Amazon: Damaged JFUR4 distributor

[HRA]

Good point Ron Kwas! Thinking about it, when I got the wrong 175CD one of its deficiencies was the fact that there was no connection for the vacuum advance. From memory there was a feature (shallow blind hole) where it should have been but clearly the casting hadn't been machined-out. I drilled and tapped the casting myself and made a hose-fitting to screw into it.

At the time I thought it was fairly obvious what needed doing in terms of depth and direction of the hole. However I don't know if the hole was supposed to be the same diameter all the way through. Maybe it should have been big on the outside to take a hose-tail but then only a couple of millimetres in diameter where it actually broke through into the inside of the carb? It's also possible that the smaller and larger diameter holes were not intended to be parallel or co-linear.

That might explain exactly what I'm experiencing. If the hole is too big I'm getting too much vacuum too soon. If the hole were smaller it would take longer to suck the air out of the vacuum regulator and advance the timing.

I'll have to have a look at this part of the -2SE when it arrives to see what I should have done!

I think I agree with you about Strombergs - the SUs I had on MGBs did seem simpler and somehow "more elegant" in concept. It may just be my impression but Strombergs seem to have lots of add-ons and adjusters/correcters to make them do what an SU does naturally. But now I'm down the Stromberg route I'm determined to get it right!

[Ron Kwas]

HRA;

I agree...as you note, any necking down of the passageway will constrict flow of the "volume pumpdown" and thereby delay any dynamic action of the vac adv...but I've never heard of that being a parameter it was engineered to...instead, I've read that the location of the vac port with respect to the partially open Throttle plate is quite precise (in the SU anyway...so I expect also in the ZS)...you may want to investigate and understand this a bit better...

Cheers

[Burdekin]

If it is a standard B20A then a good proper Stromberg 175CD2SE will work a treat with the correct needle. It is one thing less to try and sort as it does seem like you are trying to sort two issues at the one time. The carb should just bolt on and work, that is the idea of this carb. You can buy a jet section that can be adjusted, have a look in the 140 section of the forum I have a thread in there showing it but do that later if you want as this carb should not need adjusting for the B20A engine apart from fine tuning with the idle trimming screw.

If you have pinking the head may have been machined down previously, something to consider if the dizzy and carbs don’t solve the issue. The compression ratio maybe too high. Keep it under 9.5 : 1 I think is advisable for the A engine.

When I changed the old and very filthy filter and put in a new diaphragm it made a good difference and it runs lovely with the Stromberg. So if not done already replace them is a good idea I think.

Pic of mine where the vacuum is connected for the dizzy.

[Burdekin]

I was also going to say check your timing at higher revs with a timing light and see if it advances when it should. Do it with another person who can do the throttle and revs while you check the timing readings. Be very careful and keep everything clear of moving parts.

[HRA]

Thanks Burdekin - it's good to hear that the -2SE should be plug-and-play. Do you have any advice on needles? B2AF and B1AN are the specified needles for the B20A but different sites contradict each other when it comes to dates. I can't even be sure which is the earlier needle.

I also wonder why they changed the needle for this engine? If it was to cure a problem then I want the later one. But if the later needle was a compromise to get around emissions legislation then an early one sounds a better bet.

[Burdekin]

Originally Posted by HRA

Thanks Burdekin - it's good to hear that the -2SE should be plug-and-play. Do you have any advice on needles? B2AF and B1AN are the specified needles for the B20A but different sites contradict each other when it comes to dates. I can't even be sure which is the earlier needle.

I also wonder why they changed the needle for this engine? If it was to cure a problem then I want the later one. But if the later needle was a compromise to get around emissions legislation then an early one sounds a better bet.

Just get it from Burlens. I think they renamed the needles rather than changed the spec, but not sure of that.

[Ron Kwas]

HRA;

I happen to have a pair of (used, admittedly grungy, but all there) ZSs from a Volvo installation (so a dual carb configuration) in my stock...one has no Vac Port, the other has one. Here some pix and observations:

The angle of the casting boss positions the VP, when present, just outside the closed Throttle.


The VP is completely blocked at closed Throttle.


VP (clearly much smaller in ID than vac tubing and fitting) is exposed when throttle is opened by twice its thickness, so vacuum advance would be maximum just off idle, decreasing somewhat with the increasing throttle opening, and decreasing engine load, or even at WOT.

Hope that sheds some light.

Cheers

[HRA]

Originally Posted by Burdekin

Just get it from Burlens. I think they renamed the needles rather than changed the spec, but not sure of that.

Thanks again Burdekin. I've got the Excel spreadsheet with all the Stromberg needle profiles, and the two do look different. Nothing major, but possibly significant. I wonder what the story was back then...

[HRA]

Ron Kwas,

Great photos! In the first there is another "undrilled feature" in the foreground. I seem to remember drilling through at this steeper angle. If so then the hole would have ended-up too far behind the throttle. When I take that carb off I'll see if I can take some photos for comparison.

The positioning of the hole is interesting - just behind the throttle. It looks like a Venturi situation with air rushing past the slightly opened throttle and entraining air from the vacuum pipe. From that perspective I don't think that the area of the VP drilling should matter in theory, but in my experience of physics there's always a reason why theory and practice never quite line-up!

[Burdekin]

Originally Posted by HRA

Thanks again Burdekin. I've got the Excel spreadsheet with all the Stromberg needle profiles, and the two do look different. Nothing major, but possibly significant. I wonder what the story was back then...

I don’t think they make them any more, what is there is old stock. Maybe what they sell is the next best option. I have a spare B1AN needle and B1CC, not looked into if they are any different but both are supposed to be for the B20A.