Priming without pushing in the primer

W

woxof

Pre-takeoff checklist
Joined
May 6, 2015
Messages
127
Display Name
Display name:
woxof
I read this and found it interesting. Ignoring the part about what sounds to be a faulty carb.....has anybody actually ever used the technique of just leaving the primer out to assist the engine operation?

"Before i had the carburetor overhauled by a real expert, I used to prime the engine and then leave the primer open until the engine stated running well which usually took only a minute or two. I did not pump the primer, but just left it open. The extra fuel that comes through the open primer line did the job quite well."
 
I have seen a few planes where this worked ok. But most leaving the primer open seems to have little or no effect.

Brian
 
brcase said:
I have seen a few planes where this worked ok. But most leaving the primer open seems to have little or no effect.

Brian
Click to expand...
I have seen one airplane that started better when the primer was pushed in during rotation by the starter instead of beforehand.
 
brcase said:
I have seen a few planes where this worked ok. But most leaving the primer open seems to have little or no effect.

Brian
Click to expand...
I don't know about the little or no effect. I recall an incident back in the last century. This was in my National Guard unit back in the Bird Dog days. A fellow pilot fired up & went through pre take off checks. The engine quit on take off, but he landed OK on the remaining RW. No big deal, its a Bird Dog. Back in the hangar they found the primer unlatched and open. Cont O-470. Comments?
 
The procedure would provide more fuel to the cylinders that were primed. On some airplanes only one cylinder is primed.
 
Last edited:
Maxnr said:
I don't know about the little or no effect. I recall an incident back in the last century. This was in my National Guard unit back in the Bird Dog days. A fellow pilot fired up & went through pre take off checks. The engine quit on take off, but he landed OK on the remaining RW. No big deal, its a Bird Dog. Back in the hangar they found the primer unlatched and open. Cont O-470. Comments?
Click to expand...

Agree, while not something I normally try, the planes I have seen that it does work on, it does have a significant affect on them, usually running very rich.
 
One trick I was taught, which often seems to help, is to delay starting for a minute after priming, to give the fuel time to vaporize.
 
Clip4 said:
The procedure would provide more fuel to the cylinders that were primed. On some airplanes only one cylinder is primed.
Click to expand...
And on Continentals they often have the primer nozzle in the induction spider (small Continentals) or the intake runners (larger engines), not in a cylinder's intake port. They all get prime. I have NEVER encountered any engine with only one cylinder primed. Lycoming primes multiple cylinders, at least three out of four.
 
Last edited:
Palmpilot said:
One trick I was taught, which often seems to help, is to delay starting for a minute after priming, to give the fuel time to vaporize.
Click to expand...
That usually leads to fuel running down the induction manifolding and dripping out the carb airbox. A backfire during start sets fire to it and now you have a real problem. The best is to prime and crank immediately, or even during cranking as someone mentioned.
 
The unlocked primer will permit the engine to suck fuel though it at lower power settings. There are tiny springs on the check balls in the primer, and there has to be enough suction in the manifold (low MP) to overcome those springs and draw fuel though it. You can encounter higher fuel burns in cruise, or suck so much extra fuel on approach that the engine floods and dies, which is why the POH pre-takeoff checklist includes "Primer in and locked." They mean that. Take it seriously. There is a spring-loaded needle valve in the primer plunger that closes the fuel inlet/outlet to the pump cylinder. Locking the primer holds that needle firmly in its seat.
 
Funny. I was told to start instantly after prime as waiting just allows the fuel to drip out of the engine. My IR instructor scared the daylights out of me about leaving the primer open, basically stating it would kill the engine on rotation

I have no idea if any of that is true but I always double check the primer now before take off on a carb'd plane. I also crank almost immediately after priming. Maybe it depends on the engine?
 
I think Lycoming says to clean primer nozzles every 500 hours.
 
FYI: have seen more carb/intake fires due to improper primer use than any other cause. Also as mentioned in the OP about "faulty carb" one of the 1st places to look when troubleshooting a fuel related/carb issue is to check the primer assy for proper operation. Same goes for if you have to use the primer outside its normal use to start the engine which could be hiding an underlying carb issue.
 
Dan Thomas said:
That usually leads to fuel running down the induction manifolding and dripping out the carb airbox. A backfire during start sets fire to it and now you have a real problem. The best is to prime and crank immediately, or even during cranking as someone mentioned.
Click to expand...
Thanks for that information. Is the same thing true of fuel-injected engines?
 
Dan Thomas said:
And on Continentals they often have the primer nozzle in the induction spider (small Continentals) or the intake runners (larger engines), not in a cylinder's intake port. They all get prime. I have NEVER encountered any engine with only one cylinder primed. Lycoming primes multiple cylinders, at least three out of four.
Click to expand...
O-320 in some Cessna come to mind. #3 is all that is primed. Having more cylinders primed was an option.
 
Palmpilot said:
Thanks for that information. Is the same thing true of fuel-injected engines?
Click to expand...
Yes. The fuel is injected in the intake manifold right next to the intake valve, in the same port that many primer nozzles use.
 
Dan Thomas said:
Yes. The fuel is injected in the intake manifold right next to the intake valve, in the same port that many primer nozzles use.
Click to expand...

Regarding the statement below, since fuel-injected engines presumably don't have a carb airbox, is there another place where the fuel from priming is likely to drip out and catch fire?

Dan Thomas said:
That usually leads to fuel running down the induction manifolding and dripping out the carb airbox. A backfire during start sets fire to it and now you have a real problem. The best is to prime and crank immediately, or even during cranking as someone mentioned.
Click to expand...
 
