Carburettor piston spring

[3litrekiwi]

Hi, I'm still in search of that silky smooth Rover 3 litre rumoured to be lurking under my bonnet. I have improved the running quite a bit however still have a noticable hesitation just off idle that I think is a fueling issue. It is worse when the car is cold and to get any useful power at low revs full choke is required heading up the hill from home. Once under way it is less noticible and almost disappears once well warmed up.
When I first stripped the carb the piston spring, like the rest of the throat were covered in carbon. I think this was probably due to misfiring which I think is cured. One side of the spring was polished as if it had been rubbing on the inside of the dashpot and I think there is some wear on the spring. I assume that if the spring has become weaker this might be contributing to this hesitation as it would allow the piston to rise a bit faster than it should.
Burlens have the original spring (11.25 oz) and a slightly heavier (18 oz) one. The freight to NZ is a bit less than the price of two springs so I plan on buying one of each to have a play with a heavier resistance. Has anyone tried this?

cheers
Martin

[enigmas]

Do you have the correct needle fitted Martin?
Are you running the engine with a low restriction air filter...this may cause an overall lean condition.

As for intial off idle acceleration try a heavier weight oil in the dashpot...this should/will richen the mixture during snap acceleration.

Is the float level correct?

[Ken Nelson]

Hello 3litrekiwi,
Forget about the spring, needle type, and the weight of oil in the dashpot, etc. I went through 2 or more years agony driving my 3-litre with EXACTLY the same symptoms you describe and came upon several things that ALL added up to those symptoms. And, yes, once the car was warmed up it ALMOST ran perfectly back then. First off, I assume that you or someone else rebuilt the carburetor at some point? Be aware that the HD8 SU carb has also been used on Austin Healey 3000's, Jaguars, Rolls Royce and other big British sedans with 6 cylinders that need a lot of fuel. BUT as far as I can see, the Rover P5 is the only one that used the HD8 with the ECONOMIZER system. That is the little device that screws down under the fuel reservoir top cap. It has a very fine hole in the device and a steel vacuum line running to the same site that the distributor vacuum line goes and a rubber hose running to the air cleaner housing. That fine hole often gets clogged, so check it to be sure it is open (mine wasn't), and that the rubber hose isn't collapsing under vacuum (mine was). Then take the carb off the intake mount and look to see that the gasket is not blocking one of the holes in the intake housing and carburetor base (it allows the slow running circuit to send a separate fuel/air mix for idle I believe). It's easy to miss that this hole in the gasket is missing. I'm not sure if all years had this or not, but check. Finally, I discovered that my carburetor with ECONOMIZER setup also should have a 1/8" hole in the lower half of the butterfly valve. The HD8 carbs for Healey, Jag etc DON'T have this hole because they don't have the economizer, and are tuned differently. Our Rovers are tuned to idle with the butterfly FULLY closed, and fuel/air coming through this hole and adjusted by the SLOW SPEED adjustment screw instead of by opening the butterfly to a slight degree (like every other SU carburetor I've worked on for MG's, Rileys, etc). All of those errors meant that at idle speed my car was being starved of fuel until it warmed up, and had to have liberal use of choke, and backfired and carboned up everywhere numerous times after cleaning. The rebuild kit that one might order for an HD8 carb will likely NOT be set for an economizer carb, and you may well have to drill a butterfly hole and gasket hole. NOW, I might be wrong (I'm not a pro but have 40 years working on British beasts under my belt) and it could have all just been the phases of the moon, but I DO now have that silky smooth 3-litre Rover under my bonnet. Drop on by (USA) if you want to borrow mine. I hope this doesn't come across as arrogant, because I'm just so excited to perhaps be able to pinpoint exactly what you need without the 2-3 year agony! Cheers, and let me know what you find!
PS Believe it or not I've also got more war stories about that same carb on my car if you need more reasons for it not to run right for several years.

[3litrekiwi]

Thanks Ken and Vince.

