QuestionHello, I race a 63 Spitfire with a 1296 engine. This weekend I was at the track and from the start I had a problem that wasn't there a week ago. I can rev to 8,000 rpm in gears 1-3 but in 4th gear the best I could get was 6K and then revs would back off to 5200. This happened most laps although on occasion I was able to see 6800 in 4th. It felt like the plugs were fouling or starving for gas, the engine was flat. If I backed off from WOT, sometimes, I could get a few more revs, but not all the time and I'm not sure that it was relevant to the problem.
Things we checked / changed at the track.
1. Crane fireball electronic ignition
2. switched to a taller rear gear (4:54 from 3:89, this had been a changed from the week before to the 3:89 so I went back to what I know 4:54, was OK)
3. Plugs- they looked OK, light brown, no fouling
4. Fuel pump, electric, volume great
5. twin 1.25 SUs, looked lean (from pushing the side springs up and looking for a stall or a speed increase)I made them richer by clockwise a total of nine "flats" didn't make a difference to top end, started easier
6.Fuel filters
7. Fuel from 2 different sources, several tanks over 2 days and 6 track sessions
8. Compression was 150-160 on all cylinders
9. Fuel floats checked and were OK.
10. Pulled a plug wires off, one at a time, same effect from all.
11. The week before, an off course spin flattened a section of exhaust pipe. The flat section was cut out and replaced for this weekend. The entire exhaust was taken off for the repair and inspected.
I didn't have a spare, known good coil so I didn't swap that out.
Do you have any ideas where I should look next?
thanks
mike
AnswerHi Mike,
My first thoughts were exhaust back pressure but it sounds like you corrected that. High RPM problems are difficult to isolate when it runs good at lower RPM and it sounds like you have tested most stuff except I found on a higher performance engine at high speed (high load)if you have a problem then the tests must be at those high load conditions. I would do two things, put a timing light on the ignition system and run the light out from under the hood and tape it to the windshield and a piece of cardboard over the light end so you can view the flash from the cockpit while at the failed condition. Also do the same with your fuel pressure gauge to be sure you maintain pressure at high load and high RPM when fuel demand is at it's highest.
I take for granted if you are turning 8K RPM you have competition valve springs etc with the engine mods, correct?
Are you forced to use a muffler and if so is it a straight through?
If it is not a straight through, do a back pressure test. I found you can take a short piece of brake line and drill a hole just a little smaller than the piece of pipe and drive it into the exhaust pipe just after the headers and put a low pressure gauge on it and tape it on the windshield or bring it up on the door and tape it so you can see back pressure at high speed (high load) I found street nor racing engines can not handle over 1.5 PSI. back pressure. After the test you can just pull out the piece of brake line and put a short large diameter sheet metal screw in to plug the hole.
Let me know,
Howard
