Misfire after Head Conversion

[ledders]

Good morning and Happy New Year.

Recently converted the cylinder heads for unleaded on my 1972 V8 and after refitting everything yesterday we have a serious misfire and inability to rev the engine (full throttle achieves 10k revs). We are also getting serious blow back on one side of the engine. All breathers were renewed, fuel filter replaced, and fuel pump serviced as part of the upgrade.

We took great care in labelling everything we removed and ensured all was refitted in its original location.

Before we strip down the top half again does anyone have any experience/ideas on what we should/could look into first.

Many thanks

[enigmas]

Hi Ledders, if your talking about a Rover V8, the engines came with hardened seats from the Factory, so I'm puzzled as to what you did to the cylinder heads to make it suitable for unleaded!

By 10k revs I presume you mean 1000 rpm not 10,000 rpm (which is 10k)

If the engine now has lots of blowby I presume you fitted new rings but didn't hone the cyls?
As for the inability to rev properly, this could be due to a whole range of assembly and tuning issues.
These may include:
* Incorrect valve timing,
* Incorrect ignition timing.
* Incorrect preload on the hydraulic lifters, etc,

You need to elaborate on the work you or someone else carried out on the engine precisely otherwise it simply becomes a big guessing game.

[ledders]

Thanks for your reply Enigmas

Simply had the cylinder heads cleaned and overhauled with new valves and hardened high profile valve seats. Nothing to the lower half of the engine was undertaken.

Was aware of the unleaded compatible existing valve seats but the valves were worn and needed replacing anyway,

Does this help.

[ledders]

And yes - I did mean 1k revs!

[lagain]

Are the plug leads going to the correct plugs ?

[ledders]

Yes we marked them all upon removal

Thanks Lagain

[harvey]

If the valve seats have been set too far into the head then you can get the same preload issues that you get by skimming the head face too much.

[enigmas]

Harvey makes a good point. The hydraulic lifters may have too much preload and are holding the valves slightly open.

I'm assuming your cylinder head machinist guru also ensured the valve stem above the keeper and collet was exactly the same height as the OEM factory valves. ie. If the top of the valve stem is taller the rocker will add additional preload to the hydraulic lifters.

Check out the diagram below. Unfortunately if this is the case you may have to remove the inlet manifold and valley cover to check the preload or you can take a guess and fit shims of equal thickness under the rocker pedestals. Keep in mind that the oil feed from the block to the rocker gear is under one of the pedestals on either bank.

If you have a set of calipers check the amount of material at the top of the valve stem from the collet groove. Old and new should match.

Another option is to buy a set of adjustable pushrods. If you choose this option you'll only need to remove the rocker gear.

[johnwp5bcoupe]

I did discuss this yesterday with your mechanic checking the preload is a must!! after having the heads skimmed, the simple first check was to check the compression.
In theory as you have used tin gaskets again whatever they removed from the head the same thickness in packers should be put under the rocker shaft pillars "if you hadn't done anything else"! but you have had new seats and valves etc so setting the preload is a must.

[ledders]

Just a quick thanks to all for your advice. Problem rectified over the weekend via a combination of new HT leads, plugs, and lifters/tappers. Ready for the body shop now,

[ledders]

Tappets!

[enigmas]

Tappets (hydraulic lifters)? What was wrong?

or was it to do with incorrect ignition timing or faulty ignition components?

[ledders]

Morning Enigmas, it was the HT leads and/or the plugs. They must have deteriorated while the heads were off (storage was a bit damp so perhaps this was a factor).

With 55k on the clock, the engine stripped down and access to original parts we thought it a convenient time to replace the hydraulic lifters.

Thanks again