Question1972 2.5, professionally prepped: .030 over, .020 lowered deck, GP2 cam, G2 roller rocker, Carrillo rods, 9.5:1 pistons, 175cd carbs, Pertronix ignition, Lucas coil, factory fuel pump.
Engine misses at load from about 3000rpm to end of powerband around 6500rpm.
I've checked fuel psi, airfilters. Retarded distributor timing 2 degrees, new cap, new wires, new N9Y plugs.
Any ideas?
THanks so much.
AnswerHI Jeff,
All engines require the same three to run with conditions on each. From the lawn mower to the race engine.
ie; Compression, fire and fuel.
Conditions-
Compression --- sufficient compression
Fire ---- sufficient spark at the correct time for conditions
Fuel --- A combustible fuel available to the combustion chamber in a close to correct ratio to air.
It sounds to me like it was well done. So I have to look at what info you have give me. I doubt your problem is compression, at least low compression. your 9.5:1 pistons with a .020" decked block makes a higher ratio than 9.5:1
You state "under load" meaning with no load it does not misfire?
Unless you tell me otherwise I will assume that is the case.
When combustion chamber pressure climbs higher (which it does under load) it becomes very difficult for a spark to jump a gap. The ignition is divided into two sections, primary and secondary. The petronix and the primary winding in the coil is the "Primary" and the secondary winding and the coil wire, cap, rotor plug wires and the plugs are the "Secondary". (however, keep in mind that the battery, cables + & - , ignition switch, wires and engine to frame ground strap are in the ignition circuit.)
A true quick test for an ignition problem in your case would be to have it on a scope and running on a dyno. I know that is not usually available to most people so you have to do a lot of guessing and testing.
Since you have already replaced some of the secondary and that has not corrected the problem you need to look elsewhere.
You need to make some changes and test after each change. Never make more than one change at a time.
Make three test runs to confirm that the problem is consistent, then remove all of the plugs and open up the plug gap. If you are using .035" gap change all plugs to .045" and run three more test runs. Then change to .025" gap and run three runs. Let me know the results.
Another question! Do you have the tech specs of the Petronix ignitor? Does it require a 1.5 ohm coil or a 3+ ohm coil? Lucas makes both.
We have not covered fuel yet. Did you take the fuel pump pressure test at the failed mode? (3000 to 6500 RPM under load)
After you do the nine test runs with the plug gaps different and return the gaps to what you were using, turn the 3mm Allen adjustment screw for the needles in both Strombergs richer by at least one turn. Then make a test run and tell me what it did.
When testing a race engine for fuel mixture never start lean. Always start too rich and work it toward lean. Keep in mind that the most torque and horse power is from 15% to 40% richer than stoichiometric.(depending on who you ask and on what engine)
Let me know,
Howard
