Classic Mini - Misfiring and Troubleshooting the Engine - SU Carburettor Jets
Over the past two years I've worked on Paddy's (1973 Mini 1000 with a 1380cc A+ engine) mechanical and cosmetic issues including replacing a head gasket, a bent push rod, and other non-engine related upgrades. But one problem has persisted from the begining until now. He's had a nagging "miss" when cruising at normal highway speeds.
He ran OK. But not great . . . and always seemed to lack power on the top end. Now is the time to get to the bottom of the problem! Looking at on-line classic Mini forums and in talkng with other owners, I would come away with the notion that there are literally dozens of possible problems that could cause Paddy's symptoms.
THEN I came across this GREAT article that helps explain and diagnose a couple of issues that could cause misfiring at speed. http://mgaguru.com/mgtech/care/cf106.htm
"but upon acceleration at high rpm it misfires."
This would be mixture leaning out, or the spark becoming intermittent.
Adjust both carbs two full turns toward rich, expecting it to run rich at idle, then take another acceleration run. If it then runs better at high speed, you may have the wrong needles in the carbs. Pull the needles and check the numbers on the shank. If the needles are the wrong number, install the correct needles. Reset the correct idle mixture.
"Also a lot of misfire is noticed at cruising speed on the highway while running at 60- 65 mph. Nothing bad but you can feel the missing while cruising."
If it still misfires under acceleration or at higher speed with richer mixture, then it has to be a spark problem. This is not what causes it to run bad when cold, but a separate problem. If the spark plugs are old you might try new ones. If all of the electrical tune up parts appear to be in good condition, you might look for a broken ground wire between the contact breaker plate and the body of the dizzy. Also check for proper function of the vacuum advance unit on the carburetor (assuming it still had the original type distributor). These items are covered under Ignition Troubleshooting.
Thankfully the suggestion to richen the fuel mixture WORKED! The high-speed miss was gone. NOW became the task of selecting a needle that would work in my 1380cc engine. My research found an article online by Keith Calver titled SU Carbs - Quick Reference Needle Selection.
In MY case, I was starting off with an AAK Swinging Needle (mounted in photo to right). I messaged Jack Holdaway at Seven Enterprises for his suggestion. He suggested that his first choice would be the AAA needle but that they were out of those and could ship an AAU instead. The photo at the right shows the AAK, AAU and AAA needle profiles.
You can see hard it is to visually discern the profile differences in needles, Right? That's where the SU Needle Charting software is invaluable.
You simply select the carburettor configuration that you have and then enter the needle profiles that you're considering. In MY case I entered the starting needle (AAK) and then the other needles suggested by Keith Calver and Jack Holdaway - AAU, AAM, and AAA.
Here's the resulting chart:
My starting AAK and alternative AAU needles were similar but at the lower end of the lean > rich scale. The suggested AAA needle has a moderately richer profile up to Step 7 and then takes a steep climb before leveling off around Step 8 and eventually converging with the AAU needle's profile at Step 13.
BOTTOM LINE: The AAA needle was the perfect match for my 1380cc Series A+ engine fitted with twin 1.5" SU HS carbs fitted with K&N cone air filters. Paddy now easily climbs to cruising speeds of 70+ without the nagging feeling that the engine is misfiring or starving for fuel.