High rpm misfire under load

[sowen]

This has been an ongoing issue I've had with my SD1 since I got it, as the revs go over 4000 it begins to sound like it's misfiring under the bonnet, peaking around 5000rpm. I was out and about earlier and held it in gear for a little longer and it seemed to pull through, not sure of what rpm the miss quietened down at?

Since getting my SD1, I've had it running on carbs, hotwire efi and now Megasquirt, the ignition system has changed from the original dizzy to EDIS8, and the problem has persisted exactly the same. Earlier today with the help of my dad we re-checked the ignition timing, did a few runs with different advances to rule out detonation, and done a compression test which gave 135-150psi on all 8 cylinders.

Another more recent problem is there's a light top end tapping at low rpm, with the engine at operating temperature, generally at light load cruising. Backing off or pressing the throttle makes it go away, sounds like one of the lifters has lost pressure and it does it most of the time. I'm not sure if this is related to the high rpm misfire problem? I have been wondering if it's one of the rods as it does it generally only at cruising loads?

My dad suggested the high rpm misfire could be some of the valves floating, a quick Google search does show very similar symptoms, as the engine has always felt flat at higher rpms, it doesn't pull hard like my old Land Rover did. Low to mid rpm it's fine, just lacks the ooomph when the throttle is held wide open

Any thoughts and suggestions before I go and tear the top end apart?

[DaveEFI]

SD1s in fine fettle tend to kick quite hard as the revs rise. If yours doesn't, IMHO, given how much you have changed with no difference, the most likely cause is a worn cam. Especially if you don't know when it was last changed.

[ChrisJC]

Take off the rocker covers and measure the valve lifts.

Chris.

[sowen]

SD1s in fine fettle tend to kick quite hard as the revs rise. If yours doesn't, IMHO, given how much you have changed with no difference, the most likely cause is a worn cam. Especially if you don't know when it was last changed.

I'm pretty sure I had a look at the cam when the manifold was off and it looked good, lobes all intact and not rounded off. As things are going I'll be most likely having another look soon

[sowen]

Take off the rocker covers and measure the valve lifts.

Chris.

I think I've still got a bit of time to make a start doing that this evening, easier than pulling the top end apart!

[v8250]

My dad suggested the high rpm misfire could be some of the valves floating, a quick Google search does show very similar symptoms, as the engine has always felt flat at higher rpms, it doesn't pull hard like my old Land Rover did. Low to mid rpm it's fine, just lacks the ooomph when the throttle is held wide open

Any thoughts and suggestions before I go and tear the top end apart?

This was my first thought, valve float. When was the last time the heads were overhauled/rebuilt? The valve float could come from a number of worn components and it could just be that time for the heads to be refurbished.

[sowen]

My dad suggested the high rpm misfire could be some of the valves floating, a quick Google search does show very similar symptoms, as the engine has always felt flat at higher rpms, it doesn't pull hard like my old Land Rover did. Low to mid rpm it's fine, just lacks the ooomph when the throttle is held wide open

Any thoughts and suggestions before I go and tear the top end apart?

This was my first thought, valve float. When was the last time the heads were overhauled/rebuilt? The valve float could come from a number of worn components and it could just be that time for the heads to be refurbished.

I've got the engine mostly stripped down now, it appears to have never been opened up before except for removing the inlet manifold and rocker covers. Cam chain is suprisingly good, very little slack and wear on the teeth, lobes are all good and pointy still, and the cam is lined up with the crank. Only one head is off so far (ran out of time), and as I've not found anything conclusive so far the next job will be to remove all the valve springs for measurement and check the valve seats etc

[ChrisJC]

Maybe worth checking the cam timing is right with a protractor or something rather than assuming the timing marks are correct.

Chris.

[DaveEFI]

Other thing I suppose is possible is a tappet or two pumping up prematurely? But then you said you could drive through it?

Any worn cams I've seen are pretty obvious, with the wear not being even to all lobes. So a simple check with a calliper shows a difference.

If it would help, I could easily try your MS map on my SD1 - I have e pretty well standard 3.5 Vitesse engine with MS2 and EDIS.

Does an MS log show anything?

Last thing which occurred to me - what plugs are you using?

[sowen]

Maybe worth checking the cam timing is right with a protractor or something rather than assuming the timing marks are correct.

Chris.

Not a bad idea, I do still have the cam, chain and lifters from my last V8, I could compare or pop them in to see if there's a difference?

Other thing I suppose is possible is a tappet or two pumping up prematurely? But then you said you could drive through it?

Any worn cams I've seen are pretty obvious, with the wear not being even to all lobes. So a simple check with a calliper shows a difference.

If it would help, I could easily try your MS map on my SD1 - I have e pretty well standard 3.5 Vitesse engine with MS2 and EDIS.

Does an MS log show anything?

Last thing which occurred to me - what plugs are you using?

I've junked plenty of worn cams in the past, this ones nice, very similar lift on all 16 lobes. I was within easy towing distance of home and a nice long straight bit of road so held it in 1st and 2nd a bit longer than normal as it didn't matter as much if it blew at that point, almost got nothing to lose now. Plugs are NGK, replacing the Esso branded ones I had in it when bought!

[DaveEFI]

Can you give the plug number?

[Ian Anderson]

should have started engine in the dark and looked at distributor

To me it sounds like some cross arcing of spark plugs

and 4000 rpm - your dizzy could jut be adding enough advance to start it at the high revs

Ian

[sowen]

Can you give the plug number?

I forgot this evening, too busy getting the engine out and on the floor

should have started engine in the dark and looked at distributor

To me it sounds like some cross arcing of spark plugs

and 4000 rpm - your dizzy could jut be adding enough advance to start it at the high revs

Ian

It's now on coilpacks, exact same issue as on distributor. One of the last runs I did on Sunday evening was adding more advance in the upper rpm area of the ignition map and it seemed to pull a bit better. I was at the stage of fiddling with the advance map after every bit of straight road just to try to get a change in the feel of the acceleration as a final attempt before looking inside the engine.

[DaveEFI]

I asked about the plug type since when I was going EDIS, the lot on the MS forum were adamant you must use resistor plugs, so I did. But I don't know what happens if you don't, as I've not tried it. However, if you were getting same misfire with the dizzy, it's unlikely to be that.

[sowen]

I asked about the plug type since when I was going EDIS, the lot on the MS forum were adamant you must use resistor plugs, so I did. But I don't know what happens if you don't, as I've not tried it. However, if you were getting same misfire with the dizzy, it's unlikely to be that.

Forgot to check again, I think they were listed as suitable for the 4.0 and 4.6 when I bought them, the engine runs that's all I can say.

Clearly I wasn't looking hard enough at the camshaft, I guess I've found the problem now!