The Truck of Doom: An Occasional Build Thread

[Dorsai]

It doesn't look bad at all. I don't think there's anything to fear.

OK, thanks for the feedback folks. I'm especially happy to hear those circles are plugs and not passages of some kind.

For the record, I'm not too worried about current sludge; I think I drive the truck frequently enough, and for long enough distance, that I'm not generating new sludge. What we're seeing here is the legacy of the 14 years where the truck was barely driven, if at all, with only the occasional startup in the shed when my grandmother thought about it.

 

[Dorsai]

OK, thanks for the feedback folks. I'm especially happy to hear those circles are plugs and not passages of some kind.

For the record, I'm not too worried about current sludge; I think I drive the truck frequently enough, and for long enough distance, that I'm not generating new sludge. What we're seeing here is the legacy of the 14 years where the truck was barely driven, if at all, with only the occasional startup in the shed when my grandmother thought about it.

OK, I could use a little advice here...this air pressure thing to hold the valves in place isn't working at all. I know, I know...get some rope. While I'm doing that, though, I'd sure like to understand why this isn't working.

I've got the #1 cylinder at TDC. I know this because (a) the piston is at the top of its travel as confirmed by both the drinking straw trick as well as looking directly at the piston through the plug hole, and (b) the distributor rotor is pointed at the #1 plug wire. So...TDC on the compression stroke, right?

Then I apply the air. I've got a rubber hose that screws into the plug hole, and is coupled to my air compressor hose. My air compressor is nothing special - it's one of these 20-gallon units from Northern, but it's served me well so far. The tank starts at 100psi, and when I open the valve to the hose the pressure drops to 85 or so. This would be fine, except I can hear air moving through my engine. After 30-45 seconds the pressure drops to 65-70, at which point the compressor kicks on...but can never catch up with the air loss through the engine. From what I've read that pressure is good enough to keep the valves in place, but I don't want my compressor running at 100% duty cycle for the length of time it'll take me to do a cylinder.

I thought maybe I was just a little off on my positioning, so I tried rotating the engine just a bit. It never got any better, and if I got more than a couple degrees off what I determined to be TDC in either direction, the air pressure would then rotate the engine on its own, which is of course no good.

So what's going on here? Is this normal, and my compressor is just too weak for the job? Do I have crusty valves that don't seal up enough for this? Or was this a dumb idea from the get-go, and I should just stick to rope?

[85lebaront2]

OK, I could use a little advice here...this air pressure thing to hold the valves in place isn't working at all. I know, I know...get some rope. While I'm doing that, though, I'd sure like to understand why this isn't working.

I've got the #1 cylinder at TDC. I know this because (a) the piston is at the top of its travel as confirmed by both the drinking straw trick as well as looking directly at the piston through the plug hole, and (b) the distributor rotor is pointed at the #1 plug wire. So...TDC on the compression stroke, right?

Then I apply the air. I've got a rubber hose that screws into the plug hole, and is coupled to my air compressor hose. My air compressor is nothing special - it's one of these 20-gallon units from Northern, but it's served me well so far. The tank starts at 100psi, and when I open the valve to the hose the pressure drops to 85 or so. This would be fine, except I can hear air moving through my engine. After 30-45 seconds the pressure drops to 65-70, at which point the compressor kicks on...but can never catch up with the air loss through the engine. From what I've read that pressure is good enough to keep the valves in place, but I don't want my compressor running at 100% duty cycle for the length of time it'll take me to do a cylinder.

I thought maybe I was just a little off on my positioning, so I tried rotating the engine just a bit. It never got any better, and if I got more than a couple degrees off what I determined to be TDC in either direction, the air pressure would then rotate the engine on its own, which is of course no good.

So what's going on here? Is this normal, and my compressor is just too weak for the job? Do I have crusty valves that don't seal up enough for this? Or was this a dumb idea from the get-go, and I should just stick to rope?

Matthew, if the air is going through the engine, not out the ports, it's your rings that are weak.

[Dorsai]

Matthew, if the air is going through the engine, not out the ports, it's your rings that are weak.

Well, I was speaking casually there - I don't know that the air is going through the engine, exactly, I could just hear air moving. I played with it a little more after I last posted...I thought I felt air coming out above the engine. I didn't detect anything coming out of the exhaust or the carburetor, but I don't know if holding my hand over those would be enough in any case.

I've never done a leakdown test, but my last compression test was good, 160+ on all cylinders.

[Gary Lewis]

Well, I was speaking casually there - I don't know that the air is going through the engine, exactly, I could just hear air moving. I played with it a little more after I last posted...I thought I felt air coming out above the engine. I didn't detect anything coming out of the exhaust or the carburetor, but I don't know if holding my hand over those would be enough in any case.

I've never done a leakdown test, but my last compression test was good, 160+ on all cylinders.

You are doing a leak-down test, sorta. The difference is that in a real leak-down tester there would be a very small orifice, usually .040" in diameter, between the gauge at the tank and another gauge right ahead of the spark plug adapter.

