some things gone wrong!!!

[Morrisminormo]

Hi

My poor car has stopped firing on the two cylinders closest to the radiator. For the past few weeks I have had random miss fires but its been that intermittent I could never pin point it. I use my car daily and go on the motorway also.
I can get the car to start but it does sound rough, and if you rev it you can tell something is out of place. Could I have burned out valves?? as its not a unleaded head?? I am yet to remove head but I would like your thoughts. It is a standard 948cc engine. If it is not the head and it is piston/main engine problem I may buy a 1098cc engine and gear box as this may have a few extra horses that will benefit me.

cheers

[DAVIDMCCULLOUGH]

Is there any oil in the water, if you take the plugs out are they wet? Sounds like the headgasket is gone between the 2 cylinders. I would check and dont drive it until you have a good look.

[Morrisminormo]

Hi
Yes not driving it now got my "spare car" to use

I've dipped the oil and nothing there, will check plugs in the week. I will be taking the head off either way now. I obviously need to make sure I make it unleaded or go for a head upgrade. If when I take the head off there is damage to pistons I may get another engine.

Thanks for your reply ( fingers crossed it's just heAd gasket)

[bmcecosse]

Blown head gasket....off with it's head........

[Morrisminormo]

What do you reckon bmc

A 12g head while it's off??

[kevin s]

Check the valve clearances first, you may well have valve seat recession, re-setting the clearances will get it going fine for a while but you will need to get hardened valve seats before to long.

[howard jones]

Just following this thread ....excuse my ignorance but what's a 12g head ?

[Morrisminormo]

12g940 head

Just fitting a standard 940 head on a 1098cc will give about 8-10bhp more!!

So I've been told

[bmcecosse]

Indeed it will - but larger carb required too to flow the extra air!

[David W.]

Exactly my symptoms, it was the head gasket. Don't just replace it without checking head flatness. I could see a lot of light beneath a straightedge. Follow the rules about tourquing the head, adjust the valves, run it a good heat cycle, torque and adjust again.

[Morrisminormo]

There ISVs 12g940 head on eBay but the bloke said

"Dear richard_bowker,

It would bolt on but the valves would hit the block.
You would have to get the block machined/pocketed.
I have a 998cc head recon that would fit"

I wonder what the 998cc head is and is the gas flow improved?

[bmcecosse]

A 998 head is fine (no improvement) for a 948 engine - but smaller valves than a standard 1098 head . The 940 fitment is well understood......

[David W.]

"It would bolt on but the valves would hit the block.
You would have to get the block machined/pocketed."

Is that true? Is that where the 0.320 measurement comes from? Does the 940 head start with the standard 2.750 thickness?
David

[MarkyB]

The 0.320 measurement is the valve lift + a bit.
Most if not all the standard heads are 2.750 thickness.
It's worth measuring as you don't know if the valve has sunk or the head has been skimmed in the past.

[les]

I'd be inclined to put it all back as it was after checking the gasket. Get it working again before all this modification stuff. Then If you want to play at least you know things are at there best before starting. You know, that exhaust you've got could cause overheating, exhausts are not just a pipe with a box stuck along it somewhere, they are made to suit other criteria! Even more critical on a 2 stroke engine.

[Morrisminormo]

Hi les

Only two days after putting my exhaust on then this

Took the head off tonight. Doesn't look like head gasket blown. The valves on 1 and 2 are really black! I am getting a unleaded head to put back on. I was thinking about a different heads but everything seems to complicate something else. I thought about changing my engine and box for a 1098. But it's a bit drastic really

Looks like a call to ESM in the morning for a recon 948cc head

[bmcecosse]

Forget the 'unleaded' head - shocking waste of money.....for a 948 engine. If you really want one (for the 948 engine) then pick up a 998 Mini head very cheaply. Your mis-fire then is likely down to an ignition problem - try dizzy cap and/or HT leads... Any chance the HT leads were mixed up when working on the exhaust????? Check the firing order 1342 anti-clock round the cap. It's too late now - but a compression check before lifting the head would have been a good idea. So - I suggest you grind in the valves - and reassemble with a new gasket - and check the ignition system!

[Morrisminormo]

Hi bmc

I did about 200 miles in two days since the exhaust was fitted so sure ignition system is ok. Compression test would of been helpful but unfortunately bit carried away

If say two of my valves are burned out wouldn't that be due to valve seat recession?

Cheers

[bmcecosse]

Could be - but that's only because you may have failed to check/set the valve gaps correctly. Take the valves out and have a look! And two at the same time?? Valve seat recession doesn't just happen in 200 miles!

[David W.]

Do a good "autopsy" as you dismantle the head. Look carefully at the contact marks on all 8 valves and seats where they make contact. Put a straightedge across the head. Shine a light behind it and look for light leaking under the edge. Do the same for the block.