1985 FXRT Sluggish Starting When Hot

If nothing has changed recently, I'd cover the electrical connections with a wire brush and dielectric grease. Both ends of the starter - battery cable and the battery - chassis ground cable need to be clean of any oil, grease or corrosion.

First is a fully charged battery, get a battery tender.

FXRT should have a big battery 300+ CCA?

I really like ignitions with a working VOES, this aids in starting.
You can rewire the starter button to the other side of the power switch so you can get it spinning and then turn on the spark.

Have you done a voltage drop test? Make sure that the starter is actually full volts, and that the cables don't have overly high resistance. A few thoughts.

1985... I was told that HD made shorter cylinders on some. Was told that in 1987 at the HD dealer in Omaha, when I had a noise in my 1985 FXSB. Turns out the noise was the piston making contact with the carbon on the squish. Anyway I have owned 2 of the 1985's both had the crank and actually stop cranking. On the first one I did the kill switch disconnect so I had no spark to fight when cranking. The second one I adjusted the timing more to retard. It still does the stop cranking. But I hit the button and if it doesn't crank I'll stop and try again, most times it will get past it and start. They do make thicker head gaskets for this too as a fix. A good battery is necessary. Also check your VOES is not stuck should be open with no vacuum.

An easy test to see if the issue is ignition related or something else is to pull the plug wires and try to spin it over. If it still hesitates without spark then the ignition is not the issue. IIRC, the stock ignition is supposed to not fire for 2 revolutions to allow the engine to gain some momentum before firing. I don't know for sure when that started and it may have been later as a fix?? Most of the early electronic ignitions have been replaced by now though, and if it was replaced with a Dyna S or similar there is no delay in firing. If the ignition is the issue, you can replace it all with a Dyna 2Ki which is set by default to 2 revs before firing. Some others have the same feature, but I stick with what I know works and have never personally had an issue with the Dyna electronics.

In a diagnostic sense, a battery load test is step one, followed closely by a voltage drop test. Do those and report back.

Check for leaking intake manifold or seals also see if the woven ground cable is tight under the inner primary cover. Also double check your timing with a timing light and inspect your 32400-80B sensor plate for condition. The "B" in the suffix of 32400-80B stands for the third upgrade of the part. They like to melt over time. If you have a 32400-80 or 32400-80A you probably should change it. As a quick check , dismount your ignition module and turn it over and see if you can see bubbles forming in the plastic matrix. Early Evo modules had a vibration induced internal components failure. Later modules were modified with springs or buffers to prevent internal component vibration failures. Sounds like your problem is heat related.

Thanks for the pointers, everyone. I've had the battery load tested at the local auto parts store and it came back as a good battery. I also did a voltage drop down test per my service manual and things are a little murkier there. The voltage drop going from the relay into the solenoid is 0.09-0.13V, but going from the solenoid into the starter is 1.0 - 1.3V. The starter ground circuit drop is 0.09-0.13V. Additionally, since someone suggested trying a higher CCA battery I swapped in the 320 CCA battery from my Dyna and the problem doesn't manifest with this battery. I tried 10 start attempts and the starter turned the motor over just fine and the bike started each time.

The naive conclusion here might be that I need a 300+ CCA battery, but the voltage drop down test I did suggests that the solenoid isn't performing as well as it should. Did the bigger battery just paper over the real problem?

Try replacing the relay, and battery cables, if that doesn't work, new starter time. Also you could have some carbon build up on the pistons, run some Seafoam through the carb just before the next oil change.

That is entirely too much voltage drop on the solenoid-to-starter side. So it sounds like you need to isolate whether the problem is the cable or the solenoid. Run your voltage drop test across the solenoid. If the voltage drop is still 0.09-0.13V, replace the cable running from solenoid to battery. If the voltage drop is 1.0 - 1.3V, you know the solenoid has seen better days.

Got busy with holidays and stuff but I wanted to check back in here. I cleaned all the connections really well and tested at the solenoid terminal. Voltage drop was better than before but still too high, around 0.5V. I've got a rebuild kit from J&P cycles and I'll see how it does after rebuilding the solenoid.

Excellent. Let us know how it goes.