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MG MGA - Carbs or Timing ?

My car bogs down,at around 3000 rpm's, when I step on the gas. To make it run better I have to screw the mixture screw almost to the bottom. Yet,when I gunson tune the car, I get a beautifull blue colour at only nine flats down on the mixture screws but the car still bogs. Could it be the advance timing mechanism that it not advancing to match the higher rev's. Carbs are professionally rebuilt.
Thanks Gordon
Gordon Harrison

Vaccum advance was the first thought that came to mind--that and the mechanical advance hanging up. I'd check that first along with points gap.
Paul Hanley

Gordon. If you have a timing light with the adjustable feature, it is possible to check the operation of both the mechanical advance and the vacuum advance. If you do not, set the notch in the crankshaft pulley opposite the most advanced pointer and make a secondary mark at the top dead center pointer. This secondary mark is 20 deg BTDC and will allow you to check your timing using a non-adjustable timing light.

With the engine idling, apply a source of suction to the vacuum advance can, a Mighty Vac works well, and see if the timing is advanced. If not, or, if the can will not hold pressure, it needs to be replaced.

The Colour Tune does not allow you to check what is happening under load, just when the engine is not under load. Thus, it might work well for idle adjustments, but tells you rather little about how the fuel/air flows when the engine is under load. Almost all of the needles have the same basic diameter at the upper most portion, where they are at idle. The profile below that affects how they supply fuel/air when the engine is at higher rpms and under load. This is what makes a chassis dynamometer/rolling road of significant benefit. It would be of use to know what needles are currently in the carbs, especially if the distributor checks out as being correct. Perhaps you need to switch to a slightly richer needle in the crusing range. The book, "SU Carburetters Tuning Tips & Techniques" would be of benefit here. I have often found the "stock" needles not to be the best ones for performance, even with relatively stock engines.

Les
Les Bengtson

Les,
Present needles in the carbs are BB,which according to my carb rebuilder, are slightly leaner than the origional GS needles.I have a timing light,but it is not adjustable so I will try your method. I also have a dwell meter....would or can I check things out with it?
Thanks Gord
Gordon Harrison

Gord. All you can check with a dwell meter is the dwell of the points. This is the period the points are closed, charging the coil, measured in degrees of an arc. If you dwell is much more than five degrees off of the factory specification, it may indicate a problem with your points being improperly gapped and/or worn. My MGA factory workshop manual does not list a dwell specification, but, it should be about 60 deg.

The needles, in my experience, should be slightly richer than stock for best performance. Sometimes, they need to be considerably richer for best performance. The factory needles were a compromise between acceptable performance and good fuel economy. Most of the time. If you find the timing to be proper, I would suggest you contact Joe Curto, who has a website, and get the original set of needles proper for your carbs and ask him if he can recommend any, in his opinion, which might perform better than the factory needles. I would order both and test them our. This system has worked well for me in the past, and Joe is an excellent individual to deal with.

Please let us know what you find.

Les
Les Bengtson

I will call him today, Thanks Les and I will keep you psoted .gord
Gordon Harrison

Gordon,
You may want to try thicker oil in your dashpots. This provides the "accelerator pump" function on our cars.

Advance the timing until the car kicks back against the starter then carefully see how it runs. This would isolate the problem if it's timing.

Another possible cause is low float height although I'm not sure how that effects different rpms. Maybe the packing washers holding the float on are restricting fuel?

Which cylinder is the Gunson in? 1 and 4 run leaner so if you tune 2 or 3 spot on you'll be lean overall. Look at your plugs and tune the leanest of 1 or 4 first then the other. Let 2 and 3 run rich.

You may want to try tuning one carb only to see if the problem is balance. Sometimes this helps, sometimes it just confuses the issue.

Get a mate to come help. Another set of eyes and ears never hurts. Besides, it keeps you from drinking all the beer. I've heard this can limit one's tuning ability.

