Trying to figure this out about carburetors. Fuel from acc pump at start up..........October 8 2017 at 3:15 PM
Mike U. (Login mtrain)
Since the engine isn't yet running the first shot of fuel will be from the accelerator pump [or pumps if double pumper].
So, this got me to thinking as I had a bitch of a time trying to fire up the 428FE in the old Cobra this Friday.
When you hit the throttle, and the fuel is shot from the acc pump, where does it go? Obviously down into the engine, but since its pretty much a raw shot of fuel I'm guessing it [for a single plane intake] hits the floor of the manifold, then runs down to a "few of the cylinders".
I say, a few cylinders, since I can't see any way that shot, or shots of fuel could evenly be distributed to all eight cylinders.
My point is, if you're dealing with an engine that is cold nature'd, or hard to start, then wouldn't it be best to crank the engine until the fuel bowls are full, then with the starter not spinning, pump more than a few shot of fuel into the engine so that the few cylinders that DO get the fuel are somewhat primed? Then turn the engine over again until it fires?
Sorry, but the powers been out the whole day until just now so I've had lots of time to think.
Re: cold start,
|October 8 2017, 3:35 PM |
In my opinion, it's not how many cyl' get fuel, but how much of a rich fuel mixture you get . A cold engine does not have good atomization so the choke restricts air so the rich mix can ignite. EFI is similar but it triples the pulse width of the injectors to obtain a rich cold start mixture. The acc pump does give that first shot of fuel, providing there is fuel in the carb. If not, the pump has to get it there. Don't mean to sound elementary but that's how I could explain it. DR
|This message has been edited by gdr45 on Oct 8, 2017 3:47 PM|
Re: Trying to figure this out about carburetors. Fuel from acc pump at start up..........
|October 8 2017, 3:52 PM |
The fuel sits on the floor of the intake and evaporates into the oncoming air stream.
Fuel doesn't really leak into the cylinders so much. Heck even when running a good bit of fuel is simply sliding around on the intake floor, kinda oscillating in the pulses.
Cranking, the engine isn't producing enough of a signal to the idle circuit of the carburetor to properly pull the fuel mixture, yes it gets some, but a cold engine needs to be pretty rich in order to fire.
Same reason you have a choke plate, the air restriction under it causes a greater signal, thus it pulls fuel more easily.
Due to the design, certain carburetors are easier to start cold than others.
I consider the Autolite carburetors to be pretty much the easiest to start, seems I can leave one for 6 months, crank it up and the dang thing is running.
Fuel suspended intake trakt
|October 9 2017, 1:58 AM |
|October 8 2017, 3:55 PM |
with holley carbs you know it is tuned right when you give the engine two or 3 shots worth of gas of the accelerator pump wait 10 seconds, fire the engine and it should fire up just stumble a bit then catch and stay at idle.
that will happen when it is tuned right. No throttle needed for cold start.
How to tune a holley right? everyone has a way to tune. everyone has different results.
Interesting, I've never thought to wait a few seconds after the acc pump shots...........
|October 8 2017, 6:03 PM |
On pretty much all of the carbureted vehicles I've owned I have always, and usually without much thought, given the throttle a pump or two before trying to start the engine.
Of course on a vehicle that has sat for a week, or longer, I've always cranked the engine to fill the bowls with fuel, then hit the throttle a few times.
However, I've never thought to just let the fuel sit on the floor of the intake, and vaporize.
I'm going to try this next week as I have to get the Cobra out of the garage to cut/install the side glass.
That is so true. Pump, then wait.
|October 9 2017, 4:29 AM |
It makes a huge difference. I give 3-4 pumps, wait about 10 seconds, turn the key, and it pops right off like EFI. It took me years to figure that out. If you pump then immediately crank it won't fire right away. Maybe you have to give the fuel a few seconds to vaporize in the cold intake?
(Select Login johnvermeersch501)
Im curious,why would the fuel bowls need
|October 9 2017, 6:58 AM |
to fill after a weeks sitting ?? Where would the fuel have gone too ? Accel pump works off the bottom of the bowl. In the absence of a choke, crank the motor with the throttle closed , after a few pumps, to create vacuum / cylinder pull in the intake
Re: Im curious,why would the fuel bowls need
|October 9 2017, 7:51 AM |
John, when I park my truck in the parking lot of the airport and leave for 4-6 weeks, I can assure you that the fuel evaporates over that time.
