Low rpm under load misfire

[beth]

Getting back to playing with SD1 powered Landy after a break of a couple of years, except I remember it playing up before so this isn't new.

So pretty standard SD1, except for K&N conical filters directly on the end of the carbs, no elbows/etc and BBW needles. Timing is probably about 4BTDC although I haven't checked it yet but luminition kit fitted so it probably hasn't changed. Idles okay at 900rpm, hunts a little perhaps. Pootling along in high gear, low rpm, ease throttle down and noise increases but it doesn't really pull and misses with a dull thump - which if I don't do something to clear it, like back off and try again, does continue. I have a lambda sensor in the exhaust Y piece wired to a air/fuel gauge which claims the mixture goes lean as I ease the throttle open. If I plant the throttle it seems to hesitate and then pick up with lots of lights on the A/F telling me of a nice rich mixture. Carb damper pots have engine oil of 10/40, or 15/40.
Not sure if the leaning of the mixture is to blame, and what to do about it? Or if the vacuum advance/retard is not working - how to test that one? Or if I'm missing something else?...

[ChrisJC]

Is that on SU carbs?, do the pistons slide freely up and down the dashpots?

If Strombergs, are the rubber diaphragms split?

Chris.

[DaveEFI]

Getting back to playing with SD1 powered Landy after a break of a couple of years, except I remember it playing up before so this isn't new.

So pretty standard SD1, except for K&N conical filters directly on the end of the carbs, no elbows/etc and BBW needles. Timing is probably about 4BTDC although I haven't checked it yet but luminition kit fitted so it probably hasn't changed. Idles okay at 900rpm, hunts a little perhaps. Pootling along in high gear, low rpm, ease throttle down and noise increases but it doesn't really pull and misses with a dull thump - which if I don't do something to clear it, like back off and try again, does continue. I have a lambda sensor in the exhaust Y piece wired to a air/fuel gauge which claims the mixture goes lean as I ease the throttle open. If I plant the throttle it seems to hesitate and then pick up with lots of lights on the A/F telling me of a nice rich mixture. Carb damper pots have engine oil of 10/40, or 15/40.
Not sure if the leaning of the mixture is to blame, and what to do about it? Or if the vacuum advance/retard is not working - how to test that one? Or if I'm missing something else?...

First thing to do is get the idle correct.

The static timing should be 6 BTDC You need so slow the idle to about 700 rpm when using a timing strobe and disconnect the vacuum advance.
Correct idle speed is 750 rpm, and it should be stable at even under this speed.

Dunno if your lambda sensor is accurate at idle, but the idle mixture should be about 14:1 AFR. Check by lifting each carb piston in turn by a few mm. The rpm should either not change, or speed up slightly before returning to as before. If it slows down the mixture is too weak, speeds up too rich.

If you've still got running probs after this it will need further investigation.

[kiwicar]

Hi
With the change to K&Ns and shortening the inlet tract lengthyou may well have it running lean on transition from idle. What sort of milage have the carbs done? I am assuming SUs as you say the engine was SD1. I would double check the pistons lift properly then set the idle mixture using the lambda sensor then set it 1/2 flat richer then try 80 weight gearbox oil in the dash pots, see what the lambda sensor now says on transition. check the pipe to the vac advance is not blocked with gunk. the real trouble is the air filter set up you have is liable to feed hot air into the carbs when you are stationary which will then be cooler when running, you would be better off with the standard set up. If you want to use stubs and K&Ns then you want them in a cold air box from the front of the car and above all dump the elbows on the front of the carbs and fit short trumpets with inside the airbox.
Best regards
Mike

[beth]

Hmm. Didn't work on it today, went to see the traction engines at a local steam rally with Hub's instead.

Yes, it's SU carbs. Pistons slide easily in the dashpots and have as far as I know they have the standard springs.

Ah yes, I'd forgotten about the pins. I'll check that. I have no evidence the A/F meter is telling fibs, but I am aware it has limitations outside its very narrow operating band.

Sorting out the idle hunting is high on the list for sure.

I can't 'go back' to the standard set up as I don't have the parts. I just got the engine and inlet manifold. The carbs are a pair I bought from Burlen Fuel systems a loooong time ago for another project that didn't happen. So they are as new. I hear what you are saying about the hot/cold air. I'll have a think if there is anything I can do to make it more consistent.

Thanks guys I'm hoping to have another play tomorrow and will report back.

[kiwicar]

Hi Beth
Just had a thought, have you got the dreaded poppet over run valves in the throttle plates? If so you will not get a decent low idle or part throttle transition with them, solder lhem shut and remove the spring bit (or replace the butterflies).
Best regrads
Mike

[DaveEFI]

Hmm. Didn't work on it today, went to see the traction engines at a local steam rally with Hub's instead.

Yes, it's SU carbs. Pistons slide easily in the dashpots and have as far as I know they have the standard springs.

Ah yes, I'd forgotten about the pins. I'll check that. I have no evidence the A/F meter is telling fibs, but I am aware it has limitations outside its very narrow operating band.

Sorting out the idle hunting is high on the list for sure.

I can't 'go back' to the standard set up as I don't have the parts. I just got the engine and inlet manifold. The carbs are a pair I bought from Burlen Fuel systems a loooong time ago for another project that didn't happen. So they are as new. I hear what you are saying about the hot/cold air. I'll have a think if there is anything I can do to make it more consistent.

Thanks guys I'm hoping to have another play tomorrow and will report back.

My SD1 has EFI, but I did have a P6 with a similar air filter arrangement, and removing the air filter assembly did have a small effect on the idle mixture. I'd say you'd not have problems getting a secondhand set. The SD1 Club Forum has a for sale and wanted section.

I'd forgotten the poppet valves. I replaced the butterflies with plain ones.

[beth]

Well that was interesting. I tweaked the mixture just onto the rich side and took it round the block. Much more tractable and no missing on the transition at all. Just a beautiful v8 burble as the power feeds in. The idle is much more stable as well. Hurrah. Which makes me think that the 'ideal' mixture setting on the A/F meter is not the whole story.

MOT booked for tomorrow so if the guy puts it on the emissions rig it could get tweaked back down again! - I'm thinking it's CO reading is going to be a leeetle high.

It has got the poppet valves in the butterflies but sorting those is going to have to wait behind other priorities.

[DaveEFI]

Well that was interesting. I tweaked the mixture just onto the rich side and took it round the block. Much more tractable and no missing on the transition at all. Just a beautiful v8 burble as the power feeds in. The idle is much more stable as well. Hurrah. Which makes me think that the 'ideal' mixture setting on the A/F meter is not the whole story.

MOT booked for tomorrow so if the guy puts it on the emissions rig it could get tweaked back down again! - I'm thinking it's CO reading is going to be a leeetle high.

It has got the poppet valves in the butterflies but sorting those is going to have to wait behind other priorities.

What was the lambda sensor reading?

[beth]

Ummm, cant remember exactly. Doh! it was slightly rich, so I'm guessing about 13 to 13.5. Jet height screws were twiddled only about 1/10th of a turn, but it made a huge difference.