Times getting worse while track tuning!

My first time track tuning, and I'm having trouble because the times are gradually getting worse no matter what adjustment I make. I though maybe it was the prep wearing out, so I'm now tuning without prepping wheels and axles. For example, I'm working on dfw gap starting at 0.20 and going to 0.007 running 5 times for each gap. 0.20 gap my first 5 times averaged 2.6547.
0.015 averaged 2.6668.
0.010 averaged 2.6697.
0.007 averaged 2.6703.
I then went back to the 0.20 gap and the average for the next 5 runs was 2.6696. The next 5 for 0.015 was 2.67628. I stopped there because I don't know why it's slowing no matter what I do. I have changed nothing else but the dfw gap.
My car is 141.8 gm, dfw 17.1gm, 6 inch steer. Using a clean single lane best track.
Anyone know what could be the issue? I appreciate any help.

 

I’m not a pro, but I can tell you from experience that fiddling with the axles can change your drift - sometimes dramatically. Did you check that it hadn’t changed while you tweaked the gaps?

 

Clean your wheels

 

Yes, you're right. Even though I thought the track was clean, the wheels must have gotten dirtier as I tested. Everything cleaned up and don't have the problem anymore. I went back to testing with prepped wheels and all is good. My average time is down to 2.5468. Thanks!

 

Yes, I rechecked the steer and it was fine. Wheels were accumulating dirt even though I cleaned the track before. All's good now. Thanks for your reply.

 

Cleaning wheels will always help, especially when the dirt is visible. But, I have to ask, did your car go from a 2.6547 to a 2.5468? Or to a 2.6468?

 

The numbers are right. I started out testing with prepped wheels and axles for oil. First times were in the 2.53's but gradually worsened to 2.55's. There must have been too much dirt and dust on or around the track for me to test with oil, but at the time I thought my prep was wearing out. So I cleaned the wheels and axles then tested with them unprepped. My thinking was that the times would be more consistent even though slower, but the same issue occurred. I cleaned the track and wheels again, then decided to test with graphite. The graphite testing went well. When I started to see slower consecutive times, I just added a little more graphite. Maybe I can figure out oil some day, but it looks like it will be messy graphite for me.

 

Ok, I gotcha. Is this car for a scout race or a pro race?

 

We only do scout races. Became obsessed with the competition 4 years ago. We placed at council every year but never won it. This site has helped tremendously.