L84 fuel percolation

[John D.]

Re: L84 fuel percolation

Holstein, Yeah and I bet you are running 87 octane too. Meanwhile I plan on getting my 63, the LWC out of the garage and starting it up as a fathers day present to myself. Then plan on driving it around the development. No legal plates on the car though.
But first I am gonna fill the tank with 100LL. Maybe put some good lead additive in it also. Yeah next week. JD

[Dan H.]

Re: L84 fuel percolation

Holstein, Yeah and I bet you are running 87 octane too. Meanwhile I plan on getting my 63, the LWC out of the garage and starting it up as a fathers day present to myself. Then plan on driving it around the development. No legal plates on the car though.
But first I am gonna fill the tank with 100LL. Maybe put some good lead additive in it also. Yeah next week. JD

JD, good to see you're firing up LWC, wish I had the luxury of some of the good gas, can maybe get 5 gals but that will be a short thrill! The weather is cool now, 64 is running sooo nice! Will be driving up to San Jose next month, will be 85+++ so it's 'gurgle' time for the fuely boys!
Dan

[Steven B.]

Re: L84 fuel percolation

'had same problem a long time ago with the old 469. I was a line boy then and used 20/80 mixture of av. and Sunoco. It worked but got expensive then so I reduced the mixture and used a woven tubing insulation material on all fuel feeds and it worked. The material was about 1/8 inch thick, looks like cotton with a plastic or tar coating.

I guess a cool can would help, also.

[John D.]

Re: L84 fuel percolation

Crack the hood open like we did in 1965. Ran around with the hood shaking to help let the heat out.Never had the hood latched around town in the old days. Best thing to do is take the hood off period. Like an ex contributor here said. Where is Dennis(dipstick) Meanwhile I don't know how you guys can stand riding in a midyear coupe with 80 plus degree weather anyhow. Unless you have AC. Course I hate the heat. My shop temp is 67 right now. JD

[Jim L.]

Re: L84 fuel percolation

That seems like a easy way to overcome the problem, but, don't you need engine vacuum to make the Rochester FI operate. Is it some kind of an evil loop. You can't put the engine under load with no fuel, and you can't get fuel with no engine vacuum.

Engine vacuum is important at idle. A metered amount of manifold vacuum is applied to the main diaphragm, forcing the fuel meter to steer fuel to the nozzles.

Off idle, venturi vacuum, which is unrelated to manifold vacuum takes over and provides the signal to the main diaphragm.

The transition from manifold vacuum to venturi vacuum happens seamlessly when you open the throttle. There is never a moment when the vacuum signal on the main diaphragm goes to zero.

So, once the engine is rotating, open the throttle; that forces the spill valve to close and this increases the fuel line pressure. Any time fuel line pressure exceeds the vapor pressure of the fuel, the boiling stops.

Jim

[Stuart F.]

Re: L84 fuel percolation

The heat problem in the coupe is the main reason why I opted not to buy it and got the convertible instead. My buddy and I test drove two of them in the early spring of 63 down in El Paso, Texas. It was already hitting 90 in the desert SW and that was enough to convince me. And to be honest, other than economic reasons, I don't regret that decision to this day.

My car has been sitting for about 3 weeks now as we'd been out traveling, but I intend to fire it up tomorrow. In that period of time, we've entered into the hot/wet season here in Florida. Due to the humidity, I will do a complete preliminary maintenance check including the brake fluid. I at least plan to suck out most of what is in the master and replace it with fresh. I also have to crawl under it while on ramps to re-wire my heat riser open. I changed the oil not long ago so that should be good to go. Then comes a fuel fill of 91 octane Mobil along with the only thing I have access to and that is Max Lead 2000. Works for me, specially since I got my AFB singing on our crappy fuel months ago.

John, I'm still looking for the original boxes for the emblems. I use several around the house with my 1/24th display collection, but I don't usually throw anything away.

Stu Fox

[Clem Z.]

Re: L84 fuel percolation

found this on another website. Be sure to check these installations for any of the small nozzle lines touching the manifold, this will cause the fuel to boil on the way to the nozzles and result in a very erratic idle, due to the heat. Often times when a manifold is removed and installed, in order to get the socket wrench on properly, the lines are bent or moved over against the manifold for clearance. Bend the lines back and away from the manifold about 1/8 inch or so.

[Michael G.]

Re: L84 fuel percolation

I think I need to accept the fact that if I'm going to drive my car at all, I can't count on always finding Avfuel or racing fuel when I need it. That means that its likely that the car may have to run on pump gas sometimes - even if I try to use racing fuel most of the time.

In that direction, I've got a bunch of stuff on order to try and insulate the spider while actively cooling it. Without going into too much detail, my intent is to insulate each leg of the spider individually with a high-temperature, reflective insulating sleeve. Also in each sleeve will be a looped copper tube which carries coolant from a trans cooler rigged with a pump, fan and adjustable thermostatic switch.

Hopefully, I can find enough battery power to run the pump and fan while the car is off - at least until the engine cools somewhat. We'll see.

Mike

[James J.]

Re: L84 fuel percolation

I have a basic understanding of the operations and I agree in part with what you have said but, I believe that the spill ports opening and closing is controlled by the ratio lever which is directly controlled by engine vacuum.
I know that on a hot day, on a hot restart with ethenol pump gas, my engine just chugs and stalls and the throttle has no affect at all.
No engine vacuum, spill port is open, no high fuel pressure and the engine runs like crap until cooler fuel enters the system and allows the engine to smooth out and produce vacuum.

[Jim L.]

