Hot starting troubles

[Robert Lees]

Concerns regarding turning the prop backwards are usually (and correctly) attributed to engines with mechanical vacuum pumps installed.

On vacuum pumps that are getting on a bit, doing so can break the delicate vanes (they should be replaced at 500 hrs in my opinion..cheap enough).

Rob

[n6346m]

I just thought I'd add my technique of operating my L-2 without a starter for the last 22 years to the conversation. It may work or not for you. I have used it on the previous A-65 and the current C-90 the same way. I've always had a Stomberg carburetor on both engines so I can't say that I would do the same with a Marvel.
It begins with the shut-down after the engine has been running. The shut down is done according to what I anticipate for the nest start-up. If I anticipate a cold start-up for the next time, I adjust the RPM for what I anticipate the outside temperature will the next time I start. I add a little RPM to what is desired because a cold engine will not Idle at the same RPM as a warm one. For a warm re-start I do the same thing because a warm engine needs a little more air to prevent flooding. For example if normal Idle RPM is 550 I'll shut down around 750 to 800. Your engine may vary. Set your desired RPM, kill the engine and immediately, if not just before shutdown, turn off the fuel valve. Do not touch the throttle as it is ready for the next start. The next step depends on the type of primer you have. If your primer is going into each cylinder, then you're ready to go. If your primer is like mine, where it injects an atomize amount of fuel directly into the intake spider that is directly above the carburetor, as you pull the prop through each compression stroke that cylinder fills with a combustible mixture. After four pulls, all four of the cylinders have filled with the mixture and whatever the next cylinder that is about to fire is charged.
I think the reason that the Stromberg carb is particularly prone to flooding on my airplane is that the brass needle never seats properly. I've tried all the tricks and had it working perfectly on the bench only to have it start leaking again after only a few hours on the airplane. I tried delrin and that leaked and the neoprene swelled up because I use mogas. I really believe that eventually. they all leak. Whether is shows up as a drip coming out of the airbox or it leaks so slowly that evaporation equals the rate of leakage. A leaking needle means that the carburetor bowl is overfilled. It could just be that the act of adding fuel could increase the head pressure on the needle causing a leak where one may not have been there with less fuel. I just don't know.
If I have a flooded engine I leave the fuel valve off, make sue the ignition switch is off, open the throttle fully and rotate the engine backwards a minimum of 8 strokes before trying a restart. The problem with this is that you have to remember to bring the throttle back before attempting another start. I have now lost my preset throttle position and I have to guess as to what RPM to expect upon start. It is a little easier having an L-2 since I have a mark on the sidewall throttle quadrant to get a ball park setting. Natural slack in the cable and throttle linkage makes a difference whether you are advancing or retarding the throttle to get to the mark.
I completely agree with Rob's comment about rotating the prop backwards and the effects on the vacuum pumps. Most magnetos are designed to rotate in either direction since they are used on various engines with different gearing. The internal components are different of course so they can fire in the proper order.
Normal operation of an engine takes air and fuel mixture through the carburetor and the results come out the exhaust. When rotating the engine backwards you do the opposite. You take air into the exhaust pipe and the mixture comes out the carburetor. That's why you should open the throttle all the way. If I were to sell my airplane this is the way I would recommend its operation.


Bob Picard
N48923 Taylorcraft L-2B
Anchor Point, Alaska

[mikeg]

Been there and done that. Bad mag coils. Changed both and started like it used to. My problem solved. Good luck
Mike

[mikerice]

Had the issue of hard starts recently, particularly hot (A75). ended up pulling all the plugs and cleaning them well. The bottom plugs were fouled, now it starts on 2nd blade. I have recent rebuilt Slick Mags and a rebuilt carb. Turned out the plugs were the culprit. I spent a lot of time cleaning the plugs and it was worth it.

[Jim Herpst]

Ohh and I searched for a while, didn't find what I was looking for. Feel free to point me to an old post on the subject. Just fed up with this issue. Getting older now, sore muscles, and short on patience.

My bet is as you have been advised here a couple of times ..it is spark. The Eisman mags are great. The problem is the parts availability is not so great. As mentioned "new NOS parts" can be dubious due to storage of them over the years (moisture Hot rooms or worse attic space or all ) Mags when they are hot with weak parts in them tend to fail ..read coils and condensers. While all the hot start advice provided here as it relates to overly rich carbs works I am sure ...IF you have an overly rich carb issue doubtful and can be seen, smelled and once running heard (rough ) Try a known good mag

[Kerbs]

Assuming you have no mechanical issues, when starting and you prop forever with nothing happening, the engine is too rich. Open the throttle and turn the prop backwards 8 or ten blades, if the engine kicks back and reverses it's too lean prime some more.

EO

On my 85, I have a marvel with a mixture control. If I turn the engine off with mags like I used to with the 65/stromburg, it will flood every time, 100% of the time. In that case, the open throttle and turn prop backwards to empty out the carb has saved me. Even when I lean it out to shut it off, it still sucks some gas up at the end and if I prime or give it any throttle, it will flood and not want to start.

[PA1195]

A Marvel carb should (in my experience with several) dry out the fuel mixture at cutoff. But parts do wear and some fuel can bypass the main nozzle metering sleeve shutoff and accelerator pump discharge valve. One test for too much fuel is sniff the exhaust outlet. If it's rich during propping you'll smell gas being pumped through the system. After propping a PA-11 C-90-8 for years I found anything below 32* air or engine temp the fuel (from the primer or Marvel accelerator pump) would condense in the intake and lean the starting mixture. If hot starting no priming was needed. I marked the throttle to give about 700 rpm when starting. Anything slower and it might fire but not spin fast enough to fire the rest. Dual Bendix impulse mags were installed there and now on my Taylorcraft with a Marvel carb. I typically set the idle mixture to give a 50 rpm rise at cutoff in summer and winter. Winter can take up to 1/2 turn out and summer back 1/2 turn in of the mixture needle.

Gary