Amazon: Damaged JFUR4 distributor

[HRA]

Well the good news is that the tyre was an easy fix once I'd found the right tyre place. A couple of the big-name places nearby were too busy when I called-in on Friday and sounded unsure about inner tubes, so I drove out into the sticks to an independent specialist. The repair was as easy as fixing a puncture on a bike. No need to worry about making the tyre itself air-tight again and apparently there are no restrictions on where the puncture is just as long as the damage isn't too catastrophic. And it only cost me a tenner including re-balancing. I think I might invest in a spare inner-tube though - just in case I get a puncture further from home and have to rely on a less-clued-up repairer.

So to the distributor. The old distributors that Triple-S let me have both cleaned-up well and now we have one each. I've set the timing to 15 degrees BTDC, increased the octane rating of the fuel with Valvemaster Plus and I've richened the mixture a bit. It's certainly not pinking as much but I can still hear it on "spirited" acceleration.

It seems more apparent between 2000 and 3000rpm in second and third. Would I be right in thinking that that's when the vacuum advance is most active? Not that that's a reason for it to be pinking; it's not as if the vacuum regulator can over-advance.

It may also be a lean condition. As Ron Kwas pointed out, the right mixture at idle doesn't guarantee anything at higher speeds or under load. That's all the more true in my case since I'm running with a home-made metering needle in my single Stromberg.

I think my next move is to run the fuel tank down and then refill with higher-octane fuel. 97 RON should be enough. It'll cost more but it'd be worth it not to damage the engine. I might even get better mpg but since I got 30 out of the last couple of tanks that's probably a big ask.

I'm also going to do something about my Stromberg metering needle. When I got the car it was fitted with a Weber that I never managed to get running any better than "sluggish", even with the recommended jets. So I got hold of a Stromberg without fully appreciating the subtle differences between different 175 CDs. I ended-up with a version that wouldn't take the standard B1AN or B2AF needles so I spent an evening with micrometer, file and emery paper adjusting a B1G needle. By my measurements I created something between the two "correct needles - richer than the B1AN but leaner than B2AF. It runs well but maybe this pinking is trying to tell me my needle isn't quite good enough. Anyway I've got a 2nd-hand 175 CD-2SE on order and it comes with a full rebuild kit, so I'll be able to run the right needle and know that the mixture is correct throughout the rev range.

That just leaves one question - which of the "correct" needles to go for. My gut-feeling is that since I've got pinking I should be looking at the richer B2AF but I'm open to advice and/or the voice of experience! Is there an advantage to the B1AN? Or is the few thou between them truly insignificant?

[Ron Kwas]

HRA;

According to this data for B20A, which calls for a 077 Dist: Centrifugal Advance is done at 2400(Distr)RPM...so double that for Crank RPM...which is pretty high now that I think about it some more...that means the CA is never really maxed out in typical driving...!


Vacuum Advance/Retard (to confirm which you have, see: https://www.sw-em.com/Volvo%20Igniti...e_Spark_Timing
...you might want to disconnect if Retard, and if Advance, I'd temporarily disconnect and undertake some road tests to see if it will help.

Why on earth would you go to the trouble of developing your own Metering Needle profile?...I'd expect MN specified for B20A config to be suitable and again perform road tests.

Cheers

[Derek UK]

Yes, try and get the standard needle. Pinking is normally due to too much advance. Retard until it stops and mark where the dizzy is. Advance until it pinks again and mark the dizzy. Half way between the two positions will give you a usable timing position until you get the correct needle but I'd not expect the pinking return with that. If you have a strobe and a rev counter, plot the timing at the revs according to the manual.

[Burdekin]

Is this a standard B20A? Any increase in the compression ratio?

[Burdekin]

Which Stromberg carb have you got, might be worth posting up some pics of the carb and engine. I assume it’s the 175CD2SE model. Burdens sell the correct needle for the B20A, it’s not expensive, also a needle booklet. You can’t adjust that carb apart from a screw on the side (can’t remember the name) which should be screwed all the way in for older cars.

Also change the diaphragm as a matter of course with a Stromberg, it’s a 5min job.

[Burdekin]

Idle trimming screw number 18 in pic.

[HRA]

Thanks Ron Kwas! It makes sense that CA continues through the rev range. The time taken for the fuel/air mixture to burn will remain fairly constant so you need to ignite the mixture earlier in the cycle at higher revs. I suppose that in theory turbulence in the cylinder comes into play at very high revs making it more of an explosion rather than a flame-front moving smoothly down from the plug. I'm not intending to run my old engine that fast though!

I've definitely got vacuum advance - it turns the contact breaker-plate clockwise to meet the anti-clockwise distributor cam sooner. I'll see how the car drives without it connected, but I'm sure the designers put it there for a good reason.

I had to go down the home-made needle route because I mistakenly bought the wrong 175 CD. I think I got the 2 or the 2S rather than the 2SE. Anyway, it only takes a top-adjustable needle. According to the suppliers neither the B1AN nor the B2AF will fit. So I took the B1G top-adjusting needle I had and changed the profile.

[HRA]

Thanks Derek UK - that's basically what I've always done on old cars. Advance until it pinks then back it off a bit until it just doesn't. I had an MGB years ago - the distributor had an external adjustment knob to slightly advance or retard the timing without having to loosen the distributor body. It was the sort of think I played with whenever I got fuel from a different petrol station. It's a shame that more distributors don't have this useful feature!

[HRA]

Hi Burdekin, yes, as far as I know it's a standard B20A. There's no obvious sign that the compression ratio has been increased. The compression figures themselves were nothing special when I last measured them:

Cylinder dry wet
front 158 169
2 150 162
3 142 159
back 152 158

average 150 162
spread 5% 4%

I've just ordered a second-hand 175 CD-2SE so that I can run the correct needle (B1AN or B2AF?) but I'm concerned to learn that there is virtually no mixture adjustment! Do they always work as-received or is there some "setting-up" that I need to learn about?

I was expecting to be able to raise and lower the jet somehow, albeit not without taking the float-chamber off first. Surely there must be a critical measurement from the top of the jet to the bridge? Or is there some cleverness I'm missing?

What is the idle trimming screw supposed to do? On my current carb it doesn't seem to make any difference if it's right in, 2.5 turns out, or almost at the end of its thread. Is there something about the 2SE version that makes it more sensitive to this adjustment?

Sorry for so many questions but there's clearly plenty abou these Strombergs that I have yet to understand!

[Ron Kwas]

HRA;

Yes, designers put the Vac Adv there for a reason, but vac was taken from a very specific location in the carb (not just intake manifold), which your carb, being not necessarily the exactly correct configured one, might not have the vac port at, and also associated with getting the most out of the higher quality fuels (octane) which were being sold then...if you give it as much advance with today's fuels, the ping(k)ing doesn't surprise me.

I'd try a load test with this vac disconnected and plugged at carb (or wherever it is being taken from)...and with base Timing set such that 30-35 Deg was about max at 3000RPM.

Cheers

Edit: After reading your post on ZSs...I'm no expert, and I would say not having an ability to fine-tune mixture is another reason they are generally unliked (and give me a further reason not to want get to know them!)...I love SUs, among other reasons, for their complete accessibility of adjustments!