Cobra has a new friend

The final (hopefully) carb parts have arrived and I now need to find time to fit them and then an excuse to drive somewhere to test (as if I need an excuse!). Unfortunately my spare time has been taken over by servicing the newest arrival to the family, a Nissan Stagea:

If you don't know what one is, its very closely related to the Nissan Skyline, just an estate and slightly stretched floor plan. They are extremely rare in the UK, being import only (from Japan). Engine is the same RB25DET in the R32 Skyline, producing somewhere around 240-250bhp. The Stagea also has the same ATESSA AWD system which is permanent RWD then some fancy electronic controlled transfer box thingy that brings the front wheels in when you are about to die. Because of this it's a heavy old beast at around 1650kg but still manages 0-60 in 6.5 seconds.

A part of me (which I am ignoring at the moment) suspects that on a bumpy b road this car would probably be faster than the cobra.

The craziest bit - have a look in here:

No carburettor, its witchcraft!

Quick Fuel 750DP Tuning Part 2

Continuation of this thread

Had a chance to go for a good 80 mile round trip which was an excellent test of the revised carb settings - here are the results.

To recap, the car was originally extremely rich in all circuits:

Stock settings

Primary Jet: 72
Secondary Jet: 82
IFR: Unknown - suspect its .032 or .033
IAB: 72
MAB: 33
PVCR Unknown - estimate somewhere around 0.055" to 0.060"


Revised Settings

Primary Jet: 68
Secondary Jet: 82 - reduced to 78 after very short test drive
IFR: .030
IAB/MAB unchanged
PVCR 0.040"


I was hoping that the reduced IFR, primary jet and PVCR would see more reasonable AFR in light throttle cruise at low rpm, medium rpm and medium acceleration. As per the previous post, I only had chance for a very short test drive and on very light throttle it was very lean and misfiring. Heavier throttle it would clear up.

Before setting off on the 80 mile round trip, I drove a few miles locally armed with carb tweaking tools. WOT was still incredibly rich so I changed from 82 secondary jet to 78 before we set off on our longer journey. I also very quickly got to the bottom of the light throttle lean misfire - its the dreaded Holley lean tip in that is well documented. This is because I went too small on the IFR. I need a 0.031" IFR but don't have one, so as a temporary fix I compensated for this by sticking a bit of guitar wire (B string 0.016") in the primary idle air bleeds (0.071" stock) and setting all four idle mixture screws a quarter turn richer than they needed to be. This got rid of the lean stumble at light throttle but obviously is not a permanent solution as it causes the engine to load up rich at idle and almost die, and on the overrun there's way to much fuel and it was popping and banging.

The solution should be straightforward - I went too lean on the IFRs so I need to go to 0.031", so they are now on order. Good news though is that with my wire and idle screw bodge I was getting consistent light throttle cruise AFR of about 14:1. The reduced primary jet means that high rpm cruise (above 2500rpm) is now almost sensible, hovering around 13.3 or 13.4:1. Medium acceleration staying on primaries only, but getting the vacuum low enough to bring the PVCR in was showing about 12.5:1 which was an incredible improvement on the 11:1 I was seeing with the stock carb settings. This shows that the PVCR change was the right thing to do. Acceleration was now nice and crisp, not blubbery and sluggish. One jet leaner on primary will hopefully nudge the high rpm cruise towards the 13.5:1 to 14:1 range and maybe nudge the medium acceleration figure up a notch, which shouldn't be detrimental to performance. To be honest, I don't cruise much above 2500rpm so the car won't spend much time here so its not so critical.

Similar good news is to be found on heavy acceleration when getting into the secondaries. Hard acceleration ( between 3/4 throttle and WOT) amazingly showed 12.4:1 flooring it from 2000rpm, leaning out to 12.7:1 by 5000rpm which I was very pleased with. The plan here is to go one step leaner on the secondary jet which should raise that 12.4 up to around 12.6 or 12.7: 1, then I can trim the fuel curve with the high speed air bleeds to keep it flat and reduce the high rpm leanout. No lean spike on fast gear changes.

