Timing and Tune-Up
Tech Tips


Spring is fast approaching in many parts of the world, and already some people are out in their driveways trying to get the Jeep running smoothly before top-down crusing weather arrives. See the main CJ-3B Specs and Tech Tips page for several sources of specs and tune-up information on the F-head Hurricane engine, including the useful 1945-61 Universal Jeep Chek-Chart.

Here's a typical question on the old CJ3B Bulletin Board, from somebody having trouble juggling all the variables while trying to get their engine tuned: "With a T connected to the intake (incl. PCV system) I hooked up a vacuum gauge to an early 50's 3B F-head, and read 15 inches of vacuum with a shaky needle reading between 14 and 16. You can hear a skip, and feel an almost rough idle. Changed the spark plugs and wires. Plugs do foul up. Tried to adjust timing, carb mix screw, and was not much help. I seem to be missing the timing marks plate that bolts on, so I'm in need of one of those. Should I be reading around 20 inches with it connected to the rest of the engine system? And what could be my problem? It's not a bad coil, is it? I can rev the engine up OK... the vacuum drops down to 5 or so. What other tests can I try?"

Dan Norrie also tried to adjust his timing: "I just adjusted the points and set the ignition timing, with a timing light, to 5 degrees before top dead center (BTDC). Then set idle mixture to the max RPM's. The problem is when you give it the gas, it hesitates, almost stalling. Running the idle at 1000 RPM and the choke on, it operates normal, minus the high idle. Any suggestions would be great. It ran fine before, when everything was set by ear."

Tommy Wade: "I had the same problem with my 64... had to kit the carb. It had a bunch of trash in it, and the idle passageways were all plugged up. If the gas line is not your problem you may have to try this."

Brian Greif: "You could also have a bad or misadjusted accelerator pump in the carb itself. With the engine shut off, look down the carb -- you should see a squirt of gas when you move the throttle to the wide open run position."

Note: see also YF Carburetor Tips.

Cliff Todd: "I have had a similar problem with having to pull the choke to keep it running. If the fuel line is part clogged it is not getting enough gas. When you pull the choke you are giving it more gas in the air/fuel ratio, and it will run better. By cleaning out the line I got it going good. Also replaced the spark plugs and it ran like a champ. Gas fouled plugs never recover form, and at a buck for a new one I opt for a new set. However, if it ran OK before, it could be your timing mark is somehow off. You might crank it up and rotate the distributor and set with your ear. However I would check the fuel supply and put an inline filter on it -- 3 bucks max."

Ignition Timing

Early timing marksIan Fraser: "Timing is everything! While trying to awaken the Hurricane after new plugs, wires, oil and fresh gas, I experienced what mis-timing can do to one brand new muffler and one's hearing. After several cycles of attempting to start, and minutes passing in between each cycle while making adjustments, I tapped the start button on my temporary panel and heard the loudest KERBANG ever! I lost hearing in both ears (they are still ringing lightly after 24 hours), felt a flash of compression against my left lower leg, and destroyed a brand new Walker muffler.

"My friend standing next to me and making the timing adjustments claimed I darn near jumped over top of him and he's 6'3". I know I lost consciousness for a split second and total hearing loss for probably 10 seconds. Split the muffler open along the seam worse than any popcorn kernel could ever dream of popping like. Happened to see the same results couple years back in another 3B owner's garage in Ontario. I now have an identical muffler and story. Big item is danger of losing hearing, so if you're attempting to awaken an engine I highly suggest removing muffler, triple checking plug wires, setting timing and replacing rotor cap and button. Caution!"

(See also Air-fuel ratio on Wikipedia.)

Early timing marksJoel Kamunen: "These things aren't real high compression engines so they should run smooth. The early CJ-3B has the timing marks stamped onto the cover behind the pulley on the crankshaft. Try cleaning real good, and look for those stamped marks. Mine were difficult to find with the rust and crud on them. Check to make sure that your distributor advance is working and check your valve settings also. These are somewhat overlooked items that may help your engine come to life."

Tim Salkeld: "On older engines with more than stock tolerances, they sometimes do not take to being set to stock settings. Tuning by ear will often improve performance but can be very subjective. I have had good luck using a vacuum gauge and setting the distributor to produce the highest amount of vacuum at idle. Keep the idle speed constant and lock it down when you get the best reading. Then take it out for a test drive, find a hill and check to see if it pings as you drive up. If it does, then back the distributor off a little (retard) and try again. The effect will be setting to the highest advance where the engine is happy."

Keith Lowe: "I have an ignition-timing question on my 134 F-head engine. According to the manual timing should be set at 5 degrees before TDC. This sounds simple but I'm not sure if the timing marks are set to align at 0 or 5 degrees BTDC. Could anyone tell me if the timing marks (on the crankshaft pulley and timing gear cover) align at 0 or 5 degrees TDC? I suspect I can advance the timing even further given the quality of modern fuels. Anyone have any recommendations?"

Eric Lawson: "There should be two timing marks on the timing gear cover. One is for cylinder 1 at TDC and the other one is 5 degrees before TDC. On one engine that I have, there are two lines on the timing gear cover and a notch in the crankshaft pulley. Facing the front of the engine, the mark closer to straight up is the TDC mark and the one to its left is the 5 BTDC mark.

New timing marks"On another engine there is a pointer at about the 11 o'clock position (again facing the front of the engine) and a groove in the crankshaft pulley. When the groove on the pulley is aligned with the left edge of the pointer, #1 cylinder is at 5 BTDC. When the pulley groove is aligned with the right edge of the pointer, #1 cylinder is at TDC.

"If you have only one mark on the timing gear cover, I would expect the mark would correspond to 5 degrees BTDC. I adjust the engine as per the book -- 5 degrees before TDC at (I think) 600 rpm. I've also tried adjusting the timing for best low rpm performance and this occurred very close to the 5 degree specification."

