First of all i dont have a controlled fuel injected by ecu setup. it's a carb set up.

Had a drive, air fuel ratio was fine. parked car up to cool down, on firing back up engine, a hard start to 2000rpm, i heard a hissing, thrapping, psssss pss noise , increasing with rpm.

very slow on idle and increasing with rpm. it's like a puff , puff, puff, noise.

i then realised as driving home, my AFR was all over the place and I mean all over the place, sometimes not even registering at all yet the car was pulling fine, no hesitation what so ever apart from idle level had dropped alot when trying to park up. But the car would drive as normal.


So I assumed it was a leak but where?

The last time my AFR did this I installed a new conical gasket where the 4-2-1 manifold meets the exhaust and the AFR became dead on correct and stable.

I've now unbolted the two bolts that connect the exhaust to the manifold and the conical gasket seems alright. its black where it should be on the inside edge and is as "new" around the outer edges so I cannot see it could be escaping from there. I assume to leave this as it is.

I tightened the o2 sensor really tight with a long bar, but that didn't work. Now where the o2 sensor is facing there is grease and black grime on the driver side wishbone, not sure if this is coincidence.


I have a knock sensor and everything is reading fine with engine. Lifters sound alright. What else can it be?

The 4-2-1 manifold is brand new and I do not want to overtighten the 02 sensor. I have tightened the 02 sensor more tight than you would with a sparkplug. The spring washer cannot be removed from the 02 sensor.

Can you help please.

p.s just looked at the threads on the 02 and where they go in and its rust, nice and brown so can't be a leak from there.

[Edited on 8/9/19 by bikecarbfred]

think i could have wasted 4 hours for nothing.

noticed on intake manifold cylinder 4 rubber hose was kinked from carburettor to manifold so there was a leak there.

didnt realise why it would throw off the AFR as it was one cylinder with leak. also why such a noise? you could hear it when driving 30-40mph.

looks like when i did hard start with accelerater pedal down, it pushed the carbs back abit, jubliee clip must not have been tight.

anyway sound has gone, AFR now doing what it should be, nice n stable.