Engine Question

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170C
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Engine Question

Post by 170C »

My O-300B is a very easy starting engine. If the prop turns over it will start. Yesterday I flew over to buy some fuel at a airport about 40 miles away. After topping off I hit the throttle a couple of times, engaged the starter and it spun the engine, but no ignition. Tried again, same result. Did not think it needed priming, as it never does especially in warm weather. Same result. I turned the ignition off and on several times, same deal. Turned the ignition off and on again and it started. Flew back home. This morning it started as normal, flew to another airport for a seminar and when I was ready to leave it started right up. Flew to another airport for lunch and about an hour or so later when I got ready to leave I experienced the same situation--lack of ignition (that is my guess). After 2-3 attempts it cranked right up. What in the devil is going on? The only thing I have changed is using a different ignition key than the one I normally use (it is misplaced :oops:) . I haven't tried using another ignition key, but will do so to see if that could be the culprit, but the key I am using turns the ignition on effortlessly. Mag rpm drops are the same as usual. What are some suggestions?
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Re: Engine Question

Post by johneeb »

Frank,
Do you have a key start or the mechanical starter linkage? The original non-key start mag switches are easy to take apart and clean. Some key start switches have an AD for the opposite problem not shutting off when they should, that isn't much help is it?
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Re: Engine Question

Post by 170C »

Mine is the old mechanical/pull starter style. Thanks for the suggestion John.
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Re: Engine Question

Post by Bruce Fenstermacher »

Weak magneto(s). Happened to me.

Your engine is sucking air. Be sure it's getting gas. Only thing left is fire.

Your mags won't make fire if they are still grounded OR they don't spin fast enough or otherwise not capable of generating enough electricity.

You can test your mag switch to see if it is intermittently not opening by hooking a multi meter between each P lead and ground and exercising it, watching for correct and consistent reaction of the multi meter.

Next time it doesn't start, with the mag switch in the both on position, use a multi meter to check for continuity from the connected P leads to ground. You shouldn't have any.

Assuming you find all OK with the P leads and mag switch, it can only be internally in the mag(s).
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johneeb
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Re: Engine Question

Post by johneeb »

Bruce when testing the mag switch with a multi-meter do you disconnect the P-leads from the switch to insure the multi-meter does not find ground through the closed breaker points?
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edbooth
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Re: Engine Question

Post by edbooth »

Frank, disconnect the P leads from the mags,if engine starts with no problem, you have isolated it to the P leads or switch. Caution,when you do this, mags are hot all the time. Are they Bendix or slicks ? ( If I recall , on some Bendix when you remove the lead, it grounds the mag).
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Re: Engine Question

Post by GAHorn »

Frank...you wrote that you "hit the throttle" ...before attempting your warm-start. What do you mean by that statement? (I'm thinking of possible flooding.)

The actual key used makes no difference, IMO. If the switch turns...it turns.

Weak mags are indeed possible. Some mag coils, when old, will not easily start an engine when old coils are hot. A weak spark results...usually due to coil-leakage. Three A&Ps worked on Dale Faux's failure to start during a stop-over of the Wilmington convention poker-run. Another A&P walked up and diagnosed weak coils... blew cold air onto the mags with the shop compressor, and resulted in an immediate start.
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Re: Engine Question

Post by 170C »

Mags are Bendix and we're oh'd within past 200 hrs. I realize that there could still be an issue with them.

George- I pushed that throttle in a couple of times. Doubt it was flooded. I want this problem corrected before I get out in BF Egypt!
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Re: Engine Question

Post by Bruce Fenstermacher »

johneeb wrote:Bruce when testing the mag switch with a multi-meter do you disconnect the P-leads from the switch to insure the multi-meter does not find ground through the closed breaker points?
Or at the mags which ever is easier. Disconnecting at the mags would include the wires in the test which wouldn't hurt.
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Re: Engine Question

Post by Bruce Fenstermacher »

Pushing the throttle a couple of times could easily flood a warm engine cause our carbs have accelerator pumps.

However, there is no reason your engine should change characteristics from what you'd be use to all the years you owned it accept mags or plugs in this scenario.

Anyone who knows Leroy my partner knows he doesn't own anything that is not tuned like a swiss watch. He may use unorthodox methods to tune but they run reliably and consistently. A few years ago we were out on a breakfast run and the engine took to many blades to start and Leroy got suspicious and when the plane refused to start at the refueling airport 2 miles for our home grass strip he knew what it had to be. Those darn 50 year old coils in our beloved Bendix SF mags had seen better days was the diagnosis. An hour later cooler mags started the engine and it was taken home and the mags removed.

New coils where ordered from Aircraft Spruce and to our surprise when they arrived they were in vacuum sealed metal tubes marked Army Surplus and the year 1946. The key was used to unseal and open the cans and as the vacuum was released a smile came over Leroy cause he was about to install brand new (made the way they use to) coils. Our starting issues went away.

On the Bardestown KY convention trip a few years ago, twice I couldn't get the engine to start but eventually did. Reported that to Leroy who immediately removed the mags and had the rotator magnets remagnetized back to spec. (You've got to know the right people to get this done and it helps to mention tractors rather than airplanes when you visit). Couldn't have been the coils he deduced, he'd changed them out less than 5 years prior after all. Not a problem since.

I realize your not running Bendix SF type mags because the only people who do also are still using cross wind gear. And I realize your not probably going to rebuild the mags yourself. But they should be a consideration when diagnosing your problem.
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Re: Engine Question

Post by GAHorn »

Pumping the throttle was what most of us were taught "back in the good-ol-days".... and it's a bad procedure.

We have updraft carburetors (as do most aircraft engines.) When the throttle is pumped, fuel is sprayed into the carb throat ....where it immediately drains downhill into the air-box where a backfire can cause an induction-fire! (Once that occurs...the best response is to KEEP CRANKING to suck the fire up into the induction system where it'll not do harm. But ...if you do get an induction fire.... think about the air filter that likely melted and/or the airbox and the carb heat butterfly-valve that likely received a lot of heat and may have damaged the seal.... before continuing to fly.)

In other words, ....If your engine needs "priming".... then use the primer. The BEST primers inject fuel at the intake-valve...and afew of our 170's have been so-modified when their cylinders have the intake-ports incorporated. Early cyls did not. But most simply inject fuel into the LEFT intake spyder, just to the left of the carburetor. (Yes...if enough is injected, some of it also can run toward the carb and airbox.... but the majority of it remains in the induction and is sucked upward into cyls 2,4, &6. It's not the best system, but it's the one the OEM designed for the early cylinders.)

Otherwise, another method is to pump the throttle only WHILE the engine is turning during the start....such that air being sucked into the cylinders bring the gasoline vapors to the cylinders rather than run back downhill.

Frank, every day you fly your airplane it's a different atmospheric... and pumping your throttle can flood it one day to the point of being too-rich to quickly start... and not the next day. I suggest you stop pumping your throttle completely and try it "dry" for awhile to see if it really even needs priming.... but that if it does need priming to do so only while the engine is being cranked.
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Re: Engine Question

Post by 170C »

Good advise George. I don't prime my engine except in old weather but I have been giving it a stroke or two with the accelerator pump. Tomorrow I plan to test it again. I may need to open the ignition switch and clean it. We will see.
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Re: Engine Question

Post by GAHorn »

I seriously disbelieve it has anything to do with your "ignition" switch.
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Re: Engine Question

Post by daedaluscan »

I have a friend who says that "95% of ignition problems are carburation". Pretty sure that he also says that "95% of carburation problems..."

But if it runs, the mag switch makes it stop, and you get a noticeable mag drop on each single mag I would look at fuel.
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