QuestionHello, I have 1979 MGB that a previous owner removed pollution controls and
installed new cams, valves and lifters (Kent cams mild road). I am ultimately
troubleshooting starting issues (also has twin SU's and manual choke) so I felt
I should start with the timing and go from there. At 10 degress BTDC (at
1500 rpm) the car starts and runs but backfires easily and will not idle
without the choke on (even when at running temperature) if I advance from
there a FURTHER 12 degrees it appears to be better and will idle. I have the
data specs for the cams but I don't know what it means in relation to degrees
BTDC or ATDC:
duration inlet/outlet 270 degrees
inlet timing at full lift 110 degrees
Timing figures 25/65 60/30
I just can't find any reference to what the above numbers mean/are or how
they translate to dynamic timing with a strobe gun.
Can I just adjust the dizzy to high revs until it starts coughing a bit then
backing off? I'm not hearing any OBVIOUS pinking noises ...
I appreciate your help in this matter!
Lance
AnswerHi Lance,
I never seen any formula to take a cam degrees and come up with an ignition timing. I don't think that is possible anyway.
I do know that the most horse power comes from the most advanced ignition just BEFORE detonation. (low RPM detonation is the Pinging)You can have high RPM detonation that you can't hear. Both can destroy an engine in a very short time.
Any time you modify an engine in any way, you alter the map of what is a good timing for that engine. What I mean as a "Map" is the advance curve and total timing. This should be done on a dyno but most people don't have that access. The next best is a knock sensor system so you can "SEE" the detonation at each RPM as you apply load to the engine. MSD has a knock sensor display system you can add to any engine. Most hot rod builders just advance the ignition until they get the "Pinging" on a quick acceleration with a prewarmed engine. Then they back off the timing just a little. And for total timing they use a set figure that is known to be good for that engine. This is a "hap-hazard" method and many luck-up and don't destroy their engines. But usually don't get all of the benefits of their engine.
It sounds like you have other issues to address. Your carburetors don't sound like they are up to the task either. Settings, I mean not the carbs as SUs are great carburetors.
Here is a list of what controls the timing of your engine.
1. Compression ratio as compared to the grade of fuel used.
2. Temperature of the engine and outside air.
3. Fuel mixture through the whole RPM range.
4. Spark plug heat range. (the number on the plug)
5. Load and RPM
It is a complex process to get a good useable timing curve remaped when you modify an engine. And any modification will alter that map.
You need to start with a slightly rich mixture on the carburetors and a standard timing for that engine. If you had "Forged" pistons in the engine you can use the High RPM high load to see where the timing should be for total advance. At high RPM horse power drops off very quickly when you advance to the detonation point. Thus a high RPM run up a steep grade in high gear would duplicate a "dyno pull" Peek RPM is not necessary but peek torque is where you can get a good setting for ignition timing. Peek torque on a stock B is about 3500 to 4000 RPM. With a cam, you may have altered that.
You should start with a rich mixture and set the timing correct for high RPM first and then change the advance curve as you progress DOWN the RPM range. IDLE should be the last to be worked on and done with a combination of carb settings and advance curve setting NOT total timing setting. This is difficult to do without a distributor machine. You might search your area for a shop the has a race car of any kind. Dirt track car or stock car or SCCA car. These would usually be good people to give you advise as to how to set up the timing and advance curve on your engine.
Howard
