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MG MGB Technical - Sudden failure

OK, I had cabin feaver. It was 19 degrees (F) yesterday but I had to take the "B" for a spin. All went well for about 6 miles and than I heard a couple of "misses." Next, the car started to buck and finally stalled. I managed to get it started to rake home but it ran awful and I got a strong gas smell and backfiring through the carbs.

Now it starts very poorly, like it running on 1 or 2 culinders and it backfires through the carbs.

Spark seems fine, the dizzy has not moved and the plugs are full of carbon. Any ideas PLEASE?

Thanks, Tom
Thomas McNamara

Tom. Sounds like the choke was either engaged for too long a period, or that the choke did not come off as it should when released. Pull the suction chambers and the pistons off and look at how far the jets are sitting below the bridge. If they look low, check to see that the jets are properly returned to their "off choke" position--i.e. try to push them upwards. This is the most probable cause of what you describe.

Replace the spark plugs with a new set, properly gaped. I have found that after they have been carboned up, they do not work well unless you can abrasive blast, then clean, them.

Les
Les Bengtson

Thanks Les. I am at my wits end. Points are fine, dizzy is locked in place, battery is charged, carbs seem fine but it "feels" like the "B" is out of time. It barley catches. How could this happen with everything doing fine? I am perplexed. Tom
Thomas McNamara

Dear Tom. A number of years ago, my mother had to have surgery, on an emergency basis, and I would up flying out to be with her. This wound up being a several week process because I was the child who was self employed and, thus, able to stay with her until the situation became more stable. My older daughter had some minor problems with her Weber DGV equipped 77B and wound up driving my 68GT to school for a while. A trip of about five miles each way. When I got back, she told me the GT was not running well. When I took it out for a test drive, I found out that it was running very poorly. New (within the last year) SU carbs. The jets stuck in the downward position when the choke was applied and the plugs became quite fouled--to the extent that some of them were not firing properly, if at all. Replaced the plugs and the car ran fine. Noticed that the jets had not moved upwards and the mixture was excessively rich. Added an extra spring to each carb to assist in pulling the jets upwards when the choke was released and the problem never recurred.

Please check out the condition of the plugs and the position of the jets on your carbs and let me know what you find.

Thanks, Les
Les Bengtson

Tom -
As Les said. Pull the choke all the way out and release, see if both return fully upwards, several times. If they do, possibly pilot error in leaving the choke ON after take-off, or it was a one off deal. In either case, pull choke out all the way, and squirt graphite or teflon loaded lubricant on the exposed part of the jets, and release. Do that a couple of times, Extra springs not necessary, and make it hard to pull the choke out.
Take the plugs out and lay them on a heatproof place; I put them on the vise with the ends sticking out. Heat the electrodes up with a propane torch until you burn all the crap off them. DO NOT forget and pick up a hot plug - hurts! Put 'em back in and boogie.

FRM
FR Millmore

Thanks Les and FRM, I'll check the carb chokes as soon as I get home tonight. (Neat trick on the plug cleaning) Could/would this condition cause the engine to "backfire" through the carbs? When I removed the air cleaner and cranked the engine they "spit" gas vapor out. regards, Tom
Thomas McNamara

Hey Folks, more uncertainty. I removed the upper chambers and pistins. On the front carb the "collet" that holds the needle in fell to the floor. I appears to have been loose for quite some time as the end of the holding screw is quite worn.

Could this be the issue?

I tryed the choke in/out but see very little movement of the jet, if any. They are both between .065 and .073 down. These are HS4 carbs. How much should the jet drop?

I will get my wife to pull the choke as I watch tomorrow. Could the jets be stuck?


Thanks, Tom
Thomas McNamara

Tom-
"Could this be the issue?"
Good chance!
Can't have been loose all that long, certainly wasn't in Sept! And no I didn't mess with it, but I guess I should have checked the screws for tightness.

Be sure you didn't lose the spring at the top of the needle between the needle and piston - it's normally a tight fit on the needle extension. The needle holder will either have a flat that the screw bears on, or it will have a line scribed on the lower face that goes between the two holes in the bottom of the piston. The shoulder of the needle should be dead even with the face of the piston when reinstalled. Tighten the screw. Check the other one. Check both of them again. I will have checked that the needles were flush with the piston face, so you should not have to adjust anything other than putting the needle back right.

