V8 only firing on every second cylinder? Any Ideas

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frank_grimys, Apr 6, 12:48am
I have a rover 3.5L V8 in a jetboat, took it out on Taupo a couple of weeks back, went for a nice blat out kinloch way, ran fine for an hour or so, then started to miss in one cylinder, so headed home, by the time I got to shore I was running on 3 cyl (figured out through plugs).Went about eliminating issues - replaced plugs, HT leads, coil, points, condenser, dizzy cap, rewired signals from dizzy and + to coil, ignition, took apart carb and cleaned, checked float level, pressure on fuel pump.It idles fine, can rev out to 3k rpm when cold under load, but after things warm up starts missing again and running on 4 cylinders (I know from the plugs).Can rev out to redline with driveshaft dissed (No load) regardless of engine temp.Just realised last night thatit's only firing every second cylinder in the firing order, the other 4 are covered in black soot.Any avenues I could look into now, because I'm close to taking the damned thing out to the lake without bungs, so when it stops it stops for good.
c.c.d, Apr 6, 12:52am
Start with a compression test.
Then all ok check the HT leads are on correct and in the correct firing order. If has a 4 barrel carb and depending on intake manifold setup may have blocked jet on one side.Any water on plugs also !
frank_grimys, Apr 6, 12:57am
All cylinders are pushing 150-160.No water on plugs, 2 barrel 500 holley - cleaned out with the air gun and brakleen where applicable.HT leads are all in order, as if they were out of order it wouldn't idle so well, or rev out with no load.

ETA the plugs that are firing are perfect as far as colouration, and have been swapped around to check that the new plugs somehow weren't up to scratch
johnf_456, Apr 6, 1:01am
When you say replaced "Went about eliminating issues - replaced plugs, HT leads, coil, points, condenser, dizzy cap". What were these replaced with 2nd hand parts, tested parts, new parts!
frank_grimys, Apr 6, 1:03am
Brand new, it's a boat, so reliability is somewhat essential, if I replace, it's replaced with new
kingfisher21, Apr 6, 1:10am
Remove the rocker covers and check for lift on all the valves, these engines are known for very severe camshaft wear, not uncommon for a lobe or a few lobes to be gone completely.
frank_grimys, Apr 6, 1:19am
Checked lift and they look fine, I read up that they can have lobes wear circular, some pretty spectacular pictures too.If they were worn though, they wouldn't all of a sudden go from one, to two, then three etc cylinders down, and it wouldn't be able to rev out without load like it does.

Firing pattern is 1-8-4-3-6-5-7-2.My engine is doing 1, missing 8, 4, missing 3, 6, missing 5, 7, missing 2.Possible an advance isn't kicking in or something!I know the vacuum advance is poked, but it has been for all the time I've ran it without issue
frank_grimys, Apr 6, 1:22am
Runs pure glycol through the engine, and oil pressure is stable at around 2bar, neither show signs of head gasket issue
kingfisher21, Apr 6, 1:30am
Hmm, are the plugs on the missing cylinders wet or dry!
les6, Apr 6, 2:00am
try a hotter range sparkplug!
frank_grimys, Apr 6, 2:00am
Covered in black soot,

They are the right plugs for the engine, fancy bosch ones with the two points on the tip to increase spark size.
unclejake, Apr 6, 2:04am
I can't help but think that the order of the miss-fire is a red herring. If the motor runs strong when cold then your problem is related to either temperature or a pressure change.

You have replaced the obvious stuff (good call), so perhaps it is worth checking that the gasses can get out of each cylinder well.

I would also do a coolant system pressure check when the engine is hot. If you have an open circuit cooling system then it will be a bit more work to do the pressure test so perhaps a hot leak-down test would be better.

Having said all that ^, it still sounds like an ignition system problem. It seems as if the coil doesn't get a chance to build up a decent spark in time for the next cylinder. A crook alternator could explain that.

