QuestionI have a 1980 Spitfire 1500, it had a new Ivor Searle stage II 1500 with a Kent TH12 High torque cam put in with new 4 speed standard gearbox 10 years ago and has recently spent the last 12 months having the body work completely stripped, any rust cut out, welded, primed and professionally painted. I got the car back, let the paint dry in the garage for several months and have recently had the carbs tuned by a reliable specialist as the car was running quite rich on the needles, so the needles were changed for ABT needles (which as far as i'm aware are for standard spitfire engines not modified like mine).
The car seems to be choking itself if I really put my foot down, it feels like the car is being flooded with either too much fuel or to much air. When this happens you can hear the change in exhaust note, and where there would usually be a tonne of torque and acceleration the car stutters (when snapping the throttle), where as if I then ease off and accelerate gently it is quite happy reaching almost any speed as long as I am patient..
The garage swear that the carbs were correctly serviced and the car was running very well when it left, however i'm pretty sure they havn't done a good job as the carbs were "stripped" and rebuilt, yet upon checking there were no gaskets at the top of the fuel float chambers and one of the fuel float chambers is every so slightly loose and can be moved.
People have suggested a head gasket problem, I have had my coolant tested with negative results, the engine shows no sign at all of HGF. The coolant system has been flushed, I replaced the distributor, vacuum advance, pipes, cap, rotor arm and points, timing is perfect, new fuel pump, fuel filter, alternator has been tested, cooling system has been checked.
can you help?
AnswerHi Tom,
Tuning an engine that has been modified takes some work, especially if you don't have access to a dyno. Any time an engine is modified it requires a new ignition timing, ignition advance curve and a different fuel control all the way up the RPM range.
In your "Subject" line, you say "Flat Spots / Pinging".
Pinging under load is very dangerous to the engine and must be stopped as soon as possible. It is a indication of detonation that can destroy an engine in a very short time.
If your car was brought to me to tune, this is what I would do,
First confirm that the fuel is fresh and high octane. Then warm the engine up and set the RPM high (about 3500 RPM)and note that the exhaust does not show black smoke and then blip the throttle open quickly from 3500 up and have some one watch the exhaust for a puff of black smoke. The exhaust should not show black smoke until you blip the throttle then it should. If it does not, richen the carbs until it does. This insures that you are a little on the rich side but not too rich. You can't find out what your ignition timing should be unless you know that the fuel mixture is a little rich.
Without the use of a dyno and a distributor machine you will not be able to get an ideal advance curve but you can get close on total advance. If who ever built the engine did put in an advance curve to match the modifications of the engine you will be ok. Did the engine builder give you ignition timing specs? If not start by using factory specs for your car.
This is not exact but will get you into the correct area. First warm the engine up and blip the throttle open quick from an idle while you are over the engine and listen for any Pings. If it does Ping, retard the timing 5 degrees at a time until it don't do it. Write that timing down as it is you low RPM timing. If it don't Ping, advance the timing 5 degrees at a time until it does Ping. Write that timing down and retard the timing 5 degrees. What you are trying to do is to establish what the timing is at the low RPM load for Pinging. You want to be about 5 degrees retarded from that Pinging point. (keep in mind that this test REQUIRES that you have fresh high octane fuel and the engine be warmed up and the carbs be a little on the rich side.)
Now that your timing is set at about 5 degrees retarded from low RPM Pinging point, you need to find a long straight steep hill. Start up the hill and get into 3rd gear and set the RPM at 2000 RPM and accelerate up the hill until about 4500 to 5000 RPM and note if you have any Pinging or flat spots. If not then the advance curve is probably ok and the timing is ok. It does not mean it is the best it can be. To get a more optimum setting you would need to mark off a start point to go by at 2000 RPM and a finish point and note the RPM or speed that you obtain at that point. Then advance the ignition timing 5 degrees and make another run and continue to do that until the RPM or speed drops off and then back off the timing by 5 degrees and check the timing at 5000 RPM to see what total timing should be. If that timing causes you to get the Pinging at the Low RPM Ping test, then you need to change the advance curve which is difficult to do without the use of a distributor machine. If you don't get the Low RPM Pinging at this new timing than you will not need to alter the advance curve.
Keep in mind this a rough method of establishing a good ignition timing but it works.
I would be worried about your carburetor man as leaving gaskets out and the cover loose and the replacement of stock needles in a modified engine. If possible I would try to get the needles he remove to see what needles they were. Some times stock needles can be used on a modified engine as long as you adjust to a slightly rich mixture. If the needles he remove are a different ID then I would put them back in (unless they are badly worn). Then just order new needles of the ID that were removed.
Howard
