Engine mis-fire / pop noise, low revs medium to hard acceleration

Hi,

I have a misfire / pop / bang on my 3.5L V8 (SU carbs) and I am rather stuck as to where to go from here. I know that a misfire can be many things, and I also know I should look to change the distributer cap, HT leads, spark plugs, ignition coil, maybe camshaft, etc. etc. however this misfire seems not to relate to these.

The misfire occurs almost exclusively at low ish revs (1000-1400 rpm) when I accelerate with a partial - medium throttle. I expect it would do it at full throttle in the same gear at the same revs too, but I have not tried in case I blow the engine up. If I drop a gear to get the revs up, then accelerate it does not do it hardly at all. Also, you can predict when it is going to do it as the engine feels very slugish and has a 'fluttering' sound, a sound similar to one you would get if there was a hole in a manifold.

I have recently fitted a stainless steel exhaust and checked that everything is tight and no leaks I can see, and I have tried making the air / fuel mixture both weak and rich. Setting it rich seems to reduce the misfire, but not reove it fully, and most recently I have actually set the mixture fairly weak, and this too seems to now reduce it so I have no idea where to go next.

I have used a Colortune to look at the flame in the cylinder, and while the flame at various revs is a reasonable blue colour, at idle it is a very yellow flame which cannot easily be changed to blue, and therefore my spark plugs soot up when idling for any long periods.

To add to this, once I have accelerated very gently, or very hard to avoid this misfire area, the engine runs fine. Will pull reasonably well and will do easily motorway speeds on level ground (up hills are still a little lacking in power but this is not a new problem).

I can only imagine I am going to have to learn how to rebuild a carb and try to get a service kit, but before I do, are there any suggestions to help?

Thanks.

Check ignition timing first, otherwise she could just be lean.

Would timing just start to go out like that though? It used to run fine, and I was in the process of doing some upgrades as per a previous post, but now I am working just to keep it running smoothly. Would timing need adjusting after fitting a better exhaust? If so, would it need advancing or retarding?

At first thought I figured it was lean, but now ater seeing how much soot was on the spark plugs, and the amount covering my new stainless exhaust, it appears to be far too rich, unless this is a needle issue in the carbs due to the very rich idle.

What colour would a lean flame be? Currently it is a pale blue flame (not dark blue like on a household gas hob), but it has a more pronounced 'blueness' to it when lifting off the throttle and dropping back to idle, then goes yellow again at idle.

No idea on your lean colour, but I have had ignition timing go bonkers before.

Defo worth a check!

check vacuum unit works and advance weights are free to start

I would check for air leaks.

I have Strombergs on mine. they cause major misfires on acceleration if the dashpots are out of oil. It causes it to run lean and through flames from the carb if your not careful. Might be something worth checking. Popping and farting in the exhaust system can be caused by an exhaust leak drawing air into the system.
cheers

An update and more questions on this; I have replaced the HT leads, rotor arm and distributer cap and cleaned the plugs (which have only been in for a few months so fairly confident they are fine). I knew these would not fix the problem but replaced them as they looked old and tired anyway. I have put the key questions in red for ease of following this post.

After this didn't work I decided to adjust the carbs a bit more and surprisingly I have now managed to get a blue flame at idle using Colortune, however it is now definitely weak at higher revs, so have adjusted them again to somewhere between the two, not too lean at higher revs and an occasional blue flame at idle, but mostly yellow.

I have also played with the timing a little, and have found this has stopped the 'misfire'. It has not really stopped the juddering / hesitation when acclerating with part - full throttle at low revs in a high gear (3rd or higher), but it has stopped the loud bang / explosion noise that used to occur, so at least that is an improvement. I don't want to advance the timing any more because if I do, I start to hear a jingle noise at hard acceleration up hills (which I assume is 'pinking').

This had led me to suspect the vacuum advance (as previously suggested by 'fanatic') or whatever controls the timing advance at part throttle / revs may be faulty. I have read somewhere something to do with weights inside the distributer can have an influence in this area. Are these the 'advance weights' suggested?

How would I check the vacuum advance is working? I have taken the pipe between carb and distributer 'flying saucer' shaped thing off and this makes no difference on or off, and cannot feel / hear any vacuum from anywhere. A few months ago I did replace the plastic orange and black thing (I forget its name now) which I think has something to do with vacuum delay / restriction as on an unrelated problem I thought it was faulty but turned out to be ok, so I know this is ok and not relevent on this topic.

Finally, how would I check these weights are working? When you try to turn the rotor arm by hand, it moves about 8mm or so, then when you release it, it snaps back to original position, I have read somewhere this is what it is supposed to do, but does this relate to these weights?

Thanks for the suggestions so far, and just to confirm the dashpots are full, I use 10W-40 oil in these, although there is some ATF oil in there as well from a previous recommendation which from what I have read since is not ideal.

The testing of the weights is good, what you have done to test is right. To test the vacuum unit, remove vacuum hose from carby side and suck on it, can you hold vacuum or are you just sucking air, if you hold vacuum the unit is good.
You don't mention you have checked for air leaks....or did I just miss that.
How is the float levels?
Do you have good fuel pressure?
Is the fuel filter good?
Does your Landy have a electronic fuel pump in the tank? or is it a mechanical fuel pump?

