Performance Tuning Feed

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.


"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

IMG_2316Thankfully 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:

  SU Needle Chart AAK AAU AAM AAA comparisonMy 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.  


Comments from Paddy's Engine Builder - 1380cc Engine - Torque Gain

From - Bill Gilcrease - Mincomp's Corner (May 1997)

No racing to talk about at the moment, so let me talk a bit about 1380cc motors.

Engine bayI am deluged with “what’s the deal with 1380s?” or “Why?” Well the answer starts out quite simply, “bigger is better,” referring to more cubic centimeters (CCs) or in big car terms cubic inches. The basic fact is that with all things equal except fore bore size a 1380 will have about 15 HP more than a 1275. But the real advantage is the torque gain. (Torque is what pins your head back of the seat when you accelerate.) Once, again, all other things being equal a 1380 can give up to 25ft. lbs. more torque than a 1275. The real significance to the increased torque is that it allows your beloved Mini to become a rather comfortable freeway cruiser by lowering the RPM that are required at any given speed.

Building a successful 1380 is not just as simple as boring it for the larger pistons. In most every instance I have found that a cam designed for the 1275 just does not work very well in a 1380. Now I can sit here and wait for miscellaneous hate mail/Faxes/phone calls about that statement or the next one I’m about to make. In 1991 or 1992 I had one motor on my dyno playing with this cam thing and in the end I came up with a spec. that turned that 1380 to life. Up to that point I must admit that I never could see what all the hype was about but after I was done with this project I was a believer, at least in the 1380’s I was building.

I guess the whole reason I got into writing about the 1380 camshaft thing was because I just finished up a job here at the shop that was a duplicate of several that I do over a year’s span. People call me or bring in their prize 1380 that they or someone else built for them stating “I expected a lot more.” Well I advised them on what I believe the fix to be, and why.

The bottom line here is that every time I have done this cam change the customer has left smiling because he has gotten what he originally expected, and quite often more than he hoped for!


Previous Blog Post: Stages of Tune

THANKS Vintage Sports Car - Woodstock, IL!!

Paddy, the 1973 "Retro-Mod" classic Mini was in a "right pickle", as my British accented GPS would say, until he made a visit to Vintage Sports Car in Woodstock, IL in late May.   He had no compression in cylinders 2 and 3, a local shop had completely wound out the jets on the twin SU carburettors, and the suspension was badly in need of a tune-up.   Yves, Joe, and Tim took all of that in stride as they dove into the task.  

Four weeks later, Paddy is back home in Bloomington, IL and running the best he has since I brought him home from Chicago last Fall.  I always knew that he had the potential, but that he just hadn't had the sort of care and attention he needed. 

Thank goodness for automotive shops and experts like the guys at Vintage Sports Car in Woodstock, IL.   I was pleasantly surprised at the level of skill and care that each of the team members took diagnosing the problems and then finding the best solution.  Yves remained in constant contact with me by email, letting me know of each of the major developments along the way.  Digital images we're emailed so that I could easily SEE what the problems were.  

I want to publically thank Yves, Joe, and Tim for the great job that they did on Paddy!   Yves provided me with a CD of all of the images they took along the way.  In a separate post, I'll share some of those images (and stories) to demonstrate their care and ingenuity. 

THANKS Vintage Sports Car!!!




100_5708      100_5707

Far out MAN! Paddy's heading for Woodstock . . . .Illinois that is!

I'm taking Paddy up to Vintage Sports Car tomorrow morning for a head gasket replacement and tune-up.  Vintage Sports Car Inc. is a division of Historic Race Car, LLC in Woodstock, IL.  Vintage Sports Car is a separate division for their street car customers, as not all vintage car owners see their beloved old timer as a race car.

Paddy started running rough and as noted below I began the process of tracking down the cause.  I had taken him into a local "specialty" shop but they just seemed to make things worse and were not truly addressing the isssues.  It was time to take matters into my own hands.

After shooting the video below, I started my engine diagnosis with a cylinder compression check.  I QUICKLY found out that Paddy had no compression in cylinders #2 and #3.   Hmmmmm?  That's gotta be a head gasket. Right?

I contacted Bill Gilcrease at Mincomp Racing in California (Paddy's engine builder) to get his thoughts.  He confirmed my fears.  

Replace the head gasket Brian! While the head is off I would suggest that you lap the exhaust valves paying special attention to the center ones!  There are good and there are bad head gaskets, let me know if you would like me to send you the proper parts to use.  

Joe, at Vintage Sports Car, confirmed Bill's thoughts and I made an appointment to have Paddy stay with them for a couple of weeks!  Meanwhile, Mincomp is sending them the correct head gasket AND offered to give them advice if needed.

While in their shop, Paddy will get a new head gasket (obviously) but they'll also get those twin SU carburettors set correctly.   

Bring A Trailer - Paddy

Stages of Tune - Classic Mini

Mini Performance ManualHaynes Mini Performance Manual by Tim Mundy

Simply the best "must have" book for Classic Mini performance and modification info. Covers just about everything from engine mods to digital clocks. Disc brakes, steering, gearboxes, interior, suspension, body work, special editions-every mod you can think of. Haynes "Mini Perfomance" by Tim Mundy

The following excerpt helps define the various stages of performance tuning in a Classic Mini.  Paddy was elevated to the Stage 4 tune in July 2010 by Mincomp Racing in Costa Mesa, CA.

Stage 1 kits

Stage 1 kits are available from just about every Mini specialist and are a very worthwhile fitment.

The kit consists of:

  • an uprated exhaust manifold which can be either a Cooper freeflow or an LCB, depending upon the kit supplier and engine size,
  • an uprated inlet manifold, a K&N airfilter or similar,
  • plus a richer carburettor needle,
  • and a decent exhaust system which is usually a Maniflow or an RC40.

Most kits claim around a 20 per cent improvement on a 998cc engine. In a MiniWorld/Keeping Your Mini Alive (KYMA) test we managed to get 51 bhp from a 998cc Mini which was standard apart from electronic ignition using a kit from the Mini Shop. The exercise certainly backed up the claims.

Stage 2

To go from Stage 1 to Stage 2 is a case of fitting a modified head. In the case of 1,275cc engines this will probably mean retaining standard valve sizes. A carburettor needle change will usually be required too.

Stage 3

Stage 3 involves fitting a larger-valve head and uprating the carburation.

Stage 4

Stage 4 and beyond will involve a cam upgrade. This requires the engine to be removed from the car and a full engine rebuild which should include strengthening the crankshaft and balancing all rotating components.   


Book Source: - Haynes Mini Performance Manual  


Below is Paddy's 7/29/2010 Stage 4 Engine Modification Invoice

  Mincomp Racing Services Invoice