I can put a tick next to most of your suggestions except the hole in the butterfly and checking the needle. I found the needle was hanging about 2mm low when I first started my tuning journey. I will pull it out, check the stamp and have a look at the spring for wear. This time I will actually measure the diameter of the wire. The air cleaner and filter are standard. I will also check the economiser circuit again but it was clear when I last checked.

As for the hole in the butterfly, I was reluctant to go there having read the blurb about the HD8 relying on the butterfly sealing off and the idle mixture being set via the bypass circuit. But behold the graphic on page 270 of the WSM and also in the parts manual, this hole is supposed to be there! Except for the NADA engines which explains why yours didn't have this Ken but not sure why mine doesn't as we were still well tied to mother England in the mid '60s and emissions were many years from even being considered here.

The more I think about it from a flow perspective, this would prevent a shock wave of air as the butterfly opens. What I find interesting is why not just set the butterfly slightly open just like most other SU and for that matter other carbs.

Cheers
Martin

[Ken Nelson]

Glad to hear that you've got most of these things covered. I will add that I also had trouble with my needle coming loose and dropping down from the carb piston. That was another war story that resulted in getting home with a tow truck until I figured it out. And I will say that I also don't know how SU and Rover came up with the economizer, but AH, Jag, Rolls, etc apparently didn't use it so it must not have been a universally good system. Maybe Phil (the Rover Guru) can tell us more about it. If you don't have the 1/8" hole in the butterfly, I can tell you that was the final touch that fixed my carburetor completely. I think mine was missing either due to NADA, or more likely because I either rebuilt it with a kit from an AH, or the carb I was using was assembled from parts off a spare AH one I had collected. Let us know again how it ends up for you!

[djm16]

I have found that with modern petrol (at least Australian petrol) I could not get the mixture lean enough with the limited range available on the main jet. With both my P4 and 3-litre, I also had to lower the needle in the piston by 2 or more mm.

I have a Fuel ratio meter with a removable wide-band sensor that clips inside the exhaust pipe. That told me that the new economiser jet I had bought was too large a diameter (it is approximately 1mm), creating too lean a mixture. So to get the medium throttle / cruising mixture correct as well, I had to solder up the economiser jet hole and re-drill it.

No doubt I will go through all this again when the rebuilt engine finally gets back in the car.

My point is that while you can guess at the idle mixture from the sound of the engine and the smell of the exhaust, you cannot assume that WOT mixture will also be correct. The colour of the plugs is too subjective and dependent on other conditions to be a reliable indicator of WOT mixture. You really need the meter.

[Phil Nottingham]

I have not noticed that much difference in performance through modern fuels in any of our Land Rovers or Rovers apart from the P2 which does run hotter and suffers from vaporisation on restarting after a 10 min interval and also in crawling stop/start traffic after 30 mins.

As to the Economiser I really think its limited benefits of reducing mixture richness on light throttle is outweighed by its service complexity as very few people understand it or even know about it. The special banjo washer on the carb is often crushed, missing or just a plain one so this prevents proper float chamber venting if the rubber pipe to air filter is sound.  The VERY fine drillings in the vacuum block on the carb become carboned up and stops the economiser working anyway.

It is very easy to check whether the carb is venting properly by pinching the vac tube closed when it warmed up and idling. The engine should stop immediately.  Its probably less bother to dispense with the rubber tube and block off its intake on the carb elbow

[enigmas]

I think you could probably start writing a thesis on that device Phil.

[enigmas]

Jun 11, 2020 7:49:22 GMT Phil Nottingham said:

I have not noticed that much difference in performance through modern fuels in any of our Land Rovers or Rovers apart from the P2 which does run hotter and suffers from vaporisation on restarting after a 10 min interval and also in crawling stop/start traffic after 30 mins.

