Hi all

I am considering fitting an SU type carburetor to the Colander in an attempt to improve driveability. I DONT WANT TO GO FASTER!!! Previous experience with SU carbs has shown me that they can be very good when set up correctly. Several members of the Cornwall club have them fitted, so I thought I would canvas some opinion?
I did once have a Raeburn manifold and tubular exhaust but never fitted them because it was too "boy racer" for me
I am aware that a 1 inch SU can bolt straight onto the standard Ruby manifold, BUT they are like hens teeth!
The inch and eighth is apparently a good size, BUT I have a new old stock inch and a quarter, which may be too big? Theoretically I guess the carb will only open as much as required so I should get away with the inch and quarter? Anyone any experience or thoughts, maybe a rough idea of a needle to start from?
I will also have to make up an adaptor to fit to the Ruby manifold to phase the bolt holes 90 degrees and angle the carb so the float is level?

all opinions gratefully received!

Steve.

Just how fast do you want to go? How about 9 hours for the 360 miles from Falmouth to Stockport in an RN saloon with a bog standard engine including 22FZb carb? That wasn't pedal to the metal, but it was quite happy cruising at 50mph up the M5 at which it gavce 50 MPG. It was in weather conditions similar to present but Sevens seem to thrive in high temperatures. Must go now I'm off to Guildtown in the morning. Dave

Guess who clicked post instead of make change when I realised I'd misread Steve's message, then the computer wouldn't post this correction. A couple of friends have fitted 1" SUs to their cars (an RN and a RP saloon) with good results-easier operation when cold and no flat spot on pulling away, however I don't know the availability of the 1" SUs and have no experience of the 1 1/4". Me I'm just perverse in accepting the challege of running old machinery. Dave

Thanks Dave

Would like to try it for the reasons you listed ref flat spots etc. I take the point about old machinery etc and tend to agree with you, but the odd tweak here and there shouldnt upset to many people (hopefully!) and I have converted back to a manual ignition control which may cancel out the carb conversion as far as the purists go?!!!

regards

Steve.

Hi Steve,

My dad has fitted an SU to his opal which has a few engine mods (built for the 1997 jogle and did it in 18hrs 12mins), he switched to the SU last year to try to get better fuel delivery with less droplets and a more even mixture. He is trying to make the engine last longer, fuel droplets don’t do the cylinders any good as they wash the oil off the bores.
He fitted a 1 1/8” I think, he had to make an adapter plate about 3/4” thick with the hole tapered to match the manifold to the carb.
I am fairly sure he used an EB needle which I think is the standard for the 850 mini but dont quote me on it.
The car drives much smoother and definitely uses less fuel. Its a little faster too but that suits him.

Regards

Rob

Hello Steve,

Have a look at
http://speedexa7.users.btopenworld.com/index_files/Page670.htm

Its a page on using SU's on the Speedex Site.

Seems to give the thumbs up to SU's up to 1 1/4" and suggests an E2 Needle.

I have a 1 1/8" SU on my Chummy using a very simple "circular" adaptor, fitted to "Ruby" type manifold.

I think I have an M6 Needle fitted, although there seems to be a lot of variance of opinion on a what's the best there. From memory, I think Bill Williams suggested a No. 61 jet, but I couln't locate one of those.

I am very satisfied with my car from a performance point of view, but am getting nothing like optimum fuel economy. I would average mayble 40mpg (tops)around the hills down here, but used to get 50mpg on a previous A7 I owned with a 22FZ Carby.

My A7 is used as a fun car. The SU really does deliver smooth running and absolutely easy starting.

I do have a 1" SU here which I will try one day.

The guy who runs Midel Carburetters down here (The SU experts in Australia who have a close association with Burlen in UK) used to run an A7 years ago. I must ring him and see what he reckons would be the optimum needle to use.

My plugs look OK. Brownish looking with just a hint of black, but it is very hard to "read" plugs nowadays with the fuel we are forced to use..

Worst problem I have found with SU in a Chummy with Gravity Feed, is ensuring there is sufficiet head of fuel as the SU bowl ends up only marginally below the fuel outlet on the tank.

Regards,

Barry R.

My RTC has a 1 1/4" SU originally fitted to a mini.
The thing to watch out for is when choosing a needle that it will only be the top (thicker part of the needle which will ever be used, the engine will not be able to draw enough air in to get the dashpot and slide fully open, you will also need a lighter spring in the dashpot.
Somewhere I have written the details of this and the needle I settled for, but as usual it is eluding me at the moment.
Performance and economy are both very good.

Phil

My 38 Ruby had a 1" SU fitted when I first had it, and it went well. But after a leak started around the banjo which proved difficult to stop I put back a well sorted standard 26VA carb. Other than possibly a whisker less pull on steep hills there was no difference. What made an incredible difference was fitting one of Willie McKenzies distributors. Starting is instant and it runs as smooth as you can ever expect from an A7 engine, and hill climbing is good.
For the record the needle fitted to the SU was an HA.

