Sudden, massive weakening of mixture on front carb?

[GrahamFountain]

Driving along, making the common mistake of thinking how well it was going, when it started to go off on a cylinder of two, especially at mid load.
Considering the issues I've had with condensers, I assumed the old lucas one I'd pulled out of a spare had gone too and started to replace that and look for problems that might be overloading the condenser, like the ballast resistor shorted out or something. After a long sequence of failures to find the problem, including HT leads and cap, and losing the condenser screw (I was sure it fell over the right side of the engine not into the dizy, but I had to pull it to be sure) - must've buggered off into the shrubbery or wedged in a ledge somewhere.
Eventually I pulled the plugs and found the front two were white as snow.
Thing is, I'd checked the plugs about 2 days before, and thought they were all a little on the rich side, but not enough to do anything about.
I've dropped the jet on the front carb by 4 flats, and that's made it better, but not got rid - it's missing a little bit on light load but not full throttle or idle that I can tell. So I'm now waiting for it to cool a bit before I pull the plugs again and check their colour.

I also intend to lift the lid on the front float and check that, and maybe look at other fuel issues with it.

So, other than some sort of partial blockage to the front supply, are there any other peculier faults that anyone else has seen like this?

Graham

[Macleesh]

Air leak on the carb, possibly split rubber but also check the nuts are tight.

[GrahamFountain]

Just had a look at what I could think of. The levels in the float chambers seem the same, and neither needle seems stuck - they both take a little pressure to come down, but nothing compared to what the pump gives. I went to take the front chamber lid off with the pipe attached, and it spurted as I lifted it up. So there was pressure to the front carb, though that's not proof of flow. But the levels being much the same suggests that if there's enough to the back there's enough to the front as well. The front was bit higher cos of the spurt, and I ought to check by removing the lids after I've disconnected the feed lines. But I think it should have been more obvious given how tolerant of changing fuel levels constant vacuum carbs like SUs are supposed to be.

I can't see any evidence of an air leak - I had one of the grimmer nearly like the original rubber mounts separate once, and it didn't seem to affect performance much until the carb fell off the inlet manifold. That had an effect.
But I haven't given them the old WD40 test while running, and will give that a try tomorrow.

I have had intermittent fuel starvation to the front carb on one of the TR7 Sprints once - mentioned it recently. So, if it persists I'll be threading the steel tube between the carbs this weekend. But I'm fairly sure I ain't got the water in the fuel problems I had when that happened before. Filler cap's at a different angle.

The interesting thing is that after dropping the front jet by 4 flats, the front jet measures only 2 thou lower than the rear. Like I said, 4 flats didn't fix it, and I think it wants another couple if that's the main problem - not sure what 4/6th of the thread pitch is in thou, but it must be well more than 2 thou. But I can't see how the two jets could have got anything like 4 flats different, because I set them together when I did the air filters, and haven't needed to fiddle since - like I said, 2 days ago the plugs said they were both running a bit rich, but what I didn't say is the lifters said they were a little weak at idle, so I thought it was a good compromise unless I want to look for needles a wee bit fatter towards the ends or take a file to their bottoms.

Graham

[GrahamFountain]

Turning the front jet down another couple of flats has got it back to going well. I don't think I've driven it far enough for the plugs to properly colour, and I think it's now too rich cos I needed no choke as such, just a bit of fast idle - but it is unusually warm (and dry) for West Lancs.

So I think that means it's just a mixture issue.

But how it's come to change suddenly while driving is still a mystery to me. I can imagine that some muck could have been holding the jet down and it's fell out. But that seems a bit unlikely, even in my world, as I've had them jets up to the step and back down several times recently, while I had the problem with the clogged filters.

Graham

[tinweevil]

I would suspect a partial blockage in the fuel feed or the jet pipe. Checking the levels when parked won't reveal the former, the level will only drop when there's load on the engine. If there's a suitable road near you accelerate hard to try and replicate the conditions then cut the engine without lifting & coast into a layby. Then the float level can be trusted.
There's no way to check the jet pipe. Your only hope to avoid removing it is to drain the float chamber, remove the carb top and blow it through both ways. While doing that you can check for grit the wrong side of the dirt trap in the bottom of the bowl too.
I would reset the carb and find the fault rather than working around it. If it is a partial blockage it'll either clear leaving you horribly over fueled, no doubt at the most inconvenient moment possible or it'll get worse.

[GrahamFountain]

When I had the intermittent blockage in the fuel line between the two carbs on the TR7 Sprint, as I remember, that did show in the level in the front chamber just for looking. But I suppose it mightn't always.

I am going to setup the carbs again today (thank god the weather's good) and see where they set in relation to one another. And I will check that fuel pipe between the two - since the problem is only with the front carb, a flow rate limitation would have to be in there or the front float chamber valve, which I checked and am happy is working fine.

Graham

[GrahamFountain]

Looks like there's two problems at once. The main jet or choke mech on the front carb is sticking, which presumably explains the sudden change in mixture, and the ignition is going off a bit when the car's hot.