Re: L84 fuel percolation

I have a basic understanding of the operations and I agree in part with what you have said but, I believe that the spill ports opening and closing is controlled by the ratio lever which is directly controlled by engine vacuum.

The position of the ratio lever determines the mechanical advantage of the force of the main diaphragm over the force created by fuel pressure under the spill valve. With high mechanical advantage, ratio lever on the power stop, you get more fuel flow. Low mechanical advantage, ratio lever on the economy stop, you get less fuel flow. The position of the ratio lever is determined by manifold vacuum which is an indicator of engine load. Low manifold vacuum signifies high engine load which needs a richer mixture, and vice versa.

However, the position of the spill valve (or spill plunger if you have an early unit) is determined by the balancing act of vacuum applied to the main diaphragm working against the fuel force trying to lift the spill valve. This balancing act is a fundamental design principle of every Rochester FI unit.

I know that on a hot day, on a hot restart with ethenol pump gas, my engine just chugs and stalls and the throttle has no affect at all.
No engine vacuum, spill port is open, no high fuel pressure and the engine runs like crap until cooler fuel enters the system and allows the engine to smooth out and produce vacuum.

Where do you live and how hot is hot? When you shut off your car's engine, open the hood and leave it open to let heat escape. This will do wonders for your ability to refire the engine.

Jim

[James J.]

Re: L84 fuel percolation

Jim,
Thanks for the great discussion on this subject.
I had my understanding of the ratio lever backwards and your comments have made me reread the basics.
I live in New Hampshire where hot is not that hot, any tempatures in the high 70's and above have a negative affect.
The last time that I had the heat soak/percolation or what ever problem, we were at the ice cream stand. I did open the hood when it would not run, but it took approx. 45 minutes to cool enough that it would run.
This thread began as a question as to where the percolation take place and I thought I had it all figured out. Guess I was wrong.
Wherever the percolation takes place I have found that racing fuel solves the problem for me.

[Bill B.]

Re: L84 fuel percolation

I'm working on a solution, have it on paper will get back to you after I get the bugs out of my prototype. That should be around July/Aug and I'm located in Florida....So if it works down here with the high 90's, it should work just about anywhere! I'm not revealing anything else, just be patient and hope there are one or two others with a different solution if this one fails.


Bill


1961 Black/Silver/Red Int. 283/315 FI
power windows. Frame off 65% completed.
1989 White/hard top/Grey Int. daily driver 167,000 miles

"The problem with Liberalism is that eventually you run out of other people's money" Margaret Thatcher

[Stuart F.]

Re: L84 fuel percolation

Well, I really empathize with you L-84 owners. I just had my L-76 out in our 93* Central Florida heat to gas it up for the week and do a little top down sweating. It runs like a top. Not to gloat, but all the changes I made to the AFB carb, intake/exhaust system has really paid off. I sat at a number of stop lights, got in some stop and go traffic, then shut down to mix my brew at the Mobil station (93 Octane and Max 2000). It started right up and never missed a beat, no stumbles, no hesitations,etc. I even saw the temp gauge creep above 160* a few times. I'm very pleased with it. Guess I have to go back and document my changes. I still have to re-wire my heat riser back open to help with the 95+ temps coming soon.

Stu Fox

[Jim L.]

Re: L84 fuel percolation

I live in New Hampshire where hot is not that hot, any tempatures in the high 70's and above have a negative affect.
The last time that I had the heat soak/percolation or what ever problem, we were at the ice cream stand. I did open the hood when it would not run, but it took approx. 45 minutes to cool enough that it would run.

James, what you describe is consistent with a failure of the anti-siphon mechanism.

If you are opening the hood and letting the engine cool for as little as 10 minutes, it will re-fire instantly and idle smoothly without you having to do anything special. Even if you don't open the hood, as long as you go to wide open throttle before starting, the engine will instantly re-fire.

On the other hand, if the anti-siphon mechanism fails, the unit will drip fuel into the engine and flood it. Now THAT can be difficult to clear out if the engine is hot.

How about at the other end of the temperature spectrum.... when your car's engine has been off for, say, a couple of days and is cold. How quickly does it start? If it starts instantly, that's a further clue the unit is a "leaker". Normal behavior would be for the engine to turn over 8 - 10 revolutions before it catches.

Jim

[Dan H.]

Re: L84 fuel percolation

James, what you describe is consistent with a failure of the anti-siphon mechanism.

If you are opening the hood and letting the engine cool for as little as 10 minutes, it will re-fire instantly and idle smoothly without you having to do anything special. Even if you don't open the hood, as long as you go to wide open throttle before starting, the engine will instantly re-fire.

On the other hand, if the anti-siphon mechanism fails, the unit will drip fuel into the engine and flood it. Now THAT can be difficult to clear out if the engine is hot.

How about at the other end of the temperature spectrum.... when your car's engine has been off for, say, a couple of days and is cold, how quickly does it start? If it starts instantly, that's a further clue the unit is a "leaker". Normal behavior would be for the engine to turn over 8 - 10 revolutions before it catches.

Jim

Jim, on my 64, I replaced the anti siphon valve and the spider with the 'improved' version, it starts hot or cold, just a little rough when very hot after sitting. Does the 'spider' valve shut off fuel when the engine is shut down? Only 'loose' fuel would be the tiny amount in the spider itself to 'cook off'? When the 380 unit starts, it gets a shot of fuel from the main fuel pump which then refills the spider with slightly pressurized fuel to help get rid of the boiling residual fuel in the spiders. Seems like an insulating wrap to shield the spider lines would be a step in the right direction, just look ugly!
Dan