Tuesday, September 15, 2009

Adjusting Ignition Timing

I've been posting a few technical questions on a new forum I found for engine builders. I posted about how the car seems to take off at 3000 RPM but hesitates and does not perform very well below 3000 RPM. The responses I've received suggest it is because of my timing.

I posted the mechanical advance curve I posted to this blog a few days ago:
850 - 11 Degrees (my initial advance) BTDC
1000 - 13 Degrees BTDC
1500 - 17 Degrees BTDC
2000 - 25 Degrees BTDC
2500 - 31 Degrees BTDC
3000 - 38 Degrees BTDC

I was told a better curve would be as follows:
850 - 18 Degrees BTDC
1000 - 24 Degrees BTDC
1500 - 32 Degrees BTDC
2500 - 36 Degrees BTDC
3000 - 38 Degrees BTDC

I'm a little concerned about using 18 degrees initial timing. Most engines with 9.5:1 compression ratio create the most power with between 38 and 42 degrees total timing. It is recommended that all of this timing come in by 2500RPM. In order to make this happen, I need to switch the advance mechanism in the distributor from the 15L (30 degrees advance) side to the 10L side (20 degrees advance). If I then set my initial advance to 18 my total would then be 38.

I switched the internals parts of the distributor tonight. When you switch the side of the arm used, the distributor is 180 degrees out. I pulled the distributor and put it back in back in the correct alignment. It took a few minutes to get the timing set perfectly but it worked out.

My base timing is currently set at 16 degrees. I tested the rest of the mechanical advance with the vacuum line plugged. I recorded the following results:
850 - 16 Degrees BTDC
1000 - 16 Degrees BTDC
1500 - 23 Degrees BTDC
2000 - 34 Degrees BTDC

I stopped at this point as it is 9 o'clock and I needed to put the kids in bed and did not want to wake up the neighbors kids. I'll need to work on it a little more tomorrow night. I would prefer to have more advance earlier but also space out how the advance is applied. I'm not sure of the best way to do this so I'll need to research and experiment some.

09/19/09 Edit: I've been working on the timing curve a little more. I'm not sure how to change it much more as the only adjustment left after switching to light springs is to bend the tabs they attach to. However, I was able to adjust and test it and found the following:
850 - 14 Degrees BTDC
1000 - 14 Degrees BTDC
1500 - 20 Degrees BTDC
2000 - 31 Degrees BTDC
2500 - 39 Degrees BTDC

I'm not sure how I reached 39 degrees advance since I have 14 initial and 20 mechanical. This should limit me to 34.

Sunday, September 13, 2009

Still Working on the Carburetor

I still have a bit of hesitation off idle which I cannot fix. I've asked about it on a few Internet forums but have not found a solution.

I thought it could be the idle mixture screws. However, when I change them it has only a very, very small impact on vacuum. From what I've read, these should be set to where you get the highest vacuum reading.

Someone asked about a leak around the intake on one forum. I said I had already checked it using WD-40 around the intake but could not find any leaks. I walked out to look it over again today and found oil behind the rear intake seal. It was only a small amount and was not there two or three weeks ago.

I cleaned the oil off and took the car out for a 10 mile drive. When I made it home, I checked and there was no oil. I'm not sure if this would be responsible for the problem or not. Unfortunately, I would need to buy another $30 bottle of Mobile One synthetic oil and drain all the fluids again to fix this seal leak. At this rate, the supplies are going to cost more than the intake itself.