MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGA - Something missing

I have a 58 MGA that ran rough on the drive to winter storage last fall with serious missing above 2000 or 2500 rpm. I guess it was not a self-correcting problem. On startup this spring, starting was hard and idle was very rough—sounding like two or maybe three cylinders were hitting. So the question is where to start.
I admit to intermittent problems last summer with cold starts and occasional serious missing above 3000 or 3500 rpms. But nothing consistent and, until the fall when misfiring started at lower rpms, it was not a great big deal. Prior to the latest developments, I put in new plugs and points and timing was adjusted. Plug wires are good and the distributor cap is clean. I believe with some confidence the ignition is set up right and the parts are good.
I am also pretty sure the gas is good and the fuel pump is working properly. I tried to sync and adjust the carbs last summer. I am less confident on that score because of lack of experience with SUs. However, since I handled them last, the car has run fine but now has gone into this funk, as described above.
So, the obvious answer is carburetion. However, before I pull the carbs for more inspection, does anyone have other suggestions, insights or magic solutions? Thanks.
Daniel Crothers

Hi Daniel,

I once had a similar miss-firing (so I thought)experience, only to find that it was a blown head gasket between the #1 and #2 cylinders. Here is a check that might help you isolate the cylinder that maybe causing the problem. While the engine is running pull the spark plug wires one at a time starting with the #1 cylinder (be carful or your get a nice little jolt). See what effect the disconected cylinder has, then reconnect and move to the next. In my case I found that disconecting cylinder 1 and then cylinder 2 had no effect on how the engine was running. Then I did a compression test revealing the problem. If only one cylinder has no or little effect you may want to swap plug wires and or the plug with another cylinder to see if the problem moves to the cylinder you swapped with.

Hope this helps.

Ken
L Caya

Possible failing ignition coil.
Ed Bell

No need to guess. Do the diagnostic work in a systematic manner. Here's a few pages to get you started: http://mgaguru.com/mgtech/care/cf106.htm
If you get through all that and haven't figured it out yet, give us another holler.
Barney Gaylord

do a compression check first. if ok, move on.

adjust valves next.

then check your points gap again - preferably with a dwell meter, then set static timing (dwell effects timing).

Then and only then start looking at carbs.

Do the simple stuff first - set float height very carefully to 7/16" and make sure your float bowl to carb body seals are in good nick (viton parts are good here) and that the washers are there and installed in the right orientation.

Are your jets both returning all the way home after you let off the choke - reach under and push up on them to be sure.

Try setting the mixture on both carbs to be the SAME by using a vernier caliper to measure down from the bridge of the bottom of the carb throat to the top of the jet with the choke disconnected - set it to about 0.10" for starters.

How now?

Then you can look for things like a bad distributor rotor (there are lots of bad ones out there) or a failing coil (also common).

I had this issue recently, and it turned out my heater valve was trickling coolant onto my distributor cap - car was cutting out and ran rough, and eventually just died.

Replacing the valve stopped the problem, but it was hard to get all the coolant off of the distributor cap and wires.

JIM in NH

AJ Mail

I go with the adage -"most carburation problems are electrical" . Several members of our club have had just such symptoms as yours recently and it turned out to be a duff rotor arm ( newly fitted)- so I would replace that first with a known good one as that is the simplest job.
I have also had the head gasket go which gives similar symptoms as Ken says and is an easy check if you have a compression tester. I have also had the leaky heater valve like Jim describes - which also easy to spot. If it's none of those things then go through Barney's check list as listed on his website - good luck.
Cam Cunningham

Take a look under the bonnet in the dark, you may see HT electrical leaks.
Art Pearse

I knew I came to the right place! These suggestions are wonderful.

I do not keep the car at home so it will be the weekend before I can get to work.

Thank you all. I will report back next week. Until then, any other suggestions will be welcomed.

Dan
Daniel Crothers

My most recent problems were a bad rotor that died a slow death.....bad(old gas) i was told gas loses a point of octane about every month
HCR

FYI - I just ordered a new "red rotor" for my distributor from Jeff at Advanced Distributors.

The fit is perfect - very very snug on the shaft - you have to work a bit to get it on and seated, but then it does NOT move. There is no rivet to arc over or fail.

I would recommend this to everyone. My old rotor (new) was very sloppy on the shaft by comparison, and had a large rivet (which can be the source of trouble - see Barney's pages).

JIM in NH
AJ Mail

An update:

Compression is 145 to 150 all around . Checked electrical and timing and everything looked ok. Pulled air cleaners and made sure everything on and in carbs was moving. The engine started rough and ran on back two cylinders. After warming up and some tinkering with front carb, it began hitting on all four. Ha! Problem solved.

