3.5 V8 SU Carb Issues

hef19898 · Oct 1, 2020

I'll tackle the fuel filter tomorrow, there should be a new one lying around somewhere anyway.

Maybe it's just that, fingers crossed. Good thing is that fuel lines aren't that expensive neither, if it comes to that. Fuel pump would suck, so.

The pump is working, electrically that is, just fine as far as I can tell. I don't know enough about the whole ignition and fuel set up to tell if things are working together properly so. One thing at a time.

hef19898 · Oct 2, 2020

The spark plug looks like that (well, the first one I removed):

Looks ok for me, but it is the first one I saw for decades, so...

hef19898 · Oct 2, 2020

Now I am almost convinced that the mixture is too lean. With the choke fully out, the engine is running rough at the lower torque end, but revs happily up to 6k rpm on full throttle. I found a spare coil, but I won't mess with that before I changed the mixture.

kermit_rr · Oct 2, 2020

Plug looks good to me.
Your problem is definitely a lean mixture, but as said, could be for many reasons, adjustment, blockage etc

hef19898 · Oct 2, 2020

I just tried something out. That the engine is reving with pulled choke only works when the engine is cold. Now, after idling in the driveway for a couple of minutes, I have the same issues with the mixture. My neighbors are going to hate me...

Could it be that part that's causing trouble? And do I need a new gasket for the crank case?

Ratae · Oct 2, 2020

hef19898 · Oct 2, 2020

I had. Diaphragms are looking like new, I do need a new O ring for the left carb. I might even have one at home already.

It's funny that it only seems to have these issues when hot / warm so.

tomcat59alan · Oct 2, 2020

Those are Zenith carbs not SU's is that a photo of the rocker cover not the crankcase? If so I would say yes,do both of them.
These are SU's

kermit_rr · Oct 2, 2020

tomcat59alan said:
Those are Zenith carbs not SU's is that a photo of the rocker cover not the crankcase? If so I would say yes,do both of them.
These are SU's

don't think they were ever referred to as SU

You're right, completely missed the title

hef19898 · Oct 2, 2020

Rocker cover, of course. So I will add that to the list. I just hope I'm not in for a complete engine revision.

While searching for a suitable o ring in the spare collection that came with the car I found two temp compensators. Maybe I try a new one, just to see what happens. Still didn't find the o ring so. But I did find the o ring and washer for the diff casing I just ordered two days ago...

hef19898 · Oct 2, 2020

And I thought them carbs to be SUs in the beginning.

kermit_rr · Oct 2, 2020

Zenith-stromburg
They look similar to SU with the piston, but the top cover is much shorter

hef19898 · Oct 2, 2020

It is some kind of obstruction it seems. The old fuel filter was full of brown goo, including some rusted metal flakes.

Come to think of it, I had the fuel tank replaced roughly 1k kilometres ago because the old one was rusted through. Could explain it. With the new filter, I gained around 1k rpm. Now the rough running starts at just shy of 4k rpm but the engine holds the revs.

Fuel pump next, as I have neither the parts nor the tools for the carbs yet. Ah, and I fitted the axle breathers. I guess I can now claim almost one meter of waiding depth.

kermit_rr · Oct 2, 2020

the carbs should have a drain on the float bowl, try draining and see what comes out

hef19898 · Oct 2, 2020

kermit_rr said:
the carbs should have a drain on the float bowl, try draining and see what comes out

Will do that as well. And I'll replace the fuel line up to the filter / carbs as well. Those are cheap enough and with all the goo in filter I prefer to be save than sorry.

That neighbor of mine also had a great tip regarding engine experts. Turned out there is an engine revision expert more or less next door. I guess I'll visit him next week. The last thing not done is the cam shaft, and the engine didn't have an revision yet. So that's for next year, or over winter, depending on cost and money. But for now she's running already better!

Shippers · Oct 4, 2020

kermit_rr said:
the carbs should have a drain on the float bowl, try draining and see what comes out

Mine dont theyre 175cd on a factory fit 75 V8...

hef19898 · Oct 19, 2020

It officially sucks... I topped of the damper oil, enriched the mixture and cold it reved up to 6k. Under load it topped out 3.5k, yesterday that was 3k. The test drive showed much better acceleration and improved top speed from 100 to 120 mk/h. Still not where she used to be a couple of months ago, but an improvement.

That was the first half of the test drive. The second half was the same as before. Even in neutral revs top out at 3k and something. What could that be? The fuel filter is new, the pump is cleaned and when cold it, almost, works. When hot, it doesn't anymore... And I don't want to hand it over to my beloved JLR garage as that will mean another 2 months without the car and a bill I cannot afford right now. I'd love to repair as much as possible and just have them calibrate ignition and the carbs to pass the TÜV and emission test next year.

Things I touched before the issues started:

Removed and put back on the damper calipers of both carbs.

Replaced a **** relay in aftermarket transistor ignition that caused the fuel pump to shut down from time to time.

Any hints would be highly appreciated!

kermit_rr · Oct 19, 2020

right.. when cold and on choke it runs fine. when hot with no choke, doesn't get enough fuel and only manages 3k revs.
at this point have you tried pulling the choke out to see how that changes things? it will either get worse or better! unlikely there will be no change at all.
you should be able to set the ignition close enough without sending it to a garage

hef19898 · Oct 19, 2020

Pulling the choke doesn't have any real impact when the engine is hot. Is does run fine with the choke pulled or not when cold.

when it was just started, pulling the choke nice increases revs... Already before I enriched the mixture, which also increased idle rev to about 800 cold or 1,000 when hot.

kermit_rr · Oct 19, 2020

i've had a look at http://zenithcarb.co.uk/zencdcdsseries/?___store=zenith which if it is not your carb, should be very similar
when the engine is hot, you need to adjust the idle speed back to 650rpm as this is the correct number. the fact the engine speed at idle has increased, suggests it was running too lean.
when you have reduced the revs down, once again increase the mixture until the engine no longer increases speed, then decrease 1/4 turn.
adjust the idle again so it is correct, then lift the piston slightly. the engine revs should not change
it sounds like you are almost there!

3.5 V8 SU Carb Issues

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