Moss Motors, Ltd

I have fitted a pair of KN filters to 1973 TR6, since doing it runs roughly and backfires at 3500rpm and above it is fine below this, is there anything I should check ? change this is the only thing changed in the fuel system.

 

Matthew Davis

check the plates that mount to the carbs, if they are upside down they block the damper vents

I have checked that the holes are clear of the plates and the gaskets are the right way up, I assume you mean the 2 smaller holes above the bolt holes is that correct?

yes

I have now changed back to the 2 original foam type filters which are smaller both in depth and diameter and the problem has disappeared, any thoughts or suggestions from anyone?

I would try checking your fuel mixture.  The freer flow maybe changing the vacuum in the carbs and making them run leaner at higer RPMs.  I put the larger (3 1/2-4"? deep) K&Ns and have had no problems but I've been suspecting lately that I have been running rich.  I've run out of adjustment and it still seems rich.  Next step will be to readjust the float valves. 

What this experience demonstrates is just how restrictive the stock setup is. The air restriction from the original filters has created a higher than optimal vaccuum in the system, and the extra draw is pulling fuel. I'd put the K&Ns back on after tuning the carburetters correctly with NO filters on them. I think you'll experience better running and better fuel mileage as well.

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Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon