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MG MGB Technical - Exhaust problem and subsequent starting problem

Hi all,
I had some lovely, fully functional months with my MGB...and then I broke it. I was pulling off a backroad to enter a driveway where, unbeknownst to me, there was a very large 1-2 foot deep pothole. Imagine my surprise when rolling along slowly at about 5 mph and then a sudden dropping sensation accompanied by a large thunk sound. I managed to drive gently out of the hole, turned the car off, and looked the car over.

Everything appeared fine, so I started her up. She started normally and everything seemed usual until I pressed the gas. Every depression of the pedal was accompanied by a rythmic thunking noise under the drivers seat. Further investigation revealed that an exhasut bracket had come loose. Thus, I presumed, the sound was the exhaust rattling against the frame. After that, apart from the noise of the rattling, I drove home just fine.

Now I obviously didn't want to continue driving with this condition. I fully intended to look into it the next weekend; but I never got around to it.

So it's now two months later and I'm attempting to solve the problem. After replacing the bracket, I notice that the exhaust is really loose from the flange at the exhaust manifold. I made every attempt to tighten the connection, but the down pipes still wiggle.

And while adressing this issue I see some telltale moisture and white spots indicating a leak at the flange. I wanted to see just where the leak was, so I tried to start the car. Much to my surprise, she would start but I couldn't get her to idle. Unless I kept giving it gas it just wouldn't stay running. Of course then I flooded it, ended up fouling the plugs, and still couldn't see the leak.

Some new plugs did not solve the starting problem. So now I have multiple problems, or do I?

Is the exhaust problem the root cause of the starting problem? Could they be related at all?

Could I have conceivably broken the down pipes at the flange in the pothole?

Thanks in advance for your help guys.

Jeremy
1974.5 mgb roadster
photops.yahoo.com/jeremysmgb

J. Palgon

I don't think the exhaust is the reason the car will not start. That said, a leaky exhaust may/can eventually cause some exhaust valve problems. There are donut sealing rings between the exhaust manifold and the down pipes, the sealing rings are probably ruined and need replacing. On the starting problem, it sounds like you are getting fuel to the cylinders. If you have a timing light you can check for spark by clipping it to your plug wires and cranking the engine over. If the light flashes you have ignition. If no timing light remove a plug, connect the plug wire to the plug and ground the plug thread while cranking the engine, you should see a spark arc across the plug electrodes, be careful not to get shocked. If you have no spark check Les Bengtson's article on Ignition System Trouble Shooting in his web site. http://www.custompistols.com/
Click on Classic MG Sports Car Pages, Articles and Ignition System Trouble Shooting. The information there should help you find the problem.

Clifton
Clifton Gordon

Jeremy,

The following advice will not have any effect on your current problems.

In California, your B is no longer subject to smog testing. You can remove the smog pump, catalytic converter and related equipment and maybe gain a few horsepower.
S Rechter

Clifton,
Thanks for your advice. I am now quite certain the donut sealing rings are ruined. The exhaust pipes have serious wobbliness at the flange despite whatever tightening I could manage.

As for the starting problem, I am definitely getting fuel to the cylinders. I procured a timing light and it illuminates and flashes on all four wires. This, I believe, would imply I have solid ignition.

So I bought and installed new spark plugs in the hopes that they might be a simple solution. Unfortunately the new plugs met with exactly the same result. When I try to start the car it sounds as though it is not firing correctly on all four cylinders.

All was well when it was last driven and parked a few months ago. Apart from sitting in my garage, nothing has happenend to it in that time.

I've checked the valve clearances and they appear to be fine. The head was rebuilt in the last year, the gasket seems intact and the torque on the head stud retaining nuts is correct.

So what could it be? Any other thoughts?

Jeremy

J. Palgon

Jeremy: I said not running and exhaust are different problems. It's possible I gave you some poor advice. If the exhaust pipe flattened enough to block exhaust flow or the tail pipe blocked with mud that could cause the engine to not run or run poorly. I should have thought about the possibility of the exhaust being blocked when I answered the first time. Check the complete system including the resonator and muffler. If you suspect a blocked exhaust the easy way to check is to drop the exhaust pipe at the manifold. It will be noisy but if it runs OK the exhaust is blocked.

Clifton
Clifton Gordon

You might also check the manifold mounting against the head/intake manifold faces. If the knock you took has forced a breach at that point you could have trouble both with starting and maintaining a smooth idle.

