Questions on grounding and bonding on RV-6

Just bought an RV6. It has a skytec starter, 0-320 160 hp engine. It is slow to turn over, have to jog the starter to get it past the first compression stroke then it spins well enough to crank. Checked battery and it is in good condition with full charge. Positive cable looks good. Ground cable appears to go from negative terminal to firewall. I believe it is attached to a lowere engine mount bolt. It is a short cable and appears in good condition. The engine ground/bonding wire is about 10ga and attaches from then engine to the oil cooler mount. Is this a good location, is the wire too small, what have you found to work on yours? I'm think a better ground is needed? Thoughts?

Sounds marginal. Steel bolt is not a great conductor, combined with dirt, grease, and a small wire equals a lot of resistance. Try bypassing all that with a car jumper cable. I'm guessing you will see a marked improvement.

How is your ignition timing? If you only have one impulse coupled mag, make sure you are starting on only that one.

Last night I had a discussion about a Cherokee he used to fly and how difficult it was to start on both mags. Learned later that he should only start it on one mag. The symptoms he described sounds just like your post.

I believe you should have a #2 ground cable to the firewall and to the engine case. This might not solve your problem but a #10 wire is to small. Larry

LARCO said:

I believe you should have a #2 ground cable to the firewall and to the engine case. This might not solve your problem but a #10 wire is to small. Larry

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I agree. If the run isn't too long you might get away with a 4ga but a 2ga will still be best.

What kind of battery and prop do you have? A lightweight prop with low flywheel effect (wood or composite) combined with a small battery (like the 680 that is very popular) and the lightweight but power-hungry Skytech starter can combine for poor starting performance no matter what you do. That said, 10ga is too small for a ground wire and I'd start with that.

All good points so far

I would also remind you the battery can be fully charged but still be bad, It needs to be load tested. A volt meter across the battery terminals while cranking is a quick and dirty test, but not tottaly conclusive. If it's an Odyssey you can find a load test procedure on their web site, as well as a way to recover a defective battery.
Once you know your battery is good, then I'd start looking for voltage drops in the circuit. Start with the starter solenoid, these are notorious for developing high resistance.
You can also measure the voltage across both ends of each circuit, ie, put the volt meter in parallel with the total ground circuit, then the power circuit. For example put one lead on the battery ground and the other end on the engine case. Crank the engine and you should see less than one volt, any more and you have high resistance in the circuit. Same for the positive. If it is higher than a volt you can then isolate the exact location(s) of the high resistance using the same technique. So for example you can put the voltmeter across the the two starter solenoid lugs, you should have a very low voltage across it while cranking.

Tim

Just as a point of reference, my -6a has a short very thick cable from the battery to a brass bolt that goes through the firewall. Then on the same bolt there is braided grounding strap to the engine block.

I have no issues. (Well, until I left my master on that one time )

Thanks for all the input. I did fab up a 2 gauge cable and attached it to the starter mount and went with a through bolt on the firewall to relocate the battery ground cable also. Used two fender washers on both sides of firewall as extra support. It spins a lot better now. Thanks