QuestionQUESTION: Though the '70 MG_B I'm dealing with has kind of the reverse prob.; diesels when turned off, cold. not sure about 'hot', yet. Over the years have tuned & kept an eye on the timing- the mark shows up "on time".
In your answer, 6/15/2009, you mentioned that the vehicle in question might be running on an "excessively rich mixture". It caught my attention right away. Twas the first thing seen, in all my searching, that could be relevant to my Google search: "Tune-up the "1970 MGB" carburetors."
The MG is only driven 1000-1500 mi./year. She just barely failed Wash., DC . emissions test. Don't quote me but the "required" is like 1350 ppm and she got like 1400-1425, or so.
Have known my buddy the owner for almost 40 yrs., he's a close friend. I've helped him keep 2 other MGs, a '69 & a '71, purring over the years. But, never really knew how the deal with these carburetors. Not sure right now if these are "British Leyland - a Coopers Product".
Right now, I Want to know how to tweak the air/fuel mixture. Shortly thereafter I would like to know where I/he can find a good/comprehensive (inexpensive) hard copy or online resource on tuning and rebuilding the carbs. Back in the late '60s we used to use a 2-3 ft. piece of vacuum hose as a stethoscope down the air intakes on a '3 Deuces' set of carbs and balanced them by sound. Often wondered if that might be possible with these.
If you can help, thank you. If not, please let me know.
Thanks,
DPT
ANSWER: I have just published a book on maintaining MGBs in the 21st Century. It is available through the MG Car Club MGB Register. There is a whole chapter on SU carburetters. The book is called "Barrie's Notes", 80 pages, paperback.
The air filters were manufactured by a company called Coopers, but the carbs were supplied by the SU Carburetter Company, a sister company to MG. Yes, balancing the carbs can be done with a length of rubber hose used as a stethoscope. On a 1970 MGB you will have twin HS4 carbs which have a large nut underneath. Raising the nut weakens the mixture. On top, they have dashpots that need to be topped up regularly with thin engine oil (straight 20 or 10W30). Tickover is adjusted by a screw on each spindle.
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QUESTION: Thanks for the quick response. No time for specifics now, but √'d (checked) w,/mgcc: could NOT find "Barries Notes", %&^#(& ! This "follow up", re: 2nd failed emissions, would be a minor emer. were it not for it's intrusion on the annual, planned, family vacation. My Buddy won't be back 'til the 8th of August. The re-inspection deadline is the 7th. I've got to get the hydro-carbons back down and a girl friend of his & I hope to successfully take the MG-B back through for him on Fri. the 7th
Need to see a diagram or exploded view of these carbs. Actually need it now, to help me find the air/fuel mixture nuts you mentioned, "On a 1970 MGB you will have twin HS4 carbs which have a large nut underneath. Raising the nut weakens the mixture."
Here's the real bear that's got me scratching my head!? The 1st emissions failed by only approx. 70 ppm. I and the inspection tech. both suggested that all he needed to do was take it out on the highway & blow it out at 60 mph for about an hour. My Bud still wanted me to check out the system. That's all I did . But somehow I screwed the pooch!
1.) I pulled the plugs. All 4 were in good shape; indicative of lean & burning clean.
2.) Checked the points, while still in the distributor. From that angle they appeared a little burnt, no pitting; looked like normal wear.
3,) The gap was a little shy; reset it to .016.
4.) I disconnected the vacuum line to the vac. advance at the carb. & capped the port. Checked the timing. It looked a little high. It gets tricky here! The specs. call for 14º BTC dynamic (strobe) timing. There are only three tangs for reference: 1st- TDC, 2nd- 5º BTC & 3rd- 10º BTC. Ergo, you have to estimate where 14º BTC is. It looked a little high, maybe at 15º or even possibly 16º or 17º BTC. So we decided to leave it alone since it was so close to passing the first time We were hoping that just "blowing it out" out would solve the prob.
When reinstalling the vac. advance vacuum line I found a loose, large (1/4 or 3/16 in. i. d.) vacuum type line from a port, just below the vac. advance port on the firewall side of what looks like the lower throttle body. The line was too large for the port pipe as well as the 'Y' connector that it ran to at the top front of the engine. Reasonably sure it's part of the emissions control device (EEC). Don't know how the EEC is set up. Looks like it is simply a pump for gathering the fuel vapors and pumping air into the exhaust to lower the Hydro-Carbon ppm from the tail pipe. I took that line off to see what size i. d. was needed and put it back on the port, all be it loosely.
When he took it back, fri. 7/31/09, the HCs were in the neighborhood of 400 to 450 parts per million. Haven't seen the print out. These figures are what I was told.
I appreciate you taking the time,
I need HELP! As soon as you can, Please!
DPT
ANSWER: It is 7:30 am and I have just seen your question.
To see your emission control equipment, paste this into your browser:
http://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=28992
All the hoses must be clear and the pipes that they attach to must also be clear. They do tend to clog up with a white emulsion.
To see an exploded diagram of your carb setup, look here:
http://www.moss-europe.co.uk/Shop/ViewProducts.aspx?PlateIndexID=224
The adjusting nut is included as part of #17.
Adjust up to weaken, down to richen.
Don't play around with the timing, you will make things worse.
I will monitor my ID throughout the day. Get back to me if you need more help.
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QUESTION: Barrie, 8/5/09 5:00 am
Can't get the points & coil till 1:00 PM, my time. Had to order them at Auto Zone. They don't always have the top of the line parts but they're pretty good to work with. Plus they have them (parts) NOW! Today.
I realize this may be a dumb question. When you say: "Adjust up..." do you mean "righty tighty" & "Adjust... down", meaning "lefty loosey"?
Will contact you later. May be I'll have to reach you on his Mac at his place. Don't know if I can get into his email or not & I'm not sure when I can reach him. He's checking his phone infrequently on the camping trip. The camping trip is the 'war' at Penzig (sp?); you might know what that means. If not maybe I can explain it to you. In the US it's a medieval group's annual get together for battle and skill contests, archery, tactics,etc., etc. By the way it couldn't hurt to cc our communications to his email, , maybe; if you can.
I returned a rating for you; check it out (√io). Hope it's to your liking.
Thanks again,
DPT
AnswerYou will be above the carb, looking down. Turn the nut anti-clockwise (lefty lefty) to weaken the mixture or clockwise to richen.
You need to check the mixture before you adjust anything. There is a test pin at the side of each carb, but you may have trouble locating them. All they do is to lift the piston. It may be easier to remove the air filters and then you can lift the piston 1/8 to 1/4 of an inch by inserting one finger inside the air intake hole. Do this one carb at a time. If the engine speed drops, that carb is too weak. If the engine speed rises and stays up, then it is too rich. If the speed rises and then falls, it is perfect.
If it neither rises nor falls, then you need to adjust the balance between the carbs. Increase the tickover of this carb by a small amount, and then reduce the tickover back to normal by decreasing the other carb.
Also, check that you have oil in the dashpots. Undo the large plastic nut on top of the carb and look inside the tube that is revealed. It needs to be kept topped up with engine oil (at least half way up the tube). I top mine up every month.