Good luck,
Bill
Bill Eastman

Bill
I did not know there was a difference in tuning between cyl 1/4 and 2/3. Thank you for the info. Was playing with the timing a little last night and seem to be getting a better result. I think the old saying ....don't blame the carbs, may be true in this case.
I did find one serious issue last night and it was the vacumn addvance connection to the carb was only finger tight. That I am sure was not helping issues. So I will check all connections...check the timing ,as per how Les said , break out the timing light and dwell meter for further verification., completely re-tune the carbs and see what the results are.
I think the combination of out of wack timing and the loose vacumn advance are the guilty party. This weekend will tell. Thanks to everyone. gord
Gordon Harrison

Gord. I suspect that Bill meant to say that cylinders two and three run with a leaner mixture than cylinders one and four and that, when using the ColourTune you need to verify that the mixture on cylinders two and three is correct and the mixtures on cylinders one and four slightly rich.

My experience is that, most of the time, the slightly leaner mixture in cylinders two and three (due to the siamesed exhaust port not efficiently clearing out of the cylinder and leaving some minor exhaust residue in the cylinder, diluting the incoming charge) is of little consequence when the carbs are properly tuned.

The "performance" needles run slightly rich. Thus, the mixture in cylinders two and three should still be slightly richer than chemically correct and should fire just fine.

Where you have a problem is with "economy" needles which are excessively lean. These type of needles can run a slightly weaker than chemically correct mixture which, under some circumstances, can be too weak to fire well in cylinders two and three. In such cases, you commonly hear a misfiring in the exhaust note and find no change in the misfire when the spark plug to the affected cylinder is shorted out.

Les
Les Bengtson

This is for Les Bengtson, Les can you please come to
Oshkosh Wisc. and Help me tune, adjust, rework my 1956
MGA 1500 engine, carbs., timing all of the above???
Please... Sincerely, Tom Peotter
Tom Peotter

Les,
As you see, presently I have BB needles. Are these "economy or performance " needles. The manuel says GS needles are the standard, so before I re-tune should I change to GS needles and then work from there.
thanks Gord
Gordon Harrison

Gordon
I don't think you have said which car you have. 'GS' is standard for the 1500 and '6' is standard for the 1600 & MkII.
'BB' is not mentioned as either the rich or lean needle for any MGA engine! Interesting...

Pete
Pete Tipping

Gord has a 1500.

Derek Nicholson

Tom. Very much wish I could come up for a visit. Back in the mid 50's, my father was on the staff of the Air Force ROTC unit at the college in Superior. I believe it was a part of the University of Wisconson system. I started school at the "Nelson Dewey Elementary School" there and still have fond memories of the area. My younger daughter has been working at Marquette University, in Millwake (sp?) over the last two summers. It would be nice to be able to make a road trip up there sometime.

However, please be aware that I am only a humble "student of MGs", and am, by no means, an expert. There are many who are far better qualified than I. But, it would be good to visit with you, work on your car, and perhaps mine, and drink some beer. We will have to see what we can set up over the next few years.

Gord. You may remember that I suggested an SU book, "SU Carburetters Tuning Tips & Techniques". I purchased my first copy of that standard reference back in the mid 70's and still have it. I have also picked up the latest edition for my reference library. Both are quite excellent. However, my "3rd Edition, February 1975" contains "full needle charts" which my "5th edition, 1994" does not.

Needles are measured every 1/8th inch from the shoulder and the diameter of the needles, at that point, constitutes the "needle profile". These are numbered 1 through 16 and indicate the measurements at those points.

The BB needle shows:

1-.089"
2-.085"
3-.0825"
4-.080"
5-.0775"
6-.075"
7-.0725"
8-.070"
9-.0675"
10-.065"
11-.0625"
12-.060"
13-.0575"

The GS needle shows:

1-.089"
2-.085"
3-.0815"
4-.0785"
5-.0755"
6-.0725"
7-.070"
8-.0675"
9-.065"
10-.0625"
11-.060"
12-.0575"
13-.055"

As you can see, the standard, GS, needle is slight thinner from position three on down, giving a richer mixture than the BB needle. In my experience, this can cause problems such as you are having.

