I have had difficulty starting my O-540 in my M-7 for the first start in a day. It seems like it should be easy but it has become frustrating.

My process is to prime about 5 times (which seems like more than it should take), run the boost pump for 3 secs, then try and start. It usually doesn’t start up with this. So, I prime a couple of more times and it will try and catch, pop a few times and then die. Once it does start, it runs very well.

When I shut it down and restart when hot, I have found it likes a shot of primer and then starts up well.

Any helpful suggestions are appreciated.

Thanks!

Brandon

That's an awful lot of priming. The electric boost pump can help if it's really hot or to fill the carburetor float bowl, but most of us don't need it for hot starts. Magnetos or plug gaps could be contributing. I would look at magneto timing and check the plugs for wear, gap and cleaning. If they're really black, the mixture is too rich and carbon is building up on the electrodes.

I recently had a similar issue on my O-540. After ALOT of frustration we were able to track the issue down to the key switch which was easily overhauled and it starts without issue every time now.

My cold start technique is throttle open 1/4", run boost pump up to 7 (usually about 3-4 sec) THEN prime 5 times. Usually fires right off.

This spring my O-540 (B4B5) all of the sudden became hard to start. An interesting side note sometimes when I had given up on cranking and released the Key to "both" it would start. You guessed it the left mag had developed issues, we pulled it and had it repaired. Starts great again. During the start the right mag is grounded as it does not have an impulse coupling, the sick left mag was unable to start the engine. Under just the right circumstances when I release the key to both the right mag fired and the engine started normally. During the time of hard engine starting both mags checked fine during engine run ups.

It seems over the years when I have had hard starting aircraft engines from small four cylinders to 18 cylinders it usually associated with the magnetos.

Install two fresh overhauled mags both with impulse couplings and loose the strap that locks out the second mag on start. You'll like it a lot.

Thanks for all of the ideas. I’ll look into the mags and see if that helps.

Brandon

For whatever its worth my O-540 (J1A5D) only needs three shots of prime and no boost pump and usually fires up with no issues. I'm assuming based on your first comment about 5 being a lot of prime that you've tried starting it with fewer shots of prime and maybe without the boost? Good luck and let us know what you uncover.

James

James,

Yes, I did start with less shots of primer and no boost pump. I’ve always been taught to not over prime but it seems that it isn’t getting enough fuel. It has become more difficult but I can’t say if it is getting worse or if it is just the colder weather.

I’ll reply here when I figure it out.

Brandon

Have you checked to ensure that all your primer nozzles are flowing fuel when you prime? It’s not unusual to find some plugged ones. You could be over-priming some cylinders, and not priming others at all.

I never need to use the boost pump for starting. I prime about 4 times, throttle cracked a bit, hit the starter. Mags are worth checking for sure. My engine starts better with a good quality starter, like B&C, and a new battery helps too!
Good luck.

Are you sure the primer is getting fuel, and then delivering it. The plunger seals can wear or crack.
Is the fuel getting to the nozzles, vibration can crack the connection of tube to nozzle. Do you have 2 or 4 nozzles.

like others have said, pull your primer nozzles and check that they are misting fuel into the cylinders properly. it should be a nice even cone shaped spray coming out of each nozzle. if not, drop them in some Hoppes gun cleaning liquid overnight and blow them out with acetone and compressed air the next day.
another issue i've seen more than twice on 540's is the intake gaskets between the cylinder and the intake pipes. they get brittle and crack and cause induction leaks. this will make a 540 very finnicky on startup and warm up.
if these cheap and easy things check out, its in the mag(s).

Update on the starting issue.

I explained the issue to my mechanic and he thought it sounded like a primer issue like others here have suggested. He found that the nozzle in the #6 cylinder was not correct and had a much larger orifice while the other three were the correct spray nozzles. So, when priming, #6 cylinder was getting most all of the prime. The other nozzles were actually clogged since they didn’t get enough pressure to ever have much fuel run through them. This explains why it would seem that only one cylinder would catch upon starting.

It should start much better now! Thanks for the input here once again.

Below is a picture of the nozzles.

Brandon

StepOnTheBall wrote: ↑

Fri Nov 13, 2020 10:20 am

nozzle in the #6 cylinder was not correct and had a much larger orifice

Wow.. I'd say thats much larger. Looks like some sort of reducer is missing in there or something.

Glad you found the issue! Now you just need some good weather again to go fly. Looks like the next week is not that great in the area.