Hard start when hot
 

I'm having issues with hard starts when the engine gets up to operating temp. The engine starts fast and strong when cold but is very slow and weak when hot. I have a fairly new Miata battery that tests at 90%. All of the high current wires are existing Miata with a few extra engine block grounds installed. I have gone through the engine timing and verified it at 10 degrees BTDC. I installed a direct feed start switch to bypass the ignition to the solenoid on the PM starter. This works better but still hard to start. Also won't start at all when cooling fans are running. Are the Miata wires too small? Seems like it can't handle the electrical load. Any thoughts on this would be appreciated.

Starting during the heat soak period can be as simple as too small a miata positive cable or a weak ground. Your gonna have to break out a digital voltage meter and measure the voltage at the starter to see if it is dropping and how much. Measure voltage before starting then during start and see how much you are dropping to help you diagnose the problem.Your gonna need and extra set of hands to accomplish this safely. Good luck and post your results.

or double check your timing. Make sure you using the correct timing point and make sure its not bent or loose

I took some volt readings and came up with this. Attached the volt meter and read 12.5 v. Started the car cold the volts dropped to 10.5 while starting. After 15 minutes of run time. I shut the engine and restarted. The volts read 9.4 v. There is a definite difference when hot. I did rotate the timing by 2 degrees all the way to zero and there wasn't a difference in hot starts. I reset the timing back to 10 degrees. Engine runs smooth and powerfull with no ping at this setting.

Okay,it is important to point out and identify whether you are having electrical or mechanical problems. Sometimes both but I would address the voltage drop. I am not an expert but have been in the repair business for 35 years. Address one issue at a time so you will need to attack you voltage drop. You should address the most obvious and always stick to basics. Your miata positive battery cable that leads to the starter may be too small, loose or plain not adequate for your application. 1. Check that cable as heat effects the ability for the copper to flow voltage or carry amperage. A hot copper wire cannot carry the voltage and amperage when cold. So be sure you are not heat soaking the cable that runs directly to the starter from the battery. 2. Check the ground as it may not be making good contact to the body..... in addition add another ground that leads from the head or block or some part of the engine to the body. Find a large bolt and get a short thick cable and add an additional ground. It will only help. 3. You may want to run a larger positive cable from the battery to the starter... 4. Try another battery. Batteries are tricky things that may seem to work and be fine but are actually weak when you need them during a heat soak start cycle... Replace it with a know good working battery. Test equipment may not tell you good or bad condition.... you just may need to change it out with is the same or larger than what you are using now. good luck....just remember that most problems are simple. Dont shot gun fixes...Isolate the problem and address it.

Why not just try a Universal Starter Heat Insulator like the one in this link:

JEGS Universal Starter Heat Insulator - JEGS

Rookie, Thanks for the good info. I have a new battery in another car I can use as a test to verify it's not the battery. Then I am planning to go through all of the high current wires this weekend. Checking all of the connections and routining. I have noticed the existing ground wire from the battery is very small. I want to run a new ground wire directly from the battery to the starter bolt and increase the size of the battery feed wire to the starter. I am pretty sure this will rectify the problem. I will let you know how it goes.

Epantjoa, I have wrapped the starter with a similar insulator and it doesn't make a difference. I'm pretty sure it's the wirinig.

I had the advance weights in an MSD distributor "stick" in the full advance mode before causing that exact issue.

Tbone, How did you solve the problem? Did you have to replace the distributor or just lubricate?

I had to fully disassemble it. It has happened several times. It is on a 427 in a Cobra replica.

Update- My problem seems to be solved. I added a direct ground wire from the neg battery terminal to the lower starter bolt. I increased the size of the battery feed wire to the starter and added an auxilary start switch with a direct feed from the battery to the solenoid. I recently spoke with Martin at Monster Miata and he said he has found the Miata ignition switch to be problematic in delivering enough current to the solenoid when hot. Still using the Miata battery seems to be fine.

Great! Glad to hear it.....