Popping and fuel running out around trumpet

[s28ven]

Had this engine running a few times since it was rebuilt 30 years ago(not many times in the last 10 years though).

In the last 6 months the carbs have been re-built and set up although now on idle everything seems ok although when trying to get any revs it’s popping and throwing fuel back out the trumpet.

Could this be a carb issue or is it more likely to be a valve/timing issue.

Thanks,

Steven

[nigelrbfurness]

Could certainly be valve timing, maybe the inlet cam is a tooth out but also could be ignition timing. Check both. Also check that you haven't mixed up the inlet and exhaust cam sprockets!

[StressCraxx]

Just one trumpet out of four? It's maybe lean on that cylinder. Another cause can be an intake valve leaking back.

[s28ven]

https://landscapesandcontracts-my.share ... w?e=WjOVbm

[s28ven]

I’ve attached a link above with a short video. And a picture below.

It is just one cylinder that’s doing it. Looks like I will need to lift the cam cover and check the timing.

[rgh0]

looks like you have the venturis installed wrong. there should not be anything sticking out the trumpets

cheers
Rohan

[StressCraxx]

Agree with Rohan WRT venturis.

Suggest you check your valve clearances. That particular intake valve may be below minimum clearance.

[pharriso]

With Webers re-assembled like that I'm amazed it runs at all, the Auxilliary Venturis are in backwards, here's an exploded view:

[bitsobrits]

How would it even be possible to have the venturi's sticking out like that, even backwards? Are they wedged into the trumpets rather than seated in the carb body?!?

[nmauduit]

With Webers re-assembled like that I'm amazed it runs at all, the Auxilliary Venturis are in backwards, here's an exploded view:

running at low revs (till 3-3.5k) is mostly governed by the "idle circuit" (the idle circuit fuels the actual idle via the springed idle screw and the progression holes, you can do a lot just on that, drive a car home etc)

[rgh0]

Yes with the auxilary venturi holders in backwards it will only run on the idle jets.

cheers
Rohan

[Grizzly]

Have you checked the Venturi springs are in good condition and tight? if they are loose you will get a lot of fuel in and around the trumpets which leads to a very unhappy cylinder. Might also be worth checking the fuel level in the carb as that would also give a similar symptom if too high.

Do you have rootes venturi extension pipes fitted? i'm assuming you do, i know some one that had a set of these fitted with a stock air box and they made the no4 run very rich.

Also try balancing the carbs at a few different rpm's, balancing at idle is ok if you know everything is spot on but if the balance starts to go out of wack as the revs increase the throttle plates may have been reinstalled badly or the shaft might be bent causing a bad balance at anything but idle.... but that won't explain the excess of fuel.

[mbell]

Also worth checking the fuel level in the carbs, if it's too high it can run down in to the throats.

Remove the jet covers and man jets, the fuel level should be just below the drilling that goes down to the throat.

[s28ven]

Ok, so bear in mind although this car has been in the family since before I was born, I’m not a mechanic and although understand the basic principals of an engine I’ve never undertaken major engine works. My dad is a mechanic and can ask him for help (once he stop’s self isolating) although trying to do as much as I can and learn along the way.

I’ve not had the Venturi out in a number of years although my memory is that there is a long tube on one side, say 100mm and short on the other, say 10mm. So am I right in saying it looks like there has been extension tubes fitted? The carbs have just been for a full rebuild at a reputable rebuild company and then shipped back to the garage who carried out a number of bodged repairs, so it is possible they are in backwards although I really don’t think they are.

I removed the cam cover tonight and it looks like the timing is pretty much spot on going with the timing marks on the cams(I’m pretty confident the head has never been skimmed as my dad bought it when it was 4 years old and he has never skimmed it.

I can spin the camshaft follower freely (I don’t have feeler gauges at the moment) which suggests the valve isn’t being held open by the cam.

I’m thinking the next exploration should be a compression test to see if the valve is being held open by some other means? Does this seem like the best route?

The carbs look to have been fitted (by bodge it and dodge it) by horsing up the spring washers as tight as they will go, could this be causing the issues?

Thanks,

Steven

[Grizzly]

Yep the Venturi's have the rootes style extensions fitted, so are you sure the carbs are the original oem set? do they have the correct jets fitted?

Next make sure your carbs are well balanced at a few different RPM's (Idle,2000 and 3000) SK meter works great for me and they are quite cheap.... very common for the carbs to be out of balance. https://www.ebay.co.uk/itm/Carburettor- ... SwjXNaW3~j

Maybe a set of plugs, Points and ignition timing tweak might help as it is missing through the inlet but the excess of fuel would have me looking at the carbs straight away (my money is still on the venturi locating springs being weak or missing)


Might be worth giving this a read, Keith Franck's white paper has helped allot of people and it will give you a good idea of how the carbs work.

Weber_DCOE_Tuning_White_Paper.pdf

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