Cold Carburetor??

[mike7a10]

I have been fighting a problem with starting my engine. This has been an ongoing issue for months now. I have rebuilt the carb, the timing is perfect, changed all of the ignition parts (dist. cap, rotor & wires as I have elec. ignition), changed the filters, the choke appears to be working fine and is fully engaged when in the closed position. I sprayed some carb cleaner around the carb when running and I don't find any air leaks...

So yesterday I felt the carb after the engine was running for a while and the carb was cold to the touch and I think it was sweating condensation.

In "googling" this issue 2 things popped up:

1. The gaskets on the carb may need to be changed
2. One thread mentioned the manifold intake tubes could be clogged w/ carbon

But then I read that the venturi carb naturally runs cold as the gas is vaporzing so fast in the throat that the carb is cold to the touch normally. I don't recall if I have ever touch the carb just below the manifold intake before.

BUT...yesterday I adjusted the idle mix to be very rich and it started up. I am going to check it again later today to see if this was the "fix".

So anyone ever noticed that your carb is very cold to the touch and having some condensation or do you think this is indicative of a leaking gasket?

[hanleyclifford]

If the carb works OK once the engine is running it would seem you have indeed done the fix by enriching the idle circuit. You may have a problem with fuel delivery from the tank which is making initial start difficult. Maybe an electric fuel pump with "R" terminal overide is in your future? The cold carb is quite normal.

[romantic comedy]

Carbs run very cold. Some even freeze up, even in the summer. It is about the drop in pressure causing a change in temperature.

[Ball Racing]

Also the humidity, water grains in the air and barometer will influence such activity.

[joe_db]

The cold carb is normal. I estimate mine is around 50 degrees when the engine compartment air temp is over 100. Airplanes have a carb heat control to send hot air into the carb when needed otherwise it would ice up. Old cars have something like this too, but usually automatic. I bet you could see ice in an A4 carb if it was run in an open boat with no enclosure to keep the intake air warm. In my experience a damp day around 33-40 degrees is the worst.

[JOHN COOKSON]

Originally Posted by mike7a10

I have been fighting a problem with starting my engine. This has been an ongoing issue for months now.

Hard starting when the engine is cold or warmed up? Or both?
It the engine hard starts when cold but starts much easier when warmed up a lean mixture is most likely the problem.

TRUE GRIT

[mike7a10]

Who knew? (Apparently many others but me!) As far as the hard start I think adjusting the idle screw to run more rich seems to have fixed the problem as it started right up today. Thanks for the good information on the cold carburetor.

Mike
S/V Plan Sea

[Dave Neptune]

Mike, richening the idle helped for the cold starting leads me to think that the choke is not actually closing "ALL THE WAY". I'd take a good look at it.

I have seen many an A-4 with idle issues due to this. The idle gets set to rich for an easy start and plug fouling during long idle periods becomes a problem. Remember the A-4 is an "updraft" carb and it is normal for this style to be a bit harder to start cold, thus the fussy choke adjustment.
My beastie will idle all day with no problem and often on warm engine re-starts the choke still helps get her lit quicker.

Dave Neptune

[joe_db]

+1
The choke has to go 100% of the way to work. Even a tiny bit of slack makes starting much harder.

[JOHN COOKSON]

Originally Posted by Dave Neptune

I have seen many an A-4 with idle issues due to this. The idle gets set to rich for an easy start and plug fouling during long idle periods becomes a problem. Dave Neptune

If a too rich and\or too fast carburetor setting is necessary to keep an A4 idling it means something is not exactly right in the fuel and\or the igniton system.
Said another way a too rich and\or too fast idle can cover up a lot of ills.

TRUE GRIT

[lat 64]

I once rebuilt a little english Ford V-6 2300 in an old Capri. I hot-rodded the the heads wit big valves and lots of grinding in the ports. the results were an engine that could really breathe!
I used the original Weber though and I think it was a tad small. Lots of velocity and pressure drop across the venturi.
I found that on cool mornings when I would be late for work, I would jump in the car and blast off down the road driving like a teenager. The car would consistently die after about a mile. After this happened one morning, I pulled off the air cleaner and saw the frost in the venturi melting away. It ran good after the manifold warmed up, so it was not a big problem. I eventually built a "stove" to run warm air to the intake ala airplane style.

Like an airplane pilot, I prefer to have the engine warm when pulling away from the dock. I even like to get the engine warm and shut it down so the carb can be warmed up from the manifold while I go about getting the boat ready.

Russ

[romantic comedy]

Pretty much every car with a carb has a pre heater hose that goes to the air filter to introduce warm air. Usually it gets the warm air from the exhaust heat.

Many cars have iced up and broke down because this hose was not in place.

[mike7a10]

I will drop the carb this weekend and get a good look at the choke as it seems like the discussion always comes back to the choke not being fully engaged. I will post what I find out.

As always thanks for the tips. It is greatly appreciated!


Mike

[hanleyclifford]

Originally Posted by romantic comedy

Pretty much every car with a carb has a pre heater hose that goes to the air filter to introduce warm air. Usually it gets the warm air from the exhaust heat.

Many cars have iced up and broke down because this hose was not in place.

Hmmm...how about a couple wraps of that copper tubing bringing some of that hot section heat to the carb?

[joe_db]

I doubt an A4 could get carb ice in an enclosed boat. My engine room temps are plenty warm even on a 20 degree day.

Originally Posted by hanleyclifford

Hmmm...how about a couple wraps of that copper tubing bringing some of that hot section heat to the carb?

