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MG MGB Technical - Some tuning advice, please.

I'm having a tuning issue with my '67 B.

In order to get a decent idle, I have to set the carb mixture too lean. Which of course, gives me that splashy exhaust sound along with less than stellar acceleration. If I set the mixture closer to what should be average, (12 flats), idle is then rough.

I cannot get my Colortune even close to blue, bright orange to darkish orange is it.

The engine is completely rebuilt. Timing set to spec. New Euro spec distributor. Points set to spec. I had new throttle shafts, bushings installed in carb bodies, I added new jets.

Maybe I need to play with the timing? Shouln't I be able to get closer to blue with Colortune? I've also tried the manuals method of carb adjustment, with same result.

Any advice would be greatly appreciated !!
David
David Steverson

How low is your idle set? It seems that these engines prefer to run a bit rich. I let mine idle at 1000 - 1100 rpm. Much below 1000 and it is lumpy. I don't use a Colourtune, simply use the carb piston lifting pins.

Mike
Mike Howlett

What type of needles do you have, fixed or biased?
If fixed maybe the jets aren't centered. Do the pistons drop onto the bridge with a "click" when you lift and drop them?
On my '70 HS4's (fixed needles) I get a blue color although not a "bunsen-burner blue" like the directions state. Also I set the carbs by the pin method with a dwell-tach and used the colortune only as a crosscheck.
Mike
'79B
Mike Janacek

Scottish Mike,
Yes, my idle may be a bit low (tach not working). I'd guess about 800. And "lumpy" is a good description of the idle if I try to enrichen it.

American Mike,
Mine are the fixed needles. I bought new standard needles from Victoria British. Yes, the jets are centered, I made sure of that.
You're getting blue from Colortune? This is my first Colortune, although I've had numerous B's. I bought the Colortune specifically because I could not seem to get a decent idle with my mixture set where I thought it should be. With the mixture set towards that point, I get bright orange on all cylinders. When I lean it out, I get a purple-ish dark orange-ish color. ?

David
David Steverson

David,

Have you checked your float level? And your needle valves also... it sounds like you're getting fuel leaking into the manifold at low load, and such a constant leak won't enrich the engine much at high load, so if you lean out the carbs to compensate at idle then you'll get lean at high load/revs.

I had needle valves go and was wondering why my engine was idling so roughly... took off the air filters, turned the ignition on, and saw to my horror fuel pouring into the inlet manifold and also out of the overflows (next to the then hot exhaust). I had to drive it home, so I drove it all the way there at WOT to absorb the extra fuel :o)

'67 probably didn't have a fuel filter as standard (mine didn't) which may allow dirt to clog the needles.

hth,
--
Olly
Oliver Stephenson

P.S. my idle's just fine at 650rpm. Starts to get too lumpy for my liking at 500.
Oliver Stephenson

David,
Mine idles at 600-800, depending on the weather as far as I can tell, i.e. hotter=lower idle.
I seem to recall someone mentioning the type of fuel available today as possibly causing problems with the colortune colors, although I run plain 87 octane in mine.
Also remember the balance tube on the intake manifold will affect the mixture somewhat. I crosscheck mine using only number 1 and 4 plugs. All plugs are a tannish color.
Mike
'79B
Mike Janacek

You don't say what filters you are using. I recently fitted K&N individuals and did not fit the gaskets properly, as a result the gasket which should have four holes in it( two mounting and two to allow air to the bottom of the needle piston)sealed the holes to the bottom of the needle piston. The car seemed to run ok but I could never get the mixture right.
Putting the problem right allowed me to get the mixture right.
G Roberts


Oliver,
I'm OK on the carb needle valves. They are new, as are the floats (entire fuel system is new). They do not leak and there's no fuel overflow, float level is good. I have a fuel filter on the car.

Mike,
I've been running the highest octane fuel, 91 (I think). I did try some regular 87 this last time. Now, I have a bit of run-on (sometimes) when I shut the engine off.

G Roberts,
I bought Longflow air filter assemblies. I'll check that gasket. But, I think I had the air filters completely removed when I was going through all the adjusting.


Does anyone know if advancing or retarding the timing would affect Colotune or the lean/rich adjustment?

Thanks for the feedback! David
David Steverson

Hi David
I don't know if your timing will directly affect your colourtune but it certainly will affect the quality of your tickover, I would get the timing right first then work on the mixture, the only problem I find with the timing nowadays is you really have to do it by ear or trial & error as the modern fuels tend to make the book setting irrelevant.
Ron
R. Algie

What do you call a decent idle? Do you mean it is idling too high unless you weaken it? Not being able to get the idle low enough is a problem with the linkages, cable too tight, butterly not seating or bad poppet valve. Too low an idle speed will give a rough and erratic idle and a rocking engine. Too rich is characterised by a rhythmic hunting rather than roughness. There has to be a point between splashy (too weak) and hunting (too rich) where the mixture is correct or there is something else wrong. Always get the ignition right before fiddling with the carbs, <quote> 90% of carb problems are due to the ignition </unquote>. Like Mike I never use a Colortune just the lifting pins. Have you properly set up the carbs for airflow and mixture balance? It is isn't difficult but you need to do the right things in the right order. Without that the carbs will never be right, with it and left alone they will keep their tune for many thousands, if not tens of thousands, of miles despite what other may say.
Paul Hunt

Paul,
By decent idle, I mean smooth idle. My cable and linkages are correct, idle speed is not an issue.