The only time I use a leaner than standard needle is when I am setting my 68GT up for emissions testing. My best performance is obtained with an AAA needle which is slightly rich for the emissions test. I use a slightly leaner needle for the testing and can, usually, pass on the first test. But, for best performance, a somewhat richer needle than factory stock is, in my experiance, desirable.

Les
Les Bengtson

Gordon, I believe you found the culprit in the loose vacuum advance, at least that would make sense to me. Hard to get a vacuum with a leak like that. Of course, the cars are old, could be multiple factors.
Tom

Last night I had time (before the baseball game ) to verify the dwell) with my meter. Guess what...what I had thought was perfect turned out to be 5 degrees retarded. After a minor adjustment using the distributor nut, I was able to get the dwell to exactly 60 degres. What I noticed was, as I approached the correct dwell angle the engine RPM's rose and the motor started to smooth out. After the dwell was set I took the car for a toot and was surprised at the change. It still bogs a little but I will look at the carb adjustment on Friday night(big baseball game tonight).

So there are signs of improvement. Thanks Gord
Gordon Harrison

Gordon

You have me confused. As Les explained above, the dwell is an angular measurement of the period the points are open. Altering the advance/retard does not alter the way the contact breaker runs over the cam. Dwell angle is adjusted by altering the contact breaker gap - bigger the gap the greater the dwell angle and vice versa.

Steve
Steve Gyles

Slight apology to Les and all. Although the gist of my post is correct, the dwell angle is as Les described - the angle through which the points stay closed, not open. The larger the point gap, the smaller the dwell angle.

Sorry to confuse.

Steve
Steve Gyles

Steve
Many things about carbs and distributors confuse me but I work on the theory that there is always someone who knows more (than me) that can help me and everything I learn from that help makes it less confusing.So lead on please.
I can't expalain it but when I turned the micro adjustment nut on the distributor (advancing the dwell)to 60 degrees the RPM's of the motor INCREASED. Could this have being related to something else happening at the same time ....I don't know. What I do know is that I had to reset the idle speed and that a test drive after the dwell was adjusted showed a much better ride. It still bogs near the 3000 RPM level but almost 2/3 less than before. A change of needles to GS is planned and then a retune of the carbs, make me think I will solve the problem. I will let you know. Gordon

Les. I am looking for the SU CARB BOOK. Should be able to pick one up one Ebay.


Gordon Harrison

Gord. I have a problem here. The vernier adjuster on the vacuum advance can has noting to do with dwell. It moves the points plate in relationship to the points cam, thus, advancing or retarding ignition timing. More advanced ignition timing will, under most conditions, result in a higher idle speed--exactly what you describe.

The dwell, the period of an arc the points are closed and allowing the coil to be charged, is only a function of the profile of the points cam in the distributor. Thus, if by adjusting the vernier timing adjustment, you actually altered the dwell of the points, it indicates there is a problem with the points base plate being moved in such a manner that the points remain closed for a longer period of arc. This does not happen in a distributor in good condition and may point out that part of your problem is in the distributor itself.

John Twist and Jeff S. both do distributor rebuilds. You may wish to e-mail both of them and describe what is happening and request they provide a price for a rebuild or an alterate distributor.

Steve. Keeping each other straight is part of what we do around here. Certainly no need of any appology as you did nothing wrong. Sometimes, the fingers do not type what we know we are saying to them. Such is life.

Les
Les Bengtson

Gordon

This may be what you are looking for.

http://www.1978mgmidget.com/manuals_and_booklets.html
John DeWolf

This thread was discussed between 16/10/2006 and 19/10/2006

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