Ford 4500 gas engine won't go to full power (suspect vacuum leak from bad valve)

Ford 4500 · Aug 16, 2015

I'm at my wits end trying to figure out why my tractor will not go to full power. Here is some background> The tractor is a 1969 Ford 4500 backhoe with a 201 cu-in 3 cylinder gas engine with approximately 4700 hrs. I used the tractor last weekend to dig some stumps and I didn't think I was too hard on the tractor. I started it up yesterday and it started right up and idled fine however when I drove off I noticed it had barely enough power to make it up a small hill it normally has no problem with. I initially thought it was the carburetor or throttle linkage so I cleaned the carb and reset the linkage settings [Problem still exists]. Next I checked the ignition spark and timing. Spark was good and timing was at 4 degrees BTDC [Problem still exists]. Next I checked the valve lash per the manual. Intake valve lash was approximately 0.005" high so I adjusted back to spec (0.015"). Exhaust valve lash good (0.018") [Problem still exists]. I drained the gas and put new gas in [Problem still exists]. Hooked a vacuum gauge up to the intake manifold and vacuum reads 15 inches but fluctuates about 1 to 2 inches at idle??? Then when I rev it up to 1800 rpms the vacuum goes down and stays down around 11 inches. Correct me if I'm wrong but I believe the vacuum should jump back up even or greater than the idle vacuum reading as the rpms increase??? I checked for leaks around the intake manifold and carb with a propane torch while the engine was running and didn't find any leaks. I checked the engine compression and #1 read 100psi #2 90 psi #3 90psi which all seemed good readings to me. No oil in the coolant and no coolant in the oil. When I pulled the spark plugs #1 and #3 had a white chalky appearance and #2 had a little oil on it. I pulled the muffler off thinking the exhaust was restricting the vacuum but the vacuum remained at 11 inches with the muffler removed at 1800 rpms. The engine runs fine and isn't rough running like something is obviously wrong. It just doesn't have the power it had.

So now I'm stumped. I believe the cause of the low power has something to do with the low vacuum readings at high rpm and based on what I've read the primary causes are burnt/sticky valves, low compression, bad head gasket, or leaking intake manifold gasket. Based on my compression test I think I can eliminate the low compression and bad head gasket. Would the compression still be good if I had a bad valve? I suspect its a bad valve but I don't know how to confirm this without pulling the head. Is there anything I'm missing?

Jerry/MT · Aug 17, 2015

Remember when you are reving an engine with no load, it doesn't take much throttle opening to get it to max rpm so you may never achieve a near zero vacuum reading, indicative of wide open throttle. Your compression seems low. Did you do the compression test with the engine warmed up, the throttle wide open and the air cleaner off? Did you do it dry and wet? (Wet meaning a couple of tablespoons of oil in each cylinder.)
It sounds like your low power problem occurred suddenly so I'd doubt it was compression related because loss in compression generally occurs slowly over time. An oily plug generally indicates loss of oil control of the rings and that's only occurring on #2. White chalky deposits generally indicate a lean mixture.

I'd take a look at the fuel delivery. Do you have adequate fuel delivery to the carburetor? the tank strainer on the tractor may be clogged, a fuel line pinched, fuel filter clogged, dirty fuel bowl in the carb, etc. The other area to look at is a restricted air induction system ( clogged air filter, damaged/clogged intake tubing, etc) The lean mixture could be due to a leaking intake manifold as well as improper mixture adjustment. Since you checked for manifold leaks and I'm assuming you did this correctly, I'd check the mixture adjustment.

Check these things out systematically, changing only one thing at a time then restarting the engine to see the result and you'll be able to pinpoint the problem.

Ford 4500 · Aug 18, 2015

I did forget to open the throttle so I checked again and got 110 psi on #1, 100 psi on #2 and #3. This was with the engine warm, cylinder dry, air cleaner off, and throttle open. The manual does state 115 - 150 psi with a +- 25 psi variation. I'm not sure if the 15 psi below spec would make that big of a difference. I had some compression numbers written down in my manual from when I took the compression last and these were 110 psi #1, 115 psi #2, and 123 psi #3. So it looks like my compression has gone down on #2 and #3 since the last I took it (at least 2 years ago). However, if I remember these were taken cold because I couldn't get the engine to fire which turned out to be a bad coil.

