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Whoa is me, truck running poor!

Posted: Sun Oct 18, 2020 5:19 pm
by Pinzengauer
Well, last Monday morning the truck didn't want to start, but she did. I chalked it up to being Monday and I didn't want to go to work either. :mrgreen:

Truck started better for driving home, however, her running condition got worse.

So last week I put in a new fuel filter, which helped on the power loss, or so it seemed. I adjusted the valves as well as the ignition timing. It was popping less and running better.

While adjusting the valves I usually put my finger in the spark plug hole to confirm I'm coming to TDC. I felt no compression on number four, which when I bought the truck that number piston had a hole in it. So that was fixed April 2019.

Today I did a leak down test and the results are as follows.
FYI, I used a 12v air compressor for the test, normally for filling tires but that's what I have.

#1 52% leak @ 23 psi leaking past the rings
#2 51% leak @ 32 psi leaking past the rings
#4 87% leak @ 8 psi leaking out the exhaust
#3 52% leak @ 22 psi leaking past the rings

The compression tester has a "set" range where you should pressurize the cylinder until the needle reaches that target. The needle never got close, perhaps because of the little 12v compressor. However, the readings posted above would likely be the same either way, but this was my first ever leak down test so I don't know. I should add these numbers were with the compressor running.

The truck was running better Friday than it was Monday, but that's not saying much. This is a daily driver, ten miles to work and ten back.

Hoping to borrow a compression tester tomorrow.

Am I doing damage to the motor driving it? What would your next step be? Seems the head needs worked on number 4.

I'd like to do all four cylinders. Whether that means boring the jug or just new rings or what not.

More experienced mechanics, please chime in.

81 712 Ambulance.

Re: Whoa is me, truck running poor!

Posted: Sun Oct 18, 2020 5:33 pm
by Pinzengauer
Additional information;

I live at 8000'

Installed rebuilt carbs spring 2019

Weeks ago carb base nuts were loose as well as the top bolts. All have been tightened. Did not coincide with rough running.

Carbs were rejetted for altitude.

As for running rough, low power off idle, checked the fuel solenoids, they're working.

Might check the air intakes for leaks tonight, does that insulation have to be on there? I assume steyr put that on for a reason.

I plan to synch the carbs but I have the SSI on it's way.

Re: Whoa is me, truck running poor!

Posted: Mon Oct 19, 2020 3:15 pm
by Joeri
Sommething is wrong with cyl #4 :((

Re: Whoa is me, truck running poor!

Posted: Mon Oct 19, 2020 9:17 pm
by Pinzengauer

Is there a source for new pistons? There must be many piston manufacturers, I'd like to think that there's an option outside of buying used or nos parts.

Re: Whoa is me, truck running poor!

Posted: Mon Oct 19, 2020 10:08 pm
by rmel
Yes, Pistons can be had; Stock ala SAV/EI, a few different diameters
for recommended re-worked bores. Or there is custom options e.g. Aries
Pistons but then your fitting your own rings and a far more expensive route.

Based on your leak-down test, if you are sure the leak is to the exhaust,
then I'd be suspicious of the Exhaust valve -- e.g. valve seat recession.
Your probably not going to be this lucky, but try re-torquing the head
to specification, look for any carbonization at the head/jug interface
as a clue.

Failing the above, your problem may be the heads and not the pistons/rings.
If you have near zero pressure on #4 you have no recourse but to remove the
head to inspect. Then you can confirm if it's the valve and if not then it's the
rings. Then you have to decide if this is a localized #4 problem either new
rings or one head repaired. Or an excuse to do a complete top end rebuild.

If you go that route there are several important techniques that need to be
followed which are not well documented.

Re: Whoa is me, truck running poor!

Posted: Tue Oct 20, 2020 1:01 am
by Joeri
Like discribed as above i do think its a valve problem ( # 4 exhaust) so the head has to come of for inspection , this is easy to do.
If it dousnt smoke to much blue by taking oil and underpressure through the pistonrings i would leave it like it is ,but if you want to make it 100% a light sanding ( honing) the cyl. Barrels and New pistonrings would be the best solution.