Engine running despite the state of the SU

[Mark McKibbin]

When I was putting the special together I put a H1 (1 1/8") SU on the engine that I found on my shelves with the intention of sorting it later.  I have now travelled about 500Km and it ran very strongly but for other reasons the engine is out again and as I am waiting for some parts, I thought it a good time to have a look at the SU.  First off it has a damper with a weight but also has a spring also it has a 3/16" hole on the top between the Damper and the butterfly I assume for a vacuum line ...but it was running with OK apart from the plugs being very black and lastly the needle is a GG also I have now blocked the hole.

The engine is standard other than a 37 head that somewhere in the past has had about 1/16" shaved off it and a set of extractors 4 into 2 into 1

Lastly I had a 18mm bung put in the exhaust so I can put my air fuel gauge in to get it right but I do need a starting point with needle and spring or no spring recommendations.

[Dutch-Ulster rep]

When I was putting the special together I put a H1 (1 1/8") SU on the engine that I found on my shelves with the intention of sorting it later.  I have now travelled about 500Km and it ran very strongly but for other reasons the engine is out again and as I am waiting for some parts, I thought it a good time to have a look at the SU.  First off it has a damper with a weight but also has a spring also it has a 3/16" hole on the top between the Damper and the butterfly I assume for a vacuum line ...but it was running with OK apart from the plugs being very black and lastly the needle is a GG also I have now blocked the hole.

The engine is standard other than a 37 head that somewhere in the past has had about 1/16" shaved off it and a set of extractors 4 into 2 into 1

Lastly I had a 18mm bung put in the exhaust so I can put my air fuel gauge in to get it right but I do need a starting point with needle and spring or no spring recommendations.

This site might offer some help. It has a lot of information on SU carbs: http://sucarb.co.uk/h-type-carburetters

[GK5268]

When I fitted my SU (which is 1" OM-type), I tried a number of different needles and most gave rich running, I have used a GG (there are two different lengths....???), plus a 4, 6, and I set the idle mixture using Gunson's Colortune - the engine can then be revved across the range and the colour of the flame observed, I eventually settled on MO and the burn has a nice bunsen-blue colour, however, to get this, the adjustment is at the end of its travel, i.e. at its weakest point, fully up, so it is at its limit.

The car performs well and spark plug colour is ok, I would suggest that notwithstanding the bunsen-blue colour seen using with a Colortune it is running perhaps a tad rich, but I have nowhere to go with this, as I had a collection of other needles, I decided last week to see if I could effect an improvement, ones that were definitely too rich I ignored, but I had several un-used that I thought I would give a go.

I tried the 6, but found out on the road, hill climbing appeared to be less strong, but at idle the engine sounded slightly sweeter, I then ran with the long GG (it is 5mm longer than the short ones, short GG were too rich) but could not detect any real difference on the road, but starting was not as easy, 2 or 3 tugs to get it fire up, 1 is only needed normally. All adjustment was still practically at the limit, so no change there.

So, I went back to the MO, and this seems to be the best compromise, it would be nice to put the car on an analysis machine to properly determine what gives the best performance throughout the rev range, but I am satisfied with the view from the Colortune window, so, if you can get a selection of needles and try what is best for your car, I have used the Colortune on all the cars I have ever owned (all my cars have had SU's) and I think this is a good tool for home use.

As for the spring, if the carburettor was designed for this, then you should stick with this, the piston is lighter than those without the spring and if you take this out, the carburettor will still work, but you may find that it stalls at inconvenient moments as the mixture will be messed about (years ago, I inadvertently forgot to re-fit the spring on a Rover 2000SC and drove it like that for a day and it kept stalling on the overrun, once home, I opened the bonnet and there was the spring sitting on the inner wing...).

[Mark McKibbin]

When I fitted my SU (which is 1" OM-type), I tried a number of different needles and most gave rich running, I have used a GG (there are two different lengths....???), plus a 4, 6, and I set the idle mixture using Gunson's Colortune - the engine can then be revved across the range and the colour of the flame observed, I eventually settled on MO and the burn has a nice bunsen-blue colour, however, to get this, the adjustment is at the end of its travel, i.e. at its weakest point, fully up, so it is at its limit.