But not so fast. After warming, engine it still would miss occasionally and produce periodic backfire above 2500 rpm or so.

Pulled the dizzy, checked the internal wires and gave it, cap and rotor a good cleaning. After reinstallation and re-gapping and timing, spark is strong to all cylinders. Of this I am sure after accidently touching the fender (leaning crotch first, of course) while pulling the front plug lead. HT lead pulls reveals no action on front two cylinders, so back to where I started before spending half a day cleaning the distributor and adjusting my underwear.

Pulled carbs and that is as far as I got. The intermittently dead two cylinders suggest to me there is trouble with front carburetor. The backfiring suggests a timing, rotor, distributor cap or other ignition issue. The co-occurring problems make diagnosis difficult at this point so I am going to tear down the carbs this week and get them put back to specification before I go back after the ignition foibles.

If any of this sparks further thoughts (other than to use a better insulated pliers), I would love to hear them. Thanks to all.
Daniel Crothers

Check to see whether you have a sloppy distributor shaft. See whether it moves side to side in the housing. Also, check to see whether your timing chain is loose.
Mike Parker

DC if you think the problem is with the front carb, loosen the connecting throttle shaft so that rear throttle disc stays closed and then try to start the engine on the front carb which may not be possible from what you say but this may isolate the problem and direct your efforts to getting fuel and a spark to numbers 1 & 2.
J H Cole

Daniel, check your front air cleaner element, see if it is soaked with oil, and report back on what you find.
Lindsay Sampford

Lindsay,

The front element was soaked with oil. The rear was not. I noticed that but had mentally filed it away as "something to worry about later."

What do you think?
Daniel Crothers

Mike, the distributor shaft is pretty solid. As I mentioned, I had the top-end off the dizzy and the shaft was free and without wobble.
Daniel Crothers

Daniel
I had similar problems to yourself it was fairly intermittant miss firing over 3500 revs and I checked and replaced ignition parts etc after instigating a systematic testing
It turned out the rubber grommett on the float chamber- carb link on the front SU was deformed and caused intermittant petrol starvation to the front carb - new grommet problem solved
Just a thought for you
Paul
P D Camp

the front air cleaner element is the one that the rocker box vents to.

If soaked with oil, take it off (if original type) and clean with petrol and then dip in clean oil and wring out, then replace. They are supposed to have some oil in them, but not be "soaked".

Try running the car with the elements both off though to see if that solves the problem. The issue here would be the front carb is unable to breathe, so taking the element off removes that problem for the test. Air flowing through the carb is what pulls the fuel in, so no air, no fuel.

I use aftermarket Fram type pleated paper filters, which seem to work great (info on Barney's pages).

If you pull the top of the front carb's float bowl, is there fuel in it? Is it the same as on the rear? Pull the inlet fuel filter behind the banjo to make sure it is clear. Pull the float valve pin and seat to make sure they are clear - no debris - and that the pin moves freely in the bore of the seat.

Dumb idea - but check to make sure that your throttle shaft unions are all tight (1/4" nut driver). If not, the rear carb will actuate with the cable but the front will not.

JIM in NH
AJ Mail

Jim, thanks for the insights. I use the Frams and the oil soaking could be part of the problem. The front bowl had fuel, as did the back. The linkage was tight so both carbs were opening. With the air filter off on the front, gas appeared to be vaporizing properly because I felt it when the engine was rev’ed and my fingers covered the front carb bore. I have the carbs off now but have not opened them up to check for obstructions.

Paul, that is an interesting hidden problem area. I will look for properly formed rubber parts when I pull the units apart tonight or tomorrow.

Thank you again.
Daniel Crothers

It’s alive!
Distributor is back in. Carburetors are back on. Fired right up. Adjusted timing. Sounded good. Ran great.
When I tore down the front SU, the only problem spotted was left over from a prior carburetor rebuild (oops)—there was an extra cork jet seal and washer under the spring. I do not know the SU’s operation well enough to know whether that was THE problem or A problem, but I doubt it helped performance. As New Hampshire Jim pointed out, the front air cleaner had a lot of oil. That cleaner was replaced. When all was done, it ran fine. No sputters. No missing. No other rough operating issues.
Now, if someone will tell me the key to the rear seal…. It seems 6 months of sitting did not help that situation.
One more 54 year old car is back on the road. Thank you to all.
Dan
Daniel Crothers

Hooray!



JIM in NH

AJ Mail

This thread was discussed between 15/05/2012 and 29/05/2012

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.