Hi Steve,

The following may be of interest should you decide to source and fit an earlier SU carb to the one you already have. SU's were standard fitment on Morris vehicles of the 1930's and the following needle recommendations relate to those vehicles with similar engine capacities to the Seven. The early and mid 30's SU's described below didn't have the oil filled piston damper whereas the late 30's ones did. Air silencers were not fitted to the early 30's carbs, but some of the mid and all of the late ones were - this apparently affects needle choice, although these are merely air silencers, NOT oil bath air cleaners / filters. None of these carbs should have dashpot springs fitted.

Early 30's sidevalve - 847cc engine - 1" SU - Rich M.7 Standard M.9 Weak M.0.

Mid 30's sidevalve - 918cc engine without air silencer - 1" SU - Rich M.A. Standard B.A. Weak M.7 Weaker R.5

Mid 30's sidevalve - 918cc engine with air silencer - 1" SU - Rich M.6 Standard B.D. Weak M.7 Weaker P.J.

Late 30's sidevalve - 918cc engine with air silencer and oil filled piston damper - 1.125" SU - Rich M.9 Standard E.K. Weak M.O.W. Weaker M.W.

Jet size and number for all the above is .090" / 112

The above carbs will need an adapter as all have manifold flanges with vertically opposed stud holes rather than the side-draught Zenith's horizontally opposed studs - the distance between stud centres is also slightly greater on the above carbs than on the Seven. However unlike the much later SU carb you already have - possibly off a 948cc Morris Minor 1000, the suction chambers are not set at 30 degrees to the float-chamber level, making an adapter much simpler to fabricate as Barry suggests.

The late 30's 1.125" SU's are probably the easiest option to go for and they do occasionally turn up on ebay - I've seen a couple in the past 6 months or so whereas the 1" are much more difficult to find. Burlen stock most of the spare parts you'll need including new throttle spindles, bushes etc .

Regards

Jeff.

Thanks very much to all replies. Theres a lot of detail you have given me thanks.
I now think I will try the inch and quarter purely because I have one and I am allergic to spending money!
I believe I can get a float chamber adaptor or a different float chamber to allow horizontal fitment?
I will probably use one of the recomended needles and "see how it goes".
The throat in the inlet manifold is a lot smaller than the carb so I guess some tapering is needed on the inlet adaptor?
Interestingly lots of you mention starting difficulties? The car starts very well with the Zenith wether hot or cold, it idles very well too, the only issue is a flat spot just off idle as it goes onto the main circuitry. This results in a jerking movement down the road at slow speeds around town. Problem is that a "resonant" condition occurs whereby my foot moves on the throttle because of the jerking thus making it jerk more etc etc this builds up as a vicious cycle until its jerking down the road like a kangaroo!! I have made substantial improvements one way or another but cant quite eradicate it and it does detract from the driving enjoyment!

thanks to all

Steve.

Hi Barry,

Bill Williams (page 57) suggests trying a number 62 needle with an inch and one eigth SU. These are shown as being available in Burlen's latest catalogue - part number AUD 1022. Measurements every .125" from the top shoulder are as follows: .089" .085" .081" .078" .075" .0734" .0719" .070" .0685" .067" .065" .0635" .062"

Regards

Jeff

I have an excellent devoted to nothing else but tuning SU carbs, if you need any info on needle sizes etc let me know, I should be able to copy the relevant charts for you.

Phil

Hello Steve,

In respect to your Zenith 26VA......

You mention the "flat spot" just off idle.

That is what the "Progression Jet" is supposed to avoid. There is some info around on the various A7 Club Sites which talk about "the mystery jet" or something, whivch apparently arose when someone woke up to the fact that Zenith Carbs have this particular jet fitted.

Have you tried blowing out this jet, and its associated body drillings out? They are renowned for blockage.

I am also grateful to the respondent who provided the part number for the "62" SU Jet. A local SU source couldn't supply it when I last tried, but I simply asked for a "62" I will try again using the now provided part number.

Regards
Barry R.

Barry
There is a "hidden" jet that helps the idle that is a common problem with sidedraught Zenith's it is located under the carb body and is very often blocked. Mine is Ok. I have also checked the compensation jet and drillings and all seems to be well, BUT I have considered the fact that playing with the size of compensation jet could be a worthwhile exercise. Only problem is availability? I guess drilling out is the only choice? A bad practice I know but would prove a point. I am thinking of experimenting further whilst progressing with the SU project in parrallel.

Too many things to play with really!
I think I should just go for a drive at the weekend and forget about carburetors!!!

Steve.

Yes Steve, that "hidden jet" is the "Progression Jet" and its function is supposed to be to introduce additional mixture spray into the system between idling and the time the main/compensating jets take over. If you have checked yours though, that does not seem like the problem, although I can say from experience that the drilling in the carb body for the "Progression" set up can be difficult to clear.

The amazing thing about Zenith Carbs is that if you can get one to run OK and supply it with clean fuel, they never seem to need to be touched, but brother, a dicey one can drive you to drink!

Regards,

Barry R

Hi Steve
I us a red spring and a MO needle and it works well in our 36 Ruby good luck