Build 1974 FJ40 Restoration Build Thread

[FJ40Jim]

The jet sizes are actually on the fat side. Normal would be 60-70 slow and 120-124 main for the primary bbl. With the idle at 700 & 14°BTDC, there should be a lot more vacuum. The carb is vacuum powered, so with such low vacuum, it's not going to tune "normal".

I hate to say this, but it sounds like there is a problem with the cam. The engine is not being an effective air pump. The engine is rebuilt correctly, but the regrind cam is a possible suspect.

 

[JohnJL]

The jet sizes are actually on the fat side. Normal would be 60-70 slow and 120-124 main for the primary bbl. With the idle at 700 & 14°BTDC, there should be a lot more vacuum. The carb is vacuum powered, so with such low vacuum, it's not going to tune "normal".

I hate to say this, but it sounds like there is a problem with the cam. The engine is not being an effective air pump. The engine is rebuilt correctly, but the regrind cam is a possible suspect.

So here I am 5.5 years later and no further along. Everything in the string above is current with the addition of a few more datapoints below. Here are the symptoms I am observing:

• It starts/idles like a champ.
• Its down on power and torque everywhere.
• I’ve tested and posted compression, leakdown figures, all looking good and even. This is a fresh engine I built (and have since built 2 more with happy results)
• Running up through 3rd gear from a stop is OK, but lacks torque anything off ~2000 RPM
• Vacuum at idle is still ~11 inches. I would expect 14 at the low end.
• Ignition is a DUI unit, I have been experimenting with springs and weights in an effort to map out what the ignition curve is now and what it should be.
• Off-throttle results in massive backfiring. Specifically when decelerating under engine braking/coasting from ~3000 down to ~1500 it will let out a massive artillery blast from the exhaust. Not popular with the neighbors.

• AFR Observations: I have an Innovate MTX-L gauge and wideband sensor installed temporarily to help diagnose what is going on.
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• - At warm idle I have 13ish AFRs at an 800 rpm idle which is where it seems to want to be with the timing on the BB.
• - At partial throttle (~5-20%) I get lean misfiring and AFRs 15ish.
• - Accelerator pump seems to be working as initial tip-in on throttle is ~12-13 AFR.
• - At full throttle I get ~12-13 AFRs, just what I would want/expect.
• - Power is down everywhere but the motor falls on its face on ~30% throttle under load going up a hill..misfires/coughs. AFRs show ~13 but I suspect thats inaccurate from the misfiring.
• - I suspect an ignition issue (and possibly more?)

Vacuum trips: This thread documents the extensive vacuum leak tracing I’ve done: manifolds planed, block planed, 3 gaskets changed, all hoses fresh and new, boosters tested, no smoke leaks under pressure.

• Cam adventures: I sent my cam back to Delta to check their work. It took them 23 months to get it back to me. Months of calling and calling and leaving messages. I have low confidence they actually checked anything. Decorum prevents my further comment.
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• Carb conundrums: I rebuilt it 2x before sending it to JimC here. Then another time after getting it back, checking for dirt or something off…no change. Jets sizes etc are in this thread.
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• Distributor dead-ends; I have the original distributor and coil but they are 52 years old. The DUI distributor worked well before the rebuild, it seems to give good spark from start. I have tried mapping the ignition curve, also in this thread earlier.

So the truck is 99.5% done and a ton of fun but I cant enjoy it like this. I have to get to work again soon so the clock is ticking…I need your help to finish her up!

• Is it an ignition issue? The misfiring at low, accelerating rpm under load could be too aggressive a curve, meaning too light of distributor springs/too heavy weights?
• Also the backfiring off-throttle could be 1. Too much fuel under vacuum or 2. Late timing under vacuum? Or the other way around?

I’d love some help here, I suspect overlapping problems. My experience is to fix the electrical/ignition issue then the fuel trim…thanks for the help!

 

[flee]

Are the spark plugs still clean after the misfiring? Sooty could indicate poor spark...

 

[White Stripe]

Sounds like you need to degree the camshaft to verify the lobes have not been ground incorrectly. Once that is done verify the distributor has been stabbed correctly. Verify the valves are closing fully by turning the motor by hand. Then I would verify the vacuum advance is working properly.