QuestionQUESTION: hi been reading your great website and wondered if you could help someone on the outher side of the pond?
i have a problem with my mg midget 1974 1275 no emission control,the car will not tick over under 1000 rpm,tried to adjust the 2 su,s but although the front carb fuel adjuster is hard up and the spring fully compressed the plugs on all 4 cylenders are black and covered in soot,have balenced carbs,compression test was ok,but as soon as the throttle is opened up their is a puff of gas out of the front carb ventury,the car has luminition ignition fitted.
trouble is cant tell if its a carb fault or a timing fault,unfortunately havent got a timing light to check timing at the moment.oddly enough the car pulls fairly well when driven.if you could give me any clues would be most helpfull .
regards from this side of the pool mike
ANSWER: Hi Mike,
I found from working in MG dealerships that when you have a problem with the way an engine runs you MUST attend to all the details of the engine before you can adjust the carbs. You did the compression test but not set the timing. If you don't have a timing light you can still set the timing by using a 12v test light, No matter what kind of ignition system you have they all have to connect the coil to ground so it gets charged and then they must break that contact to get the coil to fire. So all you need to do is have either a test light or a volt meter to know when it breaks that coil connection on the negative side of the coil.
Connect the 12v test light up to the negative side of the coil and remove all the plugs. Remove the coil wire from the cap and connect it to an engine ground (just put the end of the wire up to any engine metal part) Now, put the car in 4th gear (on level ground) with the brake off. This allows you to easily rotate the engine by hand by moving the car. I was able to put my one hand on top of the front tire while I had the other hand with my thumb over the #1 plug hole. If that is too difficult for you to do have a friend push the car slowly forward for you. You want to turn the engine until you feel compression on your thumb. That is the compression stroke. As soon as you feel compression stop and look at the timing marks. (not an easy job on a 1275 Midget)Set the crank pulley at the timing mark (according to the sticker under the hood (bonnet) The sticker always over rides what any manual says. Your timing specs are probably a little different than ours because we had to contend with emission standards here. Now back the car up just a little bit and turn the ignition key on and by just lightly bumping the car forward by hand you can rotate the engine a very small amount at a time while you watch the test light. As soon as you see it turn on, stop and look at the timing marks. That is the present timing. So now you can loosen and rotate the distributor to get the timing exactly correct.
This is called "Static" timing. My guess is that your specs are close to 10 degrees before top dead center.
The reason you must have the timing exactly correct before you try to adjust the carbs is that you adjust carbs by the response you get when you lift the spring loaded pin on the side of the carbs and when timing is off you will get a wrong response.
Let me know when you have the ignition timing correct and I will cover the carbs. Also which SUs do you have? the HS or the HIF. The HS has the float mounted on the side of the carb and the HIF has the float inside the bottom of the carb.
I have been wanting to add a page to my web site just on adjusting SUs so I will write it up and you can go to the web site and copy it on paper. Let me know when you have the timing set and I will by then have the page up and I will direct you to it.
Howard
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QUESTION: hi Howard sorry about delay getting back to you but work commitments and all that,have now set static timing to book setting 7deg btdc [no data under bonnets of English cars that i know of, must be an export thing],i then stripped the carbs with the manifold intact to better see how the carbs work,and found one of the levers that pulls the jet down when its on choke was sticking down,with a bit of setting up i corrected the problem i am sure that this is what was causing the richness problem on 1 and 2 cylinders,i then screwed up both jets till they were level inside the Venturi and backed off 2 full turns i then took it out for a run and made adjustments until the plugs were just showing a lighter colour,i have since retarded the timing by about 2deg to stop it from pinking [slight knocking under acceleration]seems to have stopped it so your fig of 10deg was more realistic,but i am sure that the carb setup could be improved so would value your advice on the right way to set them up.
ps for your info the car has twin HS2 carbs with AAC spring loaded needles[standard with this model]their is no emission control fitted.
regards mike
ANSWER: Very good Mike, you are sharp. To put a final finish on the carbs if you are sure the butterflies are in sync by either a Unison gauge or even a small hose put in the intake when idling to listen to the "Hiss" and get the sound the same on both carbs.
Then warm the engine up and let it idle (below 1000 RPM)and lift the spring loaded pin under the edge of the pot on the FRONT carb and note the response. If the engine speeds up and stays fast, the BACK carb is too rich. If the engine slows down and dies, you either didn't get the butterflies synchronized well enough or the REAR carb is too lean. Rev the engine a little between each test to clear the engine. Never adjust the jets more than half a turn at a time.
Do the REAR pin the same keeping in mind that when you lift a pin, you are checking the OTHER carb. (unlike what most books tell you)
When the jets are set correct when you lift either pin the engine will slow down and sound like an old John Deer tractor.
Some mechanics like to use automatic transmission fluid in the top pots but I like engine oil because it gives you a quicker throttle response.
Howard
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QUESTION: Sorry again Howard its been a long time since your e mail but i have had a rather big problem with a wiring fault its been earthing out the ignition and its taken me ages to figure it out,trying to commission a car that's been off the road for so long is a trying business every time i fix one electrical problem i seem to have another!.
Anyway found your instructions a great help i have now tuned the carbs and i am amazed how little the jet nuts are screwed down[ after leveling the jet inside the venturi,the jets are screwed down by just 2 flats ] unless this is a sign the jets are worn! have yet to give the car a good run though, will make small adjustments according to plug colour.
Would like to thank you for your help and advice and wish you happy motoring, hope the weather there is better for open top driving than it is here at the moment.
regards mike.
AnswerMike, if the jets are only two flats down I would say there is a problem either with the jets or possibly the needles are either out of position in the pistons or maybe the wrong needles or as you say, the jets and /or the needles worn badly. Something is wrong there. The needles are stepped so examining them with a magnifying glass and checking their letter or number and checking the hole side of the jets may show what is wrong.
Howard