So I'm going to strip and clean the carbs and choke levers and springs, etc., and then look at the ignition.

I can't find a reason for so many condensers failing and I've swapped the coil for a new sparkrite one, so that may be a difficult problem to trace. Might need a garage with a rolling road and ignition test equipment. But the only one I know is on the edge of Preston, and they've gone into lockdown again.

Graham

[MIG Wielder]

Hi Graham, This is exactly what I was going to write ! We had a nice long trip out yesterday . The ambient was about 30deg C and as I'm running with this waterless coolant the gauge was up at 92 degC ( or 3/4 on the gauge ) and ......

Driving along, making the common mistake of thinking how well it was going, when it started to go off on a cylinder or two, especially at mid load.

Eventually I pulled the plugs and found the front two were white as snow.

Graham

.... and SWK started running on 3 pots on the road works section of the M4. It was fine on the overrun but didn't like accelerating. I tried pulling the choke with no luck . So I turned the heater on which dragged the temperature down to about 83 Deg C ( or just over 1/2 way on the gauge ) and normal service was resumed. So this morning I had the plugs out and they were all running very weak.
I reckon in my case it was fuel vapourisation in the hot summer weather.
I've just been out again and its now running fine.
I've got the original rubber carb; mounts, but I do run the high octane shell petrol. SWK is an 1850 btw. I reckon I could benefit from ducting cold air from the side of the rad; just as the 1500's do.

Tony.

[Carledo]

Ey oop, we're back to louvre weather again!

Steve

[Bumpa]

Have these carbs got waxstat jets? That was the cause of similar problems on my 1850 with HS4 carbs. Put in solid jets and all was well.

[GrahamFountain]

Personal preference I know, but I prefer the Rijksmuseum to the Louvre every time.

So the carbs are cleaned and back on and the jets are going back up neatly when the choke's pushed in (got an issue with the choke warning light, but that's cable adjustment). The jets are set 50 thou down, which is about 8 flats, and where I think the sweet spot is. Possibly not exactly right but near enough to run reasonably well in the short term. But it's still misfiring.

I only checked it would start and then pulled the distributor to swap to a condenser from the Distributor Doctor, which arrived in the post this morning. I have to say that the quality of the crimping on the tab where it clips on the points spring wasn't great and needed a bit of adjustment to sit right.

Now the car runs a bit rough, not awful but not nice neither. But the real oddity is that the strobe won't flash at all off the lead to plug 1, and flashes very infrequently off the main feed to the distributor - and when it does, the timing mark's not visible. So it's set statially. That's been giving good results I've only had to check with the strobe, so I could live with that if the car ran ok. But why the strobe suggests the spark is poor to bad, I don't know.

So I pulled the distributor only to discover there's a hole in the cap where the left side clip goes. I can't see anything wrong with the earth strap or how the condenser is fitted, so I put the best spare distributor in and the spare lead set. That distributor's got a lucas condenser that I couldn't remember if it was new. Seems it's not, because it runs badly on that set, but the strobe works again. So, tomorrow I think its down to the last new condenser.

I wonder how long a set of points would last without a condenser - I have about 5 or 6 spares.

Graham

[GrahamFountain]

Have these carbs got waxstat jets? That was the cause of similar problems on my 1850 with HS4 carbs. Put in solid jets and all was well.

No, solids.

I'm fairly sure its the condensers dying, and the carburetion issue was peripheral (much old oil and too much of Blackpool beach under there). But don't know what is killing the condensers. I have some 400 volt PCB mounting 0.22uF caps on order. We'll see if they last any longer.

Graham

[GrahamFountain]

Put the Distributor Doctor's condenser back in with a good rotor cap and all seems well - strobe works fine and no noticeable misfire on driving. So it must have been the cracked cap that was stopping the strobe and causing the misfire.

I guess now, I'll find out how long this condenser lasts - carrying my last new lucas one in a spare dizzy and keeping an eye on the plug colours as I go. If it lasts a while, I shall get another to fit in the spare dizzy - the issue with the clip for the points spring needing adjustment was minor.

If this one dies, then I guess I have to look for an electronic hall effect unit to replace the points and condenser and use a 25D rotor arm under the 44D4 cap. But I won't be happy; not having found the cause and giving up like that.

Graham

[GrahamFountain]

The 400V 0.22uF PCB caps arrived. So I've stuck one in a can from a Lucas one that died. Cos it's an aluminium can, I can't solder the earth side to the can, just lock it in place with a blob in the hole. So the wire has to go to the mounting screw and be held down by the flag ring on the end of the earth strap.

I'd go fit it, but it's the rainy season again. Actually, I think I'll stick it in the spare and give the Distributor Doctor's one a propper go. If I just got dodgy condensers from two suppliers, that should sort it.

I also thought to check that the resistance wire is doing its job when the system's reached temperature, and the coil voltage with the points closed goes from 5.8 to 6.5, which is probably mostly the battery taking a bit charge.

Graham