[supersport#69]

So a 400 with 68k original miles has never been rebuilt. If cylinders are reading between 140-170 now, what could I expect to see if the heads are rebuilt? Assuming bottom end is ok, would motor compression be close to original or not much increase?
Either way mine are having a valve job in 2 weeks so just curious if it’ll breathe life back in it.

[chiphead]

Not much unless the exhausts are burned out and not sealing or you had gasket issues. Cylinder pressure is primarily driven by static compression ratio and ring seal. A leakdown test will tell where the cylinder pressure is going. If the test determines that the valves are not sealing, then a good valve job or replacement of valves and seats will help. If the air is getting past the rings, well........dance the rebuild tango.

If the motor is weak sauce, I'd confirm the state of tune, static compression and camshaft condition if the leakdown numbers are good.

[supersport#69]

The mechanic thinks I have a burnt exhaust valve (long story but about 110) in that one so hoping there’s a fix there and improvement in others.

[i82much]

maybe take some off the heads if you want more compression?

[steve25]

A leak down test is what needs to be made baring the cylinder that has a bad valve and is only showing 110 psi.

The factory cast Pistons from motors in your mileage range that have been run with long oil change intervals ( or even dirty oil for 5000 miles ) will show .002" or more vertical ware in the ring groves beyond the needed clearance,and then that's it for trying to hold compression in the cylinders without hearing leakage into the bores / crankcase during a leak down test .

And this without factoring in scratches that may be in the bores!

[geeteeohguy]

Originally Posted by steve25

A leak down test is what needs to be made baring the cylinder that has a bad valve and is only showing 110 psi.

The factory cast Pistons from motors in your mileage range that have been run with long oil change intervals ( or even dirty oil for 5000 miles ) will show .002" or more vertical ware in the ring groves beyond the needed clearance,and then that's it for trying to hold compression in the cylinders without hearing leakage into the bores / crankcase during a leak down test .

And this without factoring in scratches that may be in the bores!

I've been telling him this for the past month but he doesn't want to hear it.
A leakdown test is the FIRST thing to do before even pulling the heads.

[STEELCITYFIREBIRD]

From experience, a fresh valve job can and will amplify any ring seal leakage.


+++ On leakdown testing!!!!

[supersport#69]

Originally Posted by geeteeohguy

I've been telling him this for the past month but he doesn't want to hear it.
A leakdown test is the FIRST thing to do before even pulling the heads.

Lol I am listening but the mechanic said he already ran the tests. He says burnt exhaust valve. I’m not going to tell him he’s wrong as he seems to know his stuff.
You are welcome to come over and see what you think . Maybe drive it around, etc too. I am not looking to waste money, just trying to make it right.

[chiphead]

Ask him the leakdown percentages and how much worse were the cylinders that had poor cranking compression numbers. Ask him to confirm ring seal was good on all 8 holes before putting $$$ in the heads. You can also tell by cylinder wall condition when you get the heads off. If you've got a ridge and deep scratches on the cylinders, I wouldn't put a bunch of money in the heads, because it's coming apart again, and soon.

Head work includes verifying guides, springs, seals, valves, seats, etc etc are all serviceable or needs replacement. Valve job won't last long if the valves have no head margin, stems show wear and the guides are sloppy. Specifically discuss things like replacing guides or using guide liners if guide work is needed. Some cheap shops will knurl the guides and tell you the guides are good. And then you wonder 5000 miles later why it's smoking like George Burns.. Consider cam swaps or other things you'd like to do, that affect head set-up.

Good head work is $$$, so be ready for the BOAT. Break Out Another Thousand.

[geeteeohguy]

Originally Posted by chiphead

Ask him the leakdown percentages and how much worse were the cylinders that had poor cranking compression numbers. Ask him to confirm ring seal was good on all 8 holes before putting $$$ in the heads. You can also tell by cylinder wall condition when you get the heads off. If you've got a ridge and deep scratches on the cylinders, I wouldn't put a bunch of money in the heads, because it's coming apart again, and soon.

Head work includes verifying guides, springs, seals, valves, seats, etc etc are all serviceable or needs replacement. Valve job won't last long if the valves have no head margin, stems show wear and the guides are sloppy. Specifically discuss things like replacing guides or using guide liners if guide work is needed. Some cheap shops will knurl the guides and tell you the guides are good. And then you wonder 5000 miles later why it's smoking like George Burns.. Consider cam swaps or other things you'd like to do, that affect head set-up.

Good head work is $$$, so be ready for the BOAT. Break Out Another Thousand.

This is right on the money. If the tech did a proper leak down test, with the leakdown tester, he will have the percentage of leakdown for each individual cylinder. That's how an engine's health is baselined. If all the other cylinders were 20% or less leakdown (10% or less is optimal) and the one was 30-80%, then you have data to work with. Normally a burnt valve will give you 80-100% leakdown in that cylinder. If one cylinder does indeed have a bad valve, and you have 30% leakdown in the other 7 cylinders, a valve job will lead to other problems. It is imperative that the condition of ALL the cylinders (leakdown) is measured and documented before going further. Can your mechanic give you any percentage numbers, and did he do a real leakdown test? (Test takes an hour or so to do getting every piston on TDC to check).

[supersport#69]

My guess is no. I think he only checked the passenger side by the low cylinder. Two were about 145, 1 was 170 and the low was 100-110.

I’m pretty sure he didn’t check the drivers side.

My guess is he blew air in the low one and listened for where it went. I’m guessing of course because I didn’t see data.

He did say he check all cylinders and retest before pulling heads though.

This is not relevant but the the part on the qjet that drops down when it’s warming up is not dropping on its own. Is there something I can do to remedy that? I started it today and it was really high and tapping pedal wouldn’t release it. I took off the air cleaner to tap it manually.

[steve25]

The on the shelf product called restore is they only thing I have ever used that got me back better compression levels in normally worn cylinders.

[Kenth]

Sounds like the choke pull-off don´t hold vacuum.

[supersport#69]

How do I fix that choke pull off so it does?

[steve25]

How did a choke pull off get into this conversation?

[Kenth]

Originally Posted by supersport#69

How do I fix that choke pull off so it does?

If the pull-off is non-functional you will need a replacement.
What is the application number on your carb?