L84 fuel percolation

[John D.]

Re: L84 fuel percolation

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

Tom, I agree with you 100%. Ditto. John

[John D.]

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

Mike, When you attempt to start the car when its hot make sure you smash the accel pedal to the floor and then hold the key in the start position until the car starts. Did you ever get a good cranking signal valve? John

[Michael G.]

Re: L84 fuel percolation

Hi John,

I tested all three CSV's I had, per your advice. The one from my engine passed the test, but, with the broken thread body I can't screw it in any more, so its useless. The replacement CSV I got back in January also passed, so, I put that one in and the engine still wouldn't start. I checked a few other things and found that the plugs got fouled when the first CSV broke. After I changed plugs, (only cut myself three times, a new record ) the replacement worked and the car started.

I also tested the original (crimped) CSV I found at the swap meet, but it failed the test. I'm gonna send that one to you as a trade-in on a spare.

Regarding my current hot-starting issue:

Thanks for the advice. Per your earlier instructions, that's what I've been doing. That procedure works fine on a hot engine, but, if the engine has been sitting turned-off for more than a few minutes it doesn't want to start until it cools off some.

We don't have any racing fuel nearby, so i want avoid that solution, that's why I'm experimenting with the cooling the spider tubes.

Thanks, Mike

[Paul Y.]

Re: L84 fuel percolation

I'm with John about the cranking signal valve. If your's is not functioning properly, the fuel might be blamed instead. I am driving mine, a 63 with FI, almost daily and I have very little of the simptoms decribed here. Maybe being mile high helps. The fuel is less than desireable, but I am not experiencing a lot more problem with hot start than I did twenty to thirty years ago when I drove it daily. I had my spider insulated then also, but it was a piece of asbestos wrapped in multiple layers of tin foil. The second skin insulation looks like it would work better that I am using now. I suggest that you test your cranking signal valve. I rebuilt my own 20 some years ago and it still works fine. My injection has a fresh restoration as of a month ago. It runs like it should and I am enjoying it immensely. I am finding that my FI works ok on the pump fuel I use in it. I just thought of another possible issue. I haven't had a tank of fuel in it for more than a week before it is used up. Maybe if it wasn't driven, I would be having more problems. Would Stable Stor work on a trailer queen? I'll keep on monitoring the problems as I am interested. Paul

[Dan H.]

Re: L84 fuel percolation

Tom, I agree with you 100%. Ditto. John

JD, my friend, this is not a solution for those of us who actually drive hundreds of miles and cannot buy Avgas, which is illegal for road use, into our cars! maybe a cooler full of dry ice is better!
Dan

[Jerry G.]

Re: L84 fuel percolation

Dan, you are correct. my 65 FI does well on race gas but it's hard to strap a barrel of the stuff on the back for long trips. i'm working on my 61 BB car and I want to use it for road touring in events like the Colorado Grand or the Copperstate 1000 so i need a better solution. My exposure to road race cars has introduced me to a very interesting product. it is a gold foil heat reflective insulation. You can see it at Porterfield-brakes.com I'm giving serious though to laying a layer of this between the intake manifold and the fuel spiders. What do you guys thinK? Jerry

[Tom P.]

Re: L84 fuel percolation

Clearly, this is an issue that can go on forever.
Insulating the fuel spider certaily may help. But the whole issue SEEMS to be resultant from today's gas formulas and not heating issues, which was not an issue when these cars were built. My FI unit is now virtually brand new and dialed in very well (as best as possible without the use of a dyno/exhaust analyzer). And the ONLY time I experience idling/low speed problems is with pump gas. Race/Avgas seems to adequately resolve the problems. Also, as implied above by one person, no, I'm not so stupid that I don't realize that not everyone has access to Avgas and that that is not a practical solution for many people. Also, I realize that Avgas is not leagal for road use. This is the 2-3rd time this has been brought up, so now it clearly comes across as a semi-threat that someone may choose to notify authorities now that they have learned of someone using 2-3 tankfulls of Avgas to try to diagnose a problem. If that's your intent, go right ahead. That's just another thorn in my side regarding the arogant, premadonna attitude of some ot the aspects of the NCRS organization.
I fully realize that the purpose and intent of NCRS is to restore and maintain these cars as close to original configuration as possible. But sometimes, it become a real challenge to do so. And one of the things that I've done to improve the operation of my FI unit is to substitute an electric solenoid valve for the cranking signal valve. It's virtually an absolute cure for the problems of a delicate cranking signal valve.

[James G.]

Re: L84 fuel percolation

Dan, you are correct. my 65 FI does well on race gas but it's hard to strap a barrel of the stuff on the back for long trips. i'm working on my 61 BB car and I want to use it for road touring in events like the Colorado Grand or the Copperstate 1000 so i need a better solution. My exposure to road race cars has introduced me to a very interesting product. it is a gold foil heat reflective insulation. You can see it at Porterfield-brakes.com I'm giving serious though to laying a layer of this between the intake manifold and the fuel spiders. What do you guys thinK? Jerry

I like the idea Jerry. Look forward to hearing about your result.

