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Post by richard on Mar 26, 2012 16:57:10 GMT
Has anyone any ideas about this:
I fitted Pertronix electronic ignition a little while ago.
After fitting, I hardly needed any choke to start the car, even in cold weather and after pulling out of the car park I can push the choke right in even on cold days.
I checked the carb settings (twin carb) with a Colortune and they seem fine: each cylinder giving a blue flame on tickover, yellow with revs climbing then settling back to blue with occasional yellow flashes.
The plugs don't seem over black but they do have a pinkish tinge which I think is due to the VSPe additive I'm using.
Is it running rich or does the electronic ignition allow more effective combustion meaning less choke?
Running an old car certainly gives you lots to think about!
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Post by Penguin45 on Mar 26, 2012 17:21:46 GMT
Quick check is to pull the plugs. There has been quite a discussion over the last couple of weeks about carb settings. The engine which I pulled apart a couple of weeks ago showed symptoms of rich running on 2 and 3; lean on 1 and 4!
David and Tommy are really the people for this - definitely Premier League when it comes to carbs and timing!
Chris.
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Post by tommydp on Mar 26, 2012 21:36:44 GMT
I believe it's normal for cylinders 2 and 3 to run a little richer than 1 and 4, due to the design of the cylinder head. As 1 and 2 share an inlet port, like 3 and 4, the cylinders in the middle (2 and 3) will rob fuel from the corresponding outer cylinder because they (2 and 3) are closer to the carb. I could be wrong here, there are different opinions on this.
You would expect less of this effect on a twin carb, but I'm not sure. I can't remember much difference on the plugs on the 1800 S and Marina TC I used to have. I had a look at the plugs of my 1800 today, as it performed well again! Then number 2 and 3 were slightly richer than 1 and 4, ie correct! And the piston tops were dry soothy! On Thursday it misbehaved, then 3 and 4 were rich and 1 and 2 weak, and all piston tops were wet! You gotta love that car:-)
Chris, I wonder if your plug reading, reffering to the pictures, could be due to using the exhaust manifold from a single carb? I'm not sure but believe the difference between the cylinders were a bit more than you would expect, even though the inlet ports are siamesed.
When it comes to carb adjustment, I would say it could be running a little rich if you don't need to use the choke when setting off from cold. Anyway, it's how it performs when at operating temperature that matters. If it pulls and idles well and smoothly and doesn't smell of unburnt fuel I wouldn't bother fiddling with the twin carbs. An electronic ignition kit will provide a healthier spark I suppose, so would effect the combustion of the engine.
Regards, Tommy
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Post by dave1800 on Mar 27, 2012 0:30:13 GMT
Yes, it is usual for cylinders 1 and 4 to run weaker than 2, 3. I have been doing a bit of research and as Tommy says it is due to the sharing of inlet ports (siamesed ports) for cylinders 1+2 and 3+4. If you examine the firing order, 1-3-4-2 and rewrite it as 2-1-3-4 you will see that if you look at the shared inlet for cylinders 2-1, the sequence in the manifold becomes suck, suck, nothing nothing. (And the same for 4-3). Now if you look at the valve timing you will see there is overlap where inlet valves for cylinders 2-1 are both open at the same time. Because of the effect of the exhaust valve for cylinder 1 also being open for part of this overlap the effect is to weaken the charge for cylinder 1 especially at high rpm and high load. It would appear that this effect is more pronounced with the MGB / 1800 S camshaft because there is more valve overlap so more opportunity for this "charge robbing". Whether the twin carbs makes this more or less of an issue isn't clear (to me). The siamesed exhaust for cylinders 2-3 on the other hand has regularly spaced "blows" 180 degrees apart. As far as the ignition is concerned, if it is slightly advanced (2-3 degrees) this would offset the slower burning that occurs with a cold engine ie effectively retarded until it warms up - and need less choke. But, the most likely reason for you not needing the choke is a rich mixture as Tommy noted. Before setting the mixture, check the air filter isn't clogged, check and adjust the tappets if necessary and the ignition timing. Hope this hasn't done your head in! Regards David I believe it's normal for cylinders 2 and 3 to run a little richer than 1 and 4, due to the design of the cylinder head. As 1 and 2 share an inlet port, like 3 and 4, the cylinders in the middle (2 and 3) will rob fuel from the corresponding outer cylinder because they (2 and 3) are closer to the carb. I could be wrong here, there are different opinions on this. You would expect less of this effect on a twin carb, but I'm not sure. I can't remember much difference on the plugs on the 1800 S and Marina TC I used to have. I had a look at the plugs of my 1800 today, as it performed well again! Regards, Tommy |
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Post by indianajones on Mar 27, 2012 4:47:05 GMT
Interesting topic, I seem to have the same thing going on with my crab. Seems to need less choke than it did before my carb rebuild. Perhaps a tad too rich? I have set it at operating temp as per the manuals, so when you lift the piston about 1mm the revs slightly rise. (Note I also have electronic ignition)
-Andrew
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Post by richard on Mar 27, 2012 18:27:45 GMT
I haven't noticed any difference in richness between the cylinders, maybe the twin carbs reduce the effect. All the plugs look OK. I'll recheck the timing, tappets etc
Interesting comment David about slightly advanced ignition timing. I recently had the distributor rebuilt and the advance curve calibrated. I think it advances more on the S models to run on 5 star fuel. I use unleaded Super with octane booster to try to get somewhere near and then set the timing just behind where it begins to pink. Just thinking maybe this is why it needs less choke.
Thanks
Richard
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Post by richard on Mar 27, 2012 18:29:48 GMT
Ps. This forum is great. I no longer feel like the only person in the world pondering over the troubles of a 40 year old landcrab!
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Post by indianajones on Mar 27, 2012 19:04:48 GMT
Oh this site has been so great for me. I would be so lost if it wasn't for the lads here.
On setting the timing, how do you know where it's best to set it? i.e. based on gas etc
-Andrew
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Post by richard on Mar 27, 2012 20:11:23 GMT
As far as I understand, set the timing as per workshop manual with a strobe. Then, in top gear, accelerate quite hard at about 30 mph. If you hear any pinking(pre-ignition-a sort of tinkling sound) then stop and retard the ignition a bit with the adjuster on the side of the distributor. You're aiming for the most advanced setting you can without pinking.
I find that different quality fuels mean the timing needs tweaking to get it at this point quite often.
Avoid driving it for long if you are getting any pinking because it will wreck the engine!
Hope this helps
Richard
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