Regards
Roger
Roger T

Clifton and Roger,
Thanks guys. I suspected both of these possibilities as well. I was originally thinking that the jolt upwards on the exhaust pipe might have jarred the manifold against the head causing a poor seal. If this were the case, it'd be easy to see why I can't get it running. But I checked the mating faces and gaskets and they appeared to be fine. After retightening the manifolds, I still have the same problem.

The exhaust didn't take in any mud, nor did the pipe flatten in any way. It just simply has come loose at the flange. Nonetheless, perhaps there's a major blockage. I'll try to check it tomorrow.

Thanks again,

Jeremy

J. Palgon

Jeremy,
Firstly, thanks for the feedback. Another thought comes from your comment that she starts but only runs if you gas it - then it floods. I wonder, any chance that jar ( a 1 to 2 foot pot-hole would generate a considerable jolt) might have upset the carbs? Gaskets (mounting or float bowl), balance settings or even caused some structural break or crack?

Regards
Roger
Roger T

When you changed the plugs did you get the leads back on in the right order? 1 3 4 2 anti-clockwise? making sure 1 has the rotor pointing at it when it is at the top of its compression stroke? If the shock did jar the float valves open to cause flooding it should have been apparent right away in the journey back home. It's possible the 2 month non-usage since then is a contributory factor. If you have the original style of SU fuel pump you can tell this by turning on the ignition and listening to the clicks. Once the pump has pressurised the system it shouldn't tick more than once per 30 secs. If it does it implies that either the float valves are leaking or the one-way valves in the pump itself could be leaking.
Paul Hunt 2

Low vacuum is a possibility. I had a problem where the charcoal canister became saturated with fuel, car would start but not idle.

Disconnecting the canister - carb (HIF type) pipes cured it until I dried out the charcoal.
Phil Collura

Roger,
The carbs were my next thought. But, as Paul mentioned, it probably shouldn't have run well on the way back home if that was the case. It may very well be a carb problem, unfortunately I am not comfortable toying around with my dual SUs. The local MG mechanic is very proficient at adjusting them, and it never ran better than after he last adjusted them. If it turns out to be carbs I may have it towed back to him.

Paul,
Good thought about the spark plug lead order. I too was wary of that mistake, so I triple checked it. They're definitly in the right place. I do have an original points style SU fuel pump; and after some initial clicking it goes quiet for long stretches. So I think the system is well pressurized.

Phil,
This is an intreguing notion. I never even thought of anything remotely like that being the culprit. What precipitated your problem? And did you simply dry out the charcoal by letting it sit open?

Thanks everyone,
Jeremy
J. Palgon

Well guys,
I've exhausted all things I am able to or feel comfortable testing. I retightened the manifold to the head, checked the ignition again, checked the valve adjustment, and still no luck.

I now think that it's either a carb problem, or I have poor compression. The latter option would be aweful and surprising because the head was recently rebuilt and I did manage to drive it home just fine. I do not have a compression test gauge, but I'm going to have that checked out shortly.

I realize now that what I described as not starting/failing to idle is more of a not firing on all cylinders thing. It sounds as though only two or three are firing consistently--producing a sad and uneven sound compared with it's usual happy runnning self.

I'm going to enlist the help of some friends in the local MG club. I have no doubt that I've simply described the problem poorly, or overlooked something; and I therefore imagine they'll locate the problem fairly quickly.

Thanks to everyone for all the help.

Jeremy
J. Palgon

Jeremy,

I can understand your disappointment but don't be too hard on yourself. We have all been there and, no doubt, will be back again at some point. Its all part of the fun associated with these little cars. Worth it all when you get yours 'on song' - you will see.

Just one thing you might check - the timing. Its a long shot but possible your distributor may have shifted and/or there may be an issue with the rotor. There have been suspect batches of rotors and the jolt/drive home/turn off and/or recent starting attempts may have highlighted a problem. Commonly the 'beak' breaks or dislodges.

Good luck with this and we would love to hear that (when) you have solved the problem and are again enjoying your car.

Regards
Roger
Roger T

Jeremy- I actually had the saturated-canister problem on two MGs., '73 & my '74.

In both cases the carb had flooded, and raw gas got into the canister. After that, the engine would start OK, but run OK only if I kept the RPMs above 3000. Any RPM lower, the engine would die.

