First post for ages! The Cobra was still plagued by the instantaneous lean spike when you very slightly touch the throttle. I'd almost tuned it out by just running everything else rich and sort of tried to live with it. Finally decided something had to be done and I wondered if I had too much primary transfer slot showing, which is a common thing I found from a few quick Holley and muscle car forum searches.
My transfer slot exposure at idle was a little bigger than the normal recommended max 0.040" but not far off. I set the front and rear idle to show even slot exposure and put it back together to go for a test drive. While I was at it I thought I may as well put a fresh metering block gasket on as the old one had been on and off the carb a few times and was looking a bit tired.
Went for a drive and good heavens it was terrible. Blubbery, big lean spikes touching the throttle, massively rich everywhere. Spent a whole afternoon driving and tinkering with idle mixture/idle throttle position and air bleeds. It just would not make sense and respond how I would expect. I sort of got it half decent and gave up.
Next day went for a long drive. Again very bad running, fat and blubbery as if it was massively rich. AFR gauge was showing high 12's on cruise but the car felt very very sluggish and was bucking and jerking at low revs. Lazy to accelerate and generally piss poor.
Got to my destination and thought right then lets have a look. With the engine off I looked down the throats just to check the throttle positions at idle and check I hadn't made a complete bodge and I noticed the top of the right hand side (UK driver's side) primary throttle plate was wet with fuel. I then noticed bubbles coming out of the timed vacuum port that sits just above the throttle plate in the closed position. Hmm! Then I started having a good look at everything and noticed that the clear plastic vacuum line that goes to the vac advance cannister was full of fuel. Very odd! Over the next 5 mins I observed more bubbles coming from the vac port and the throttle plate being constantly wetted by a dribble of fuel. The primary fuel bowl was slowly emptying itself.
More investigation required! I took the primary fuel bowl and metering block off and had a good old poke around. I was particularly interested by the hole in the gasket that didn't match up with anything. On closer inspection the hole just very slightly exposed the fuel feed channel from the right hand jet (circled in red below)
The red circle on the metering plate below shows the fuel channel that the mystery gasket hole slightly uncovers.
Looking at the gasket above you'll see there's an impression in the gasket where its been next to an opening on the other side, i.e. the main carb body. Looking at the body that was easy to identify, the oval opening circled in red below:
By attaching some hose to the timed vac port and blowing through it, I discovered that oval port marked in red above is directly connected to the timed vac port, which means there was a direct path from the fuel bowl - through the metering block, through the mis-placed hole in the gasket and straight to the vac port, all below the fuel level. This is why it was dribbling out even when the engine wasn't running. Just imagine the complete chaos this was causing to the fuel mixture and distribution in the carb! This explains why it was running terribly. Even with my single plane manifold, cylinder 5 and 6 were probably getting a completely awful mixture and the other cylinders getting a compromised but just about ok mixture.
My lambda sensor is on the other exhaust bank so that read somewhere in the normal range and didn't show up this problem.
Because this mystery extra hole was only on one side, I was able to flip the gasket over so it now blanked off the oval hole in the body. The mystery extra gasket hole now slightly overlaps the left hand fuel feed, however in the carb body its just a blank wall so it can't go anywhere.
The metering block has locating pins so I had to cut a new hole in the gasket to line up with one of the pins. I have no idea what this extra hole is for but the gaskets I bought are genuine Quick Fuel gaskets. I don't know if they are front/rear and I've got them mixed up, I thought they were all the same so I'll check that later.
I reassembled and turned the car on but didn't start it (fuel pump on) to check there was no fuel dribbling from the port. Success! and no fuel in the vacuum pipe. A promising start. Fired the car up and started to twiddle with the mixture screws. Didn't take long to find the peak vacuum, about 12.5 to 13 inches at about 950-1000rpm. The AFR was in the usual range of mid 13's so nothing unusual there. I left everything else the same. Previously I had the IFR at 0.031" for a reasonable 2000-2500rpm cruise mixture (on transfer circuit) but I had recently put the IFR to 0.032" to cover the lean tip in problem. Previously this had resulted in an OK but generally rich (high 12's) low rpm cruise that I just lived with to cover up the lean tip in.
I got the car on the main road to cruise and it was really really lean at around 15-15.5:1 AFR when cruising between 2000 and 2500rpm, leaner at the bottom end. I stopped in a handy layby. I didn't fancy changing the IFR's at the side of the road but I did have a selection box of air bleeds. It had 0.073 in so I swapped to 0.069 idle air bleeds. Small adjustment to the idle mixture again for highest vacuum and got back on the road again.
I know that reducing air bleeds makes the circuit start sooner as the fuel can be drawn with a lower air flow (imagine holes in a straw), so reducing the idle air bleed would slightly richen the over all transfer slot AFR and would make it more active sooner. The result was an 1800-2500 cruise mixture of 14 to 14.5:1. Below that and it all went a bit rich so I think there is a bit of overlap from overly rich idle mixture merging into the start of the transfer slot but I can sort out that later. There also seemed to be a little bit of a gap between 2500 and about 2700 when the mains really start flowing so perhaps the main idle bleed needs reducing to make the mains come in a little sooner to take over when the transfer slot finishes.
Even better - the lean tip in from moving the throttle a couple of mm has completely disappeared! I'm not sure exactly why this is, because the mystery extra hole in the gasket was allowing fuel in, not air, so it wouldn't have been acting as an air leak. Maybe there's some internal channels and holes in the metering plate or the carb that made it behave that way but now its amazingly completely gone! It's transformed the way the car drives. Just a small touch of the throttle and instead of a lean spike and a bog, the mixture just smoothly transitions to mid 13's and the car accelerates smartly. Super pleased!
Edit: Thanks to some knowledgeable people at
cobraclub the mystery gasket hole was identified as being the standard Holley position for the timed vacuum port channel that feeds out the side of the metering block. On Quick Fuel carbs its in the baseplate. It looks like QFT supplied me with standard Holley gaskets which aren't compatible with my QFT metering block. Its interesting that the QFT main body (with QFT logo cast in the body) is the same as a Holley and retains the timed vacuum channel to the metering block, even though it is redundant.
