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MG MGB Technical - Compression test results

1974.5 RB MGB. Weber carb and Petronix conversion. Bought car in July 04. Noticed #3 plug would soot up, others nice and gray. Was advised on these threads to conduct a compression test. Finally bought a decent test kit on eBay (Actron). And the results are:
#1- 130
#2- 135
#3- 135
#4- 130
It seems that everything is OK (I think), what is causing #3 to soot up? It doesn't skip on acceleration or cruising, the idle is a little lumpy though. Ideas or comments appreciated.

John; The rear carburetor is probably running rich. Synchronize and adjust your carbs and you will probably be ok. You can get carburetor adjustment information on Paul Hunt's web site:
Click on fuel and then SU Carbs.

Clifton Gordon

Yes or could be a weak spark if the ignition, and plug leads in particular, haven't been attended to recently. I'd give any recent purchase a thorough service with new plugs, points, condenser, leads, distributor rotor and cap.
Sometimes originality has to be sacrificed as I found with 2 nice looking period plug caps on mine which measured 600K and 2M2 (the others were 12K and 20k, all should be circa 10K). Still ran but those 2 sparks were weak.

As a quick excercise you can use a tube and equalise the "hiss" at idle by listening close to the carb inputs . Then both carbs are atleast drawing the same amount of air . This should even out the tickover , and may cure the sooting .Yuo may need to put one up and 't other down.
It would still be worth buying a colourtune .
S Best

<<Weber carb and Petronix conversion>>
Just wondering what set up you have John, which Weber and what's a Petronix?

Well, It's obvious I didn't carefully read your post as I was thinking SU's. Rich, a Petronix is a points replacement electronic gnition that installs inside the distributor.

Clifton Gordon

Sorry, it's a Weber DGV downdraft, (wish it was a DCOE), and as Clifton said Petronix is an electronis points conversion installed by the PO. How do those numbers sound? If I recall the low compression engines should run about 130 to 140 on a compression test if the engine is in good shape. It is unknown if the engine was ever rebuilt. I am wondering if I have a vacuum leak somewhere in the intake or exhaust manifold, causing the #3 plug to get sooty.

No offence intended, S, but quick exercises are no good. Once you start fiddling with twin SUs you are far better off setting them up from scratch and knowing you have the mixture balance *and* the air balance correct. Also I have to say that while two colortunes might be usable (although I never had much luck with one on a single SU carb engine and have used the lifting pin ever since), just one will need swapping from No.1 to No.4 several times during the setting up process. This is because the carbs are interdependant and changing one has a small effect on the other two cylinders which means their carb has to be tweaked slightly to compensate. It can take two or three iterations of this to get them balanced correctly. Can't see a vacuum leak causing a sooty plug ... unless it is in No.4 intake causing you to overadjust its carb in compensation. But again that would only be with twin SUs, unless the Weber has seperate throats and jets for each pair of cylinders also, but then again the fixed jets in a Weber would mean it could only be over-rich when idling and not running? Easily checked anyway by squirting carb cleaner of propane gas around the gaskets and listening for a change in idle.
Paul Hunt

John. Did you do your spark plug test properly? Warm the engine up, install a brand new set of plugs, take it out on the interstate, or other highway, run it for at least 20 minutes, take the car out of gear, coast to the side of the road, cut the engine, pull the plugs and read them?

The DGV means that the basic carb adjustments apply to all cylinders. However, cylinders 2 and 3 have a shared exhaust port which makes them run slightly leaner than cylinders 1 and 4 which have their own exhaust ports. If you had anything which reduced either the power of your spark (bad wire, bad terminal, bad plug, not fully tightened plug, etc.) that might cause an inefficient burn which could result in soot. (Excessively lean mixtures also show shoot and high hydrocarbons at the tail pipe.) A slight air leak can also cause a miss in number three cylinder which may show up as a sooty plug.

A proper plug test followed by a little time on an engine analyzer (allows you to check out sparking voltage and cylinder balance), then some playing aroung with possible sources of air leaks (carb base gasket, intake manifold gasket and any hoses running into the intake manifold) should give you an idea of where the problem might lie. But, it is also possible that you are getting some oil burning which, if sufficientl light, will also blacken the plugs, but does not look all that different from the blackening caused by an excessively rich mixture. The soot from an inefficient burn looks slightly different than that of an excessively rich mixture and you will have more of a gasoline smell to the plug. Excessively rich is a dry, dull, fluffy black. But, these are the extremes and what you see in real life can be significantly different that what you see in photos take 50 years ago and still being printed. Les
Les Bengtson

I guess I didn't do the spark plug test properly, I just checked the plugs as I removed them for the compression test, the car was warmed up however. I will try the technique you described with NGK plugs. I am running Bosch platinum right now. The wires were replaced this past fall and #3 showed the same symptoms prior to the new wires, plugs, rotor and cap. I will keep searching.

