B21FT Cold Start System

[D924]

I finally got round to my project of putting B21FT fuel metering assembly on my supercharged Porsche 924 2.0 with an adjustable needle valve in place of the FV.

It's working brilliantly when it's hot, AFRs are perfect and consistent but it is a BEAR to start cold.

Can someone give me a brief overview of how the cold start systems on the B21FT worked please? I have a recollection that it has a WUR that reduces cold control pressure like most K-jetronic cars, but also it increased the FV duty cycle to a fixed high number for a cold engine. Is that right, and is there anything else?

If anyone has the factory warm/cold control pressure numbers for the B21FT as well that would be useful as I can't find them. I'm running 3.0 bar fixed control pressure currently. Not sure if the B21FT had boost enrichment WUR or if that was all done with FV as well.

 

[OttoB]

I cooked B30 and Capri 2.8i K-Jet combo when I was younger and inexperienced. To understand K-Jet I implemented several pressure gauges and noticed that cold control pressure was 1,5-1,8 bar. It went up to 3.something rather quickly, say driving a few minutes.

Do you have cold start valve and impulse relay?

 

[D924]

I cooked B30 and Capri 2.8i K-Jet combo when I was younger and inexperienced. To understand K-Jet I implemented several pressure gauges and noticed that cold control pressure was 1,5-1,8 bar. It went up to 3.something rather quickly, say driving a few minutes.

Do you have cold start valve and impulse relay?

I've cold start valve with a switch to bypass the Thermo-Time Switch (for hot start problems).
I only got the engine started once I unplugged the Cold Start Valve. I think it's probably flooding.

I really like K-Jet, in the same way that people like Swiss watches when a $10 quartz will keep better time. It's just a lovely system of hydraulics and totally analog operation.

What's an impulse relay?

 

[hiperfauto]

The thermo time switch regulates how long the cold start injector is on based on engine temp and cranking time. You may be leaving it on too long if it's flooding.

The WUR is critical to starting a cold engine as it works like a choke in a carbureted engine. Here's the pressure specs for a B21FT.

The impulse relay allows the cold start valve to work on a warm engine.

 

[D924]

So B21FT hot control pressure (no vacuum) is far lower than mine. 145-175 Kpa vs 300Kpa on mine.

Which means I've adjusted my needle valve (frequency valve) far too rich to compensate to get a decent idle (like your Terminal 11 mod).

Which means I need a massive increase in cold control pressure. Sound about right?

 

[hiperfauto]

The 145-175 Kpa figure is for a warm regulator and cold engine under acceleration. Once the thermostat valve is warm engine vacuum (or lack thereof) doesn't affect the control pressure. It's strictly used to add fuel under cold acceleration. Return pressure should be 345-375 Kpa on a warm engine.

Which means I need a massive increase in cold control pressure. Sound about right?

Remember, control pressure is the amount of fuel being returned to the tank. When the engine is cold less fuel is returned so that the engine gets more fuel. Control pressure should be lower on a cold engine.

 

[dl242gt]

Think about it like this. With the higher control pressure of a warm engine there is more pressure pressing down on the plunger. The low cold control pressure allows the plunger to be higher for a given airflow. Which is more fuel.

If you are trying to do some performance stuff. Try to study the Group A engine setup. They used what appears to be a decreasing rate fuel pressure regulator or it was an adjustable regulator for the control pressure. I think on the Group A engines they lowered the control pressure under boost. That is also how the B21ET functions for boost enrichment. The boost pressure on the control regulator lowers the control pressure. B21FT boost enrichment is done with the lambda controller going to a high duty cycle to enrich the mixture. There is a boost pressure sensing switch on the firewall of a 240 turbo to signal the controller.

 

[D924]

Cold starting:
Now I've raised the cold control pressure to 2 bar, it is no longer flooding and starts properly with AFR of 12-13 cold idle instead of <9. Sometimes. Other times it's insanely lean (20+ AFR showing on gauge) will barely run, dies if I open throttle. This eventually clears, given maybe 10-20 seconds, and it then idles nicely at 12-13 AFR. I think it could be two things:

1. Control pressure is for some reason "stuck" at system pressure (5.5 bar control pressure would indeed give this kind of AFR) to begin with.
2. Plunger is stuck at the bottom of its travel, and frees off after some attempts at revving.

 

[D924]

I also have a hot start problem because my "clever" needle valve just dumps all the fuel pressure to tank immediately. I need to put a check valve in line with 2-3 bar cracking pressure,

https://www.ebay.co.uk/itm/385407943028 maybe something like this? I assume the amount of pressure the FV "bleeds off" from the lower chamber is a tiny, a few PSI at most, right?

 

[TIPSP]

The fuel accumulator and the check valve on the pump are supposed to hold pressure. There is also the little oring inside the fuel distributor that controls your system pressure (return). That can bypass pressure making hot starts difficult if oring is bad. I noticed on mine the plunger doesn’t full go to the bottoms unti it runs for a second. Have to tap the throttle and hold it. It’s got some gummy crap outside probably an older leak. I have to try cleaning it again.

I have a digital wur now and have been trying to control the warmup cycle and tune it. I have the running tune working great, but the stupid box won’t let you lean out warm up cycle enough when it’s cold I can only get it around 11afr. Above 60* it’s fine around 12.6afr then it gets to my target afr for idle and driving.

 

[302ftw?]

The fuel accumulator and the check valve on the pump are supposed to hold pressure. There is also the little oring inside the fuel distributor that controls your system pressure (return). That can bypass pressure making hot starts difficult if oring is bad. I noticed on mine the plunger doesn’t full go to the bottoms unti it runs for a second. Have to tap the throttle and hold it. It’s got some gummy crap outside probably an older leak. I have to try cleaning it again.

I have a digital wur now and have been trying to control the warmup cycle and tune it. I have the running tune working great, but the stupid box won’t let you lean out warm up cycle enough when it’s cold I can only get it around 11afr. Above 60* it’s fine around 12.6afr then it gets to my target afr for idle and driving.

Could you start a thread documenting you experience with the digital wur? Very interested in this device

 

[D924]

I found this thing called the Push Valve (on far right of image) which is part of the metering head and basically shuts off the WUR return once system pressure drops by a certain amount (allowing the spring to extend, closing the push valve).

I don't see why the WUR would actually need to have its return shut off - the WUR itself acts like a check valve, if system pressure < set control pressure there should be no flow. In theory... I guess it would allow the rest pressure to remain at the full 5.5 bar instead of dropping to 3.5 or whatever the warm control pressure is.

However I'm thinking I can plumb my needle valve up to the WUR return port instead of the main return port and get the behaviour I want (not leaking down through the valve when the engine is turned off).

Believe the WUR return port is the one to the left of the main return port?