SU Carbs ... HELP!!!

crunchy62

I could really do with some help with sorting a problem with my MG Midget.

The car is gone nuts and accelerates up to between 5000 and 7000 rpm on its own!

I've checked everything including electrics and carbs and I just can't figure out what's wrong.

One other suggestion is a vacuum leak but I can't find anything.

I can't get the car to a mechanic and it's not drivable and anyway it seems really hard to find a good one.

So if there's anyone out there who really knows their stuff with SU carbs and would be interested in taking a look (for a fee) I'd love to hear from you.
Thanks

savagethegoat

are they the type where you have to top up with oil?

JP 1800

Where are you based? you can pm me if you like.

jca

What year is the car? Does it have a brake servo? If so, blank the servo connection off at the manifold and see what happens. The servo diaphragm could be leaking or allowing brake fluid into the engine. Also check if it has a closed crankcase ventilation setup that the pipe or control valve aren't allowing a big air leak. If you have the later su carbs with the emissions valve built into the butterfly check that this valve isn't stuck open. A last thing to check is that there isn't a broken advance spring in the distributor allowing the the timing to advance uncontrollably.

cvc01

Sounds like a possible distributor problem, if you want to pm happy to discuss

crunchy62

Cheers for the replies.

To answer the questions asked:

savagethegoat - Yes, they are topped up with oil

JP 1800 - I'm between Naas and Blessington

jca - It's 1974 so fairly basic. I checked the vent pipes from rocker cover to carbs. No emissions valve. Accuspark electronic
ignition fitted

cvc01 - see last answer.

Just to add that I removed, cleaned carbs. Everything seems okay with them. I set them for the recommended base settings for tuning. I've checked the timing although I've no strobe light so it's probably not 100% but still close enough for the car to run okay. I checked the distributor cap for visible cracks, fitted new HT leads, checked and cleaned plugs and firing order. Checked the fuel pump also.

jca

Is there a servo? Did you check for a punctured float? Any chance of you posting a video? Does it start racing immediately you start it?

crunchy62

No brake servo

Car won't start without choke and sometimes idles rough for a couple of seconds and then revs like crazy.
Floats are fine. Also ran the car with fuel pump disconnected and a bit of fuel in the float bowls ... same issue.

Will try to get a video together if I can get an assistant to help

jca

Does opening or closing the throttle make any difference at all when it's racing? Unless there's something badly amiss in the distributor it still sounds like a major air leak on the high vacuum side. What is the inlet manifold gasket like? Any broken bits or pieces missing, has a blanking plug fallen out?

crunchy62

Just a note to say that I got it sorted. In the end it was really a case of going back to scratch with all the fuel and electrical possibilities and feeling my way along. No point going into all the detail but sometimes it's not a good idea to strictly follow workshop manuals. As these cars get older they develop their own little quirks and a more feel your way approach seems best. Anyway cheers for the help. Now if anyone knows someone who could paint a back wing it would be great?

unfit2006

Hi Crunchy. I 've been following this thread as I have owned a few MGs down through the years.
What was the problem in the end ?

jca

What was it? Flooding carb? Dodgy distributor? Massive air leak? The suspense is killing me....

crunchy62

This was really a case of assuming one thing and then doing something else based on that assumption and making matters worse!
So I had a mechanic service the car a while back so I assumed the timing was fine ... it was still out. In fact probably still is as I've no timing light but I set it as best I could and it's better.

The plug leads were wrong ... although I figured this out early on. How that happened I don't know but it did. Worth mentioning that the Haynes manual shows No. 1 in the wrong position!

The car wouldn't start without choke and the plugs were sooty. This confused me as I assumed that the mix must be rich whereas it was really the result of using too much choke. Why did I need to use the choke? Well it turned out that setting the carb jets to the recommended starting point for tuning (2 turns down) was not enough to start the car and the mix was actually way too lean. (So much for following the manual!) I cleaned the plugs, turns the jets down a few more turns and tried starting without choke ... it started. So I knew then that it was really down to the mixture not being right and then managed to get the car running fine.
I know this seems very simple and obvious and it is but as I said when you follow the manual and things don't happen, you assume something is wrong or you alter something else and you end up going down a garden path!

Do I feel a plonker ... no. You'd have to have been there!