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MG MGB Technical - Still can't get my car to run right

I've checked and rechecked my timing and carbs. The spark plugs are telling me its not running rich, and yet its drinking petrol still and is running rough. Its doing at best 20mpg (UK) which I just can't live with and it doesn't seem to have that much grunt. OK its not slow but its not as fast as its felt before. Its a 1978 MGB GT with twin HIF4 carbs, and is otherwise stock aside from a 123 distributor and flamethrower coil. Oddly as well, when I remove the oil filler cap with the engine running, the idle speed drops. Is this normal? The other problem is there's a gap between the exhaust and manifold unions below the carbs which I need to sort out. Its there though cause the exhaust has no flexibility and the engine rattling has obviously worked it loose and pulled a stud out of the manifold. Ill slacken the exhaust mountings and reposition it and put a nut and bolt through so thats fine but would that be what is causing all this trouble? This evening Im going to adjust the tappets and recheck the timing. What else should I be looking at here? I mean Im properly stumped. Its not a complicated engine and this should be well within my mechanical expertise but I just cant rectify the problems. Im half a heartbeat from taking it to a mechanic to sort out, but I would prefer to get this sorted out on my own, and get that warm fuzzy feeling inside when all is well thanks to me, instead of my wallet.
Ross Kelly

Are you saying you have a exhuast leak at the manifold and the engine head? Or at the manifold and the twin exhuast pipe?

Either way this is a exhuast leak and will cause rough running and also poor gas mileage, especially if its at the manifold and the head. This can cause you to burn out a valve also if ran to long.

Get that sorted out where there is no leak and go from there.

Matt
Matt Kaser

Its at the bottom of the manifold where it joins the twin exhaust pipe. The thing is though, when I put my hand down there (When the engine is cold obviously) I dont feel any exhaust gases coming out. Would it be more likely to suck air in though?
Ross Kelly

Ross,

if the needles in the carbs are not worn and are selected right for the air cleaners you use, i would check the o-rings of the choke mechanism at the sides of the HIF carbs first. If they are suspect to leak fuel (happens due to age), renew them! Keep the dampers toped up with oil as discribed in your workshop manual.

With the exhaust, there is a cradle that prevents the downpipe from oscillating. It is positioned at the bottom of the back plate of the engine and prevents the problem you discribed. If it is not there, you should add one, otherwise the connetion betwen the manifold and downpipe will become leaky soon again.

With the differnt idle at closed or opened oil filler, check out whether the right (vented) filler cap is installed. All to often the non vented version from the Mini and some MGB versions is used as an exchange. In the drivers handbook there are recommendet intervals to renew this vented filler cap. Seems that someone has had the idea to use the wrong one, as it is cheaper, fits the valve cover and learly looks the same.

Hope this helps

Ralph

Ralph

I fully rebuilt my carbs only a few weeks ago with a proper SU rebuild kit. All the seals are brand new. The needles are stock too. I'll be replacing the needles with the appropriate ones for K&N air filters though. They're the AAA ones, aren't they? I replaced the cap with one I bought in Halfords which looked pretty much the same and fit ok, but I'll stick the old one back on instead cause obviously its causing problems. I don't really understand what you mean by a cradle at the base of the exhaust. Whats holding the exhaust in place at the bottom is a P shaped wrap-around clamp attached to the body. It doesn't allow for any movement of the exhaust at all though. I would have though the mountings should have all been rubber.
Ross Kelly

Look in the parts books to see what you are missing for exhaust mountings The exhaust should not be mounted solid to the body. Check the color of your plugs after letting it idle for a few minutes. A high float level or leaky float valve may be causing some flooding at low rpm's. The rpm drop when you take off the filler cap may be normal. If the rest of the emissions equipment is in place the carbs are drawing vacuum on the crankcase, air flow into the crankcase is trhu a hose from one aircleaner, when you remove the cap you increase air flow into the crankcase which will lean out the mixture in the intake manifold.
John H

There's no emissions equipment on a British spec car (thank god... I've enough troubles as things are without more sh*te to go wrong).
Ross Kelly

While the British cars were not smogged, you may still have a positive crankcase ventalation system.
John H

OK I'll look into that.

