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MG MGA - Setting Up The Carbs

I am in the process of renewing the jet assembly, seals, needle etc. Memory fades as you get older and I was checking how many flats of the adjusting nut for the initial set-up when my brain came alive and I remembered how I did it about 15 years ago.

If the mixture was about right before taking the assemblies apart first remove the dashpot, piston and needle. With vernier calipers measure the distance of the jet below the bridge. When the new jet assembly is in place turn the adjusting nut to achieve the same measurement. It's as good a starting point as any. I seem to recall last time it was spot on and required no further adjustment when going through the tuning process.

Steve
Steve Gyles

Steve, 1.5 turns open is a good starting point. Barney's site below is a simple way to get basic setup. I recommend using the teflon O rings rather than original cork main jet packing. Lifting the needle using the lift pins accurately sets idle mixture, for modified engines rolling road tuning is the way to go - timing and A/F ratio can be monitored through the rev range.
http://www.mgaguru.com/mgtech/care/cf120.htm
Mike
Mike Ellsmore

Mike

I got my new assembly from the manufacturers - Burlen. Listed as SU-Perdry. It has the appropriate O rings, not cork. I am doing my initial set-up with my verniers later today/tomorrow. I will let you know how that compares with your 1.5 turns. Sounds a bit of a long way in for an initial set-up if you have no previous datum. Not a lot of scope for weakening the mixture. Or do you mean 1.5 revolutions rather than 1.5 flats? Revolutions I can agree with (ish) - that's 9 flats although the archives talk of 7.

Steve
Steve Gyles

Yeah 1.5 revolutions (turns) not flats. Should have said 540 degrees to avoid confusion.
Mike
Mike Ellsmore

Steve,

This is exactly how I was told to do it years ago when I joined the site.

I seem to recall the starting point was .078" but I could be entirely wrong

"If the mixture was about right before taking the assemblies apart first remove the dashpot, piston and needle. With vernier calipers measure the distance of the jet below the bridge. When the new jet assembly is in place turn the adjusting nut to achieve the same measurement. It's as good a starting point as any. I seem to recall last time it was spot on and required no further adjustment when going through the tuning process."

Tysen McCarthy

No 'Like' buttons on this site Tysen, so glad to know it worked for you. I reminded myself by sort of cheating. Seems I posted a similar process back in about 2004. Interesting how info gets lost with the passage of time. I would likely have gone back to basics but am glad that I have yet to attack one of the carbs so can take the measurement.

Very pleased with the one carb I have done. Very smooth operation.

Steve
Steve Gyles

Browsing through the archives I came across some excellent set-up tips from Don Tremblay back in 21 Aug 2001 which I will attach below. I use both of his methods for balancing and for setting the jets, although I will admit to just counting flats for adjustment after doing the vernier measurements. I guess I will revise my method now.

Steve


"Carburettor set up tips or: Carburettor tuning made mindless

Synchronization: I believe in "real time" and that means sychcronizing the carb's with the aircleaners on. Think of the advantages-never mind having the aggravation of having to take those beauties off!

Leaving the air cleaners on is also the easiest,fastest and most accurate way of setting the sychronization. First off, buy yourself a SU toolkit. Moss still sells them as P/N 386-300 for less than $20.00. Very worthwhile investment that will last a lifetime and will do wonders for your MGA's performance. It's a very simple device with two tubes and two bent whiskers. It also comes with a few other goodies plus instructions. Here's how to use it:

1.)Remove the brass dampers and insert the tubes down each hollow piston rod.

2.) Attach the whiskers so that they are pointing at each other and are at exactly the same level.

3.) Use a 1/4" socket-loosen on of the hexnuts that connects the carb's together.

4.) Start engine and adjust the idle screw so that the engine rpm reached is 3500 and both whiskers are pointing exactly at the same level.

5.) Tighten the 1/4" hex nut

6.) Lower the rpm so that the engine is idling at 850-950 rpm.

7.) You're done!