As to the Economiser I really think its limited benefits of reducing mixture richness on light throttle is outweighed by its service complexity as very few people understand it or even know about it. The special banjo washer on the carb is often crushed, missing or just a plain one so this prevents proper float chamber venting if the rubber pipe to air filter is sound.  The VERY fine drillings in the vacuum block on the carb become carboned up and stops the economiser working anyway.

It is very easy to check whether the carb is venting properly by pinching the vac tube closed when it warmed up and idling. The engine should stop immediately.  Its probably less bother to dispense with the rubber tube and block off its intake on the carb elbow

Is there any hard and fast technical evidence (Rover or other literature) that proves the 'economiser device' actually improves fuel consumption at constant cruise speeds Phil.

[Ken Nelson]

Here is a link to a SU Carburetor Manual that has a discussion of the working of the "Additional Weakening Device" www.pieterbolhuis.com/rileyclub/wp-content/uploads/2014/02/Book-No.137-SU-Carburreter-and-Fuel-Pump-Workshop-Manual-1958.pdf
and another link to a forum site for Holden and superchargers that describes the function of this Economizer system forum.fefcholden.club/index.php?topic=23061.80

[djm16]

A very long time ago, possibly on the P4 forum, I pointed out that both 3-litre and P4 carbs consantly flooded, despite trying to reset the float level. It was apparent that the brass float only barely floated, and could not overcome the petrol pump pressure (original double sided pump on 3 litre and modern SU electronic pump on the P4). Adding a lightweight spring under the float fixed the problem in both cases and allowed me to reliably set the petrol level in the main jet.

I noted on searching the net that the specific gravity of modern petrol was a little lower than petrol of the 60s and 70s, which would explain my observations.

[3litrekiwi]

This is probably why the replacement needle valves have a viton seal rather than the original lapped stainless steel one.

[3litrekiwi]

I got around to checking the carb butterfly today and mine does have the 1/8 hole so is correct. Also pulled the needle and it is the correct UR one for the car. BUT! The dashpot damper "Oil cap" is incorrect and has a hole in the top so is vented and these carbs are supposed to be non vented, there is an internal passage. With this open to atmosphere it is a vacuum leak and quite a well disguised one. I put a bit of aluminium tape over the top to seal the hole and the car drives much better! I will buy a correct one but in the meantime this will do. I am hesitant to get too cocky as many things on this car have needed several attempts but I think this one might be solved. Thanks for all the advice.

[Brendan69]

You could just melt the vent hole closed with a soldering iron and then smear some black silicoon over it.

Regards,

[enigmas]

Araldite!

Would a miniscule vent hole in the dashpot damper really make that much of a difference in engine performance Martin?

[3litrekiwi]

Looking at the SU site, they are pretty clear that the vented and non vented are not interchangable and I was pleasantly surprised that the flat spot off idle and when cruising at 30mph seems to have gone. If the economiser can make a difference then this might too. With the hole open, putting a finger over it changed the idle speed, not much, but the engine note definitely changed up and down. The real proof will be when the engine is cold. 

[Ken Nelson]

That is interesting. I can't remember if mine is vented or non-vented. I'll check, but my car currently is "out of service" with a Volvo power steering box conversion almost finished (I hope!)

[Phil Nottingham]

It does make a difference as does the length etc 

[3litrekiwi]

Jul 11, 2020 8:18:33 GMT enigmas said:

Araldite!

Would a miniscule vent hole in the dashpot damper really make that much of a difference in engine performance Martin?

Hi Vince, apparrently not, or at least not the whole difference! The improvement was only temporary and then the old slight hesitation off idle and surge at slow cruise came  back but sort of intermittently. 

So to the economiser. I gave my local old classic car expert a call and his view is the economiser is more trouble than it is worth. So I thought a run with it blocked would test the advice. I was a bit dubious as the wisdom here is that it is required. I am loath to state problem solved again but the engine definitely runs better with it sealed. The hesitation off idle is definitely gone which makes sense as the mixture will be richer if the pressure in the float bowl is higher.