According to the shop manual, the initial adjustment for the jets is 12 flats from jet head flush. At that point, I get an idle that is very rich for my car, i.e. visible exhaust, engine movement (lurching, jerking).

As I move each jet adjusting nut up to weaken, the lurching improves until I reach a point where I have a smooth idle, lifting pin methods seems to indicate mixture is good (I think), but has definite splashy sound.
OK, splashy sound indicates too weak. If I try to enrichen from that point, idle smoothness starts going away.

I have checked my timing, dwell, valve adjustment several times. The distributor is new. The engine is completely rebuilt. I have no manifold leaks.

I can tell when I drive the car that the adjustment is too weak.
I bought the Colortune because of this whole issue. All it's told me is that I cannot get anything close to blue.

I suppose the orange means too rich?

Thanks for your post.
David
David Steverson

Forget the colourtune. Take the car out, give it WOT for a mile or so at high revs, kill the ignition and coast to a halt. Remove a plug and inspect its colour. Adjust your jets until doing this reveals a nice light brown plug, not too dark and certainly not white. Once you have this set up right, if your idle is too rich, then you have problems elsewhere regardless of what's new. New and rebuilt in the classic MG world are frequently much worse than original.

Good luck!
--
Olly
Oliver Stephenson

P.S. you should shift into neutral as you kill the ignition just so you don't cover the plugs in fuel.
Oliver Stephenson

12 flats down shouldn't be way too rich but close to correct, needing only slight adjustment from there in my experience. I'm no expert on Colortune but I understand you should be able to richen to yellow or orange and weaken to Bunsen blue, the brightness decreasing as you weaken still further. The flats, colour, shaking, and several other things make me think you have some other problem. Added to which, you cannot really tune twin carbs with a single colortune without continually moving between cylinders. The carbs are interdependant and changing one will require a smaller compensating adjustment on the other, you can't just set one then set the other and be correct, and this is irrespective of what you use to judge the mixture.
Paul Hunt

I have been using a Colortune since the 1960s, and with the changes to the nature of what is called gasoline here in the US over the last 30 yrs, I find it is not worth much as a tuning aid. It will still tell you if your mixture is rich or lean, but it is not the "spot on" tool it might have been years ago. At least that is my opinion.

I have a very different method of tuning for the final mixture, altho, as Paul says, following the factory shop manual procedure (an excellent version of which is outlined at Paul's site, http://www.mgb-stuff.org.uk/) is a must up to that point. The mechanical setup of float heights, linkages, ignition timing, etc, is often grossly underrated by many mechanics, and you really can't do a decent job unless you follow it.

Where my process varies is at the point of setting final mixture. I have not been blessed with an ability to discriminate by ear the drop in RPMs by using the lifting pin - never, not in 35 yrs of futzing with these carbs. But I have learned that a vacuum gauge can help me to visually get a similar indication of each carb's mixture adjustment.

I hook up the vacuum gauge to a tap in the intake manifold as close to the center of the balance pipe as possible. It probably makes no real difference, but it satisfies my sense of equality of vacuum influences. I then run the engine and set the mixture of each carb (they are mechanically independent of each other at this point) so that both together give a vacuum gauge reading of approx. 15.5" to 16.5" Hg at idle (about 1000 rpms for a 68, 18GF engine).

This means that I raise or lower each carb's jet in turn until I see the vacuum approach my target level. It then becomes a balancing act between them, but in truth, the amount of adjustment to what I consider correct from the initial start point of "X" flats down or .060" drop of the jet from the bridge, is really only a flat or two, and usually up (or leaner). I am able to see pretty easily a rise or depression of overall vacuum as I twiddle with each carb independently, and I just twiddle away until they both produce TOGETHER the vacuum level given above, always moving towards the higher end if possible.

Once this "balance" has been reached, I cross check with the colortune, and usually, it simply verifies that the color is the same in both, a light blue sometimes splashed with yellow or a yellowish white. That seems normal for this engine, and reflects the less precise control a float valve has on idle mixture than EFI and computers do.

As I have stated elsewhere, I get even burning, decent spark plug color (a light tan or gray) and mileage of about 30-34 mpg flying down the interstate in O/D at 70-75 mph and Normal operating temp shown on the temp gauge. I take that as meaning that this method at least gets me into the right ballpark if not exactly a homerun. FWIW.