The car performs well and spark plug colour is ok, I would suggest that notwithstanding the bunsen-blue colour seen using with a Colortune it is running perhaps a tad rich, but I have nowhere to go with this, as I had a collection of other needles,  I decided last week to see if I could effect an improvement, ones that were definitely too rich I ignored, but I had several un-used that I thought I would give a go.

I tried the 6, but found out on the road, hill climbing appeared to be less strong, but at idle the engine sounded slightly sweeter, I then ran with the long GG (it is 5mm longer than the short ones, short GG were too rich) but could not detect any real difference on the road, but starting was not as easy, 2 or 3 tugs to get it fire up, 1 is only needed normally. All adjustment was still practically at the limit, so no change there.

So, I went back to the MO, and this seems to be the best compromise, it would be nice to put the car on an analysis machine to properly determine what gives the best performance throughout the rev range, but I am satisfied with the view from the Colortune window, so, if you can get a selection of needles and try what is best for your car, I have used the Colortune on all the cars I have ever owned (all my cars have had SU's) and I think this is a good tool for home use.

As for the spring, if the carburettor was designed for this, then you should stick with this, the piston is lighter than those without the spring and if you take this out, the carburettor will still work, but you may find that it stalls at inconvenient moments as the mixture will be messed about (years ago, I inadvertently forgot to re-fit the spring on a Rover 2000SC and drove it like that for a day and it kept stalling on the overrun, once home, I opened the bonnet and there was the spring sitting on the inner wing...).

Thanks for that, so a GG may be OK then or at least close, I think my problem was the open hole where the vacuum nipple would have gone caused me the richen it to make it idle but at the same time it would have made the rest of the range rich also I will toss the spring as the damper is weighted.

---

When I was putting the special together I put a H1 (1 1/8") SU on the engine that I found on my shelves with the intention of sorting it later.  I have now travelled about 500Km and it ran very strongly but for other reasons the engine is out again and as I am waiting for some parts, I thought it a good time to have a look at the SU.  First off it has a damper with a weight but also has a spring also it has a 3/16" hole on the top between the Damper and the butterfly I assume for a vacuum line ...but it was running with OK apart from the plugs being very black and lastly the needle is a GG also I have now blocked the hole.

The engine is standard other than a 37 head that somewhere in the past has had about 1/16" shaved off it and a set of extractors 4 into 2 into 1

Lastly I had a 18mm bung put in the exhaust so I can put my air fuel gauge in to get it right but I do need a starting point with needle and spring or no spring recommendations.

This site might offer some help. It has a lot of information on SU carbs: http://sucarb.co.uk/h-type-carburetters

Thanks I will bookmark that site although they do not mention weighted dampers.

[Michael D]

you might want to read my post and. the replies thereto in the topic 'what have you done with your Austin seven today' as 1.25. inch SU carburetor running issues are being discussed there too. Including spring, damper, needle and settings. Methinks you've done wisely to blank off the hole that was meant for the advance vacuum line as that hole will have messed up your mixture by drawing in false air. Your plugs having been black is testimonial to a too rich mixture (although others may called it plug fowling by oil...) I like mine coffee brown but we're still tinkering (and probably fit a Ricardo head) so I cannot yet comment further.. but my 1.25 SU is on a blue spring and a No 6 needle (which at this moment may be a bit rich) and I'm happy to report it usually starts at first attempt with a bit of choke for starting only (but then again it is / was not in a cold and damp lockup )

[Jeff Taylor]

When I was putting the special together I put a H1 (1 1/8") SU on the engine that I found on my shelves with the intention of sorting it later.  I have now travelled about 500Km and it ran very strongly but for other reasons the engine is out again and as I am waiting for some parts, I thought it a good time to have a look at the SU.  First off it has a damper with a weight but also has a spring also it has a 3/16" hole on the top between the Damper and the butterfly I assume for a vacuum line ...but it was running with OK apart from the plugs being very black and lastly the needle is a GG also I have now blocked the hole.

The engine is standard other than a 37 head that somewhere in the past has had about 1/16" shaved off it and a set of extractors 4 into 2 into 1

Lastly I had a 18mm bung put in the exhaust so I can put my air fuel gauge in to get it right but I do need a starting point with needle and spring or no spring recommendations.