Ingvar: "My Kaiser F-head has the raised single mark on the timing chain cover. There is a corresponding notch on the crank pulley and so the theory is to just set the marks to be adjacent each other with a timing light - right? Okay, but do I have to pre-set the timing light setting to 5 degrees before top dead, and then move the distributor until the marks line up, or is the raised mark already compensating for the 5 degrees and I just leave the timing light setting at zero degrees and line the marks? My gut is the latter and I have set it that way but I have a lag in throttle response."

Eric Lawson: "If you leave the timing light set at 0 degrees or you have a timing light that doesn't have this feature, align the mark on the crankshaft pulley and the 5-degree before TDC mark that is on the timing gear cover. If you set the timing light for 5 degrees advance, then align the TDC center mark on the timing cover with the mark on the crankshaft pulley. Looking down at the front of the engine, the 5 BTDC mark is "leftmost" of the two marks. The two marks are only about 1/2 inch apart so if you have found one, it should be easy to find the other one."

Joel Kamunen: "I time mine first by sight with a timing light and then, with the distributor tightened just enough so I can move it by hand, I will drive the Jeep and slightly move the distributor until it runs the best. Then mark that spot and tighten the distributor clamp."

Ingvar: "Thanks for all the advice on timing. I tried the suggestion about turning the distributor and listening for highest RPM before the following plateau. That seemed to do the trick quite well and my baby is running very smoothly without any pinging. One thing I still notice however, is that if I am going down a hill the Jeep lurches and sort of chatters. As soon as I hit the gas this stops and all is beautiful again, until I have to lay off the gas that is. Any ideas?"

Jim Sammons: "Do as Joel says -- the timing marks are reference points. The engine will not always run the best at the 5 degree setting, due to wear in the timing gears. I always set mine by ear. TDC is a reference point for setting the timing when rebuilding or repairing the engine. You cannot always trust the 5-degree mark to make your engine run its best."

Brian Greif: "Joel is right about timing these. If you have a hesitation you might check the weights in your distributor -- I have found them frozen with rust not allowing the timing to advance past the initial timing of 5 degrees."

Vacuum Leaks and Uneven Idle

Dan Fedorko: "The F-head engine in my 63 3B does not idle at a constant speed unless I give it a lot of gas (pretty fast idle) by keeping the choke out or pulling out the throttle. The engine cycles between smooth and rough idle and will eventually die unless I am there to pop the gas pedal to increase the engine speed during the rough times. It does this with two different Carter YF carbs. It will run smoothly for 5-10 seconds and then real rough for 5-10 seconds. I have played with the timing to no effect. What could be wrong? Am I missing a tooth or two on one of the timing gears? Could it be my distributor?"

JC: "It sounds like you have a vacuum leak -- the tip is that you have to keep giving it gas. Check all your lines, and the base of the carb."

Steve: "I'd check these in the following order: Good fuel pressure from the pump, and no debris or crud in the pump's glass bowl. Good carb to manifold gasket. (Use WD-40 as a leak-check around the carb base while idling to see if it makes any difference.) Proper idle screw adjustment. From the sound of it, you may be running too lean at idle. I adjust mine out to keep the engine running. From there, slowly turn the screw in until the engine begins to sputter. Then back it out 1/2 to 3/4 of a turn. Make minor adjustments from here to get the best idle you can. If the timing is set correctly, it should have nice, crisp throttle response off the line when the idle mixture is correct. Correct float level. This is one of the most commonly overlooked items with old carbs. For the $14-15 bucks the overhaul kit costs,it is worth it. There should be a new float in there, or at least a float gage and new metering rod. If the float is set correctly, it will cure a bunch of symptoms. I think you can safely rule out the timing gears. When they fail, they do so catastrophically and the engine will die, period. A chain and gears may slip a tooth and keep running, but not a gear set. I forgot to mention the vacuum line on the PCV valve, and on the wipers too. Try capping off one at a time to see if the idle improves, as JC noted! This includes the vacuum advance line, and the vacuum chamber on the fuel pump, if any."

Scott Blystone: "You can speed up vacuum diagnosis as follows: Disconnect all vacuum at the intake manifold by unscrewing the line that enters the manifold right beneath the carb on the passenger side -- plug the hole tightly. If this helps, then the leak is somewhere in that line. Put a new fuel filter on (or clean bowl). Does it ever stutter when driving at decent speed? If not then fuel pump is probably OK. Someone suggested WD-40 to check for leaks, but carb cleaner is better -- at idle, aim the plastic tube at potential leak areas and give quick, small spurts -- with vacuum system sealed off as described above. Start by working around the carb base spraying small areas, then do the gasket on the bowl of the carb. Sure sounds like a vacuum leak to me. You might pull and clean your PCV valve as well."

Wes Knettle: "All the above are good starters. When eliminating vacuum leaks don't forget the throttle shaft bushings. If she's worn excessively the shaft will vibrate around, increasing and decreasing airflow at the throat. Sticking valves will cause this as well. You said you swapped carbs so I'd look hard at the other areas. Look at intake, vacuum, and valves. Is fuel pressure at idle adequate?"

There was a happy ending, as often happened on the old CJ3B Bulletin Board. Dan said: "Thanks to everyone who gave me advice. The vacuum leak was at the base of the carb. I will have to get a new gasket."

Thanks to all the contributors, and to Ed Wilson for editing the material. -- Derek Redmond

Note: for detailed service instructions, see the Jeep Universal Service Manual. A reproduction is available for sale at many Jeep parts dealers.

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Last updated 3 March 2001 by Derek Redmond redmond@cj3b.info
All content not credited and previously copyright, is copyright Derek Redmond