Another point(s): For damper oil, I use "Marvel Mystery Oil". ATF is a close second. I find motor oil to lack response-especially if the temp. drops below 70 degrees ambient. Also look at the top of your damper brass hex nut. SU improved the response of the damper's by removing stock from the bottom of the damper and denoted the modification by stamping an "O" on the top if the brass damper. I believe that the change was made sometime around 1960.

Mixture: To get the mixture correct both carb's or if you have three have to have both of then set EXACTLY at the same mixture. Counting flats on a hex nut or using a "Colotune" just does not give accurate results over the operating range of the engine. Instead, I prefer to start with a very accurate measured depth of venturi to the top of the jet. To do this, you have to go and and buy a dial vernier. They are not very expensive. Their accuracy is to within one thousandth of an inch. A good almost spot on starting point is a depth of .070" (The total range will be plus or minus .010 from this point) measured from the aluminum cast horizontal bridge of the carb to the top of the brass jet. Once you start using one (sold at Sears, Home Depot etc) of these things, you will see how inaccurate any other method is in comparison. The main thing is to get both jets EXACTLY the same. after your initial setting go for a good run (2500-3500rpm) and see how the vehicle runs. Go home and shut the engine off immediately-do not idle. Pull plug number one and number four. The color of the electrode must be brownish/tan. If more brown lean out .005" on both jets. If more tan-enrichen .005" on both jets. By playing around in .005" increments, you will find the best performance for you engine. It's hard to convey what the exhaust note will or should sound like. All I can say is that it is a very pleasing sound and you will know it when you have achieved perfect mixture. For my 1600, it is set at .068" (very fine tuned) down from the cast bridge. I am running .030" over pistons with a street (BP170) piper cam and a lightened flywheel. I also have a recurved distributor. Forget about the pushing the plunger up 3/32" of an inch. It belongs in the stone age. Forget using the Colortune except as an diagnostic tool. Forget listening to the exhaust or reading plugs unless you have a measureable baseline of analysis. The dial vernier is the only way. It's what the pro's use and I thought that I was the originator until I visited a vintage race course."


Steve Gyles

Steve,

That is exactly the post! I have that printed out and in my binder.
Tysen McCarthy

In case some people are unfamiliar with the SU syncronizing tool that Don refers to I have attached a picture.

I use it all the time. Brilliant. Far better than the suction kits you put over the intake or the listening to the hissing.

Steve


Steve Gyles

To Steve's point its important to remember that the piston's in the barometric chamber on the SU's are matched to each other.

they get switched all the time by DPO's

The way to test is remove the spring tape over the holes at the bottom where the needles are and put in the chamber. Turn the chambers over (with your finger over the rim to prevent the piston from falling out) they should descend at approximately the same rate.
Tysen McCarthy

Hi All
Just setting up engine and seen the thread regarding the carbs. Just ordered the SU kit from Moss and wondering if you need to empty the oil out of the pots before inserting the syncronizing tools. Couldn't see it in the above comments. Doh!!! Hope the kit comes tomorrow so I can take the car out in the sunshine.
Thanks Brian
Brian Paddon

Steve I have just read the article you posted by Don Tremblay, I was interested to read the part about setting the jet height using a clock gauge, as it is important to have the jets exactly the same height, some years ago I was working with some service engineers from Norton motor cycles, these bikes were fitted with twin HIF SU carbs, this set up was both carbs were completely separate and also both the exhausts were separate, when it came to set the carbs I asked one of the engineers if he would like to use my CO machine to set them, he replied no thank you we set the jet height using a clock gauge which was fixed in to a dash pot piston, and then inserted into the carb body,he then told me this way they will be exactly the same and this is the most accurate way to set the mixture,I asked him if he minded if I checked it after he had finished as I told him this would not work,he agreed, when the engine was up to normal running temperature I placed the probe from the CO machine into each exhaust and checked the reading from each, and they were a mile out from each other
Andy Tilney

Brian

As I have my carbs apart at the moment there is no oil in the pots and as synchronisation is the first task when all bolted back on the engine I will not fill them til that job is done.