Thanks 'rangy35' for coming back on this.

When you say remove the vacuum pipe and suck on it, should this be direct from the carb, i.e. including the black and orange plastic valve thing in the pipe, or after it?
The reason I ask is that I bought a new valve thing thinking my old one was blocked as I could not suck anything through the filter alone, but it turns out it is supposed to be like that, therefore if I suck through the pipe with that in place, surely I would just create a vacuum in that small bit of pipe between where I am sucking from and the valve?

I forgot to comment on the air leaks, where would common places be for air leaks and how is best to check? Obviously the engine fan blows a lot of air around the engine bay so the 'bit of paper' trick would not work. I cannot hear any obvious leaks but again, over the engine noise this is not very conclusive.

I am not sure what 'float levels' are, unless this is the amount of oil in the dashpots? In which case they are full.

The fuel pump is in the tank I think, so probably electronic, and I changed the fuel filter about 3 months ago so should be ok I think. Again, I am not sure how to check for fuel pressure however.

Just to update my last reply;

I have tried sucking on the pipe from the point after where the black and orange valve connects and it seems to hold vacuum but has a faint smell of fuel / oil (my sense of smell is not great at identifying things) so not sure if this is normal or not. It also will not let you blow down the pipe so whether this is good or bad, again I am not sure.

Naa that is good, the vacuum unit seems fine.
The float levels is the chambers where the fuel is held for the carbs to draw fuel from to supply to the engine, running lean or having low float levels will cause a lean fuel mixture and that will cause pop noises.
If you are happy with the fuel filter then I would suggest to do a fuel pressure test, I have no clue what the spec is from Land Rover but a good rule of thumb is 3-5psi.

Have you done any work on the fuel system recently? Or did this just start all of a sudden? If I am not mistaken there should be a restricter in the fuel return line, is that still there?

To find air leaks you have to use a liquid and while the engine is running. I don't know if I should tell you what I use because if you are not careful you could cause a fire. So take a spray bottle with a adjustable nozzle and fill with water, test spray and adjust your spray pattern to a stream. Then spray all the joins on the intake system, keep water away from the distributor and coil but spray it along all the joins and shafts on the carbys, where the carbys mount to the manifold, any where you can think false air can enter the engine. If there is a air leak the water will seal the leak momentarily and the engine will run really smooth if that is the case then that may be your cause for the pop noises, dont stop there you may have more air leaks than one.

Good luck and keep us posted.

When you suck on the pipe to the vacuum advance, with the dizzy cap off you should see the mechanism move.
I had a problem recently where my vacuum jammed meaning the timing was out at low revs.

A compression test is worth doing as heads tip and blow across cyl and into valley cooking oil- hence later heads don't use outer bolts and outer bolts recommend 25nm now

When my 3.9 V8 had this problem it turned out to be the o2 (oxygen)(lamba) sensors.

I changed everything from the fuel filter, plugs, leads, maf sensor, you name it, after a couple of hundred quid thrown at it, it turned out to be the lamba sensors.

John

Seems to be running ok (ish) for now, just having trouble with carbs at the moment, this may be related to this problem or not, will see what happens.

So the float levels must be inside the carbs then? Can these be adjusted from outside or do I need to strip the carbs down?
I am thinking of buying a carb rebuild kit from S.U. and fitting the new parts, I just need to learn how to do it first then will buy the kit and have a go.

I am not sure how to do a fuel pressure test but will look in to this also. I have not done anything with fuel system before this issue, it basically appeared after fitting my new stainless exhaust. After fitting the exhaust, I was adjusting the mixtures on the carbs to get the best running / power, then suddenly started getting the bad misfire. When I adjusted the mixture back again, the misfire / pop noise was still there and got worse, to the point I was at earlier in this thread. After adjusting the timing, this pop has gone but the rumbling / hesitation remains in a high gear at low ish revs ~1100 - 1500.

I have re-balanced my carbs now which has made things smoother, however the rumbling remains and I cannot get the idle right now. It either idles too high, slowly rising up to 1500 rpm when I come to a stop while driving, or drops down and idles very low at <400 rpm. I am trying to adjust the carbs to find an idle which works, but does not seem to be working, another reason why I am thinking of buying a carb rebuild kit.

I have no idea if there is a restricter in the fuel line, again I will look into where this would be and then see if it is there.

I have not checked air leaks yet but I will try the method you suggest and see how this goes.

I will also try the vacuum advance with the distributer cap off, just to make sure it is actually doing something.

I did do a compression test shortly after buying the Land Rover, and all cylinders seemed reasonably close together, but I will check this again to make sure nothing has changed. I have heard about these outer bolts on cylinder head causing issues, and unless there has been some historic work done on this engine I do not know about, I assume mine will be tightened too much as most things on the engine seem fairly original. While cleaning / changing spark plugs recently, I have noticed a reasonable amount of oil on the one for cylinder 7, not sure if this is related to these cylinder head bolts but is another area I need to look at.

I am fairly certain I do not have a lamba sensor on my engine, however if there is an older, likely mechanical version of this on these engines I will look into it and check this also.

Thanks.