Welcome to our resource for MG Car Information.



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 MGB Technical - American Export Model Engine Issues

I have a 1972 mgb roadster which was an American export model and has an 18V584Z engine. Originally this car had an evaporative loss system fitted, HIF4 AUD493
carbs (ABD needles) and a 41370 25D distributor. The original carbs and distributor are worn, the evaporative loss system removed and the car was not running correctly anymore so I opted to try and put it back to "standard" UK settings. To do this I reasoned that standard carbs (ACD needles in particular) and a standard distributor would suffice.

I still cannot get the car to work well above 3000rpm as at that point (40+mph) it can lose power entirely at times, occasionally backfires and generally feels down on power barely making 65-70mph.

I was wondering if I'm wasting my time trying to make it work with "standard" settings and I really need the ABD needles and special carb? The only components I can imagine being different in this engine to a "standard" one would be the head and/or camshaft. Does anyone have any information on the american export models which would give any details on changes specific to the 584Z other than the distributor and carbs?

Some other information in case it helps:

* It has standard timing (10 degrees static, 15 degrees at idle)
* Increasing the timing by another 10 degrees does make it run better but its still rough at 3,000rpm and still doesn't feel right.
* Compression on all cylinders is 225psi - comparatively high?
* Its had replacement coils, leads, plugs and distributor and carbs
* Valve timing is correct
* Valve clearances are correct
* Disabling the vacuum advance helps a bit and at least makes the
problem more reproducible.

I have tried to glean what information I can about the 584Z from books, manual and the internet but its a comparatively rare engine so any advice would be welcome!

I have been having issues with the performance of the car for over 9 months now and its really
wearing me down, I'd like to get it sorted!
Richard Purdie

It sounds like you could be suffering from fuel starvation. Have you checked the pump?
Dave O'Neill 2


I agree with David that fuel starvation is worth looking at.

I don't know the details on that engine, but if your compression gauge is correct, I don't think the engine is in stock configuration. I think stock compression would be more like 130 psi for a low compression model and 160 psi for a high compression model. So, you may be dealing with an engine that has been souped up, and trying to get it to stock specs may not work out.

If the compression really is that high, I might suspect a non-stock cam has been installed. If someone was working on it without knowing that it had a non-stock cam, they might have aligned the timing marks as if it were stock, and thus thrown the cam timing out.

It's not clear to me if you replace the carbs or rebuilt them and changed the needles. I can't help, but someone who can may want to know.

Also, besides the evaporative system, the U.S. spec car had a bunch of other emission control components. I don't understand them well, but having some of it gone, and some if it present might cause problems.

C R Huff

The pump is an "electronic" one and seems to give reasonable pressure so I don't think its fuel starvation.

The cam is aligned as per the timing marks and I did check that at TDC (checked with a dial gauge) the inlet and exhaust valves on cylinder 1 (or 4) were switching over.

I did entirely replace the carbs after trying to rebuild the originals and have tried various combinations of the needles since then.

All the emission control equipment that I know of has been removed.
Richard Purdie


these engines were rated 8:1 CR, home marked versions were fitted with 8.8:1 CR pistons.
Chamber volume of this head is quoted to be 37ccm on 8:1 Cr cars, while 8.8:1 engines of this time used the ones with larger inlet valves but hat a chamber Volume of 42.5 ccm. The main diference was in the deeper dished pistons.(16ccm via 6ccm if i remember it right)
There is also a difference in ignition vac. The US version used manifold vac while the 8.8:1 CR cars made use of ported vac at the rear carb.

With my ex US B (which suffered of the same symptoms you cuote), i changed the dizzy for one out of a MG 1300 FWD (same as Mini 1275) and changed to ported vac at the carb.
This 'tune up' finished with the problems of backfire and the engine then was able to excellerat as it should.
The needles were changed to the No. 5 version in the HF 4 carbs and K&N Filters fitted.
Estimated BHP after this canges 78-80.


As Ralph says American engines after 1970 were low-compression so you aren't going to get exactly the same results as a UK engine anyway. As far as distributors go that is a moot point. From 67 onwards American distributors and engines were specced more and more to meet emissions limits rather than performance, but even UK specs are largely irrelevant today as the fuels are so different and without spending a lot of time and effort on a rolling road the best you can do is set the timing for no pinking at any combination of revs, load and throttle opening. That goes for *any* distributor on *any* engine, although some combinations will be further off the ideal cureve than others and give poorer running as a result. Also carb and inlet manifold vacuum sources sould make no significant difference in driving as they are both the same off-idle.

Timing *must* be done with inlet manifold vacuum disconnected and the source port plugged, and it is best to do the same with carb vacuum as well as otherwise you can get grossly retarded timing.

Apart from excessive pressure causing flooding it's fuel pump *flow* that is the most important. With a pipe removed from a carb and directed into a container it should deliver *at least* one Imperial pint per minute and in practice close to two, in a steady stream of pulses, with minimal bubbles. Electronic or not makes no difference, all SUs work the same way and can have the same problems in use.

Backfiring out of the exhaust (if that is what you are getting) implies loss of HT, and that can be caused if the distributor has been modified or has the wrong rotor, so that the rotor moves away from the cap contacts under extremes of vacuum advance, you do say that disconnecting the advance improves things somewhat. There can be many other causes for loss of HT, of course.

How did you check valve timing? It wasn't long ago that someone bought an MGB with a modified engine that just wouldn't run, and the upshot was some hot-shoe kid had changed the camshaft and completely buggered it up.
Paul Hunt


Richard said he had 225 psi cranking compression. If his gauge is rignt, doesn't that indicate that it is not a low compression engine now even it it used to be?

C R Huff

You've hit the nail right on the head. With a reading of 225 PSI, I'd expect a Compression Ratio of at least 10.5:1. Definitely not an Original Equipment engine! However, it may be a poorly-done attempt at building a high performance engine. I'd tear it down and investigate the combustion chamber volume, crown-to-deck clearance of the piston, and above all, the specs of the camshaft lobe and its timing. After that, since the distributor has been replaced, I'd check out the ignition curve to make sure that it compliments the valve timing. Afterwards, a good rolling road session should discover the correct needle profile. However, if it is a true performance engine, a stock exhaust system would be so restrictive that the engine won't run right, no matter what else is done.
Steve S.

225psi....something wrong with that...coupled with rough running I'd be looking at the camshaft. You can measure the valve depression with the valve cover off using a steel ruler and make sure the depression is even across all valves. My 78 MGB bought with only 37K 5 years ago had a completely buggered cam...worn like you would not beleive...amazed it ran at all. You also cannot trust the timing marks on these...the harmonic balancers rotate the marks out of alignment...often!


The 225 psi is not necessarily wrong. It might be wrong, but it might just indicate that it has been modified. I don't remember what my Sprite runs, but it was at least 225 last time I checked it. It is not stock, and I did not check it with more than one gauge.

But, I am with you on checking the cam timing. If the 225 is correct, Richard may have a real nice running engine once he gets it sorted, though getting enough octane for it may be a trick.

C R Huff

This thread was discussed between 26/11/2008 and 30/11/2008

MG MGB Technical index

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