I adjusted the tappets today and I have to say they were ridiculously loose. On some of them there was nearly 1mm clearance I'd say. Anyway the car certainly sounds better now! I adjusted the timing too which was out quite a bit. Im still not happy though. The idle is very rough. I'll get the exhaust sorted out and see how I fair out from there. One thing though is the spark plugs all looked fairly clean so I wouldnt say its running rich at all. Would the exhaust be causing all this trouble?
Ross Kelly

Ross. The gap between the header pipes and the exhaust manifold is bridged by a donut, two on cars having twin header pipes. Thus, there should be a gap between the two flanges. Look at a workshop manual, or the Moss catalog, to see how the system is set up.

An engine analyzer and an exhaust gas analyzer would make a world of difference in the time, and money, it is taking with your trouble shooting activities. There must be a shop in your area which would provide these basic services for a reasonable fee. Much easier to make progress when you can find out what the exact problem(s) is/are.

Les
Les Bengtson

I work in a car dealership. Im going to use their exhaust gas analyser there in the next few days but I dont want to go in and use it and for me to fiddle with the carbs trying to get the mix right if its something else throwing it off.
Ross Kelly

If the timing was retarded, it could have quite an effect on your gas milage.
John H

Well its set to TDC, which is whats recommended with the 123 distributor. Today on my lunchbreak in work I brought the car up to the workshop and did a compression check on it and checked the emmissions. The foreman said that the car would pass the national car test emmissions test for fuel injected vehicles, so it was absolutely perfect. The compression from cylinders 1 to 4 was 140, 135, 135, 130. Since I adjusted the valves its much quieter too. So why the hell is it using so much fuel?? I just can't figure it out!
Ross Kelly

Do you have something silly like a siezed brake caliper!

Carl
C Bintcliffe

No I replaced the brake pads on the front, and its not pulling to any side. The wheels don't get hot. I calculated that Im getting a maximum of 22 mpg (UK gallons). Its appalling! Around town I must be getting around 15mpg. I just can't live with this level of fuel consumption, especially considering the price of fuel here at the moment. I would have expected something in this region for a jensen interceptor or a jaguar e-type, but not a 1.8 litre MG.
Ross Kelly

If your not burning it you must have a leak somewhere. They can be hard to find as a small leak in a hot location will cause the fuel to vaporize before it can be seen. With the car cold and not running you may want to turn on the key and check the complete fuel system for any type of leak. Also snug up all the connections in the fuel lines. Be sure to check the top of the fuel tank as they are prone to rust.
John H

Yeah I must get under the car when the key is turned on to see what happens. I was told that the fuel pump when the ignition is on but the engine not running should tick every 30 seconds or so. Mine ticks every 3 to 5 seconds, and when I turn it on in the morning it pumps at full speed for about 3 or 4 seconds. I believe the fuel tank was replaced during the restoration.
Ross Kelly

Ross. Like John H, I believe you may have some form of fuel leak. If the pump is ticking every three to four seconds, there is a problem somewhere. It should not do this.

When leaving the ignition circuit on for more than a few seconds, disconnect the wire going from the coil to the points type distributor. If you have an electronic points replacement system, disconnect the wire going from the ignition to the coil and insulate it (it is live). If this procedure is not followed, and the points are closed (or the points replacement simulates this condition) current is flowing directly to ground and it is possible to fry the distributor and significant amounts of wiring.

Since you only think the fuel tank has been replaced, I suggest you empty it and remove it from the car for inspection. They rust through from the top and a leaky tank will, significantly, reduce you fuel mileage.

Look at the connections going from the fuel tank to the fuel pump and from the pump to the hard line going forwards to the engine compartment. I have seen a number of leaks in this area, both from bad/insufficiently tight fuel lines and from the banjo fittings not being sufficiently tightened to the pump.

Les
Les Bengtson

Ross,
I don't believe the 123 dizzy should be set to TDC as a dynamic setting. Yes, as a static setting to get it close enough to fire up, you do set the engine to TDC and then rotate the dizzy body till the LED comes on through the hole, but you should also check the timing with a timing light, with the vacuum advance disconnected, at the proper rpm, etc.