Every FI car I ever owned, (over 15) had it's own personel little quirks. Some started right up. Others would start and then die........start and die. Some would never start when hot. In each case it took time to diagnoise the problems. Engine compression, ignition point wear. Distributor oiling problem where the lower seal went bad. In fact, 90% of any fuel injection problem I ever had was distributor related.....not the unit........not the CVS. The other 10% was engine wear issues. Once the injector was rebuilt it would run forever.

MY POINT HERE IS WHAT IS THE REST OF THE ENGINE PACKAGE ?
Often engines are rebuilt using different camshafts, flat top pistons, modern head designs. All neat stuff, but item that change the Rochester Fuel Injection operation.

It all goes back to 14 parts air, one part fuel. Stoemetric. If an item affects that, then the engine runs badly.

When was the last time you adjusted your solid lifters? Or don't you use them any more?

Remember Rochester built 29 different model units for the Corvette from 1957 thru 1965. In 57 and 58 and 59 they had 5 different units each year for different engine calibrations. And they had three different nozzels alone for different applications.

Gasoline has changed greatly since 1957-65 days. But there are a whole lot of other items to consider about poor operation. And this forumn gives us all a chance to share opinions, but each will refer to that persons individual engine package and knowledge of how to fix it.

[James J.]

Re: L84 fuel percolation

As far as I know, my engine is all original, no signs of it ever being rebuilt or even out of the car. When the tempatures are in the 70's and below the FI runs great on pump gas with no problems starting, restarting or idling. Once the tempatures go into the 80's and above, restarting and idling are a real problem. Upon a hot restart it just sits and chugs for a while, seemingly until the cooler fuel from the tank eventually reaches the FI unit.
The race fuel that I purchase in the heat of the summer contains no ethenol and all of my problems clear up.
To me, it seems apparent that the lower boiling point of the addition of ethenol causes the fuel in the spider to evaporate and is the main problem.
I had the FI rebuilt by Jerry Bramlett just last year and it runs like a top on pump gas, but only in the cooler weather.

[Dan H.]

Re: L84 fuel percolation

Dan, you are correct. my 65 FI does well on race gas but it's hard to strap a barrel of the stuff on the back for long trips. i'm working on my 61 BB car and I want to use it for road touring in events like the Colorado Grand or the Copperstate 1000 so i need a better solution. My exposure to road race cars has introduced me to a very interesting product. it is a gold foil heat reflective insulation. You can see it at Porterfield-brakes.com I'm giving serious though to laying a layer of this between the intake manifold and the fuel spiders. What do you guys thinK? Jerry

Great idea Jerry, many years ago we used to cut /notch table hot pads for cooking pots and put them on top of the FI adapter plate to insulate the spiders better. Sort of a poor mans solid base gasket. Tom's use of the electric CSV is also a excellent idea, along with making sure everything is done to ensure all is well with the FI unit etc. Having been stuck in stop and go LA traffic and running on 4 cylinders is no FUN! I changed to a 160 thermostat and added 2 quarts of 'water wetter' coolant which has so far prevented a similar situation. When I bought my car 5 years ago it had 1/2 tank of avgas in it. It was the best it ever ran, when I put my 1st tank of California gas in it, it was awful. No amount of adjusting etc. would fix the rough running. Have been doing little things ever since and it runs very well now on our tree hugger gas. Runs much better when we fill in Oregon or Navada though. Let us know any other helpful hints, they add up and help!
Dan

[Jim L.]

Re: L84 fuel percolation

To me, it seems apparent that the lower boiling point of the addition of ethenol causes the fuel in the spider to evaporate and is the main problem.

Not to split hairs or to put too fine a point on it, but the main problem is a design issue with Rochester FI: At idle, it's a low pressure system that readily allows fuel to percolate at normal engine temperatures. Idle fuel pressure is approximately .1 PSIG, clearly not high enough to prevent the fuel from boiling.

Find a way to increase the fuel pressure above the vapor pressure of the fuel, and percolation will stop.

Jim

[James J.]

Re: L84 fuel percolation

Unfortunatly there is no fuel pressure, high or low, when the engine is off and the hot intake manifold boils the ethanol/fuel from the spider, making restart a real problem.

[Jim L.]

Re: L84 fuel percolation

Unfortunatly there is no fuel pressure, high or low, when the engine is off and the hot intake manifold boils the ethanol/fuel from the spider, making restart a real problem.

True enough. However, if, at the moment the starter energizes, the fuel were to become pressurized (like in a modern EFI system), the boiling would instantly stop.

This is easy enough to verify with the experiment I described previously: As soon as the engine starts, regardless of how roughly it's running due to fuel percolation, put it under load. That will drastically increase the fuel pressure and the percolation ceases.

Owners of 250 HP/PG cars are actually lucky. If they can train themselves to left foot brake, then they can stop fuel boiling by simply holding the brake and applying slight throttle while in gear.

Jim

[James J.]

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.

[Dan H.]

Re: L84 fuel percolation

Jim, my friends 250 HP 57 would actually boil the fuel in the fuel bowl, you could hear it gurgling. Got a solid base gasket from JD and it really helped, no more gurgling. Still have to floor it to restart when hot but it does 'clear out' nicely now.
Dan