L84 fuel percolation

[Michael G.]

Is the actual fuel percolation in an L84 engine taking place in the spider tubing? Can I eliminate it by cooling the spider? (I've already tried 100 Low lead aviation fuel without success.

[Bill B.]

Re: L84 fuel percolation

Michael - Check previous thread yesterday 10% Ethanol & Percolating Fuel??? & a must read Sept 21, 2008 Clem Zahrobsky thread: "Keeping Fuel Injection Cooler" this should give you some good ideas...

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

[Michael G.]

Re: L84 fuel percolation

I read most of the postings regarding percolation and there are a lot of good ideas there, but I still wonder if anyone has ever determined if the problem is limited to percolation within the spider tubes, or whether its also an issue within the fuel meter or fuel lines from the filter?

Thanks, Mike

[James G.]

Re: L84 fuel percolation

John DeGregory and Jerry Bramlett have written a lot on this subject. 108 leaded racing fuel is the best fix. Car will run 15 degrees cooler, idel better and has gobs of power like the old days. Hard to carry it around.

a 25-50% mixture will work well.

[Tom P.]

Re: L84 fuel percolation

The modified FI unit on the SB400 in my 56, is for the most part, made up of 62 components (air meter, fuel meter, spider and nozzle lines). The plenum is an early finned top (probably about a 4800) which was cut in half by Bill Thomas about 1961 (between the center fins), hogged out and welded back together. As I mentioned in another discussion, I've been having increasing problems with idle-very low speed rpm. I've recently put in 10gal of 100-110LL Avgax instead of race gas because its about half the price/gal. For all practical purposes, ALL the problems have gone away. Just recently, I filled it up with 91 octane (highest level available at pumps in Okla) NON-ETHANOL and some of the problems have returned!!!! Saturday, when I go out to the airport to work on our bomber, I'm going to buy some more Avgas and see if the problems go away. If so (which I expect), then that will more or less confirm that the problem is in fact today's poor quality gas!

Now, is the percolation occuring in the FM, spider, nozzle lines, nozzles, or where?? I don't know!! But the initial use of race gas and then Avgas seemed to cure the problem. The last time I had almost 100% Avgas in the 56, I let it sit in the driveway for 15-20min, just idling with the hood closed and it never hinted that it was going to have any of the previous idling problems. So, I'll repeat the test this weekend.

[Stuart F.]

Re: L84 fuel percolation

I seem to recall my 58 Fuelie engine that I had in my 50 coupe always was tempermental in the heat of the summer, even back in 1958 using Sunoco blue straight (101 or 103 octane ?). I always thought it was the nature of the beast. In traffic, it would stumble and idle poorly even back then. My overview of threads on the subject today tells me that the newer series units (63 thru 65) were greatly improved, but still a problem now more likely related to the gas.

I never made any changes or mods on that 58 unit to try to improve it like I have done over the years with carbs. Guess I was in awe of it and was satisfied to just hear it whistle with that 57 open element air cleaner I had on it. Used to scare the commandos to death!

Stu Fox

[Jim L.]

Re: L84 fuel percolation

Now, is the percolation occuring in the FM, spider, nozzle lines, nozzles, or where??

Here's an experiment that helps answer the question: When the engine is running rough, start driving (traffic conditions permitting, of course) and give reasonable throttle. The engine will immediately smooth out and run well.

Why?

Because the fuel in the lines between the fuel meter and the nozzles is now pressurized which raises its boiling point to something above that of its current temperature.

On a really hot day, you can do the opposite experiment and induce percolation: While coasting down a slight grade at modest speed, lift your foot off the throttle. If there is enough engine heat under hood, fuel will percolate and you can feel the engine cut off and on randomly.

No, neither experiment solves the problem. Taken together they just reveal where it is.

Jim

[Tom P.]

Re: L84 fuel percolation

Here's an experiment that helps answer the question: When the engine is running rough, start driving (traffic conditions permitting, of course) and give reasonable throttle. The engine will immediately smooth out and run well.

