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MG MGB Technical - '74 MGB missing, sluggish and low power.

Since my last post I have put in new plugs, wires, rotor (proper type), changed the oil and checked fuel flow to carbs, which is normal. My oil pressure has become low (after warm up,it gets down to 10 psi at idle and roughly 50 psi at 3000 rpm) and when I take it up to 3500 rpm the oil pressure gauge needle bounces up and down between 50 and 75 psi.
When driving in all gears the car misses and loses substantial power. It is worse in 2nd and 3rd gears, but the problem is in all gears. It seems to be worse as the car heats up. It misses and sputters with substantial acceleration. When first started when cold a bit of smoke comes out of the exhaust and I smell something that resembles carb cleaner. The smoke is only temporary at initial starting and goes away as does the smell.
I've run out of ideas. Any thoughts would be appreciated.

How do the plugs look after running?...

Carb diaphragm?

Guys....the plugs look good. Tan color rather than sooty and no diaphragms in the carbs. Austin.

It is going to sound funny but try this..
Disconnect your exhaust system at the manifold and drive it a short way noise and all. I had something like that when a mouse or bunch of meese decided to make a home in the front section of the exhaust system. I had to replace it as I could not clean it. Baked mouse may smell different.

If you can reproduce the misfiring with your head under the bonnet clip a timing light onto the coil lead and each carb lead and watch the flashes, looking for any irregularity which will indicate HT problems. Have you checked dwell? Timing including centrifugal and vacuum advances? Plug gaps? Carb setup?

Oil pressures are a bit low, Workshop Manual quotes 10 to 25 idling, 50 to 80 running. That is for a 3-bearing so you would expect a 5-bearing to be a bit lower, but mine are usually 40 and 60. Bouncing of the gauge is 'normal' and varies from car to car, it is caused by the individual characteristics of oil flow, viscosity and relief valve. The bouncing you have could be the valve sticking in its bore, which could also be the cause of the generally low pressures. You say 'has become', do you mean it has suddenly dropped from something else?
Paul Hunt

I think the smell gives it away - anti-freeze. Run a compression check to see if you have a cylinder or two low. That would explain the miss and poor performance also. May even have some in the oil which explains the oil pressure oddity. Hope I'm wrong.
Norm Tucker

Ok gentlemen...after checking all previous issues and driving it today after sitting for a while it started right up and ran beautifully. I drove hard and fast for about 1/2 hour and came back home and parked. After sitting for 1/2 hour or so, I pulled the manual choke out and tried to start it again and had a difficult time getting it to turn over. Turned it off, tried again and it would not turn over. Finally, the 4th time it started. I then drove out of the driveway and it sputtered and spit going relatively slow. I took my foot off the gas and then accelerated quickly and it jerked and sputtered. I pulled the choke out, accelerated a bit, pushed the choke back in and after a couple of spurts and spits it accelerated fine, quick and ran fantastic. Then after driving around again quick and at high speeds for a 1/2 hour or so I came back, turned it off and let it sit for about 15 minutes. I tried to start it again and the same difficulty starting occurred. Could it be the choke? Could it be sticking or either something not sending enough gas into the carbs? Any thoughts?
Thank you very much.
M Rodgers

The smoke you mention, is it white and steamy? Check to see if it worse on a closed throttle, i.e., blip the throttle and watch on over-run. Check to see if you are using water.
Allan Reeling

Allan, the smoke is white but not steamy. More like oil smoke and goes away quickly. I also smell gas when it's difficult to start. When I turn the key and push the accelerator all the way to the floor the 4th or 5th attempt it will start with quite a bit of sputter, but after taking it up to 3000 rpm or so it seems to be fine. I'm stumped.
M Rodgers

By the way, I'm now registered my name appears as M Rodgers rather than Austin. I'm trying to get it back to Austin but am having difficulty.


Are you using the choke when the engine is warm? If so, that might be why it is hard to start. You should only need the choke for a cold engine.

C R Huff

Have you looked a the choke cable when having the hard starting/bad running problem. If it ran well for a while then after stopping/restarting is running bad, I think that eliminates timing, valves etc.

Given what you've said in the most recent post I'd be looking at the carbs.........particularly the choke.

BH Davis

Hi Charley. I have only used the choke when cold. After running for a while and the engine is hot I've tried to start with no choke engaged.

Hi BH...what do you think I should look at with the carbs? They are non-diaphram type and damper oil is filled. I will take a look at the choke cable, or have someone pull it in and out while I look. What should I be looking for?
Thanks for your responses, guys!

Look at the linkage. Make sure all screws and clamps are tight and can't vibrate loose.

Look at the choke itself on both carbs. While the car is running see if you can get a change in engine rpms or performance while jiggling the choke linkage....first close to one carb and then close to the other.

You can do the same with the throttle linkage.

Look at the butterflys and make sure they are not loose on the throttle shaft.

Runs well hot as vs cold just make me think carbs.

BH Davis

Hi Austin,
Do a compression test to rule out head gasket problems, which is what it sounds like to me.
Allan Reeling

Thank you for all your responses, guys.
Allan, I've been told by a reliable mechanic that it may be blockage on one of the valves preventing it from rising and dropping completely. With the mouse nest residue something may have blown in from the carb. Could that be a possibility? Could I see the valves by just taking off the valve cover and perhaps cleaning the area out? Any other thoughts anyone ? The mouse nest in the carb thru the air filter I believe is the culprit and causing all these issues. I'm just not sure where to fix it.
Thanks very much.

Partly depends what your mice make their nests from!!! Bits of grass, twigs etc, I would have thought would have been incinerated by now. Looking at your valve clearances might give you an idea as to whether the valves are seating properly, unless you have re-set them, since the problem started. But again a compression test, wich is very, very simple to do, will tell you if a valve is not seating properly.
You should be able to see a non seating valve, looking under the rocker cover, and turning the engine over by hand. A non seating valve won't rise as far as the others. But this is more likely to be caused by a bent or burned valve/seat, than animals!!
Allan Reeling

Hi Allan. Thank you for your response. The mouse nest was mostly paper from the air filter and some soft material which appeared to be insulation. It was literally jammed between the air filter housing and the opening to the carb, with most of it inside the side hole of the carb holding it in the "up" position. After I pulled all of it out that I could, the carb piston slowly moved back down and appears to be normal. I don't have the ability to do a compression check on my own as I don't have the proper equipment, but I will take it to someone who does and have them perform the compression check. I have run out of other scenarios as to what could be causing this problem and hopefully there wasn't major damage done while driving with this mouse nest in the carb until I discovered it.
Any other thoughts would be greatly appreciated.
Thanks again...

I don't know how related this might be but I'll describe a somewhat similar situation that I had with my TF-1500 recently and the fix that took care of it.

I had a persistent high speed miss which nobody seemed to be able to correctly diagnose. I suspected that it might be a distributor problem as I pretty much tried everything else to no avail. Finally, I pulled the distributor and sent it to Jeff Schlemmer at Advanced Distributers. A couple of days after sending off in the mail, I got a telephone call from Jeff and his first words were "we gotta talk". You know, the kinda talk you don't want. Jeff stated that my distributor had been rebuilt before and that there just wasn't enough left for him to do anything with. After we talked for awhile about how I drove the car etc. He said that he'd like to try something and that was that he wanted to rebuild a 45D for me with a Pertronics ignitor. I said "go for it", and a few days later my "New" distributor arrived in the mail. I reinstalled it in the TF and it fired right up and that car has never run that good during my custodianship.

Just something to think about.

Cheers - Dennis
D L Rainey

This thread was discussed between 20/08/2009 and 17/09/2009

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