3 or 4 cylinders... a mystery, or is it?

[Peeeetay]

1600
009
Recent top end rebuild.

At idle: removing HT lead from dizzy for cylinder 1,2,4. causes lumpy running as you would expect.

Removing HT for cylinder 3 makes not a blind bit of difference to engine noise.

Compression test came back good, changed spark plug 3 times, and swapped HT lead. Checked points gap, timing and replaced dizzy cap.

At HIgher Revs: If you increase the revs by a few hundred RPM removing HT lead 3 has the effect you would expect, noticable difference in engine noise and lumpy running.

What gives? I've asked a few people, one person suggested that this isn't unusual. He says dubs often run at 3 1/2 cylinders intil revved a bit when the full 4 kick in, but this sounds a bit odd to me. Any ideas?

How it drives: On a run, up hill and down dale, the bus runs great, I've been to Arran and back and all over the uK this summer and it has run nicely through out. Pulls well and doesn't feel underpowered even with a full load up hill. The only slight flat spot is when initially pulling away. You have to give it a bit more revs than feels quite right or it can sometimes stall.

Cheers,

Pete

 

[Peeeetay]

No one any clues on this?

Advice very much appreciated!

 

[lhu128l]

Twin carbs ? If it is it could blocked idle jet

 

[Trikky2]

Seeing as you have already done a compression test and changed the plug wire there's only two things left to check.

1. The plug cap has a resistance in it, only a few ohms. Use a meter to check you have continuity (or swap it for another one). The resistance can go open circuit and hence gives a weak spark at tickover.

2. You may have a minor air leak on the intake gasket giving you a weak mixture at tickover when the vac is high. Try squirting some carb cleaner or similar where the manifold connects to that cylinder. If it affects the tickover there's a leak.

 

[Peeeetay]

Hi, thanks for the help guys.

lhu128l - No mate it's single carb, cheers though.

Trikky2 - I'll check both of those thank you. By plug cap, do you mean the bit on the end of the HT lead? Or the top part of the plu its self? I've tried 3 different plugs so I am pretty sure it's no the plug.

Thanks again, both possibilities sound like an easy enough fix!


Pete

 

[Trikky2]

Hi, thanks for the help guys.

lhu128l - No mate it's single carb, cheers though.

Trikky2 - I'll check both of those thank you. By plug cap, do you mean the bit on the end of the HT lead? Or the top part of the plu its self? I've tried 3 different plugs so I am pretty sure it's no the plug.

Thanks again, both possibilities sound like an easy enough fix!


Pete

I mean the Bakelite end of the lead that plugs onto the spark plug. More commonly known as "plug caps".