Good luck mate.
les6, Apr 6, 2:05am
the load on a boat motor is not the same as car!something not right!check the points and even try the old condenser!have heard of faulty ones from new!
gadgit3, Apr 6, 2:05am
Whats the mechanical advance like! still free to move!
bwg11, Apr 6, 2:43am
frank_grimys wrote:
All cylinders are pushing 150-160.No water on plugs, 2 barrel 500 holley - cleaned out with the air gun and brakleen where applicable.

Probably a long shot and I am not familiar with the Rover V8, but is the inlet manifold just a gallery type feeding all cylinders from both chokes of the Holley, or is it more a sophisticated design to separate the inlet tracts with each choke feeding alternate cylinders. If it is the latter, I would look for leanness one one choke, will would allow to rev out without load, but not with load.
elect70, Apr 6, 3:25am
Do comp testbeforereplacing any parts , bet head gaskets blownjust had to do my disco 3.9 & exhaust valveTop gasket setex SCACL$180valve $40 .well known for it .Cam wear doesnt happeninstantly on themgradually looses power over long period .Not cheapfornew 1, I looked at aearly RRhe spent $4.5 G oncam , liftersrings & bearings at w/ shop
74nova, Apr 6, 4:13am
Try another complete dizzy.
matthew_129, Apr 6, 4:23am
When it starts doing it can you pull off the HT leads one by one and check for spark at each of them! Sounds like a fueling issue or vaccum leak to me
bellky, Apr 6, 4:30am
I think the plugs are the problem somehow.

Don't laugh at me but our ride-on mower was doing exactly the same thing; new plug fixed.
gmphil, Apr 6, 4:31am
so not firing on one side or head ! gasket gone on that side
NZTools, Apr 6, 4:53am
Black soot on the plugs denotes too much fuel. Running fine when cold and gettting worse as it warms up also leans towards too much fuel,. (think leaving the choke on)
Check the inlet runners of 8, 3, 5 and 2. Do they all lead to the same barrel on the carb!
kingfisher21, Apr 6, 5:00am
How is the Holley mounted to the manifold also, these engines normally have twin Strombergs. Has the standard manifold been machined!
hi-ling, Apr 6, 5:08am
/\ /\ from what I remember from these, 1 barrel of the carb feeds no's 1, 4, 6 ,7, and the other feeds nos' 2, 3, 5, and 8, so I would say its a carby problem. Either try a different carb or get that one cleaned out.
frank_grimys, Apr 6, 5:10am
For those that haven't read the full thread, I have compression tested and it was fine.The cylinders are numbered odd numbers down one side, and evens down the other, so it's not 4 cylinders on the one side not firing, glycol and oil are perfect colouration, compression in all cylinders correct, so short of taking the heads off and examining them (a big job if it's not the gasket that's the issue), it's not likely the head gasket.

I have already stated swapping the plugs around the cylinders and ending up with same cylinders firing.

The carb doesn't choke, and being a two barrel, one feeding each side and having the firing pattern it's having it's near impossible to be fuel, especially backed up with the firing cylinders that have as much opportunity for fuel as the others coming out perfect in colour.

If the new rotor arm doesn't work tomorrow I'll certainly be looking into a new dizzy.Altenater is running the same volts and amps as it was before these issues started, and the signal is hooked through the ignition, as it always has been (not correct wiring, but it works).

Mechanical advance is free and smooth.

Black soot is also a sign of a retarded spark or incomplete combustion, which are more likely as it was a sudden f out, instead of a gradual issue building
frank_grimys, Apr 6, 5:15am
Just bolts on top, one barrel feeds 1,3,5,7 the other feeds 2,4,6,8.And since it's two firing on each side I just can't see the fuel being the sudden issue.Within the space of 5 minutes it goes from revving out to 3k rpm, only just on the plain, to hitting 2.5k rpm and spluttering about around 13km/h.Running right I should jump out of the water and start porpoising in a couple of seconds