Certainly faster in the summer when that asphalt is well over 120degrees here in south Georgia.
No big deal, but it certainly makes an electric fuel pump a nice upgrade.
The gasoline around here evaps pretty quick......If the vehicle sits for approx a week,
|October 9 2017, 1:22 PM |
I did say about a week, but often its a bit more time than that. However, even after a week the fuel bowls have lots a bit of fuel due to evap.
When I rebuild a carb, I let it sit in the Berrymans for about a week, then move the main parts of the carb to a bucket of gasoline for cleaning off the acid [or whatever Berryman carb cleaner is made].
I can tell you that I've seen a 5 gallon bucket with a loose plastic lid, sitting full of gasoline, loose more than an inch of fuel over a three day period [again that is with an unsealed lid covering the raw gasoline].
Update: "Its dead Jim". Looks like the gas has lost its spunk......
|October 9 2017, 4:29 PM |
I tried to start the engine today, and observed the fuel going from the squirters into the engine, and waited a good ten seconds, or more, and not even an attempt was made by the cylinders to fire.
I was able to start the engine last week, but it was popping, and backfiring pretty good, so it looks like the fuel is too tired to fire [I know that a week shouldn't make that much difference, but I would bet that its the issue as I had a hard time starting, and keeping the engine running last week].
Too tired to fire?
|October 10 2017, 8:28 AM |
I have cars that sit all winter and fire right up. I'd suspect some other issue, either timing or spark. I drove the Falcon yesterday - dual carbs, sitting for about 3 weeks. Switch on the fuel pump, three pumps, lit right off.
Well today I squirted a little fresh gas down the carb, and still no workie. So,
|October 10 2017, 3:06 PM |
I pulled all of the plugs on one bank, and they were soaking wet with fuel.
I cleaned the plugs, and cooked them a while on the open flame of the stove top [wife is out shopping today].
Put the plugs back in, and still no start.
Now I'm really starting to get puzzled as the engine did fire, and run last week.
I checked for spark on a few random plugs, and all was well there.
Then I check compression on a few random cylinders, and compression was fine [no backfires last week so it really shoulnd't have jumped time, but now I know it not the timing].
The only thing I can think of is that the engine really flooded with bad fuel.
I pulled ALL of the plugs, cleaned them, and left them out for tonight so the fuel in the cylinders can evap.
I'll try to fire it again toward the end of the week.
However, I do believe gasoline is different in different parts of the US, and at different times of the year [ie winter gas], and I have personally ran into fuel going bad just sitting for a year. The last time I drove the Cobra the engine was starting to have symptoms of bad fuel. I just didn't have time to take it to the station, and add some fresh fuel at the time.
Before you reinstall the plugs..
|October 10 2017, 3:43 PM |
Squirt a few shots of engine oil into the cylinders, about a tablespoon's worth or slightly more and crank the engine without the plugs to work the oil into the rings. If the cylinders are gas washed the oil will increase compression somewhat and allow the engine to crank faster [less ring friction]. Been there...done that.
I will RJP. I had that problem on my very first hot rod. I remember the,
|October 11 2017, 3:49 PM |
I was just a kid at around 18 years old, and the engine was a real Boss 302 with a single Holley 750 dp carb on a tunnel ram.
As with my hot rods now, that car/engine would sit for a while until I could have time to play with it.
The compression was around 13:1 so the old Ford starter had a bit of a hard time cranking the engine.
Anyway, I tried to start the engine one day, and nothing. I was too young to figure it out other than I figured it was something truly horrible.
I had every mechanic we delivered to [I worked in autoparts at the time] come out to give it a shot. Nobody could figure it out as it had a Mallory HiFire box with a Mallory duel point dizzy [which was throwing everyone off with the problem].
Last, was an older mechanic my fathers age who told me to squirt a little oil in each spark plug hole, crank the engine without pumping the throttle, then squirt a little fresh fuel [back then the fuel lasted waaaaaaaaaaay longer before going bad] down the intake, and give it a try.
Well, as you guess, it fired right up!
Lesson learned prolonged starting attempts with a carbureted engine can wash down cylinder walls, rendering the piston rings ineffective.