Help with 2F Compression Test Results (FJ40) (1 Viewer)

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wyolands

wyolands

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just purchased a rough 1981 fj43 and did the compression test.
1- COLD engine
2- Removed all spark plugs
3- Cranked engine with full throttle
4- Were at 6,800' elevation

results:
120 in all cylinders
spark plugs were clean but were black from soot
Weber carb:
145 main Jett's
170 mix jet

thanks for any suggestion, i heard you have to add to the 120 number ? since our elevation is so high
 
I'm also at 6,800.' 103 was my highest dry, 88 lowest dry - junkyard 2F about five-years-ago. Now, the motor seems much stronger, or having less deposit-congestion. It has excellent torque and mpg, when the engine is warm. It probably needs more cylinder cross-hatch.

Your 120 lbs is like 149 at sea-level. Excellent motor.

Which Weber? 32/36 DGV or DGEV is a bad carb, worse at altitude.
 
Grayscale said:
I'm also at 6,800.' 103 was my highest dry, 88 lowest dry - junkyard 2F about five-years-ago. Now, the motor seems much stronger, or having less deposit-congestion. It has excellent torque and mpg, when the engine is warm. It probably needs more cylinder cross-hatch.

Your 120 lbs is like 149 at sea-level. Excellent motor.

Which Weber? 32/36 DGV or DGEV is a bad carb, worse at altitude.
Click to expand...
good to know thanks, it has the 38
 
charliemeyer007 said:
I agree seems like a good motor. Do the wet test for compression using Marvils MO, let it soak in for a few day after whirl it over to distribute the oil around the rings. Hook up a direct read oil pressure gauge and see what that shows.
Click to expand...
okay i'll try that thanks
 
After checking valves (first order of tune-up procedures), ignition maintenance and carb/fuel checks, there isn't much wrench-turning that is left to do. However, I think that most F/2F will benefit from cleaning in two areas, the intake and exhaust ports in the head, and the piston rings. Also, a borescope for cylinder cross-hatch (cylinder-hone) or peek into the head ports, will tell you tons about the engine's condition and its needs.

When oil gets too hot, lets say it is leaking thru the valve guides more than it should (head intake/exhaust ports), or, it made its way thru the oil control rings due to cylinder wear, ring wear, or because it was too high a viscosity oil for the ring tension, it polymerizes, or becomes like seasoning on a piece of cast-iron-cookware. Really, the solution is to do an engine tear-down. Piston rings that are stuck to the piston groove don't perform correctly and can lead to even more blow-by. A valve caked with thick layers of baked-on 10W40 isn't going to breathe thru the port properly. And who knows if valve-congestion makes valves stick, or overheat from glowing carbon burning? However, there is a detergent package in the crankcase oil, and additives are commonplace in gasoline fuel. So, fresh gasoline is always a must. I like the Techron additive(s) for gasoline. I no longer have a Chevron filling station to buy from, so I buy bottles of additives. I did Lucas last summer, right now Marvel Mystery Oil (super inexpensive) is in the fuel tank. Techron is either for long-term-storage, or they make one for use right before an oil change; I always forget which is which. Because oil has detergent, I do an oil change (w/ an conventional API-SP) about every three or five tanks of gas, as it is probably fuel-contaminated (blow-by, or rich-condition from a cold or hot carburetor) or the oil is water-contaminated (blow-by and resulting H2O-condensation). BTW, high-viscosity oils have longer hydrocarbon chains and are more subject to thermal break-down, so I stick to a season-appropriate viscosity (referring to the Owner's Manual). I never use oil additives (Marvel Myster Oil is in the fuel tank-only). I often change the oil without also changing the oil filer. I refuse to just think that oil is oil and gas is gas when there are resources being made available to the consumer that claim to address a common problem - engine deposits, but, I could be wrong about all this liquid tune-up stuff for running old engines, as I'm no expert.
 
charliemeyer007 said:
I agree seems like a good motor. Do the wet test for compression using Marvils MO, let it soak in for a few day after whirl it over to distribute the oil around the rings. Hook up a direct read oil pressure gauge and see what that shows.
Click to expand...
If you park it so the spark plugs are on the up-side of the hill, gravity might distribute the MMO over more of the ring set.
 
Grayscale said:
If you park it so the spark plugs are on the up-side of the hill, gravity might distribute the MMO over more of the ring set.
Click to expand...
Pistons run vertically, parking on level ground should distribute any fluid evenly

20240408_125901.jpg
 
wyolands said:
just purchased a rough 1981 fj43 and did the compression test.
1- COLD engine
2- Removed all spark plugs
3- Cranked engine with full throttle
4- Were at 6,800' elevation

results:
120 in all cylinders
spark plugs were clean but were black from soot
Weber carb:
145 main Jett's
170 mix jet

thanks for any suggestion, i heard you have to add to the 120 number ? since our elevation is so high
Click to expand...

Great to hear you're diving into your 1981 FJ43! Compression test results can indeed vary with altitude, so let's break down your results and suggestions for moving forward.

