3.5 V8 SU Carb Issues

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hef19898

hef19898

Well-Known Member
Posts
590
Location
Germany
Hi there,
a cursory search didn't return anything, so here is my problem.

A while ago, my beloved RRC started to have starting issues. They were solved by toping up the carb dampers every once in a while, the left one more often. Two weeks ago, it started to starve for petrol while driving. Only during the first, say, 20 km or so. Around the same time, the fuel pump started to act up.

Visited my local garage today. First diagnosis is, that it's most likely carbs and fuel pump. Could also be the wiring or something with the contactless ignition (don't get me started on the horror show that goes for wiring on that car). Estimated time to get the fault properly isolated is around two full days. At 125 bucks per hour.

Now the plan is to chase down the wiring of the fuel pump, maybe get a replacement pump and refurbish the carbs myself. The invoice of the ignition from the previous owner said that it is "self regulating". Ah, yeah, and replace the spark plugs.

I do have the Workshop manuals and repair guides (even original LR ones). But is it difficult to refurbish SU carbs? And is there someone selling refurbished ones, as I am quite confident to get them of and reasonably confident to get the back on but much less so to put them back together?

And what can it be, besides fuel filter and stuff? It got a new tank less than 1k km ago, if that can have something to do with it.
 
hef19898 said:
Hi there,
a cursory search didn't return anything, so here is my problem.

A while ago, my beloved RRC started to have starting issues. They were solved by toping up the carb dampers every once in a while, the left one more often. Two weeks ago, it started to starve for petrol while driving. Only during the first, say, 20 km or so. Around the same time, the fuel pump started to act up.

Visited my local garage today. First diagnosis is, that it's most likely carbs and fuel pump. Could also be the wiring or something with the contactless ignition (don't get me started on the horror show that goes for wiring on that car). Estimated time to get the fault properly isolated is around two full days. At 125 bucks per hour.

Now the plan is to chase down the wiring of the fuel pump, maybe get a replacement pump and refurbish the carbs myself. The invoice of the ignition from the previous owner said that it is "self regulating". Ah, yeah, and replace the spark plugs.

I do have the Workshop manuals and repair guides (even original LR ones). But is it difficult to refurbish SU carbs? And is there someone selling refurbished ones, as I am quite confident to get them of and reasonably confident to get the back on but much less so to put them back together?

And what can it be, besides fuel filter and stuff? It got a new tank less than 1k km ago, if that can have something to do with it.
Click to expand...

Setting the basics up properly to start with is best, then any issues are more noticeable ...

Read this for info:

https://www.lrukforums.com/resources/tuning-twin-su-carbs.32/
 
Damn, that is a good resource, thanks! I just realized one thing... I don't have SUs, I have ZenithSZenith by the looks of it.
 
Yes and no. It's a great resource, even if I can't use it. Maybe I upgrade to SUs. Edelbrock and Holly are too expensive and I have no idea how that would work with the snorkel...
 
hef19898 said:
Damn, that is a good resource, thanks! I just realized one thing... I don't have SUs, I have ZenithSZenith by the looks of it.
Click to expand...


You'll need a carb adjuster kit and a carb balancer to check the air flow through each carb, as the adjustments are done internally on the carb..

Also check the diaphragms are ok, replace any that are damage or torn disconnect balance linkage while tuning...


Try this:-

http://www.bmcno.org/tech tips/zenith stromberg tech tips.htm
 
Man, it always amazes me how fast you get useful answers around here.

So, I pit the adjustment tool on the shopping list, will have a look at the fuel filter, check the fuel pump wiring, top up damper oil and see what I get. Next step would be carb adjustment. Maybe I replace the fuel lines while I am at it. And check the spark plugs first!

Slowly I am turning into an almost full mechanic if it continues like that... Front axle, diff and clutch won't be late I am afraid...
 
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kermit_rr said:
Have you got a pic of your carbs? You said they needed oil, only ever heard of that in SU carbs
Click to expand...

Strombergs have dashpots the same as SU's ...

iu
 
Just out of curiosity, can the high consumption of 18 l on 100 km have something to do with the same problem?
 