Prior to sealing it off, I measured the jet and a 1.6mm drill will slip in easily and a 1.7 won't fit so my jet is much bigger than Phil's 1.0. This would make the cross sectional area 2.5 times that of a 1mm hole. If 1.0 is correct then 1.6 should be dropping the pressure in the float chamber much more than the design value whatever that is!

I wonder whether at some point the jet has been blocked and the smallest size in a basic imperial drill set being 1/16 has been used to clear it.

So, the plan tomorrow is to make a little insert with the smallest drill I have, drill the jet out and solder the insert in. I can start with 0.7mm so the car is going to get that first and I'll do enough miles to be sure it is making the difference and the I'll go up 0.1, rinse and repeat until the surging comes back. Given the poor standard fuel consumption, finding the sweet spot seems worthwhile if possible.  

[enigmas]

Well you're obviously quite skilled on a lath if you can turn-up such a small insert Martin. I've upsized jets on motorcycle carburettors successfully with fractionally incrementally larger drill bits and even soldered-up main fuel jets to up-size them but what you're planning on doing, that's definitely up another level.

I presume you're talking about the needle jet Martin. Wouldn't it be easier to simply purchase the correct size jet?

[3litrekiwi]

Too much flattery Vince. The jet I am talking about is the one in the economiser. The main jet is new so that will be correct. The only problem I have is going fast enough for the tiny drill size. The correct speed for brass would be several thousand rpm and my lathe doesn't go that fast. Just have to run at top speed and be very careful.

[3litrekiwi]

For my engine it seems 0.8mm diameter is pretty close to the right size jet in the economiser and I seem to have found the smooth six under my bonnet! No surging at any speed and the flat spot off idle is gone.


Thanks for the advice guys.

[Ken Nelson]

I am returning to this thread on the forum, because I continue to have the type of drivability problems that Martin has talked about here. My car starts well when cold, but until it is fully warm it demands a lot of choke and revs to keep from stalling out on take-off. Once going, and once fully warm, it runs fine. But it is very difficult to restart the car when warm after a short stop at a store, and seems to do best with about 1/3 choke setting and the throttle held to the floor as it cranks longer than it should. Once it starts it again runs fine, but I remain embarrassed during this entire performance in a luxury car. As Phil has said, any mileage saved by the Economizer is insignificant, and I'd gladly remove it, etc if possible. So Martin or Phil, what is the best solution again to these problems? Thanks again for any advice!

[djm16]

Is there any play in the butterfly spindle? If so, the mixture will tend to be weak at idle. The correct fix is the modified spindle seal kit (I have the kit but have not got around to fitting it yet). My temporary fix is a little felt washer between the external arm on the leading end of the spindle, and the carb body. It keeps dirt (and air) out of the spindle bearing.

[enigmas]

This is one hell of a thread and to my mind the economiser device just seems like a big headache considering the changes in the formulation of petrol (DJMs pertinent comments) and the mechanical changes associated with older engines.

Surging at cruise speeds is generally a lean condition and I'd be looking at the needle 'station' at that particular rpm. Motorcycle tuning skills with needles and slide type carbs are useful here, although the SU needle is controlled by vacuum and not a cable. To overcome this difficulty and to gauge the needle location, fit piano or TIG wire within the dashpot tube. You'll need to zig zag the wire so that it is a firm fit within the tube. Then hold the engine at the specific rpm and have an assistant mark where the wire sits in relation to the top of the dashpot at that particular RPM. You should now be able to extrapolate where the needle sits in relation to the jet. Then perhaps some skilful tweaking of the needle's diameter at this point will cure the lean surge.

Another issue with flatspotting of the engine from idle to initial throttle opening is the transition port. Although the SU used on the 3 litre 6s feeds through a hole in the butterfly I'd want to take a closer look at the transition port to see if it is functioning to full effect. The bleed hole in the butterfly I surmise must weaken the vacuum draw on the transition port causing a momentary flat spot during transition.