Main Fuel Screw and Idle Speed

[ally406]

I have an odd problem on my 406 with Bosch and TD04. Car is running great and I'm now trying to dial in some more fuel. I'm using the idle adjustment at the back of the pump to bring the idle speed back down after turning the fuelling up (I usually go with 1,000rpm since that's decent with cold veggie). However, often when I start it, or dip the clutch at a junction, it will start to chug at 500rpm. A blip of the throttle and it will usually come back to normal idle speed. This will persist until I return the fuelling and idle adjustments back to standard, so I'm in the position of not being able to add more fuel   Only other adjustment is a ground LDA, otherwise it's an unmolested pump. Anyone seen this before?

[johnny625]

Driving a stage 1 tuned xud with a Bosch I can tell you that often when turning the max fuel screw the throttle arm stop screw needs to be wound out somewhat. I do refer to the stop screw on the back side of the LDA on which the throttle arm should rest when the engine is idling.

Try doing this when the engine is at normal running temp (not super hot, nor stone cold)

-Adjust the max fuel to the desired amount with the engine running and tighten its lock screw.

-Slacken the lock nut on the throttle stop screw and wound out the screw good two or three turns just to make sure it doesn't affect the idle RPM (make sure the throttle cable has enough slack for the throttle arm to 'follow' the stop screw.

-Slacken the lock nut for the hot idle on the back side of the pump. Adjust the idle to the desired RPM (tends to idle the best between 875 and 900 RPM at least in my case). Tighten the lock nut for the hot idle screw.

-Now, slowly wind in the throttle stop screw until you just hear the idle rpm's rising. Then back off the screw 1/2 to 3/4 of a turn and tighten its lock nut.

After this you should have rock solid idle and anti stall. When you rev the engine to about 3000 rpm and release the throttle abruptly the engine should within 2 to 3 seconds smoothly transition into idle, without any shaking or sounding like its gonna stall.

If you still have your problem than your bosch pump is probably knackered or worn internally.

[ally406]

That's some great info Johnny thanks! I'll hopefully get back to it this week and try your suggestions. It's also crying out for a gov mod.

[ally406]

Hi Johnny, I've made some progress.

I wanted to get the gov mod done first before messing with the fuelling, so I did this last weekend. With a lot of trial and error I have the car idling nicely at 950rpm using the method you describe, and it's holding it's fuelling until about 4,600rpm which is great. I'm still not able to increase fuelling past standard but I think I have an idea why.

I'm now thinking that the buffer that the throttle arm rests on is my problem. This is pretty much acting as a stop screw which is why I think I've been having trouble adjusting the fuelling. I'm of the opinion I should remove it and try increasing the fuelling again. It's the part I've boxed in red, thoughts?

[johnny625]

Yes, that's the damper...you can adjust it using the nut and the screw on the throttle arm so it can allow the arm to go more backwards. Failing this you can remove completely. Many VE pumps don't have the damper installed and they work just fine.



On a second thought if you are using veg your issues might also be related to it...sometimes veg can cause the governor and the weights to become 'sticky' and the anti stall might not be kicking in as it should when you dip the clutch. I would suggest adding a bit of petrol to clear things up or even running a tank of diesel.

[ally406]

Everything's nice and free, it's an 80% veg 20% misfuel blend.

Thanks for the advice, that's the fuelling wound in as far as the idle adjustment allows. Getting 20psi at 3,500rpm with not much smoke at all. Needs more fuel which will probably involve bumping the throttle arm round on the shaft, but that will be for another day!

[Martin306]

Everything's nice and free, it's an 80% veg 20% misfuel blend.

Thanks for the advice, that's the fuelling wound in as far as the idle adjustment allows. Getting 20psi at 3,500rpm with not much smoke at all. Needs more fuel which will probably involve bumping the throttle arm round on the shaft, but that will be for another day!

That's still a pretty heavy mix, it could still be temporarily sticking internally when it's running.

I know what mine was like inside (Thanks Samass and his dodgy diesel) but I just took it apart and washed it out with clean diesel and it's been fine ever since.