280i - setting the mixture via the duty cycle lead

[m2500tvr]

Hey, Folks:

Anyone have the number for setting the 280i's Bosch K-Jetronic fuel injection with a dwell meter via the duty cycle lead? The number was 23 degrees on a 6-cylinder dwell scale? 24? 26? I don't recall. I came across this same procedure in a VW fuel injection book. The number was 46, as I recall, on a 4-cylinder dwell scale. I'm curious to give this mixture setting method a try. I've got a CO meter to check against.

Regards,

- SJ
'67 Lotus Elas S3
'76 TVR 2500M
'85 TVR 280i

[jeff m]

I'm not really sure re your question but I'll have a go...
There is no injection timing as it's a CIS system, so I'll assume you are asking about the 80%/20% frequency valve that goes closed loop once the engine is hot. (oxygen sensor/lambda)
There is a pulse sig that is sent to control the ratio.

This is really considered part of the emission set up.

Hope I'm close!....... if not re post

Jeff

[m2500tvr]

Hi, Jeff:

Thanks for your reply. One way to adjust the mixture with a K-Jetronic fuel injection system is via the frequency valve. With a dwell meter attached to the duty cycle lead, turn the mixture adjustment screw until the dwell is cycling between 35-45 on a six cylinder scale. I just got refreshed on the scoop. I'm going to give it a try. I'll check the results against my CO meter.

- SJ

[jeff m]

I'd not thought of going that route, I was waiting for a spare WUR to come my way so I could tap it and put in an adjustment screw.

What CO are you aiming for?

[m2500tvr]

jeff m said:

I'd not thought of going that route, I was waiting for a spare WUR to come my way so I could tap it and put in an adjustment screw. What CO are you aiming for?

My brother had an SCCA IT prepared VW Scirroco that, of course, used the K-Jetronic system. Tuning the motor while leaving the stock PI in place was an interesting experience. The limiting element to the system is the air flow sensor. It will only admit so much air, so when found that at some point cam profiles would get wild enough to require the engine to inhale more air than could be delivered by air flow sensor, and you couldn't get a correct mixture. You can usually deliver plenty of fuel, just not enough air to mix with the fuel in the proper ratio.

You're right, the part of the K-Jetronic system we're talking about is described as the emissions control system. It's that feedback loop of the system (the Lambda Control System Action Circle) that lets us adjust mixture via the frequency valve duty cycle lead on the distributor. And, really, this loop will try to keep the mixture in the range it's designed to provide.

I've not played with the mixture on my 280i ever. This'll be the first time for me. What CO value is a good question. I know what the book says about tail pipe CO values for the US (0.2%) and Euro cars (1.25% plus/minus 0.25%). I suppose I'm just going to play around with it and see. I'll have the dwell meter connected so I can watch the frequency valve, and I'll have the CO meter at the tail pipe at the same time. I'll turn the mixture adjustment screw and see what happens I should probably be good and adjust the valves before I do all of this, but I'm really not looking forward to doing that...

Cheers,

- SJ
'67 Lotus Elan S3
'76 TVR 2500M
'85 TVR 280i S1.5 dhc