So if you are truly on TDC on the compression stroke and you don't hear any air escaping through the exhaust, carb, or radiator neck, then it is all going into the sump - which you should hear in the dip stick tube, PVC port, and oil fill on the valve cover. And that means it is going past the rings, meaning they are worn. Not necessarily bad, but worn.

If it was me I'd rotate the engine while watching the valves and make sure that when it is at what you think is TDC on the compression stroke that both valves are closed on #1 cylinder. If you find that you were already at that spot then you may need the rope trick to keep the valve up if the compressor can't keep up.

But now you are wondering how worn your engine is. I'd buy a leak-down tester, maybe something like this tester from Amazon, and check. But you already know that the engine runs well, so perhaps you don't really care how much leakage you have. Just run it in that case.

 

[ArdWrknTrk]

Well, I was speaking casually there - I don't know that the air is going through the engine, exactly, I could just hear air moving. I played with it a little more after I last posted...I thought I felt air coming out above the engine. I didn't detect anything coming out of the exhaust or the carburetor, but I don't know if holding my hand over those would be enough in any case.

I've never done a leakdown test, but my last compression test was good, 160+ on all cylinders.

I was going to say you've just performed your first leakdown test.

But, what is the SCFM rating of your compressor?

Have you tried putting a spoon of oil in that cylinder and seeing how long it takes to blow out?

With the valve cover and/or pushrod plate off there may be too many places for air to escape that it will make a noticeable breeze.

 

[Dorsai]

But now you are wondering how worn your engine is. I'd buy a leak-down tester, maybe something like this tester from Amazon, and check. But you already know that the engine runs well, so perhaps you don't really care how much leakage you have. Just run it in that case.

You're right, I am wondering that. I do care how much leakage I have, although I'm not particularly worried about it at this stage - as you said, the engine runs well. I had it in my mind that leakdown testers were more expensive than that which is why I've never done one. I'll probably buy that one, but not until after I'm back from OK...I'm already getting side-eye from the wife every time a new package shows up, and that's without her inspecting the credit card statements since I started working on the truck in June.

Oh, and thanks for the excellent explanation of what's (probably) going on. I see rope in my immediate future.

But, what is the SCFM rating of your compressor?

Have you tried putting a spoon of oil in that cylinder and seeing how long it takes to blow out?

I don't seem to have the manual for my compressor, so no idea on the SCFM - but it's surely quite small, this is a very basic compressor that was given to me as either a birthday or Christmas gift (I don't remember which). And no, I haven't done the oil thing...the goal of this effort is to get less oil in the cylinders, so I'll save that test for later.

 

[Gary Lewis]

But now you are wondering how worn your engine is. I'd buy a leak-down tester, maybe something like this tester from Amazon, and check. But you already know that the engine runs well, so perhaps you don't really care how much leakage you have. Just run it in that case.

You're right, I am wondering that. I do care how much leakage I have, although I'm not particularly worried about it at this stage - as you said, the engine runs well. I had it in my mind that leakdown testers were more expensive than that which is why I've never done one. I'll probably buy that one, but not until after I'm back from OK...I'm already getting side-eye from the wife every time a new package shows up, and that's without her inspecting the credit card statements since I started working on the truck in June.

Oh, and thanks for the excellent explanation of what's (probably) going on. I see rope in my immediate future.

But, what is the SCFM rating of your compressor?

Have you tried putting a spoon of oil in that cylinder and seeing how long it takes to blow out?

I don't seem to have the manual for my compressor, so no idea on the SCFM - but it's surely quite small, this is a very basic compressor that was given to me as either a birthday or Christmas gift (I don't remember which). And no, I haven't done the oil thing...the goal of this effort is to get less oil in the cylinders, so I'll save that test for later.

I think you are on the right path, Matthew. Don't worry about the leakage as the engine runs well.

As for the leak-down tester, you might shop around a bit. That one has some reviews that aren't flattering. But you want one with two gauges if possible so you can set your air compressor far away and still know what the inlet pressure is and be able to adjust the regulator to something consistent from cylinder to cylinder, like 100 psi.

[ArdWrknTrk]

I think you are on the right path, Matthew. Don't worry about the leakage as the engine runs well.

As for the leak-down tester, you might shop around a bit. That one has some reviews that aren't flattering. But you want one with two gauges if possible so you can set your air compressor far away and still know what the inlet pressure is and be able to adjust the regulator to something consistent from cylinder to cylinder, like 100 psi.

Worse comes to worse I'll be glad to loan you my leakdown tester.

PM me your address, and I'll put it in the mail.

[grumpin]

Worse comes to worse I'll be glad to loan you my leakdown tester.

PM me your address, and I'll put it in the mail.

Bought one of these back in the mid 80’s, so I think the longevity is good. I used to use it a lot.

Got some adapters awhile back so I could use it on my vehicles. Haven’t used it on my vehicles yet though.

https://aircraft-tool.com/shop/detail.aspx?id=2E-KIT