So in summary:

From stock settings

Lean out primary jets 4 jet sizes
Lean out secondary jets 4 sizes
PVCR reduced from unkown size (somewhere between 0.055 and 0.060") to 0.040"
IFR reduced from unknown size (somewhere close to 0.033 or 0.034" to 0.030"

Low rpm cruise - improved from 12:1 to 14:1 but needs slightly larger IFR to cure lean tip in.
Medium rpm cruise (2500rpm and above) imrpoved from 12:1 (or even lower!) to approx 13.4:1
Light accel (primaries but above power valve vacuum) improved from around 12:1 to 12.5:1
Medium accel (primaries only but power valve activated) improved from 11:1 to 12.5:1
WOT improved from around 12:1 or even richer, to between 12.5 and 12.7:1

MPG with my old Edelbrock Thunder 650 was about 18mpg. With the stock calibration on the Quick Fuel, that dived to around 10mpg. With the current revised setup I am now getting around 15mpg but I was enjoying the car so much that quite a lot of our 80 mile journey was with my foot buried. I think perhaps 16 or even approaching 17 is possible with one more step lean on primary and secondary jet and a lighter right foot.

Quick Fuel 750DP Tuning Part 1

Continuation of this THIS THREAD

To recap, the first drive with the new carb was incredibly rich and obviously needed some work, but I wasn't really surprised, and wasn't too worried as everything is adjustable.

Based on the fact that low rpm and mid rpm cruise was incredibly rich, with WOT also definitely on the rich side I leaned out everything.

Stock settings

Primary Jet:       72
Secondary Jet:   82
IFR:                   Unknown - suspect its .032 or .033
IAB:                   72
MAB:                33
Pump cams:       Pink/Pink

Getting fuel when needed clearly wasn't a problem so I left the pump cams alone. Revised setup was as follows:

Revised Settings

Primary Jet:       68
Secondary Jet:   82
IFR:                   .030
IAB/MAB         unchanged

After several days of being busy, I finally got the chance for a drive. The result - idle screws were now functioning in the 3/4 to 1 turn range, an improvement from 1/2 turn as before. My low speed cruise below 2000rpm was now mid 13's, with very very light acceleration it would shift to the low 14:1 range momentarily and then settle back down to mid 13's with a constant throttle position. This is great!

Harder acceleration, (but still above PV vacuum level) got the mains on which lowered things a little towards the 13:1 area. Harder acceleration (PV active) and it nosedived towards 11:1. WOT was still incredibly rich at between 11:1 and 11.5:1, sometimes dipping as low as 10.8:1 but I had left the secondary jet unchanged so this wasn't too surprising. My aim was to get cruise/light accel sorted first.

This all suggests that the power valve channel restrictor is too large. When the PV is active below 6.5" vacuum, a shed load of fuel is getting dumped in so these were the next target. I stripped the front bowl off and took a look a the PV. I was a little worried as the Quick Fuel SS series does not come with adjustable PVCRs so I would have to find some suitable wire to stick in to reduce the area. Imagine my surprise and happiness when I removed the PV and saw these:

Lurking underneath the PV valve orifice (the big hole in the middle) are two adjustable jets, the same 6-32 size as the other adjustable restrictions in the rest of the metering block! I was slightly suspicious of this carb from the start, as SS series should have red anodised metering blocks and this one is clear. I think what has happened is someone has specified a carb for a particular application then for some reason decided not to buy it, but the shop had already made it up, so they sold it on (to me!). I tried to find out what PVCR size a 750-AN and found anywhere between .055 and .062 depending on the source on the internet. I found some handy bits of wire of various diamters and determined that these PVCR's were approx .055", or somewhere around 1.5mm. 

I already had some unused restrictors that were the original IFR's (.032") that I replaced, so I drilled these out to .040" (1.0mm) and used them as new PVCR's. I figured if I started small it would be obvious what the effect was - I'm hoping the effect will be leaner on part throttle accleration, then I can always drill them progressively larger if I went too far lean.

I reluctantly left the secondary jets at the stock 82 size so I could properly evaluate the effect of the PVCR change. Only make one change at a time! The next test drive will be interesting.

As an extra note, at the end of my test drive yesterday for the last 1 mile or so, on very light throttle cruise the AFR was off the scale lean and it was misfiring all over the place. On application of more throttle it would clear up and run normally so I had to do the last mile nailing it then coasting. I'm slightly confused as to what the cause could be. I pulled all the plugs and they are all a little dark from the rich mixtures I've been running but otherwise ok. All plug leads check out fine, timing is locked down and unchanged, vac can is working normally. Can't think of anything else at the moment other than some crap managed to get in the carb and blocked a channel. I've stitched it all back together ready for the next test drive so I'll see if the problem has gone away.

Later edit - this was resolved with fresh fuel,  it got to the bottom of the tank of fairly old fuel.