Palmpilot said:
Regarding the statement below, since fuel-injected engines presumably don't have a carb airbox, is there another place where the fuel from priming is likely to drip out and catch fire?
Click to expand...
More than once, I've walked/taxiied past a 172SP with fuel dripping from the cowling and a young pilot cranking what looks to be a hot engine (i.e. the infamous fuel injected hot start).
 
Dan Thomas said:
Lycoming primes three of the four cylinders in the O-320. I have never, as a career mechanic, on plenty of Cessnas, ever encountered one with only one nozzle. Where did you see one?

View attachment 95847

From the Lyc parts manual for the O-320 series, page 124 of the .pdf: https://www.lycoming.com/sites/default/files/O-IO-LIO-320 Parts Catalog PC-103.pdf
Click to expand...

Just do a Google search for O-320 with one cylinder primed and you will find discussion on it. http://www.cessna172club.com/forum/ubbthreads.phpubb=showflat&Number=228138
The 172s with 0-320-H2AD only had 2 primed from the factory.

Edit: Looked online at some old parts manuals. The Cessna 175 serial number 55001-56672 has one primer. Page 274. http://www.aeroelectric.com/Referen...essna 172 & 175 Parts Catalog (1956-1962).pdf
 
Last edited:
Clip4 said:
Just do a Google search for O-320 with one cylinder primed and you will find discussion on it.
Click to expand...
So I did that, and I could only find one guy, with a 1968 Cessna Cardinal (177) who said his was a single-primer system. I looked up the '68-'77 177 parts catalog, and it shows only the three-cylinder priming system for all serial numbers. That means that someone in the last 53 years has taken two of the nozzles and lines out of the system. It does not mean that the airplane came with only one cylinder primed. The parts catalog, as issued by Cessna, is an FAA-approved document and is used to confirm that systems are correct.

https://www.mountpoint.org/N3212T/CessnaCardinalN3212T/Manuals/Cessna177PartsCatalog68-76.PDF

Pages 432 and 433 of the .pdf.

The 177 uses the same O-320-E2D that the 172s used, and has the three-line system as per Lycoming catalog.

What one sees out in the field is not always what is supposed to be there. I have found plenty of things missing or other things substituted for the correct part, and that constituted an airworthiness violation if there is no STC for it and it's classified as a major mod.

One primed cylinder would make starting a real pain.
 
Palmpilot said:
Regarding the statement below, since fuel-injected engines presumably don't have a carb airbox, is there another place where the fuel from priming is likely to drip out and catch fire?
Click to expand...
There's a drain at whatever low spot there is in the induction system. In the 172SP, that's in the air filter housing. Some engines have drain lines off the intake runners to drain the fuel out through a tube at the firewall so it doesn't make a mess and fire hazard in the cowling.
 
Dan Thomas said:
So I did that, and I could only find one guy, with a 1968 Cessna Cardinal (177) who said his was a single-primer system. I looked up the '68-'77 177 parts catalog, and it shows only the three-cylinder priming system for all serial numbers. That means that someone in the last 53 years has taken two of the nozzles and lines out of the system. It does not mean that the airplane came with only one cylinder primed. The parts catalog, as issued by Cessna, is an FAA-approved document and is used to confirm that systems are correct.

https://www.mountpoint.org/N3212T/CessnaCardinalN3212T/Manuals/Cessna177PartsCatalog68-76.PDF

Pages 432 and 433 of the .pdf.

The 177 uses the same O-320-E2D that the 172s used, and has the three-line system as per Lycoming catalog.

What one sees out in the field is not always what is supposed to be there. I have found plenty of things missing or other things substituted for the correct part, and that constituted an airworthiness violation if there is no STC for it and it's classified as a major mod.

One primed cylinder would make starting a real pain.
Click to expand...

I posted you a Cessna parts manual link with the page showing the diagram. Granted it would be a pain.
 
Briar Rabbit said:
I have seen one airplane that started better when the primer was pushed in during rotation by the starter instead of beforehand.
Click to expand...

That's the only way my o-300 will start. First flight of the day, pull primer out, start turning motor over as you're pushing the primer in. If you do it that way, it starts quicker than my fuel injected pickup does.
 
Clip4 said:
Edit: Looked online at some old parts manuals. The Cessna 175 serial number 55001-56672 has one primer. Page 274. http://www.aeroelectric.com/Reference_Docs/Cessna/cessna-maintenance-manuals/Cessna 172 & 175 Parts Catalog (1956-1962).pdf
Click to expand...
Yes, it has one primer, in the intake manifold upstream of several cylinders. Continental did that. The 175 used the Continental GO-300. If you look closely at the picture, that line doesn't go the the region of the cylinders; it runs down under the engine to the intake manifold on one side right above the carb.

The O-200, like all the small Continentals, is similar. It has a four-way manifold with ONE primer nozzle in it that feeds all four cylinders.

upload_2021-4-26_21-16-13.png
 
One thing I've noticed is that no matter how I prime the engine, it won't start if I don't turn on the mags! :redface:

(This was in a Citabria, whose starter and mag switches are not integral.)
 
Once in the dark, I started up an Archer just to taxi to a fuel pump and didn’t get the primer locked in before starting. It ran...but poorly.

Pump right before start or maybe during if you’re coordinated enough. I can’t think of a good reason to do anything else.
 
You must log in or register to reply here.
Back
Top