Mystery misfire

[JerryL770]

Thank you Mark.

We will do our best to go through your list - next week.

Jerry

[Series1 Stu]

Here's a radical idea to try.

Remove the distributor and re install it 180° out. Then see if the misfire moves to cylinder 6. If so, the problem is in the distributor or distributor cap. If not, then it's the leads or number 1 cylinder top end - fuel or valves.

Regards

[Hugo]

I think he said he'd tried a different distributor (and cap & leads). And that carb also feeds nos 2 & 3, so it's hard to see how it can be a fuel fault. And I believe he said he'd tried a different inlet manifold.
What intrigues me, unless I have misunderstood, is that it will run on no 1 (plus 6 perhaps?), but only when 2-5 are disconnected. To my mind that suggests there is something weird going on with the HT leads. I don't know if he has tried my suggestion of running a separate HT lead outside the bundle to no 1. If that doesn't produce a result, I give up!

[JerryL770]

The plot thickens

A change of the inlet cam (straw grasping) produced on change or improvement.

It has been proven that No. 1 cylinder is firing as should by the process of removing the plug cap, installing a plug in the cap and running the engine holding the tip of this plug over the top of the plug in the head and observing a nice fat spark across the plug in the air with it then jumping to the top of the plug in the head, from whence it must be sparking OK in the cylinder, as we know that plug functions OK also.

No dice. It still misfires

[mgcjag]

Are you sure its fireing when its on its compression stroke.... Steve

[JerryL770]

Are you sure its fireing when its on its compression stroke.... Steve

We will check that.

[mgcjag]

Hi Jerome....any news on this yet... Steve

[hedde]

Does the inlet manifold has a vacuum connection at the rear, for brakes? Did you block it off?
Otherwise the first cyl can get a lean mixture.
Had this problem testing my engine on a workbench
Hedde

[lowact]

Is the exhaust manifold clear? If the exhaust is somehow restricted the cylinder charge will be too lean to ignite (reliably). For same reason check exhaust valve proper functioning also?

[JerryL770]

It remains a mystery

The exhaust is clear. It still misfired with the exhaust removed.

It's been on a old Crypton Tuner which has proved the ignition is OK.

I don't know about any vacuum connection at the rear of the manifold. The manifold has been changed without effect so maybe a leak in the pipe?? Will have it checked, if so.

EDIT. Yep, this last one has been checked out and the port blanked off - still no proper go....

[JerryL770]

Solved at last

Just to re-cap, the car came, known to have a misfire and the head was removed to check it out and odd valves were found, some slightly bent. So new valves were purchased and the head sent to the engine shop for refurbishment which included some new valve seats.

When it was rebuilt, the misfire persisted and everything was checked: ignition, dizzy, electrics, carbs and manifolds including the vacuum pipe connection at the rear of the manifold being blanked off. The misfire persisted.

The head came of for checking and No.1 combustion chamber was clean (others black) showing non firing here.

Everything went back on and on checking the valve clearances it transpired that No. 3 pot clearances had closed up 12 thou So this was corrected but the misfire continued. Lots more head scratching and changing components to no avail.

Then it was found.

Turns out that when the rear vacuum connection was blocked off, the misfire continued but it was likely that now it was No 3 misfiring.

A start to check everything again, including blocking off the rear vacuum connection showed that now indeed there was a leak coming in there, not from the pipe but a hole in the vacuum tank. So the misfire was cured by blocking off that connection at the back of the inlet manifold. Transpired that the original blanking of this inlet occurred after the No 3 cylinder valve clearances had closed causing a misfire to continue.

We are a family of happy bunnies

[mark10337]

Not only you breathing a sigh of relief... most of us also! Really pleased for you that you found and cured it.

[Mich7920]

Thanks for this interesting feed back.
Issue not easy to find !

[Hugo]

I was under the impression all vacuum inlets had been closed off?

[JerryL770]

Thanks for the kind words.

Hugo, I pointed out that when the vacuum connection had been blanked off and the engine re-tested, it was still misfiring because the valve clearances on cylinder 3 had closed up, most likely because the new seats had settled further into the head. Although this got corrected, at the same time the vacuum pipe was reinstated as this was thought to be OK and so a misfire continued to make its presence felt.

Only when going back to the start and re-checking everything did the leaky vacuum tank make its presence felt as there then, wasn't another fault.

[Hugo]

Hmmm... That would worry me a bit; if the valve seats are that mobile, they might decide to make a bid for freedom. Sounds like they weren't fitted properly in the first place if you have lost all the valve clearance. I have to say this all sounds a rather convoluted diagnosis - not saying you're wrong, but I'm rather a fan of Occam's Razor.