I also remembered that when I did the compression test I neglected to open the carbs fully. Would that cause my results to go up, down, or totally screw up the test?
Cyl 1- 130
Cyl 2- 135
Cyl 3- 135
Cyl 4- 130
Thanks for the advice.

You will get lower readings with the throttles closed. All figures quoted are wide open. Sorry for earlier post when I had my SU hat on. .I suggested just equalising flows , I did not want to suggest leaning out to get no3 looking good , and then have you burn a hole in no 4!There is a plug cut technique where you switch the engine off at whatever speed/load regime you want to investigate coast to a halt then pull the plugs, if you let it idle for a second all the info is lost and you just see the idle state .I have never tried this myself , and agree with earlier comments that plug colours are different with todays fuels .
S Best

Question on colortune. I was told that these will not work if you are running unleaded fuel. Anyone have the answer?
Bruce Cunha


Others have pointed out the difference in porting that leads to slightly different mixtures between cylinders in these engines. The SU manifold has a large balance tube to help even this up. I'm not familiar with your carb and manifold but assume it is a case of a single adjustment which will effect all cyclinders. Fuel-wise you may have to accept a difference if the induction is not equal, and therefore set it up so the weakest cylinder does not overheat. With one carb, logic would suggest 2 and 3 may get more fuel, but this may be emphasised in choke conditions so get on the freeway for a few miles then compare plugs.

Re the plugs, I tried Bosch platinum in a modern (then) fuel injected engine when they came out 20 years ago and was not impressed. I've tried them and NGK precious metal type in a vintage 4 stroke motorcycle and was similarly unimpressed. I've seen several postings on the net which suggest their use in older engines in particular is not recommended. I would fit a set of NGK BP6ES which are superb imho.

By all means run the comp test again with throttle open but you will probably only find a few pounds difference, if any, at cranking speed. I find no difference with constant velocity SUs. In any case it is a comparative test and would seem to have done its job.


I used a colourtune with unleaded very recently, switching it between all 4 cylinders after setting the carbs manually. It worked absolutely spot-on, showing a blue flame with slight orange tinge at the best setting. Thinking it may not be particularly sensitive, I then varied the jet settings to watch the result and just 1 flat on the jet nuts was obvious in the flame colour and engine note. The downside of course is that you are only setting up at the bottom end but for an engine on standard settings which is known to run OK when set up this way I would thoroughly recommend it, if only to see your own work in action!
I would say it is a visual aid for someone who knows how to set the carbs otherwise - it will not make the inexperienced into an expert - but you can easily see the spark and flame on a B and it would be a good tool to compare John's cylinders with.

While tinkering I made sure all the bolts on the exhaust and intake manifolds were tight. I had a hell of a time trying to reach the bolt between the 2 and 3 cyl. I could only put a 3 inch wrench on it but did manage to tighten it a quarter of a turn (I need better wrenches). I will see if this helps the #3 plugs sooting problem. Thanks for the advice all. The car performed great on a 30 mile early spring evenings cruise though.

John. In my opinion, you are worrying over a very small problem, if any problem exists at all. I would suggest that you take your car into someone having a modern engine analyzer and having them check out your engine. My old Sun engine analyzer is about the size of a large shelving unit. My American car mechanic has one which is only slightly larger than a clip board. Mine you make notes on what the scope and gauges read. On his, he prints me out a complete set of readings from his shop computer. When I rebuilt the engine for the 79 LE, which my daughter had to have, then turned out to be too short to drive, William suggested that I bring the car over and he would "scope" it on his engine analyzer and print out the results for me.

Now, we not only know the engine was showing good both on the engine analyzer and in driving, but, we have a record of what the results were at the time of test. We have used these, over the years, to keep track of the condition of the engine and what is wearing at what rate. Thus, while I believe your engine is in good condition and, unless a properly conducted spark plug test is performed, will continue to believe such, I also believe you would benefit from spending a few dollars to have the engine put on an analysis machine and to have the results printed out. If you, like I, intend to own your car(s) for decades, rather than months, complete documentation is your friend.

