Engine primer
Moderators: GAHorn, Karl Towle, Bruce Fenstermacher
Engine primer
I always thought it was better for the engine to pump the throttle before starting. Then I was corrected at the fly-in inJanuary in Bryan,Texas that I was completely wrong. Since I hadn't used my primer since the engine was rebuilt in 2010 I didn't know that the primer wasn't even hooked up! Now, how many cylinders does the "spider" fill. The service manual for pre-1962 100 series models goes everywhere from one cylinder to all six. Mine is a 1950A. Thanks in advance. Don Nolen a.k.a. " Luvjet"
Re: Engine primer
Hi, Don!
When you pump the throttle, the carb accelerator-pump sprays fuel into the throat of the carb, but if the engine is not already cranking as you pump the throttle, that fuel drains down into the air-box where it pools and can be a fire hazard if a backfire occurs (and can damage your airbox and air filter assy.,...all of which should be inspected if backfire should ever occur.)
You'll have to confirm this by inspecting it but, the usual situation is the primer shoots raw fuel into the left manifold, which means that cylinders 2,4,6 get a whiff of fuel when the engine is cranked. The right side manifold ("spider") has a plug in it at the same location and could be used also for priming if you connect the primer lines to it also. (My own right manifold plug was removed and used for a manifold pressure guage.)
Anyway, in the original configuration the primer shoots raw fuel into the manifold downstream (which is actually above, but between the carb and cyls) of the updraft carb and air box and is therefore less likely to create a hazard.
Some airplanes with this model engine have been altered so that a primer line is introduced at each cylinder intake valve. Some cylinders have a small plugged hole on the upper surface by the intake valve (outboard of the spark plug). These plugs can be removed and a primer line installed at that location, but it's not a common installation and was not done this way originally on the 170 series.
Unless I was based in the far "nawth" (instead of next to the flip-flop-and-thong-attired-college-girl pathway to the river like you are) I would not bother with such a mod, opting instead to use pre-heat for the few times you are plagued with sub-60-degree days.
Another consideration for all is that an engine start which is made without a (fire extinguisher-equipped) outside observer (marshaller) might have developed an intake fire without much indication to the pilot, so be alert for that if you've pumped/primed prior to start. A deadened "poof" sound is an indication of a possible backfire, and it's usually advised to continue cranking the engine to "suck up" the fire into the intakes to prevent damage, i.e., "don't stop cranking" until you're confident the flames have been extinguished.
And if you DO have an outside observer, it's a good idea to have a predetermined hand-signal that a fire is observed so that the observer can safely approach the fire to extinguish it. (The signal commonly used is a palm-down hand waving rapidly left/right, held low so as to indicate fire beneath the airplane. I hope it's obvious that the observer should not approach the plane until the prop stops cranking, and the pilot should cease cranking if the observer approaches the airplane.)
I know many if not most of you know these points but it's not a bad idea to repeat them from time to time in order to maintain vigilance. Sorry if it seems didactic.
In order that everyone understand the importance of a "marshaller" on the aircraft ramp, please take a short-course on correct hand-signals to be used on the airport ramp. I'll be expecting everyone to use the correct "British" procedures at this summer's convention in Gulf Shores: https://www.youtube.com/watch?v=Vz7FI_S0_NE
When you pump the throttle, the carb accelerator-pump sprays fuel into the throat of the carb, but if the engine is not already cranking as you pump the throttle, that fuel drains down into the air-box where it pools and can be a fire hazard if a backfire occurs (and can damage your airbox and air filter assy.,...all of which should be inspected if backfire should ever occur.)
You'll have to confirm this by inspecting it but, the usual situation is the primer shoots raw fuel into the left manifold, which means that cylinders 2,4,6 get a whiff of fuel when the engine is cranked. The right side manifold ("spider") has a plug in it at the same location and could be used also for priming if you connect the primer lines to it also. (My own right manifold plug was removed and used for a manifold pressure guage.)
Anyway, in the original configuration the primer shoots raw fuel into the manifold downstream (which is actually above, but between the carb and cyls) of the updraft carb and air box and is therefore less likely to create a hazard.
Some airplanes with this model engine have been altered so that a primer line is introduced at each cylinder intake valve. Some cylinders have a small plugged hole on the upper surface by the intake valve (outboard of the spark plug). These plugs can be removed and a primer line installed at that location, but it's not a common installation and was not done this way originally on the 170 series.
Unless I was based in the far "nawth" (instead of next to the flip-flop-and-thong-attired-college-girl pathway to the river like you are) I would not bother with such a mod, opting instead to use pre-heat for the few times you are plagued with sub-60-degree days.
Another consideration for all is that an engine start which is made without a (fire extinguisher-equipped) outside observer (marshaller) might have developed an intake fire without much indication to the pilot, so be alert for that if you've pumped/primed prior to start. A deadened "poof" sound is an indication of a possible backfire, and it's usually advised to continue cranking the engine to "suck up" the fire into the intakes to prevent damage, i.e., "don't stop cranking" until you're confident the flames have been extinguished.
And if you DO have an outside observer, it's a good idea to have a predetermined hand-signal that a fire is observed so that the observer can safely approach the fire to extinguish it. (The signal commonly used is a palm-down hand waving rapidly left/right, held low so as to indicate fire beneath the airplane. I hope it's obvious that the observer should not approach the plane until the prop stops cranking, and the pilot should cease cranking if the observer approaches the airplane.)
I know many if not most of you know these points but it's not a bad idea to repeat them from time to time in order to maintain vigilance. Sorry if it seems didactic.
In order that everyone understand the importance of a "marshaller" on the aircraft ramp, please take a short-course on correct hand-signals to be used on the airport ramp. I'll be expecting everyone to use the correct "British" procedures at this summer's convention in Gulf Shores: https://www.youtube.com/watch?v=Vz7FI_S0_NE
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
Re: Engine primer
Thanks George! I'm gonna get this fixed once and for all. My mechanic is on it but it really is confusing as to the "spider" and all that part. Thanks for the input. I'm gonna come see ya soon. You are just up the road from me.
Re: Engine primer
When I had the stock engine in my airplane, one of my earliest mods was to install a six cylinder primer spider and prime each cylinder at the intake port. Made it start much better and smoorher in cold weather. I highly recommend it.
BL