Bob Muenchausen

David, I also have 67 B with only 52,000 original miles, which i bought about 6 months ago. Engine in great condition and never been touched as far as I can tell. I have exactly the same problem as you and have spent many many hours trying to achieve the smooth idle. I have reset and rechecked the timing over and over again. The carbs are set at 12 flats down. At this point I have a lumpy idle even at 1000 rpm. Definately running rich. Colortune shows orange. On the road the car runs great. Far faster than my 74 with full smog. Great pick up at all speeds, fun to drive, with a full ear to ear grin all the time. When I adjust the carbs, i.e. lean them out to achieve a very smooth idle to about 6 flats, the car runs terrible stumbling all the way. At 6 flats the colortune shows perfect blue, idles great and runs crappy. If I ever find the perfect settings I'll let you know, please post to the list if you find the optimum settings as well.

Running rich but smiling all the way,

Andy 67 B
Andy Preston

Timing is not often the problem, if set correctly to begin with, but needles and linkage adjustments often are.

Just because an engine runs ok at idle doesn't automatically mean it runs great at rpms past that. One thing I have seen several times is that air flow balance between carbs can appear to be spot on while the linkage system is pulled up tight by the return springs at idle. But soon as the gas pedal is depressed and the linkage comes up off of rest on the stops, suddenly the balance can go away and you can sometimes see that the air flow in each carb then varies between them from that point on up in rpms.

Why does this happen? Sometimes it is because of worn linkage and worn throttle stops. Other times it is because of worn throttle shafts and bushings, causing the linkages to shift when torque from the cables applies force in a different direction and from a different point/angle than the return springs. Not everyone has return springs for the cable pulley as well as the individual carbs and I think this is also where some of this twisting may come from.

This is one reason why I use the "SU tool kit" with its two widgets and wire thingies to set and check balance for the carbs. If the wires are set to even with the motor off, you can actually watch the movement of both pistons up through the rev range, once it is started, continuously, rather than in steps as with a tool like the UNISYN. Being able to see a continuous visual display of how the pistons/needles rise can tip you off as to a shift in balance as the throttle is pulled off its stops, and can help you to isolate a mechanical problem that causes shifts in performance as you drive the car. A sniggling point, perhaps, but one too often overlooked.

Also, if it runs great rich but lousy once leaned out, I would question the choice of needles. Richening the mixture by lowering the jet in relation to the needle at idle also changes that relationship at most other rpms/vacuum levels as well. This accomplishes pretty much the same as a needle change. I am probably wrong, but I too notice a slight gain in power from my engine when it is warming up and the choke pulled out - effectively doing the same thing as lowering the jet at idle once it is warmed up. However, once my engine is warmed up completely, the power differential goes away, and what was a small benefit during warm up becomes too rich a mixture once the engine is at a stable operating temp. FWIW
Bob Muenchausen

Like Bob I always check (and recommend) the air balance off-idle even though the workshop manual doesn't mention it. There seems little point in having a nice balanced idle if it is crap when driving!
Paul Hunt

David, what needles do you have in your carbs. If they are the rich ones, perhaps that's the problem at idle.


andy 67 B
Andy Preston

David- I'm thinking bowl leak, piston wash. Using a colortune on 1 and 4 alternately, a K&N 02 sensor, and a "hot wire" sniffer up the tailpipe wazoo, I can get those pesky SU's humming in a few intense minutes. I've found once setup with this boffo overkill, the carbs remain "tuned" till next smog check. Take the air cleaners off and inspect the jet holes for raw gas leakage with a strong light while running. Happened to me last year with new rebuilt carbs, and lunched the engine vis a vis: " Classic 4 corner skirt scuff" . Broke my heart I gotta tellya.
vem myers

I have an MGA with H4s. I thought that you shouldn't rely on flats. You should set the depth of the jets from the bridge with a caliper.

Using a small funnel on a plastic tube to listen, the difference in hissing between the carbs is pretty noticeable. Have you released the throttle shaft linkage and set the cars seperatly? I also use the carb kit with the wires.

If I'm redundant with the other answers, please forgive, but I gotta get my ft hub back on and get out there while the sun is shining.

Fred Horstmeyer

I currently run ABD needles in my HIF carbs on a 73B. It pulled well until I recently installed K&N filters and removed the Cooper cans i.e. the "Bob filter" design. It now exhibits symptons of being too lean when accelerating even though the idle is smooth and it passes the 'pin lift' test. This appears to be consistent with the experince of others who have made this change. I have ordered a pair of AAA needles which are richer in mid range but essentially the same as the ABD at idle - based on information from the WinSu program. My expectation is that the idle will be the same but there should be more power under acceration. I am away for the next week or so but will feed back results once the needles have been changed and a test run made.

BarryQ
73B
B.J. Quartermaine

This thread was discussed between 24/05/2003 and 01/06/2003

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