I pulled the fuel filter and put in a fuel pressure gauge which shows good pressure to the carb. About 5-6psi at idle (700 rpm) and 8-10 psi at 1800 rpm. Still no increase in power.

When I pulled the air filter for the compression test I checked the intake tubes and couldn't find any blockage. A friend of mine suggested spraying all possible air leakage paths on the carb and intake with carb cleaner to see if the engine bogs indicating a possible leak path. I tried that this evening and no smoking gun.

I'm also going to open up the carb to clean it and to see if there is anything broke or missing. I cleaned off the spark plugs and reset the mixture to the rich side and let it run awhile. When I pulled the plugs again #1 and #3 were still white and chalky and #2 had a little oil and smelled of gas. I found an older set of plugs that I saved when a did a tune up 2 years ago and those plugs did not have a white chalky appearance. Lean mixture from air leak or not enough gas from carb???

I think my next step is to do a cylinder leak down test to see if I'm getting leak by the piston, head gasket or valves. I'm trying to think what else to check but nothing is coming to mind. I appreciate the reply. Go Ore Diggers!

Jerry/MT · Aug 18, 2015

Do a wet compression test and note the difference between wet and dry. A big change in compression in all cylinders will indicate worn rings. You'd might also also see some blowby from the crankcase vent .A no change in one cylinder will indicate a leaking valve. Your compression test indicates a problem with #1.

Why did you pull the filter to check fuel delivery pressure? Check the pressure downstream of the filter with the filter in place . That's how you are running the engine. You obviously have a lift pump to get those fuel pressures.

Is your spark advance working? A stuck centrifugal advance will short you on power at higher speeds. To check this you'll need a timing light.

I gather you went to Montana Tech!

jimmysisson · Aug 20, 2015

Sure it's the engine? Could it be the clutch won't let you move?
Jim

Ford 4500 · Aug 24, 2015

Sorry for the delay I was out of town for a few days.

So I did a leak down test on each cylinder at TDC. Both the exhaust and intake valves on each cylinder are seating/sealing correctly as I was not getting any leak-by. All the leak-by was going thru the crankcase indicating that I do have some piston/ring wear. I set my inlet pressure on the leak down gauge at 100 psi for each cylinder and got 73psi on #1, 75psi on #2 and 78psi on #3. This roughly equates to 25% pressure loss on each cylinder. I did not notice any leak by from cylinder to cylinder or bubbles in the radiator. This indicates to me the head gasket is good. I didn't get to the wet pressure test yet but the results of the leak down don't seem to bad to me.

I left the fuel gauge in place of the fuel filter to verify I had enough pressure to the carb and to eliminate the fuel filter as a source of the problem. I rebuilt the carburetor and didn't notice anything broken, however, I am missing the "sintered bronze plug" that goes between the air horn plug and retainer. I made one similar from the head of a brass bolt and made sure it was sealed with a piece of gasket material. I started the tractor and it idles fine but still would not rev higher than 1900 RPM's. When I adjusted the fuel mixture I noticed the engine didn't show any signs of running rich out past 5 full turns of the fuel mixture screw. The RPM's significantly dropped below 1/2 turn in. I don't think this relates to anything with my issue but just throwing this info out there if someone picks up something wrong.

Next I checked to make sure the distributor was advancing. I unplugged the vacuum line to the distributor and set the timing at 4 degrees BTDC. When I plugged the vacuum line back in the timing jumped to 6 degrees BTDC indicating to me the vacuum advance was working at low RPM. I increased the RPM's to 1200 RPM and the timing advanced to 18 degrees BTDC, I increased to 1400 RPMs and the timing advanced to 25 degrees BTDC. When I opened it to full throttle the engine only got up to 1900 RPM's however the timing stayed at 25 degrees BTDC. Is this normal??? Shouldn't the timing keep advancing as the RPMs go higher or is there a limit? I couldn't find any specs for this. I opened the distributor cap and didn't notice anything broke or missing. Thoughts?

I don't think its the clutch as I know the engine would easily rev to 2000 rpms before I started having this issue. Now I open to full throttle and it jumps to 1900 RPMs for a few seconds then drops down to 1600 RPMs.