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Clean Idle

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Learning my way through through this - graduated from Webers.

The plan is this - I will be pulling the engine from the car and tuning it on a dyno once I get it to idle cleanly. I'm disinclined to spend a day in the bay simply getting it to idle - I want to be beyond that and able to log good usable information while I'm spending both time and money at the shop.

I'm having difficulty getting my engine to idle.

The engine is a destroked 1 liter DOHC Rover K-Series, which I will be running at Bonneville.

75.5 mm bore, 55 mm stroke.

I'm using the Holley EFI system with individual throttle bodies - no airflow sensors.

Normally aspirated.


O2 sensor.

Temp sensor.


43 psi fuel pressure.

Target horsepower is 135 @ 9500

CR is 14.5:1.

43 mm TBs - 4 individual.

Head flows 151 cfm on intake @ 28 inches H2O - 131 exhaust.

The injectors I was sold are Bosch 36 lb/hour. My calculations indicate they should be about 21, but availability dictated the larger size.

Fuel map parameter is set at 14:1 across the board.

Timing is 31 across the board - again - I just need to get it to idle and run cleanly for now.

The above parameters are properly set in the program, and I can get it to run, but even when it finally gets to temperature - which requires constant throttle manipulation - it will idle for about 10 seconds and die.

I have not been able to create a data log consistent enough to give me an insight into precisely what's going on.

I pulled the plugs and found black soot - clearly during these start-up attempts, it's running rich.

If my diagnosis is correct, I'm thinking I need to decrease fuel, but I'm not sure which way makes the most sense. I'm thinking I've got three options:

1. I can go in and adjust the "on" time of the injectors in the program.

2. I can dial back the fuel pressure - although to where, I'm not sure, and would welcome a suggestion or a method.

3. I can try to locate a set of 20-25 lb. injectors - which appear to be a more appropriate size.

Any other information I should be including?

Where should I start?


Chris Conrad

No need to change injectors. And leave your fuel pressure at your 43 PSI target.

Normally, you avoid changing the fuel table until the engine is warm, but with a new installation, you just have to give it what it wants. What were your Lambda readings while it was warming up?

Have you verified the ignition timing (ie, you are getting 31 degrees of timing since that's what's being requested (make sure there are no compenstations).

Have you tried setting ignition timing to something more typically of idle (say 15 degrees)?

Why do you think it dies - fouled plug? Runs lean? Engine safety system (oil pressure, coolant temp too hot, etc)?

I'm new to this forum but I second David's comment about lowering the idle ignition timing to ~15deg. I work on a lot more on modern cars which idle near 10-15 degrees. You may need to increase your idle air flow so that the car doesn't die with the lower timing but that should allow the ECU to add more timing (up to 31 degrees) in the event that idle speed drops and the engine needs to make more torque to recover.

Normally iding does benefit from a lot of ignition advance, but by pulling it back the engine will require more air and thus it may be possible to better tailor the fuelling with larger injectors.

Do you have the water temperature, and air if used, sender correctly set up in the software?

As the plugs are sooting up, badly, I'd suggest pulling the fuelling in 20% steps. As David suggested, I'd leave the fuel pressure as is as it will aid atomisation. If you do have access to a set of smaller injectors, that will immediately reduce fuelling, so if you have access to a set you can always swap them out later - oh, on that, you do have the set you have correctly identified (if it's an option) in the software?

Are you running sequential or non-sequential as I was wondering if part of the problem may be the injectors being 360 out?

You surmise the sooting is from the warm up, but you didn't mention anything about swapping in fresh 'plugs when it was warm and reading them - that would definitely be something you should do - maybe a grade or two warmer JUST for the initial idle tuning.

You mentioned having to manipulate the throttle - any reason you didn't simply increase the idle screw for a 'idle'of 2k or so, or was this done in conjunction?

At this point, I would be concentrating on just getting a reliable idle, even if elevated, as everything else can be done from there.

[edit] Forgot - you do have the correct lambda sensor selected in the S/W?