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Author Topic: F-Head Running Strange  (Read 3098 times)

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Offline Shark49

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F-Head Running Strange
« on: June 29, 2025, 11:12:53 AM »
Engine ran before I started my restoration.  Got it all put back together and running but it seems when I give it steady throttle it sputters and seems like it doesn't have power.  I set the points timing best I could.  It will idle fine but when I try to drive it sputters bad.  Almost like when it gets up to temp it doesn't like it.  I double checked wires and everything is in order.  Recently rebuilt carb by Sal at Vintage Jeeper.

I ran compression across all 4 cylinders and all are right at 92psi.  This is the low end but at least consistent and doesn't appear to be blown head gasket.  Or is it?  Nothing in the oil or coolant to indicate that either.    Is it the valves?  Are they easy to check?

Kinda at a loss on what to do next.
« Last Edit: June 30, 2025, 03:58:35 PM by Shark49 »
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Offline Rus Curtis

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Re: F-Head Running Strange
« Reply #1 on: June 29, 2025, 04:54:02 PM »
How did you set the timing?

To set or check the valves, remove the top valve cover and side valve cover.
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Offline Shark49

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Re: F-Head Running Strange
« Reply #2 on: June 30, 2025, 03:57:59 PM »
I found TDC and did the standard move the dizzy until it sounds good by ear.  It will rev up just fine sitting but doesn't like to drive and hesitates really bad.
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Offline scoutpilot

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Re: F-Head Running Strange
« Reply #3 on: July 01, 2025, 04:33:38 AM »
What is the condition of the distributor? Dirty inside? No shaft wobble? Advance weights move freely? Advance weight springs in good shape?
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Offline Rus Curtis

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Re: F-Head Running Strange
« Reply #4 on: July 01, 2025, 08:14:42 AM »
Quote from: Shark49 on June 30, 2025, 03:57:59 PM
I found TDC and did the standard move the dizzy until it sounds good by ear.  It will rev up just fine sitting but doesn't like to drive and hesitates really bad.

I believe moving the distributor until it sounds good is fine tuning TDC.  Not the recommended 5 degrees before.

I did this exactly - once.  Your timing needs to be advanced off of TDC.  This allows a better power arc when the engine is at higher rpm.  I was proud of my attempt to smooth the idle out, but it created a scenario where the distributor was behind the engine when running down the road (same symptoms as yours).  Recommend setting Before Top Dead Center (BTDC) should fix this. 

There's more than one way to set the timing.  I haven't mastered the vacuum method yet.  I used a timing light on #1 spark plug wire.

After ensuring timing set to specs, scoutpilot has more issues needing inspection (I had the shaft wobble too).  Are you dripping oil into the lube port for the distributor (just a few drops).
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Offline scoutpilot

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Re: F-Head Running Strange
« Reply #5 on: July 02, 2025, 05:58:04 AM »
What is the Octane rating of the fuel you are using? How old is it?

5º BTDC is for 67 Octane. The higher the Octane Rating the sooner the spark has to happen because higher octane takes longer to burn completely. There is more to it than that and I would be happy to speak with you on the phone.
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Offline SteveKfl

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Re: F-Head Running Strange
« Reply #6 on: July 02, 2025, 08:39:59 AM »
5 degrees BTC for "67 octane"???  Who sells that?  I'm curious... What would be "approximate" settings for local "87 octane regular" w/10% Ethanol, and "90 octane  REC-GAS" no Ethanol (my choice)???  Thanks!
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Offline scoutpilot

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Re: F-Head Running Strange
« Reply #7 on: July 03, 2025, 04:01:47 AM »
67 Octane fuel is no longer available. I recommend the 90 Non-Ethanol. The best way I know to properly tune the motor is to use a vacuum gauge and a dwell/tachometer. https://oldjeepcarbs.com/thread/1321/f4-134-vacuum-testing
« Last Edit: July 03, 2025, 04:03:39 AM by scoutpilot »
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Offline Shark49

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Re: F-Head Running Strange
« Reply #8 on: July 04, 2025, 01:21:59 PM »
Im gong to check the vac on it and go from there.
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Offline Shark49

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Re: F-Head Running Strange
« Reply #9 on: July 06, 2025, 09:26:01 AM »
y'all are going to love this!! So I figured out what it was.  The little bracket that holds the choke cable was loose.  When I gave it throttle it was pushing the choke and choking the engine.  Haha.  I fixed it but not happy with the bracket but they are impossible to find I guess.
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Offline Rus Curtis

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Re: F-Head Running Strange
« Reply #10 on: July 06, 2025, 05:01:01 PM »
Outstanding!
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Offline SteveKfl

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Re: F-Head Running Strange
« Reply #11 on: July 07, 2025, 09:07:02 AM »
If the bracket pictured is the issue, can you add something like tape around the cable housing, or maybe caulk at the location allowed to cure before tightening and testing, to take up some of the "clearance" and stop the movement... if "clamping down" is the issue???
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Offline Mark W.

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Re: F-Head Running Strange
« Reply #12 on: July 07, 2025, 12:04:04 PM »
I run 87 oct E-10 in my 134L I run 12° BTDC timing. I see no reason to bother with a higher octane fuel. The engine does not have anywhere near the compression ratio to use it.
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Offline Rus Curtis

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Re: F-Head Running Strange
« Reply #13 on: July 07, 2025, 02:03:06 PM »
Quote from: Mark W. on July 07, 2025, 12:04:04 PM
I run 87 oct E-10 in my 134L I run 12° BTDC timing. I see no reason to bother with a higher octane fuel. The engine does not have anywhere near the compression ratio to use it.

Mark,
How did you settle on 12 degrees?
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