Insulating Fuel Distribution Lines

[KiloWhiskey1]

I hate problems like this. Hard to figure out why it's happening!

Two issues that I've encountered that caused back-firing that may be worth considering:

1. Small intake leak
2. Timing slightly off

[Don Patterson]

I also had the same problem with rough idle after landing on hot days.
What made an improvement on my I0-360 was changing the spring in the Flow divider to a stronger one. I was told that the Cessna sky-masters changed these springs on the rear engine because of the lack of cooling. I believe it allows the pressure to build in the lines and help prevent this problem.

This made a noticeable differance in my idling on hot days. I still get some But not as bad.

[N427EF]

I hate problems like this. Hard to figure out why it's happening!

None of this is a mystery.
It is simply a fuel system installation with too many components and fuel hoses too close to heat sources. Often a gascolator unnecessarily added FW
to make matters even worse.
Among the fixes suggested, I believe a fuel system installation with as many components as possible installed aft of the firewall "cool side" is the best general approach to a trouble free fuel delivery system.
I am aware that this method of fuel system installation is not popular with everyone but it works for me.
I did also install a constant flow return line to ensure a constant flow of "cool" fuel supplied to the fuel controller. Again, not popular with too many people but highly effective, very simple, almost no cost and needs no attention nor does it rob you of speed or horsepower.
My IO-540 runs very smooth at 600 RPMs hot or cold anytime with avgas or
91 octane mogas E10.
I may not have the engineering credibility of some but I do have the proverbial
" proof in the pudding" with my installation.
BTW, my RV-8 had the same installation and never coughed or sputtered hot or cold or any other time.

[KiloWhiskey1]

Ernst,

My post must have struck a nerve with you. Sorry about that.

I've always been of the opinion that when troubleshooting a problem, never rule out things until you know for sure they're not the issue. Kinda like your tag line in your signature, "don't believe everything you know".

My suggestions were probably off the mark, but easier to rule out than some other theories.

Keith

[N427EF]

Did not mean to sound the way it did, my apologies.

Certainly does not hurt to check everything, especially intake leaks which I think go untreated in many cases due to the nature of intermittent rough running especially at low RPMs.

What is apparent to me is that a fairly large number of Rvs exhibit the symptoms of fuel boiling and most likely because the airplane is already built
and running, redesigning a fuel system is not on the top of anyone's list.
Other fixes usually somewhat cure the problems and all is well or so it seems.

[Tim Lewis]

Originally Posted by Don Patterson

I also had the same problem with rough idle after landing on hot days.
What made an improvement on my I0-360 was changing the spring in the Flow divider to a stronger one.

Where did you go to find out which spring was appropriate for your installation? I have similar behavior in an IO-540.

[RV7ator]

Been through this recently on a 375. Worked with Airflow Performance, who was most helpful. The condition was mitigated, not cured, though.

It's overheated fuel vaporizing, and for me it happens in the servo. Using clear tubing segments for viewing, clear fuel in, bubbles out of the servo. Perhaps the spider runs also added to the problem, but the bubbles already existed so the engine ran very rough. Heavily insulating the very short fuel lines FWF did no good. Even shunting around the hot engine pump direct to the servo didn't help. Nor did shunting around the high pressure aux pump direct to the engine pump. It's the huge 10:1 pressure drop between the servo inlet to the outlet that sets you up for bubbles with very slight heating. I installed the 4lb spring (2 is standard), but no obvious improvement, but it can't hurt, either.

And all servos are not created the same. The initial installation was a Bendix RSA. That thing would bubble if you looked at it wrong. AFP exchanged it for a Silverhawk - which is the same design - and now operations are usually smooth unless it's more than five minutes from touchdown to shutdown, or >15 minutes idling from cold (such as leaving OSH).

AFP swears they can set their calendar by the phone calls about rough running when spring turns to summer. The entire situation is a good reason for using a carb if you don't want to put up with it for whatever perceived advantages FI means to you.

John Siebold

[N427EF]

John,
Does your system have a purge valve ?

I ask because if you do, you could very easily plumb a constant flow return line into it.
Tee'd out of your mechanical fuel pump outlet and return fuel via the purge return line.
For the cost of a hose and a T with an orifice the size of a #65 drill hole you can see if this would solve your problem. It's worth a try and if it does not work for you can easily restore the system to its current configuration.

[Don Patterson]

Tim,

I ordered the spring from a guy that rebuilds Bendix fuel injected systems in Illinois a few years ago. Don't know if there are stronger springs for the system or was the spring in my system just not strong enough any more. I have an older Bendix system.

Don