Bruce. I do not know "the answer", but my experience may be of use to you. I purchased a ColorTune device to assist in setting up my carbs, both twin SU and single Weber DGV. Here in Maricopa County, Arizona, all cars within the county must face an emissions inspection prior to registration. All MGs, 1967 to 1980, must be inspected on an annual basis. To me, this means one of my four MGs must be inspected every three months. Hence, I have investigated both the ColorTune and the gunson's CO meter with an eye to allowing me to meet the Arizona emissions testing standards.

My experience is that the Colortune (Colourtune), which is supposed to allow the carbs to be tuned until you get a "bunsen blue" (a blue-white flame) does not work with the fuel available to me. We use a blended fuel having either gasoline and ether (MTBE) or gasoline and alcohol, depending on the time of year. Both are "unleaded" fuels and neither will show a "bunsen blue" when adjusted while the ColorTune is in place.

My experience with the Gunson's CO meter is similar. When I have tuned my cars using the Gunson's meter, they have been tuned to the required emissions standards. The emissions testing station is three miles from my house. Hence, not a great deal to go wrong between adjustment and testing, right? The results at the emissions testing station bear no similarity to what I am showing on my Gunson's CO meter. If I show a 1.6%CO reading, the emissions testing station might show a 0.1% reading with excessive hydrocarbons. My Gunson's CO meter has never shown any form of correlation with the test results obtained at the testing station. My American car mechanic has set the Weber carbs on my 77 and 79 MGBs. He, as a favor, was kind enough to allow me to use my warmed up Gunson's meter while he was using his professional quality five gas analyzer. His readings, of interest, were CO and HC, the two readings which are measured by the emissions testing stations. My Gunson's machine only read CO, which, interestingly enough, is all the MGs of this era were required to meet at time of production. (Funny how increased standards seem to happen, what?) The Gunson's machine was at distinct odds with the mechanic's machine. The emissions testing faucility's machines agreed with the mechanic's machines more closely than they agreed with the Gunson's machine (by a wide margin--mechanic's machine showed .01% CO and the testing station showed .00% CO, within the accuracy range of both machines).

Thus, my experience with both the Gunson's Colourtune and the CO meter have not been good and I would not recommend either to the hobbyist.

Les Bengtson

Les, All,

Some interesting comments about the Gunsons gear.

I gather the Analyser uses Gunsons own system and heard long ago, when they introduced emissions testing over here, that it could produce results which don't correlate with other machines. You may find this mentioned in the manual.

A Colourtune setting of blue/white is too lean in a non-emission car running SUs. I'm not sure how emission equiped vehicles would find it. As said, mine comes out Blue/tinged Orange using Texaco 95 RON unleaded in UK and I've had similar results for 25 years.

Any others tried Colourtune?

That raises the question of the local fuel. I've been doing a bit of research into this whilst having problems setting up my car. In the end it wasn't the fuel causing pinking and running on, but it was interesting and explained other issues such as 3 instances of rubber seals in the fuel system swelling up and leaking with unleaded when they'd been fine with old fuel.

Lead's been gradually disappearing over here and no doubt the fuel formulas have been changing, largely un-noticed except by us old car users. I used to think of fuel additives in terms of a few ounces per gallon, it may have been with lead, but apparently not so with no-lead formulations. 180 proof Ethanol is 106 octane and is used both as anti-knock and petroleum substitute. It's oxygenating and helps clean burning. There are quite a lot of new cars around, E85, that are already set to run on ethanol fuel and US Midwest fuel may already be 15% alcohol according to one site. 10% is common. Toluene is 114 octane and added in large doses, up to 45% is allowed in some fuel specs. Formula 1 cars ran on 84% toluene in the turbo era.

A couple of very interesting sites,


I've done OK with the Colourtune with SUs and unleaded, although I've always taken the colours on the box as "diagrammatic" and adjust across the range from rich to lean to get a feel for the scale before deciding what is right. The only time it's been difficult was when I've used it on SUs with very worn throttle shafts that made the idle terribly lean or with needles that were wrong for the engine. Still, it made a good diagnostic tool in those cases, as it has with other problems. One proviso is that you need almost total darkness and a very clean plug to get a good view of the flame to spot the more subtle colour changes.
Steve Postins

My experience of using the colourtune has been similar to Steve's. I would add that a pair of them are good for getting the 2 carbs the same.
David Witham

This thread was discussed between 06/04/2005 and 07/04/2005

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