Any suggestions before I take the engine apart?

[ethel]

A couple of days ago there was a bang and the engine ran roughly, down on power, wouldn't idle, would only run with throttle pressed - and lots of dark smoke coming out the back. I was only a couple of hundred yards from home so I made it back revving the engine and slipping the clutch -uphill all the way, of course. It was running like I imagine it would on only 4 cylinders.

So far I've checked the timing marks on the crank and cams - they're still aligned and the cams move when I rotate the crank, the rotor arm still goes round as well and is pointing more or less to no2 cylinder when no2 is at tdc. I've checked the cylinder compression, there's one on each bank that is <120 psi and all the others are 145-155. With oil in those two low cylinders they come up to nearer 150. Everything seems to spin around smoothly when I crank it for the compression test - even the oil light goes out. The spark plugs were very sooty, but it had just been running on the choke.

Given the loss of power I was expecting a more dramatic failure, can anyone suggest any other tests before I start work to take things apart - I still haven't worked out what the noise was.

Thanks, John.

[tonyh]

Sounds like all it needs is a good tune up and maybe the carbs need checking.

[Phil S]

if there are no other nasty noises with it running start with all the basics, carbs,timing,plugs,leads, if they prove ok then go deeper

[richardthestag]

bang could have been fuel in the exhaust - backfire ?!? or was it a loud bang from the engine?

cheapo rotor and caps can cause all manner of weird issues. mainly because spark doesnt quite make it all the way to the plug sometimes

Check both carb diaphragms also for splits, run the engine with the air filter elbows off, when you rev the engine the piston inside the carb should rise and then drop. If it doesnt rubber diaphragm is likely cause

[71Stag]

Swap the coil out they dont take the heat that well being sat in the V and will give you a loud bang and lots of smoke and I agree with Richards comments on the diaphragms they also will give similar symtoms

[wilf]

+1 for carb diaphragms. Also it does sound like it was flooded, so needle valves too.

[ethel]

Thanks for the suggestions, I'll look deeper into the carbs for a start, I had checked that the external linkages were ok, but not inside the carbs. I guess the loss of power with maybe a bit of a shudder from the drivetrain could have made me think the noise was louder than it was.
I'll let you know what I find.

John.

[ethel]

The diaphragm on the lh carb was cracked around the outer edge, just inboard of the rim that's clamped in place, time to order a new one.

John.

[MandM]

Mine ran like that when it was running on one carb due to the other being blocked with crap etc, give them a good clean, and change the fuel filter, before going to deep

[ethel]

While I'm working on the carbs, I thought I might as well replace the o rings on the needle/mixture adjuster to save topping up the damper oil. One came apart ok, on the other the socket for the adjuster hex tool has rounded off - it's been getting worse over the years - and won't turn, so I cant release the needle. Do you think I should try to drill out the adjuster from the top, or just sacrifice the needle and punch the complete assembly up through the damper tube. I think the second option might be easier.

Of course if I fit one new needle then it might make sense to fit both..... and new jets...and do a complete overhaul!

John.

[wilf]

I would do the latter, and replace with new parts.

Beware of score marks or "dings" from the star washers on the i/d of the damper tube, as they can cause the damper to hang up, with some very weird effects on running. (actually, not dissimilar to what you posted, now that I think of it)

[Kithmo]

To me, a bang could mean a spark is occurring at the wrong time, igniting unburnt fuel. Either when an exhaust valve is open (a backfire in the exhaust), or when an inlet valve is open (a backfire in the carb or air filter). I would look closely at the distributor cap for tracking marks (thin black lines) between the contacts and check the position of the rotor arm is correct and it has not slipped round, check its key is still intact where it fits onto the distributor shaft.

[richardthestag]

While I'm working on the carbs, I thought I might as well replace the o rings on the needle/mixture adjuster to save topping up the damper oil. One came apart ok, on the other the socket for the adjuster hex tool has rounded off - it's been getting worse over the years - and won't turn, so I cant release the needle. Do you think I should try to drill out the adjuster from the top, or just sacrifice the needle and punch the complete assembly up through the damper tube. I think the second option might be easier.

Of course if I fit one new needle then it might make sense to fit both..... and new jets...and do a complete overhaul!

John.

If you remove the grub screw completely

Then from the underside of the piston you can see a slot that the grub screw goes into on the needle assembly, It is possible to use a jewellers screwdrivers to unwind the needle from the adjuster screw. Just hope that it is not so tight that it turns when you turn the needle

Then use a drift to knock the adjuster and star washer out of the top and rebuild

http://www.southerncarbs.co.uk/are good for rebuild stuff as are other suppliers

[wilf]

This might help:

https://bowtie6.com/other/zenith-stromberg-carburetter-repair/

[ethel]

This might help:

https://bowtie6.com/other/zenith-str...retter-repair/

A very useful link, thank you. I just need to find a tube to go over the needle and inside the damper tube that I can use to drift the whole assembly up the tube, though I'll soak it with some carb cleaner first.

John.