The jets move down about 1/2" when the choke is pulled fully out, hard to see how you got it started if they don't, in this weather. And they did work a few months ago.

It's quite possible that the jets did drop when you started the car, then the needle fell out blocking the jet in that carb from returning. That would explain your symptoms of running very rich and fouling the plugs. But then the needle fell down the jet and, once the needle fell down in the jet, that carb would have been delivering zero fuel, which would certainly account for poor running.

FRM
FR Millmore

Hey FRM, thanks again. As I read the yellow book, it shows a seperate choke circuit, (jet), for cold starting. Do the jets still drop in the HIF when choked?

The "collet" that was loose has a slot in the side about 3/4 of its length. I do not see a scribed line on the bottom. I am thinking that the slot needs to be at a certain position of rotation but I can't find a reference in any of my books.

The holding screw has a really buggered end so I will order one today. Moss shows a collet, (needle holder), and screw as seperate parts. We'll see if thay match.

Please let me know your thoughts.

As an aside, how is your daughter doing in school? I thought we might see you come through the area from time to time. BTW, we are getting 12 t0 16 inches today.

Thanks to all, Tom

Thomas McNamara

Tom -
Make up your mind: "These are HS4 carbs" " Do the jets still drop in the HIF" ! I can't remember what you have.
HIF do not drop the jets, choke is a separate thing entirely.
The screw end goes in the slot to locate the needle in the correct orientation; the depth is still set as for the HS. The screw end is turned down to fit in the slot, are you sure it's damaged?

Kid doing great, happy and crazed with intellectual delight. She took one of the Mazdas, so she has wheels, which means I'm not commuting - if I do get up I'll stop and beat you into shape!
I seem to be up to about 8-10" since it got dark last night, and it's coming down steady - way more than forecast called for, I may get really buried!

FRM
FR Millmore

Hey Les & FRM, sorry for the mis-information. They are HIF with the seperate choke circuit. I am loth to adjust anything as FRM had the car running wonderfully.

I scrounged a set of carbs from a friend of mine and I'll check the retaining screw at my earliest opportunity.

Does the HIF have choke issues? I've already replaced the floats and with new ones and the float valves with vitron. This solved a flooding problem 6 months ago.

FRM, happy to hear that your offspring is doing OK. If she has your mind she will probobly be an engineer. Does she know thart this is her time? I did not, I was too young and too dumb. As they say, youth is wasted on the young!

Stop by, with a little warning, anytime. Now, back to shovel snow, again & again and .....

Any other ideas on my problem? I'll be re-assembling tonight if I have the steam. Regards, Tom
Thomas McNamara

Tom -
Yes HIF have choke issues, which are very obvious but fortunately rare, There's a little o-ring in the valve and if it's missing or bust the things run astonishingly rich.
She knows, and is taking full advantage. No engineer though, but she did figure out that being broke is NFG, so double major, Business Admin to go with her Art History. She "will have to study abroad", and then do whatever. Idea is to make enough $$ to go comp rallying!
Getting deeper!

FRM
FR Millmore

Hey Les, FRM & fellow travelers,

I filally got some time to dig into the problem the car is having with backfire through the carbs and terrible running. The carb needle was a red herring!

I have found that the 4th valve back has an iaaue. The pushrod is jammed alongside of the rocker! I am not sure how I can repair this yet, first lunch and then back to the garage. I guess I'll start by draining the cooling system in preperation of loosening the head bolts.I thought this might help someone else. Regards, Tom
Thomas McNamara

Thomas,

If it were mine, I would pull the pushrod and check it before I started thinking about pulling the head.

Just back the adjuster out, slide the rocker sideways against the springs on the rocker shaft, and pull the push rod out. Give it a twist as you lift the rod to separate it from the cam follower so that you don't lift the follower out of the block with the oil suction.

Also, I would spin the rest of the push rods and see if they are bent.

Charley
C R Huff

Thanks Les & FRM. Too late, I loosened te headbolts, removed the rocker and found two bent puh rods. Now, I know a better way. :( Thanks, Tom
Thomas McNamara

I got exactly the same result that was due to a stuck exhaust valve after a bad valve job, machinist had left a too narrow clearance in the new guides.
Guy RENOU

This thread was discussed between 07/02/2010 and 14/02/2010

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