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MG MGA - MGA performance problem solved!

I was not completely truthful in my last post. While the MGA truly runs better with the recurved distributor, the recurve did not fix my problem. The motor acted rich no matter what I did. "No matter what" included:

Recurve dizzy (see previous post)

Replace needles/jets in carbs. Centered, recentered, centered again.

Replace float valves with new Viton tipped (old were similar)

New spring loaded viton tipped float valves

Grose jet in front carb- finally fixed very slow seep past valve

Reset float levels, timing, valves, fuel pressure, front shock oil level (just kidding).

Just before Wheels and Wings, I got her running a little better but still down on power. Found out later that I forgot to tighten carb linkage so front carb didn't open. Fixed that, ran rich again.

I pulled the filters to get better access to the mixture mechanisms. Reset to baseline and adusted till all was good, replaced filters, and took a spin. Ran like crap. Did it again but was too lazy to put the filters on- ran great.

Problem- front air filter wouldn't flow. Since it is modified to fit the crankcase vent, it was always on the front. I had a set of Frams of unknown original applicaton but new and fit well.

I put my old "small dog excluders" back on and got to enjoy a couple runs before storage.

Funny thing- I "checked" the filters multiple times. I looked throught them, blew air through them, meditated over them- all the standard stuff.

Such is life.

Bill
Bill Eastman

Psst, Bill - how's the Jag going?
Bill Spohn

I found the same problem when using paper filter elements. The front one would "oil up" from the valve cover vent. I started carrying a spare filter element so that at the first sign of a problem I could replace the oiled one.I have since changed to K&N filters, and no longer have that problem.
Ed Bell

I also found a problem with the paper filters. My front one doesn't oil up, but I have to set the mixture with them on the car. If I set it with them off and then put them on it will be running way too rich. Obviously they are very restrictive. I have bought some K&Ns which I intend to put on over the winter or in the spring. Do you think I will have to change the needles as many people say? There should be plenty of adjustment on the jets, so I wonder why you would have to change the needles. I thought the theory of an SU carb was that once it is set at idle it is correct througout the range. Will the K&Ns run lean at higher RPMs even if set correctly at idle?
Ralph
Ralph

Ralph,
The K&N's will flow more air, especially at higher revs, so the mixture needs to be enriched to avoid damage.
That said, if you never run your engine hard your (worn?) needles will probably do fine. Ideally you should have your engine fine-tuned on a rolling road by someone knowledgeable about SU's and their needles after fitting the K&N's.
Willem van der Veer

The needle profile is based on everything involved in airflow, from filters through the engine and out the tail pipe. Anything that changes it will require a different profile. These changes are not linear or predictable. Some things, like exhaust or cam changes, or head mods, can result in "bumps" in requirements, which could be either positive/rich or negative/lean at various points on the needle. If you are very lucky, the new requirement could be OK with the old needle profile, which means it wasn't right to begin with - not uncommon, but it rarely self-corrects. In general, restrictive filters get more restrictive as flow increases. This means that the profile needs to be leaner at higher flow. Less restrictive filters like K&N don't do this as much, so require relatively richer needles as flow increases. The difference in needles is small to zero at low throttle openings, and this is where mixture is adjusted, so the correction cannot be done with adjustment. Worn needles can get you big problems. The wear is all at the low flow end of the needle; attempts to correct mixture via jet adjustment require raising the jet to correct excessively rich idle. When the needle is above the idle range, the mixture goes way lean, since the needle is not worn in the operating range above idle.
It is most important to understand that the terms "richer/leaner" as commonly used in describing needles reflect the mixtures given at greater throttle openings RELATIVE TO IDLE.
Ralph's understanding of SU theory is correct IF the correct profile needle is used.
FRM
FR Millmore

This thread was discussed between 25/10/2005 and 26/10/2005

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