Also, much though I appreciate the HIF series of carbs, they are prone to float problems. If the fuel inlet valve is not sealing properly at the right float level, excess fuel will run down the intake manifold into the engine. Since it is not atomized, it might not show up as rich on the plugs, but it will drink excess petrol and it will run rough. Perhaps you might disconnect the fuel feed from the first carb, plug it effectively with a bolt or some such (perhaps a pressure gauge?) and turn the ignition on. If the fuel pump stops ticking every few seconds, you might be onto something.
David "I miss my HIF4s" Lieb
David Lieb

I did in fact set the timing dynamically to TDC but I didnt disconnect the vac line. I tested it at around 800rpm. I'll try disconnecting the vac line tomorrow.

Les, the 123 distributor goes into standby when the ignition is on but the engine isnt turning so the coil isnt active then.

I got a full fuel line replacement kit from Sussex classic car parts but until now I only used the ones that go in the engine bay. I must get in there and replace the rest. Is there a way to see if the tank is leaking without having to remove it?

I replaced the needle valves as part of the carb rebuild I did so I would doubt the carb is overflowing but Ill certainly check it anyway to be sure to be sure. ;) I really hope its not that though cause if Ive to take the carbs out again Ill actually scream like a little girl.

So tomorrow then Ive a bit of work ahead of me. Ill be delighted though if I get to the bottom of this poor fuel consumption problem.

Thanks for all the help, advice and indeed patience with me on this guys. I really appreciate it.
Ross Kelly

Ross,
TDC is ONLY used for statically timing the 123! It should NOT be dynamically timed to TDC.

http://www.tdcperformance.ca/pdfs/123MG.pdf has a lovely chart at the end indicating the dynamic settings for the various advance curves. Which curve are you running? Please note that the proper way to time these is to find your setting in the chart, disconnect and plug the vacuum advance line, rev the engine up to the rpm in the chart and set the advance. Best done with a timing light with a programmable advance knob. I am sure you have a friend with one somewhere...
David "with a 123/Mini on his midget" Lieb
David Lieb

Ross. No, there is no acceptable method of testing the fuel tank without removing it from the car. At that point, you can do a visual inspection. If the upper surface looks good, that is sufficient. If you find holes, that is sufficient. If there are pits and they have been painted over, the fuel tank needs to be pressure tested to see if it is leaking. I have also seen leaking around the seams, but that was a unique situation that I have not seen since.

Needle valves have been known to be bad from the factory. Floats have also been known to have holes in them and not rise upwards with sufficient force to shut off the needle valve. Removing the suction chamber and piston, followed by switching on the ignition switch, would allow you to see fuel being forced out of the jet. It should be drawn upwards by the suction produced by the pistons (cylinder) and if there is any fuel being forced out of the opening in the jet, there is one problem identified.

Figure out the proper timing for your engine. As David notes, you are using the "get it started" figure, not the final figure. You want about 32 degrees before top dead center (BTDC) at about 3,000-3,500 rpm. Then, check what you have at 1,000 rpm and make a note of that figure for future reference.

Paul Hunt's website, "The Pages of Bee and Vee" have some information on the proper distributor mechanical advance curve for your engine. You need to try and get as close to that as possible. Leave the experimenting until we have your fuel consumption problem sorted.

Someone mentioned that part of the problem could be retarded timing. (Forgive me for not running though the whole thread and noting their name--it should be noted.) Both over advanced and overly retarded timing will cause problems with fuel consumption and cause over heating. Overly advanced timing means the pistons have to force their way upwards against the burning mixture, with damage to the pistons and the bearings and excess heat generated. Overly retarded timing means that the mixture is not burning in an effective manner and that the mixture is still burning as the exhaust valve begins to open, resulting in burned exhaust valves, seats, and excess heat being injected into the cooling system.

So, I see two areas which need to be investigated. First, you need to get the ignition system dialed in properly.

Second, you need to investigate the fuel system from the tank forwards. Do not forget to inspect the seal around the pick up tube/fuel level sending unit. I have seen the seals installed incorrectly when a new tank was installed. Very poor fuel economy and puddles of fuel on the garage floor. Ditto with fuel pumps and not having the bolts holding on the banjo fittings secured tightly enough.

As you say, much to work on.

