TVR Tasmin, KJet Pressures Again

Sigh, It seems like my life is a never-ending cycle of messing with KJet fuel pressures.

But hopefully, this will help

My quality German made KJetronic pressure testing setup arrived. Huge thanks to MissingParts on eBay, as this is some proper quality gear, and for less than I paid for the other setup.

Nicely crimped ends, with quality rubber and fabric braided hoses

The seller has a great sense of humour. “For oldtimer cars”

I hooked the new setup up, which was a lot easier with the banjo fittings, rather than the generic screw fittings

Now it was time to confirm the readings from the old gauge. Previously the cold control pressure was stone dead on the gauge, at near enough zero psi. This gauge, not so much.

We want cold pressure here, near the green line (depending on ambient temp), otherwise it’s too lean when cold

So, what about the system pressure, which wouldn’t go over 5bar, no matter how thick of a shim I fitted? Off the gauge; over 6bar

And we want that at 5-5.5bar, on this green line. Too high and the control pressure will be too high also.

Well damn. No wonder it still wasn’t happy.

I pulled the two additional shims out, and we dropped to 5bar system pressure. Better, but not perfect.

I knocked the adjustment on the WUR to get the control pressure down to the required half bar, but the car wasn’t running right.

The next step was to step back, reset everything and presume that everything I had done with the old gauge was buggered.

Because the system pressure was a tad low I wanted to add my small extra shim and see how that increased pressure (i carefully measured it when I made it, to add 0.5bar). Hah, perfect!

I found this amazing step by step guide, which I followed and it worked perfectly. Big thanks to the author of that.

The first step was to make my WUR adjustable. This is so that if I knock the pressure adjustment down too far I can use a nut to pull the pin back out, instead of having to remove and disassemble it to tap the pin back out by hand. I drilled and tapped the pin to M5x0.8 and using a screw, nut and washer, made it adjustable. The guide that I used is here. The basics of the mod are that you screw the screw in tight (or Loctite it in so it cant turn) and then tighten the nut down to pull the pin out of the body. To push it back down you wind the nut completely out, against the head of the screw and then use a punch and hammer to tap the screw/pin down again.

It would be a lot easier to use if the screw didn’t have a flange as there is limited space when mounted on the car, but its what I had on hand.

With the WUR apart again I replaced the O-Ring for the diaphragm and flipped the thin metal diaphragm to the other side to even up any wear.

Following the guide, I found that my initial pressure with no springs/strip was OK, and the pressure could be increased to the correct level by hand. Good.

The next test had me checking the heater works, which when holding it in my hand with power applied, I could confirm it did get warm to the touch. Good.

Next was to test the pseudo-warm pressure by reassembling the WUR with the springs, but without the heater/strip. This applies pressure to the mexican hat and diaphram, to emulate the warm pressure. This should be 3.5bar or HIGHER.

I had just under 3bar. Not enough. This is where I had to get creative and work out how to adjust this. On other WUR, there is an adjustment screw under a brass cap on the base. You drill through that cap, and there is a hex screw to raise or lower the platform the springs sit on. I tried drilling what I thought was the cap, but turns out the base for my platform is actually a pin pressed into the housing. Its the recessed circle with a hole drilled in it.

So, with callipers in hand, I tested/measured to see if I could use a hammer and punch to also adjust that like you do the cold pressure pin. Sure enough, some careful whacks of the adjustment tool, and I had raised the platform, thus increasing pressure on the springs. 4bar is perfect.

I fully reassembled the WUR, heater/strip and all, and reinstalled on the car to test/adjust the cold pressure. I got this easily down to a solid 0.7bar (within the margin of error for the ambient temps, I didn’t want to mess around too much getting it lower)

I connected the heater and watched as the pressure slowly increased. The heater and strip were working perfectly.

I got it near the required 2.9bar, but it was still a tad low with the engine running at temp, so I used the one last adjustment available; tapping the main circular unit on the WUR down. This is the part that the two fuel hoses bolt onto. This is also pressed into the body, and like the other two adjustments can be carefully tapped down with a punch. This takes very little to increase the pressure, but be very careful not to punch it down too far or the WUR had to come completely apart again to tap it back out. I alternated tapping the punch on both sides, where the arrows are pointing.

This resulted in a nice 2.9bar when warm. Excellent.

After some tweaking of the idle and CO screws, the results were immediate and obvious. The car ran and idled better than ever, including idling under 2000rpm for the first time. I set the idle to about 1000rpm, which is higher than factory spec, but it felt happier there than the 800-900rpm recommended.

The only thing left to do was to put on my big boy pants and try taking it for a run. The last time it was on the road it constantly tried to die on me and left me blocking intersections. Not ideal, and no wonder I was nervous.

This time, it started and was driving perfectly. It was pulling strong and felt good… until the hesitation kicked in again at high RPM under load.

The exact same issue as before I rebuilt the Kjet system.

I came home with mixed emotion. The car ran and drove well unless I got on the throttle. Kind of a win, but also a fail.

Mrs Petrol and I were discussing the issue and she reminded me that her old Alto used to buck and hesitate if the fuel was too low…

Surely that’s not the issue? Nooooo

If you look at the design of the tanks, the feed to the pump is at the front of the tanks, toward the front of the car. It’s possible that under acceleration the little fuel in the tanks was sloshing away from the outlet and starving the pump. Maybe.

The orange arrow is pointing to the fuel outlet on that tank, the other is on the other tank in the same location.

Well, I guess I limp the car to the gas station and chuck some gas in and see what happens.

Well, what does happen is that if you don’t have both fuel caps open when you try to put any fuel in the tanks it all comes rushing back out and pukes down the side of the car and onto the ground. Oops.

I added 20L to each tank, at great expense, and guess what, my Wife was right (like usual), the hesitation is completely gone, and it will happily rev out to redline under WOT. Amazing.

The sound and feel of the car is crazy. It’s so loud, but makes a great noise. You sit so low, but the car feels big. Even when it’s not moving everyone is breaking their necks to see what it is. It’s not a car for the shy.

It does leave me wondering a little, if I had just filled the tanks when I first got the car, would it all have been OK anyway? The main issue I had before I rebuilt it all was that hesitation issue. In saying that, it’s starting, running and driving better than it ever has since I got it, so the work was well worth it regardless.

I’m stoked. Still some things to tidy, but it is almost ready to go for a WOF check.

Parts Used

Copper Washers for Metering head and WUR – M8x12x1mm (smaller), M10x14x1mm (larger)

Please note these parts are specific to my car and may vary. Please check before ordering.