So, now I'm totally lost... I topped up the dampers, around 5 shots with one of those small oil cans each side. The right damper pushes easier than the left one.

Turned the keys and immediately heard the fuel pump ticking. Engine started as easy as ever (I don't have a direct comparison so). No black smoke, revs up happily, idles at 600 rpm when warm and without choke, around 800 with the choke a quarter out... Can't be that easy, can it? Please say it can!
 

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hef19898 said:
So, now I'm totally lost... I topped up the dampers, around 5 shots with one of those small oil cans each side. The right damper pushes easier than the left one.

Turned the keys and immediately heard the fuel pump ticking. Engine started as easy as ever (I don't have a direct comparison so). No black smoke, revs up happily, idles at 600 rpm when warm and without choke, around 800 with the choke a quarter out... Can't be that easy, can it? Please say it can!
Click to expand...
both carbs should act the same
https://www.google.co.uk/url?sa=i&u...ved=0CAkQjhxqFwoTCPj4g7bFiOsCFQAAAAAdAAAAABAF
 
Thanks! Seems I have my work cut out for me this winter... Is it normal, so, that carbs loose oil over time to the point they are empty? Or is there a gasket worn out or something?
 
There is a small 'O' ring in the bottom of the dashpot oil tube. If these have failed then your oil will leak out.
Have a watch of this:

You don't need to dismantle the whole carb or remove from the engine. Just take the top off & lift the piston out. Mark it all so it all goes back in the same position.
From 31.50 where he removes the needle assembly & then the adjuster from the piston. (The adjuster is held in place by a star washer so banging it on the bench in the hope it will fall out won't achieve anything, needs to be pushed out. Don't forget the star washer on reasssembly)
Once he's got it out he points out the 'O' ring.

Comment on the Stag Owners Club that they need to be replaced with the proper ones not a 'near enough this'll do'.

The adjusting tool is used when adjusting the needle when the carb is assembled, the knurled part is held while you turn the allen key bit.

ETA: https://bowtie6.com/other/zenith-stromberg-carburetter-repair/
 
Last edited:
That is some very good info. Just had a chat with my neighbor, nice guy who had a Jeep back the day and did all the maintenance himself. He even called a mechanic yesterday he knows. The mechanics theory was faulty valves in the engine / exhaust tubing going to the air filter where the engine pulls and burns oil. Which it does, especially after standing for a while. Engine oil is still ok since the last service, even if on the high side. It is clean, so.

The current working theory now is the following: The carb damper oil is leaking into the engine, no problem when run regularly, a bigger problem when not as all the oil has to be burned at once. With the carbs empty, the whole mixture is off. Either running to rich, or too meager. In the latter case, the engine ends up stalling. How the fuel pump figures into this, no idea yet.

The engine is running just fine with topped up carbs, so it seems reasonable. The short term fix is topping up the carbs and running the engine ever so often. Long term fix is to replace the worn parts in both carbs. If it is just that, I am more than happy.

Maybe I pump out the oil from the carbs before the vacation, so...
 
Sort your carbs out before you do anything else, you know there is a problem with them & fiddling about with other stuff when they may be the problem is pointless.
When you check them you will also be able to see if there are any tears in the diaprhagm - which will result in a rich mixture & poor performance (already mentioned in jamesmartin's link)
You've already said it runs fine with oil in them.
In your OP you said "Two weeks ago, it started to starve for petrol while driving."
"The extra rich mixture required for acceleration is produced by the upward motion of the air valve being retarded by the oil damper in the piston guide rod. This condition allows engine vacuum to draw proportionally more fuel than normal from the jet. Should there be no oil in the dashpot, only a normal running mixture can be produced, and acceleration will be very poor."
http://www.mossmotoring.com/back-basics-intro-zenith-stromberg-carburetors/

Black smoke is generally indicative of excessive fuelling.
Oil burning generally blue.
Blue smoke on a V8 at start-up is often worn valve guides/valve guide seals but in your case may be the oil from the carbs.
 
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