[JOHN COOKSON]

Mike

You can always take the back flame arrestor off and try choking by holding your hand over the carb throat while you use a remote starter switch or someone else in the cockpit starts the engine. This should help narrow down whether you have a choke issue or not.

How do you have the choke adjusted? The movable arm needs to swing through its entire range and not hit the fixed arm or anything else such as the outer cover in the hold down bracket.

Also, is the poppet valve on the choke plate somehow not right?

TRUE GRIT

[lat 64]

Originally Posted by romantic comedy

Pretty much every car with a carb has a pre heater hose that goes to the air filter to introduce warm air. Usually it gets the warm air from the exhaust heat.

Many cars have iced up and broke down because this hose was not in place.

Yes, that was always my understanding. It could be mine was missing. There was no evidence of it on my car though. Not a trace. Often those things got dysfunctional long before the engine wore out. I restored quite a few to meet emissions inspections on the old junk I drove.
The two-stage Weber it had was really quite reliable and had a well-designed automatic choke.

As I wrote, I have the habit of warming up the engine on the boat and then turning it off to let everything warm up. This is because It typically shuts down and dies during the first few minutes while I am puttering about. I am wondering now if I am responding to something like carb ice.
I think the possibility is real. We have a cool harbor. It's can be 45 deg. in the morning some days. It never gets really warm down there until afternoon on a midsummer day. Also dampness is an issue too. I take many steps to ventilate to avoid a moldy boat.

You got me thinkin'

R.

[joe_db]

Take the flame arrestor off and look. 45 degrees and 60% humidity is about the worst possible combo for carb ice.
I know a damp 45 degree day caused so much carb ice in a Cessna 150 I had to taxi with it on* to make it to the runway without icing up

* carb heat bypasses the filter, so we normally do not use it on the ground.

[lat 64]

Originally Posted by joe_db

Take the flame arrestor off and look. 45 degrees and 60% humidity is about the worst possible combo for carb ice.
I know a damp 45 degree day caused so much carb ice in a Cessna 150 I had to taxi with it on* to make it to the runway without icing up

* carb heat bypasses the filter, so we normally do not use it on the ground.

I'm a couple of months out before I can do this check, but I can easily find carb-ice conditions this spring. I looked again at your chart in post# 5. It looks like I'm very often the blue zone at least. I am understanding that "glide" power settings is low manifold pressure? That is, a greater pressure drop across the venturi? That would be just like when I am running the engine at a high idle with no load at the slip.

I'm now excited to look into this. It fits the problem of shutdown I have been dealing with since I owned the boat.

I'll have to get my camera ready and try to make this happen so I can see if it's real or just my over-thinking it.

Good thread,

Russ

[joe_db]

Glide = low power. The bigger the pressure drop - i.e. less power and the throttle plate mostly closed - the bigger the temperature drop.
Your manifold pressure gauge would have a "high" reading if it is the Isspro or similar because it reads inches of vacuum and high number = low pressure
Airplane gauges are backwards and read absolute manifold pressure.

Originally Posted by lat 64

I'm a couple of months out before I can do this check, but I can easily find carb-ice conditions this spring. I looked again at your chart in post# 5. It looks like I'm very often the blue zone at least. I am understanding that "glide" power settings is low manifold pressure? That is, a greater pressure drop across the venturi? That would be just like when I am running the engine at a high idle with no load at the slip.

I'm now excited to look into this. It fits the problem of shutdown I have been dealing with since I owned the boat.

I'll have to get my camera ready and try to make this happen so I can see if it's real or just my over-thinking it.

Good thread,

Russ

[lat 64]

Originally Posted by joe_db

Glide = low power. The bigger the pressure drop - i.e. less power and the throttle plate mostly closed - the bigger the temperature drop.
Your manifold pressure gauge would have a "high" reading if it is the Isspro or similar because it reads inches of vacuum and high number = low pressure
Airplane gauges are backwards and read absolute manifold pressure.

Thanks, I think I have it right then.
Just the other day I was explaining Bernoulli's principle to a teenage girl. Finally made sense to her when I used the perfume atomizer example. Just find a person's point of reference and you'll get the ideas across.

You pilot's owe a lot to Bernoulli I think.

R.

[mike7a10]

OK...let me start by saying I'm not a mechanic. Having said that I have been pretty successful in keeping my old A4 alive and kicking including a total rebuild a few years ago with a lot of help from this website.

That being said...my hard start problem kept coming back to the choke as I had previously said. I dropped the carb as promised and everything seemed fine. I was about to reinstall it and I decided to pull out a few A4 manuals I have in the boat.

I looked, and looked (you get the picture) and suddenly it dawned on me that the choke butterfly valve looked wrong. I had installed the choke valve upside down with the top of the valve oriented "forward" (the poppet valve was at the "bottom").

After reinstalling it right-side-up (poppet valve "up") and oriented with the top of the valve oriented towards the back of the carb, the motor started right up.

Installing the choke valve backwards had caused the spring to pull the choke shut. Now the spring pulls the choke to open.

I tested it several times after the "fix" and it is running great.

I was overjoyed and feeling pretty dumb at the same time. I seem to be getting used to this feeling as an A4 owner... duh! So thank you again to everyone. Your collective advice helped me to figure it out.

Mike
S/V Plan Sea

[hanleyclifford]

You got a lot of company - the older I get the more often it happens. Sign in shop: "DOLTO" (Don't overlook the obvious).

[lat 64]

Nice work,

I voted a green thumb 'zup on your last post. This is not an obvious repair. I can see myself doing the same thing. (I know, that's not saying much)

Russ