That all said i periodically check the balance and never bother about any slight overspill. It soon burns off. I top up when job is done.

Steve
Steve Gyles

Andy

I guess the difference here is that the MGA carbs feed into a common gallery so I can understand the reasons for setting both the same.

Steve
Steve Gyles

Andy

On reflection, Don's process does raise an issue he did not cover. He said to stop the engine immediately after a run and check the No.1 and No.4 plugs and then describes the actions for both carbs depending on the colours. But what he does not cover is if the 2 plugs are of different colours. This would then result in different adjustments such that they are not exactly the same - contrary to his theme throughout the process.

I agree with the bulk of his process on the initial setting up. It is simply applying good engineering practice to neutralise manufacturing tolerances of the various bits.

Steve
Steve Gyles

One thing I seem to remember in the dialog around Don's article is that you should check #1 and #4 plugs after you've given them a run up the RPM's not just at idle.
Tysen McCarthy

Steve after renewing the jets, a base setting of the jet height adjuster nut fully screwed in and then out 12 flats is a good starting point, it is not important to have both jets exactly the same as after adjustment to set the mixture they may well end up at different heights. in theory yes they should be same height, but in practise this is not always the case, after setting the carbs if you want to double check to see if you have set them correctly a CO machine is the best way then you can check the CO at idle and make sure it is weakening off after its initial rise at higher RPM
Andy Tilney

Out of interest. 1800V. H4 Carbs. What needle for general running?

I have been running a 6 up til now.

Steve
Steve Gyles

Thought I would bring my progress up to date. My car has always run well so I am only talking finer tuning here. It has always tended to run on the richer side. The needles in the replacement carbs acquired during the rebuild in 1997 were No. 6. I knew no better at the time and took it as standard. Following various posts on this forum I did try out the FX needles but did not like them with my 1800 engine. They lasted about 2 weeks and I reverted to the No.6.

When I did my tune-up about 6 weeks ago I ran out of adjustment on the front jet (nut fully wound in) and still I was not happy. So I had a chat with Burlen down in Salisbury (SU manufacturers). They supplied me with their Super Dry jet assemblies and GS needles. Cost me a bomb. I spent ages being meticulous in assembling those jets, centering them and measuring their depth to a thousandth of an inch of each other on the vernier calipers. Then went for a run. Slightly rich on both. Took both pistons out and leaned off both jets by 5 thou. Went for a run. Rear plug perfect front plug still slightly rich. Piston out again on front carb and leaned by 3 thou. Perfect.

What a difference. Correct coloured plugs. Correct choke starting. Great performance. One very satisfied owner. It will be interesting to see if it makes any difference to my MPG.

Steve
Steve Gyles

Steve,
Can I ask where you bought the SU synchronising tool please? MOSS have what looks to be the correct tool but it isn't listed for H4s.

http://www.moss-europe.co.uk/tool-kit-su-carburettors-gac6101x.html

Many thanks
Richard
Richard Atkinson

Richard

Yes, that's the kit.I got mine from the MGOC. It looks like No.3 on this link: http://www.mgocspares.co.uk/acatalog/MGOC_SPARES_CARBURETTORS__MGA__301.html#a12216W_2dZ015

Similar price to Moss. The tubes are an interference fit in the damper cylinders so should read across a number of carbs. I have heard of owners making up their own with a few pieces of wire bent to suit. The tubes are just the icing on the cake.

Steve
Steve Gyles

Thanks Steve. I too use the vernier gauge method to get the initial set up. The calipers do need to be the correct size to sit across the bridge. I had one old gauge (not digital) that was too wide.
Richard
Richard Atkinson

Steve. Interesting, but excuse my ignorance - what are Super Dry jet assemblies?
Bruce.
B Mayo

Bruce

Standard assemblies but with the 'rubber' type seals instead of cork. Seem to be ok. Went together easy enough. No leaks and I hope won't be attacked by current fuel - not that I have had any problem with that up til now.

Steve
Steve Gyles

This thread was discussed between 24/05/2017 and 19/06/2017

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