perhaps your 'rebuilder' mixed up the throttle plates? It's a bit of work, but it may be worthwhile to move the plates around to see if the fits are improved.

Do keep track of where they started:)

[Blue Streak]

So quick update, the carb body has been sent off to Burlen for rebushing, new spindle and throttle plate including a vacuum test. When it returns I’ll take a photo and measure the tolerance so hopefully we’ll get a definitive answer to the question.

[mgcjag]

As per my post above(and Burlan)...they should close completely......or you get air leaks.....look close at the plate and you see that it has a beval edge to create the seal......Steve

[Allrand]

I think I see your problem, the throttle plate for that carb is facing the wrong way. Note the circular stamping or machining mark isn't visible on the one that fits well. You can't simply rotate it by 180° because of bevel angle would face the wrong way. Rotate it 180° and flip it back to front, it might achieve a better fit that way.

[Blue Streak]

Final Update on this one. I sent the carb body to Burlen to see if they could help. The restoration team tried a few different options including new shaft, throttle plate including slightly oversize versions but to no avail. In the end they were able to provide a good second hand version which you can see from the light allows no light past, will clamp a 1 thou feeler gauge all the way round the throttle body where the plate closes and will hold a sufficient vacuum to pass their end of line test. So the moral of the tale is no visible light, and should be able to clamp a 1 thou gauge all round to be able to pass the vacuum test and thus seal well enough to allow the idle screw to operate properly. Thanks everyone for your input.

[Blue Streak]

Just to clarify the picture is of a good second hand carb body with a new shaft and throttle plate fitted. It appeared that the carb body I had had possibly been somehow distorted so it was no longer round hence the throttle plate didn’t fit.

[Blue Streak]

Just put the carb back on tonight and………success! Idling now able to be brought down to 700rpm using the idle air screws so must have a nice seal at the throttle. Interesting how what looked like a small gap (Less than 2 thou mainly 1 thou) could have such an impact from one carb. Hope this might help someone with a similar problem.

[dal2.0litrefrogeye]

Just being nosey is "throttle plate " an American terminology? Where as us in the UK call them butterflies
Kind regards nosey of Orpington