Compression Test Results Analysis​

  1. Your Observations:
    • 120 psi in all cylinders: This is consistent across all cylinders, which is a good sign. Compression consistency indicates no significant internal engine damage (e.g., valve or piston ring issues).
    • Elevation of 6,800': Atmospheric pressure decreases at higher altitudes, which lowers compression readings compared to sea level.
  2. Altitude Adjustment:
    • Atmospheric pressure at sea level is roughly 14.7 psi. At 6,800 feet, it's closer to 11.3 psi, about 23% lower.
    • To estimate your sea-level compression:
      • Adjusted Compression = Measured Compression ÷ (Elevation Pressure ÷ Sea-Level Pressure)
      • Adjusted Compression = 120 ÷ (11.3 ÷ 14.7) ≈ 156 psi
    • This means your engine would likely show around 156 psi at sea level, which is in the normal range for a 2F engine (145–170 psi when new).
  3. Sooty Spark Plugs:
    • Black, sooty plugs usually indicate rich running conditions (excess fuel in the mixture).
    • Your Weber carb setup, combined with the high altitude, could be running rich because:
      • Jets may be oversized for the elevation.
      • Altitude reduces air density, which requires leaner jetting.

Recommendations​

  1. Address the Carburetor Jetting:
    • The Weber carb jets you listed (145 main jet, 170 air correction jet) might be oversized for 6,800 feet elevation.
    • Suggested starting point for jetting at 6,800':
      • Main Jet: Try 140 or 135.
      • Air Correction Jet: Try 180 or 190.
    • Fine-tune these values with a wideband O2 sensor or spark plug reading after adjustments.
  2. Recheck Compression After Tuning:
    • Perform a compression test again after addressing the carburetor and ensuring the engine is tuned properly.
    • If results are still consistent and within acceptable ranges, your engine's compression is likely healthy.
  3. Evaluate Timing and Ignition:
    • Ensure the timing is properly adjusted for your altitude. At higher altitudes, advancing the timing slightly can improve performance.
  4. Monitor for Excessive Oil Consumption:
    • While your compression numbers are consistent, keep an eye on oil consumption. Worn rings or valve seals might not show obvious signs in compression tests but can still cause issues.
  5. Check Valve Adjustment:
    • Ensure the valves are adjusted to spec, as incorrect clearances can affect compression and overall engine performance.
 
JaxFJ said:
Great to hear you're diving into your 1981 FJ43! Compression test results can indeed vary with altitude, so let's break down your results and suggestions for moving forward.

Compression Test Results Analysis​

  1. Your Observations:
    • 120 psi in all cylinders: This is consistent across all cylinders, which is a good sign. Compression consistency indicates no significant internal engine damage (e.g., valve or piston ring issues).
    • Elevation of 6,800': Atmospheric pressure decreases at higher altitudes, which lowers compression readings compared to sea level.
  2. Altitude Adjustment:
    • Atmospheric pressure at sea level is roughly 14.7 psi. At 6,800 feet, it's closer to 11.3 psi, about 23% lower.
    • To estimate your sea-level compression:
      • Adjusted Compression = Measured Compression ÷ (Elevation Pressure ÷ Sea-Level Pressure)
      • Adjusted Compression = 120 ÷ (11.3 ÷ 14.7) ≈ 156 psi
    • This means your engine would likely show around 156 psi at sea level, which is in the normal range for a 2F engine (145–170 psi when new).
  3. Sooty Spark Plugs:
    • Black, sooty plugs usually indicate rich running conditions (excess fuel in the mixture).
    • Your Weber carb setup, combined with the high altitude, could be running rich because:
      • Jets may be oversized for the elevation.
      • Altitude reduces air density, which requires leaner jetting.

Recommendations​

  1. Address the Carburetor Jetting:
    • The Weber carb jets you listed (145 main jet, 170 air correction jet) might be oversized for 6,800 feet elevation.
    • Suggested starting point for jetting at 6,800':
      • Main Jet: Try 140 or 135.
      • Air Correction Jet: Try 180 or 190.
    • Fine-tune these values with a wideband O2 sensor or spark plug reading after adjustments.
  2. Recheck Compression After Tuning:
    • Perform a compression test again after addressing the carburetor and ensuring the engine is tuned properly.
    • If results are still consistent and within acceptable ranges, your engine's compression is likely healthy.
  3. Evaluate Timing and Ignition:
    • Ensure the timing is properly adjusted for your altitude. At higher altitudes, advancing the timing slightly can improve performance.
  4. Monitor for Excessive Oil Consumption:
    • While your compression numbers are consistent, keep an eye on oil consumption. Worn rings or valve seals might not show obvious signs in compression tests but can still cause issues.
  5. Check Valve Adjustment:
    • Ensure the valves are adjusted to spec, as incorrect clearances can affect compression and overall engine performance.
Click to expand...
thank you, okay i switched to 135 main 190 air. WOW what a difference runs amazing i bought the High altitude weber carb kit from JT Outfitters they were extremely helpful

Thank you everyone
 
What are you using for an idle jet(s)?

By no means do I think that it is a substitute for a dyno test, but, I'm curious about power-output as seen in mpg. Can a tuned Weber 38/38 perform akin to the Aisan? My electric choke Weber, which I ran on my pickup, would make me nearly deaf with high start-up RPMs during t-shirt weather in stop-and-go driving; could the electric choke be the cause of spark plug suit?
 
Grayscale said:
What are you using for an idle jet(s)?

By no means do I think that it is a substitute for a dyno test, but, I'm curious about power-output as seen in mpg. Can a tuned Weber 38/38 perform akin to the Aisan? My electric choke Weber, which I ran on my pickup, would make me nearly deaf with high start-up RPMs during t-shirt weather in stop-and-go driving; could the electric choke be the cause of spark plug suit?
Click to expand...
i'll go look at what i used and get back to you asap, it total transformed the FJ i haven't tracked the MPG but from what i can tell it doubled it i almost thought the gas gauge was sticking, The other day it was 1 degree outside and it started i was amazed.
 
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