Why?

Because the fuel in the lines between the fuel meter and the nozzles is now pressurized which raises its boiling point to something above that of its current temperature.

On a really hot day, you can do the opposite experiment and induce percolation: While coasting down a slight grade at modest speed, lift your foot off the throttle. If there is enough engine heat under hood, fuel will percolate and you can feel the engine cut off and on randomly.

No, neither experiment solves the problem. Taken together they just reveal where it is.

Jim

Jim,
That's EXACTLY what my unit does on today's pump gas--------------but not on the Avgas!

[Michael G.]

Re: L84 fuel percolation

With 100 LL AVgas, I don't really have a problem with the car unless I've driven it a while and turn it off. If I re-start it immediately, there is still no problem, but, if I turn it off and let it sit for a few minutes, it is hard to start. When it cools off a bit, it starts immediately again.

I assume this is a result of "heat-soaking" the spider tubes when the engine is off, so, I'm going to try insulate the tubes and also actively cool them when the engine temp is above 140 F (with the engine off). I'm rigging up a system to do this. This may not work if the fuel in the fuel meter (or other lines) also needs to be cooled, but, we'll see if percolation in the tubes really is the big problem.

If it is effective, I'll let you guys know how I did it (I'd patent it, but there's not enough fuelies out there to cover the legal fees)

Thanks, Mike

[Stuart F.]

Re: L84 fuel percolation

I have reason to believe that some form of insulation could be used effectively based on reverse experience I had with my 58 engine during the winter; I used to throw an old wool army blanket over the engine to keep the Injection unit warm, or at least from freezing. It would start at 0 degrees or above, but not if it was say 2 degrees below. At home in a detached garage, I used a heat lamp. They sure are fussy buggers.

Stu Fox

[Jim L.]

Re: L84 fuel percolation

I assume this is a result of "heat-soaking" the spider tubes when the engine is off, so, I'm going to try insulate the tubes and also actively cool them when the engine temp is above 140 F (with the engine off). I'm rigging up a system to do this. This may not work if the fuel in the fuel meter (or other lines) also needs to be cooled, but, we'll see if percolation in the tubes really is the big problem.

I'm very curious about what you are planning to do for active cooling. About the only thing I can imagine is some form of Peltier device. Is that what you'll be using?

Jim

[John H.]

Re: L84 fuel percolation

I'm very curious about what you are planning to do for active cooling. About the only thing I can imagine is some form of Peltier device. Is that what you'll be using?

Jim

Jim -

Peltier technology is fascinating - my '08 Dodge Avenger R/T and my wife's '08 Sebring Limited both have the heated/cooled cupholder well in the console, and that's a Peltier device; push the rocker switch one way and it cools (blue light), push it the other way and it heats (red light). Yes, they work, but it's most effective when the O.D. of the cup/bottle closely matches the I.D. of the well.

[Michael G.]

Re: L84 fuel percolation

Hi Jim,

Hadn't really considered a Peltier device, but, from what little I know about such devices, the power requirements might be a bit much for a 63 Vette electical system to handle. There's also the issue of what to do with all that heat and/or where to put the heat sink. Maybe after I get an electical/chemical engineering degree

My idea's much more basic, I'll let you know if it works.

Mike

[Jim L.]

Re: L84 fuel percolation

The Peltier devices certainly are inexpensive enough. On line I found a 300 Watt device for $25. It was far from unique. There were several with that capacity in that price range.

It looks to me like the "interesting" challenge in adapting one to the spider and nozzle lines of an FI unit would be managing the heat given off by the hot side, which is adjacent to the chilled side. Quite a packaging problem.

Jim

[Jim L.]

Re: L84 fuel percolation

My idea's much more basic, I'll let you know if it works.

Mike

Mike,

Please do. This is a problem that holds my attention. If you solve it without resorting to expensive fuels you'll have the gratitude of every owner of a fuelie that is actually driven.

Jim