First time it happened, I had no idea what the problem was, but I had seen engines with a significant vacuum leak behave the same way.

By luck, I happened to disconnect the canister pipes to the carbs, and son of a gun, the engine ran fine.

The bottom of the canister is screw-on, removing it revealed the charcoal was wet with gas. Let it air dry, never had a problem since.
Phil Collura

Hi all,
Progress has been made and my most recent car saga is almost resolved. Thanks to everone who offerred help. Here's how it went:

So I had the car towed over to the large garage type shop of an MGOC friend. We spent the whole day troubleshooting, exploring and rechecking the various possibilities for fault:
1. Ignition
2. Timing
3. Fuel delivery
4. Air leak/poor seals
5. Vacuum leak
5. Carb set-up

At first it appeared that the plug on number 1 lacked spark. But that turned out to be an anomaly, as it worked just fine a minute later and ever after. We also swapped out the distributor rotor just to make sure that wasn't the problem.

The timing and the fuel delivery were definitely good. Then we did a leak down test to check for an air leak. Two cylinders tested very good (almost no leaking), and the other two were slightly lower but acceptable.

Next we plugged the intake manifold outputs to the vaccuum advance and the anit-run on valve--to test if the hoses had invisible cracks. This resulted in no change.

Then my good MGOC friend suggested something I would never have thought of: water condensation in the gas tank as a result of winter storage. This seemed unlikely considering it's been in my garage here in California. However it has been the coldest winter around here in decades. So we added Isoheet (a gasoline additive meant to eliminate the water in the tank) to the gas.

Unfortunately, even after waiting some time and forceably running the engine to try to clear out the potentially watery gas, still no luck.

This left the dreaded option of carb trouble. My friend and I were both loathe to fiddle with the carbs because the local magical MG mechanic has set them up so wonderfully. But it seemed to be the last possible option. My friend, himself no stranger to carb adjustment, adjusted the carbs while I forcibly kept the engine running. Eventually he managed to get the car to idle. A little more tweaking and it almost sounded normal again. A couple test drives down the road seemed alright, if rough.

Working on the assumption that there had indeed been water in the gas, I filled up with fresh gas down the street. Almost immediately afterward the car revved much higher, suggesting that our assumption had been correct. Then we adjusted the carbs and the idle again. Now everythign seemed almost normal.

So I set off down the freeway a few exits to my local MG shop to pick up replacement exhaust donut ring seals. On the way the car almost totally died. Once at speed it was alright, but occasionally sputtering. But every time I had to slow down to 20 mph (or less than 1,500 rpms) the car would stall and refuse to start again without full choke and a foot buried in the gas pedal.

Once at the shop, the proprietor told me I should have started the troubleshooting after installing the new donut seals. He said that an exhaust leak could also act as an intake under certain conditions --muddling all attempts at troubleshooting.

I managed to get the car back to the shop, sputtering and all. After a little more carb tinkering, it was running and idleing again--but with a really strange engine fluttering-revving. If just left to idle, the car would rev up to 2,500 rpms, then cease revving, then fall to 800 rpms before sputtering and revving up again.

This reoccuring pattern was thoroughly confounding until we noticed that the the charcoal canister was expelling large volumes of gas (something that had not been occurring before). In short order we found that the back carb was overflowing massively. Further delicate exploration revealed some junk stuck in the carb float valve. After clearing the blockage and readjusting the carbs yet again, some final semblance of normality was restored.

Only then did I finally change out the exhaust donut ring seals. My old ones were clearing ruined, and the new ones went in easily. Finally, I thought, everything was restored to good running condition.

The car ran well enough driving home, but the exhaust was rattling against the frame (despite being solidly tightened down at the flange). And that's where I'm at now.

Hopefully the exhaust position just needs slight repositioning. Beyond this, the car is simply much less smooth than before. So I guess it's time for another trip to the magical MG mechanic for final tuning.

Jeremy

P.S. I forgot to mention that my driver's side rear shock absorber link seems to have broken while being towed (on a flatbed) to the shop. So I guess I have some more work ahed of me.






J. Palgon

It might be a long shot but what about that big drop you had jarred up your gas tank and you sucking in crap that got kicked up and is clogging up your carb, making it stick open and flood and run like sh*t. Could of jarred the fuel pump as well, and shook up some crud.
James

This thread was discussed between 03/02/2007 and 27/02/2007

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