Les
Les Bengtson

Les,
He isn't even using the "get it started" numbers if he is then dynamically timing it to TDC! He is probably a good ten degrees retarded right now; no wonder it is running rough and eating petrol.
David Lieb
David Lieb

So ideally what should I set it to at 800 rpm idle without vac advance? Around 10 degrees before top? The markings on the chain cover indicate 0, 5, 10, 15, 20 degrees before. Am I right? I dont have a timing light with programmable advance. I can set it statically to TDC. The way I set timing statically is to remove no.1 spark plug, put the lead on it and leave it against the engine block, then rotate the engine until the mark on the pulley lines up with the first pointer on the chain cover, then slowly rotate the distributor body until the spark plug fires.


Jesus... I just had a thought. I might have fluffed this completely. My timing pointers are at the top of the pulley wheel, it being a late 18V engine. Ive been using the pointer thats on the left as you look down on it as TDC but considering the engine turns clockwise, am I wrong?? Is it the mark on the right thats TDC, then all marks before this are 5, 10, 15, 20 degrees before? This could explain a lot.
Ross Kelly

In my experience removing the oil filler cap on a UK spec car causes the idle speed to *increase* as it effectively introduces a vacuum leak.

You seem to be fiddling with bits and pieces and keep finding things wrong, you won't get anywhere unless you do a full set-up from scratch of valve clearances, plug gaps, points/gap dwell, timing, checking the functioning of the vacuum and centrifugal advance mechanisms, and only when all of that is correct and fault free can you move on to the carbs. They must then be setup from first principles for airflow balance and mixture balance. Few words, but it takes much longer in practice to do all that properly and carefully.

The last pointer the pulley cut passes is TDC and it should be the biggest. Everything before that is BTDC, usually in 5 degree increments. As this engine is an unknown quantity you also need to confirm that the engine really is at TDC when the marks say they do. The most accurate way to do this is screw a pointer into the plug hole of No.1 deep enough to touch the top of the piston before it has reached its highest position. Turn the engine by hand one way or the other until the top of the piston just touches the pointer, and make a mark on the pulley in line with the TDC mark on the cover. Turn the engine the other way so the piston goes down and back up the other side as it were, until the top of the piston just touches the pointer again, and make another mark. TDC is in the middle of those two marks. If that doesn't coincide with your pulley mark, eithe rit is the wrong pulley, or the pulley is delaminating (it is a metal-rubber-metal sandwich to act as a harmonic damper) and the outer slipping round relative to the inner. If that's the case there is no point continuing with that pulley - new marks or old - as it will continue to slip.
Paul Hunt2

I have been trying to work through various problems methodically and Im learning as I go, while using what knowledge I have from previous vehicles as well. Carb problems were basically sorted out when I replaced the carbs but the fuel consumption problem remained, which threw me a bit. The timing has been set horribly wrong which doesnt help matters and Im going to try to put that right this evening. Ive set the tappets and theyre fine now. U have been getting through the jobs. My overdrive is working perfectly now which is great. My new carbs are in and emmissions are good now too. Ill balance them again this evening though. I'll also check to see if my timing marks are accurate, and I'll report back tonight when Im finished.
Ross Kelly

Ross. As Paul notes, the Top Dead Center mark is the upper most mark. Each of the marks that are lower, and to the left, represent five degrees Before Top Dead Center--ignition advance. Static timing should be about ten degrees BTDC to get the engine started, with the final timing set using a strobe light.

If you do not have an adjustable strobe light, align the timing mark on the harmonic balancer with the 20 deg BTDC mark on the timing chain cover. Make a second mark, on the harmonic balancer, in line with the TDC pointer. This new mark is 20 degrees advanced of the factory mark. Run your engine up to 3,000 rpm, vacuum advance disconnected and plugged, and time the engine to 12 deg BTDC using the SECOND mark (the new one you have made--20 deg + 12 deg=32 deg of total advance).

Drop the engine speed to idle, reconnect the vacuum advance and set the idle speed to between 800 and 1,000 rpm.

Information of static and dynamic timing is available on my website, www.custompistol.com/cars/ and should be of help to you.

When I build an engine, I run a thin line of paint from the inner hub to the outer periphery of the wheel, in line with the factory timing mark on the periphery. If the harmonic balancer should begin to delaminate, it can easily be seen by the lines moving out of relationship with each other.

Les
Les Bengtson

I set the timing today statically to TDC properly. Already there's an enormous difference in the running of the car. Its much smoother, sounds better and pulls harder. Theyre seriously gutsy engines! I am always amazed at the amount of low down torque these little engines have. Theyre like a big diesel engine in that respect!

