The V8 Owners Forum -

Long-standing problem with the SD1 unit in my Lightweight.
Hope someone can give me a steer.

Standard engine, with so-called 'hi performance lifters' installed in 1992.

Won't stay in tune.
Standard H1F6 carbs, although I'm not necessarily pointing the finger at the carbs. I rebuilt them a few years/8,000 miles ago with a kit from Southern Carbs.

Ever since I owned it seven years ago, the mixture has tended to drift. When I bought it, was showing over 10% CO.
Back then, I'd retune it, it was OK for a few months, then it would drift up or down. Maybe back towards 10%, sometimes down to less then 1%.

Over time, the interval between tunes has got shorter. In the beginning it would hold maybe 6 months, then three months then one, now it won't even hold overnight.

I've checked the float height. I replaced the (perfectly good) floats.
I have a Facet pump and a pressure regulator with gauge, set to 2.8psi.
I sleeved the fuel pipe across the back of the engine in case of ethanol evaporation.

Renewed the carb/manifold gaskets with red Hermetite.

I still suspected an air leak so the valley gasket was replaced. (The cam and lifters looked OK.)

However, it's no better, running lumpy, won't idle and the car is simply not drivable.

The weird part now is the carbs. If I cover the nearside intake, the engine almost stops - as it should - but if I cover the offside, the engine speeds up!!

Tearing my hair out - would appreciate any ideas please?

Ivor

Had this happen once and it was a sticking dashpot piston in one of the carbs.
Also 2.8 psi seems a bit low.

Thankyou.

I believe for SUs, the correct pressure is 2.6psi, I upped it a bit. I will however check.

The dashpots and pistons are clean, no gum. Could polish them with Solvol perhaps. And of course there is no guarantee after all these years that they are a matched pair any more or even belong to this pair of carbs.
It had 13 owners since from 1983 to 1999, bit of a case of pass-the-parcel, which makes me wonder how long this problem has been going on.

Off the original topic, but how do we rate the SUs against a 4-barrel? Is there any advantage on a standard engine?
I ask because I have a new Edelbrock 500 and a JWR Offenhauser manifold (dual port) which for a road-going auto should be a good combo, but I never convinced myself that it was worth the bother of fitting them. Given the current woes however, perhaps I should reconsider?

V8

Hi Ivor
Have you checked you have oil in the dash pots?
Also have you checked the diaphrams do they have a tiny hole in them?
Does the engine rev well and if not have you checked or even have a fuel filter .
These can block over time and if you are running a Facet barrel style electric pump these have a filter in the base which can get mucky quickly
With regard to the 4 barrel I would say got with it without question.
Better economy and performance so win win situation in my opinion.

cheers
P

Mmm, yes, I think currently ATF in the dashpots. Had been using 20/50. Iirc the original SU oil was SAE 20.

Inline fuel filter quite new. The pump is also pretty new, and the fuel tank.

This problem has survived all the component changes and renewal, so far.

No diaphragms, that would be Strombergs I believe, so at least one thing less to go wrong.

Ivor

If covering one carb at idle is very different from the other. and they are balanced, sounds like you have an internal leak or blockage somewhere in the carb. So less fuel is being drawn up through the jet.

You can see fuel being drawn up past the needle with a torch in subdued lighting. If that looks different between carbs, might be a clue.

Definitely sounds like an air leak. Are the butterfly spindles worn in the carburettor casing? Try spraying some easy start around the place and see if it affects the idle speed.

Chris.

You could also get another pair of carbs. It would quickly tell you if the carbs were the problem.

Chris.

Thankyou for all that.
I'll have another delve into the carbs and report back.

V8

ChrisJC wrote:You could also get another pair of carbs. It would quickly tell you if the carbs were the problem.

Chris.

That's what I'd do, BUT I would not get HIFs, go for the older stuff off a P6 or SD1.

Before changing the carbs I would do the basic checks like compression test, spark plug leads etc.

It may be you have leaking exhaust valves which have got worse over time and gradually making the lumpy engine worse.

If things are different on each bank...with carbs feeding each bank. You need to ensure you're measuring the exhaust gases per bank also.

Do both read the same ? A combined view out a single tailpipe will only confuse matters.

Also, what are hydrocarbon readings like ?

unstable load wrote: That's what I'd do, BUT I would not get HIFs, go for the older stuff off a P6 or SD1.

SD1's had HIF carbs from the very first cars, and P6's only had HS6's before the HIF's and they were hopeless.

The lifters were fitted 24 years ago!
Has the engine been refreshed since then?

mgbv8 wrote:The lifters were fitted 24 years ago!
Has the engine been refreshed since then?

Not to my knowledge. Although no history worth speaking of.
The lifters pump up quickly. If it weren't for this weird problem, I'd be quite happy with it, it lugs my Land Rover around pretty well.

Mind you, I do have another engine. 25,000 from new. Bought it for another car now gone. I'd need to change the cam if I put it in the Landy - has a Piper 270, which was fine for the Caterham where it lived.

V8