Hi Mark,

As a long term owner of a 1936 Morris 8 Tourer (Australian's know them as the 8/40 Roadster), it's more than likely that the SU H1 (1.1/8") carb you have was originally fitted to either a 1939 to 1948 - 918cc Morris 8 Series E or a 1948 to 1952 - Morris Series MM Minor - i.e. the early Minor with the 918cc 8hp side-valve engine. There should be a cast number on the side of the carb body - if from either of the above vehicles it would be 3457 or 3457/1. 

These carbs used a weighted suction chamber piston and therefore were not fitted with a piston spring. They did however have a brass (oiled) damper acting within the hollow piston shaft. The 3/16" hole you mention was not part of the original carbs specification, so probably a much later addition perhaps to fit a vacuum gauge ?

The jet size is 0.090" and when new and fitted to a Morris 8hp side-valve engine the recommended needles were:
Rich: M9
Standard: EK
Weak: MOW

Obviously with the mix of today's modern petrol and the fact the carb's fitted to your Austin 7 these needle specs may no longer be applicable to your needs. 

Here's a link to Burlen's Parts Lists for the Morris 8 Series E and Series MM Minor. Bear in mind that the exploded colour illustration is essentially generic and of a later SU carb - i.e. it shows a piston spring - however I believe the Full Parts List is of the correct 1930's specification and doesn't include a piston spring within the parts listing.

http://sucarb.co.uk/carbspec/carburettor...s/id/1848/

http://sucarb.co.uk/carbspec/carburettor...s/id/1849/

http://sucarb.co.uk/carbspec/carburettor...s/id/1896/

SU also produced a 1.1/8" downdraught carb fitted to some Wolseley cars in the 1930's / 40's - for obvious reasons (the piston being horizontal) these are fitted with a piston spring. 

Just to add to what Gary was saying about his SU 1" OM-type carb - these carbs were originally fitted to pre 1934 Morris Minor's and early MG's - the needle specs for the 1933 Minor were Rich: M7, Standard: M9 and Weak: MO - the needle he's currently using.

[JonE]

We are in a sad position of not having the excellent mintylamb SU needle comparison site up and running, unless anyone has a new address for it? I have a few graphs plotted for 1 1/4 SU needle comparison, but none for the 1", and wish I did!

[Jeff Taylor]

Hi Mark,

Here's a 1950 exploded parts diagram of the SU H1 (1.1/8") carb with weighted suction chamber piston and brass (oiled) damper as fitted to the Morris 8 Series E from 1939 to 1948 - note the vertical inlet manifold flange fitting. The 1" OM-Type that Gary is using has a horizontal inlet manifold flange fitting as per standard A7 Zenith side draught carbs. 

   

[Austin Carr]

We are in a sad position of not having the excellent mintylamb SU needle comparison site up and running, unless anyone has a new address for it? I have a few graphs plotted for  1 1/4 SU needle comparison, but none for the 1", and wish I did!

Post #8 on The Mini Forum ( http://www.theminiforum.co.uk/forums/top...ison-site/ ) maybe useful.

Post #8 reads -"There is another site here which gives all the dimensions of needles. It's not as visual as the minty lamb website however. You can copy the codes into excel and plot them yourself"

http://www.teglerizer.com/suneedledb/

[Bob Culver]

Aspects of SU were covered at length on the old website. It may be appropriate to rake over a few points. Following refers primarily to the horizontal versions.
There are several variants and major components can be interchanged, notably pistons of different types and weights. So when comparing needles it important that details stated so like considered. And that the needle orifice not worn. The body and piston of 1 and 1 1/8 seem the same size, just the output dia differs so needles presumably similar for same pistons and cars. The needle calibration beyond the lift range is only in use when enriched for starting.
Early carbs had heavy brass pistons and no dashpot. The piston spring generally only appeared in mid 1950s with ohv Minor and semi downdraught. I dunno how the piston of these compares with earlier.
I found with the common initial postwar 1 1/8 dashpot model the standard EK needle was fine, the listed “weaker” option hopelessly so.
Part operating the enrichment whilst running is a test for mixture. Plotting needle shape can be much more informative than just comparing figures.
1 1/8 seems seriously oversize for a standard car but very many run these with satisfaction. (The horizontal models look more period and with no vent rib can be more easliy polished!)