I also took a look at my carbs and got them balanced up. They weren't far off but a little bit of twiddling helped things. I took off the dashpots and pistons and turned on the ignition to see if the float bowl was overflowing. Its definitely not. I took off the fuel line from the front carb and plugged it and turned on the ignition again. Still, the pump was ticking around every 3 seconds. There was no fuel spilling onto the ground. I checked all the banjo fittings and unions around the pump and all the fuel lines along the length of the car. Not a drop. They were all bone dry. What I did notice though was that the sides of my fuel tank, as if running down from the top, were moist. The dirt on the tank was moist and the moisture smelled of petrol. So one can only assume it is petrol and is leaking out when I take a corner or go over a bump, so that would say to me that my tank is bunched. The bit I dont get though is why this would be causing the fuel pump to tick at faster intervals than its supposed to. I would have thought that when the car is stationary so long as the pump can take in fuel and the pipes get pressurised then it should just tick slowly. Anyway its new tank time I reckon. I wonder if I could get a stainless steel one. It would look cool from the back of the car apart from anything else.
Ross Kelly

Ross,

I believe there is a check valve between the tank and the pump. If the check valve bleeds back into the tank, the pump will click to keep the pressure up. So, they can click without leaking.

Did you check to see if your TDC mark is actually TDC?

Charley
C R Huff

There are check valves in the pump on both suction and pressure sides in most. They are accesable by removing the fittings from the pump body assuming you have an SU pump. They may have some dirt or grit in them as your leaky tank is both letting fuel out and dirt in.
J Heisenfeldt

Ahaaaa! I was wondering that. I couldnt figure out why my pump was ticking away merrily even when the fuel line was blocked. That also explains why when I turn on the car in the morning the pump ticks like mad for a few seconds and then slows down. Evidently the petrol is going back into the tank. So really I'd need to be taking out my pump and cleaning it when I'm fitting my new tank. I ordered a new tank today from Sussex classic car parts as well by the way. Then, once I have that fitted that should be the last of my "engine" problems.
Ross Kelly

It's the one-way valve on the suction side of the pump that causes the pump to keep clicking if it leaks back, the other one stops fuel draining back from the float chambers, although with the valve on the suction side and the pump chamber full of fuel there will be no space for any fuel to drain back from the carbs anyway so it (the one on the carb side) seems a bit superfluous.

The tank will be nothing to do with the pump clicking (unless dirt is getting in and affecting the pimp valve as previously noted). Sounds like the top has rotted out, quite common, dirt gets on top from the wheels, that gets wet and salty, and kept in contact with the paint and metal rots through. With a new tank (other than stainless which are pretty expensive) give it a couple of coats of Hammerite smooth, then loads of Waxoyl underbody seal over the top before fitting.

Are you saying you set the timing statically to TDC and left it there? It should be 6 to 10 degrees BTDC static depending on engine and distributor, then fine tuned to the correct figure dynamically. If it's still on TDC and you think it pulls well, just wait till you get it right!
Paul Hunt2

Ross.
I had the same problem with high fuel consumption and could'nt figure it out either. Last christmas i had all the rear end resprayed and when the bodyshop removed the fuel tank they found a hole in the top the size of a letterbox! Water gets in between the boot floor and the top of the tank and (unseen) rots the tank.They said this is common in MGBs. The car now gives good MPG. Just a thought.

Chris.
Chris82

The pimp valve? Isn't that something that they routinely install in modified cars? lol. Just joking. I'll pick up a spray can of waxoyl next week so Ill be ready to go when the new tank arrives. I should have some hammerite at home. Its green, but then so is the car so it doesn't really matter. Is the pump difficult to remove?
Ross Kelly

Paul,
The 123 line of electronic distributors has an LED inside. The proper procedure for statically timing this distributor is to put the engine on TDC and rotate the distributor body clockwise until the LED shines through the proper hole. It is set up so that this will NOT make it fire at TDC, it will be about 10 degrees advanced. Just close enough to fire it up and set it right.
David "I heart my 123" Lieb
David Lieb

OK, forgot this was a 123.
Paul Hunt2

This thread was discussed between 06/05/2008 and 12/05/2008

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