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Old 10-09-2014, 06:48 PM   #1
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What octane to use

I recently bought a 1988 345 Airstream Motor Home with a ford engine. Should I use a higher octane gas? When I turn off the engine it tends to knock. When I bought the motor home the owner was using a gas additive. Wouldn't be better to use a higher octane?
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Old 10-09-2014, 06:59 PM   #2
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Hi,

So when you turn the ignition switch "off", the engine runs-on, like "k-knock, K-NOCK-KNOCK-KNOCK, sputter, swoosh? If so, what's happening is combustion is taking place after ignition ceases at the spark plug (gap). You can have such if there are "hot spots" in the combustion chamber, such as excessive carbon buildup, possibly combined with a larger than spec throttle opening or vacuum leaks, which would allow off-idle amounts of air to enter the engine, even though you're not on the throttle.

To answer your question, a higher octane fuel would, if anything, have no or a marginal effect on what's going on.

If any of this makes sense, you might want to discuss with your mechanic.
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Old 10-11-2014, 06:50 PM   #3
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The engine timing could be off. Have your mechanic check it.


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Old 10-11-2014, 08:28 PM   #4
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Your idle speed could be too high also.
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Old 05-04-2016, 05:57 PM   #5
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High idle, advanced timing are both the most common causes.
My racing buddies clean out their combustion chambers by running a hot engine at medium RPM and pouring water into the throttle body/carburetor inlet to steam-clean the chambers. (I don't know if that has any validity or not... I wouldn't do it to my vehicles.....and my racing buddies are already brain-damaged in my opinion.)

Gas additives are usually ethanol...which is already present in modern gasoline.
Wanna know if higher octane will cure the problem..??? All you have to do is buy a tankful to find out.
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Old 05-04-2016, 06:19 PM   #6
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Quote:
Originally Posted by Boxite View Post
High idle, advanced timing are both the most common causes.
My racing buddies clean out their combustion chambers by running a hot engine at medium RPM and pouring water into the throttle body/carburetor inlet to steam-clean the chambers. (I don't know if that has any validity or not... I wouldn't do it to my vehicles.....and my racing buddies are already brain-damaged in my opinion.)

Gas additives are usually ethanol...which is already present in modern gasoline.
Wanna know if higher octane will cure the problem..??? All you have to do is buy a tankful to find out.
^
X2

Carbon 'hot spots' are causing the run-on, possibly made worse by bad timing.

Try spraying some Blue WW solvent into the carb/throttle body.

Water Injection has been use for years. It does work


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Old 05-04-2016, 07:00 PM   #7
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What octane to use

http://www.misterfixit.com/diesling.htm

http://workshop-manuals.com/ford/f_3...itioner_(tsp)/


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Old 06-08-2016, 11:49 AM   #8
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^///Water Injection has been use for years. It does work


Bob
I'm quite familiar with water-injection systems on heavy aircraft, having flown a few of them. But that is not for the purpose of cleaning carbon from combustion chambers, as has been described.
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Old 06-08-2016, 11:58 AM   #9
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Oldsmobile and buick offered a rocket fuel injection system.... it was simply a water injector post carb. The "jet fuel" was nothing more than blue distilled water! The steam was actually a horsepower booster.
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Old 06-13-2016, 09:32 PM   #10
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Oldsmobile and buick offered a rocket fuel injection system.... it was simply a water injector post carb. The "jet fuel" was nothing more than blue distilled water! The steam was actually a horsepower booster.
That's correct!... but it was used for the same reason as aircraft and it was a water-methanol injection system for increased power and cooling..... not for carbon removal or run-on.
http://blog.hemmings.com/index.php/2...obile-jetfire/
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