Dellorto barrel imbalance

[The Cyclist]

My Webers do exactly the same, with the flow on cylinders 1 & 4 being greater than 2 & 3. This seems to be a common pattern, and happening with both Dellortos and Webers. Surely that means it's not a carburettor issue.

I don't have a confident proposal, but I wondered if it's related to us using tubular exhaust manifolds as 1 & 4 have longer pipes than 2 & 3?

[alaric]

Hi. Thanks for mentioning this. Its clearly not a carb issue but I needed to test it - at least I have spare shiny carbs now haha.

I did swap the carbs over - switched front and rear with my original carbs - and the flow pattern moved with the carbs. So that fooled me.

I am going to try a gasket compound on the adaptor plates which locate the o rings. Mine have little holes in them... so you never know. It can't do any harm. I'll stick the o rings in place and that will hopefully fill any holes.

Apart from that I'm stumped now. The car does run, but it's got a regular rough idle due to the barrel imbalance. So I could use it, but it's spoiling the experience for me.

I'll let you know if my encapsulated o ring plates work.

Sean.

[2cams70]

Have you done what promotor said right at the beginning of your posts and measured your cylinder compressions?

Also have you checked other things that might cause a rough idle - mixtures, ignition, etc. If your engine is running lumpily it may be at a running slow point when the intake valve opens on those cylinders hence you will get a low reading on just those cylinders because the engine at that point is not sucking as hard

[alaric]

Hi 2cams. Thanks for helping out.

I did check the compression ratios and found 200psi in each. I'm hoping that means the engine is OK.

I've overhauled the ignition system and set the timing with a strobe. The light is nice and stable i.e. not jumping either side of the timing mark. Re mixture I have a colortune and have deployed that a few times to set the mixture at idle. The jets possibly need changing but are now new. It needs a rolling road to get the best jets I think.

I'll check again though. It looks like the centre barrels are flowing more air. That pattern did change to being the outer two barrels flowing more air when I swapped over the carbs.

I think the sooner I get it to someone that knows what they are doing i.e. not me, the better. I'll try the gasket seal though. If that works I'll be buying rounds of beer at my local pub haha.

Sean.

[The Cyclist]

I had a chat with my local old car specialist & he suggested that over years, twisting one end of the throttle spindle might twist it very slightly - which would cause the observed issue. He suggested you can see if the butterflies are seating by testing whether they hold petrol.
I suspect that's probably what I need to do.

[alaric]

Hi all. Well I do have such fun with these carbs...

I took the new carbs off the car. On testing them I discovered the pump jets were blocked. So I went back to my original carbs and have found the same. There's a varnish like sludge in the float chambers of the old carbs and the pump on the one I've cleaned out was full of it.

Is that just that the fuel is too old, or have I used a bad file type? If the latter can someone advise the UK fuel that I should be using? Thanks.

Needless to say I've abandoned getting the car to Castle Coombe tomorrow. I'll get there but later than planned.

All the best.

Sean.

[StressCraxx]

Hi all. Well I do have such fun with these carbs...

I took the new carbs off the car. On testing them I discovered the pump jets were blocked. So I went back to my original carbs and have found the same. There's a varnish like sludge in the float chambers of the old carbs and the pump on the one I've cleaned out was full of it.

Is that just that the fuel is too old, or have I used a bad file type? If the latter can someone advise the UK fuel that I should be using? Thanks.

Needless to say I've abandoned getting the car to Castle Coombe tomorrow. I'll get there but later than planned.

All the best.

Sean.

If you are finding sludge in the float chambers and the accel pumps, it's time to drain the fuel tank and have it cleaned. Replace your fuel filter and the fuel screens at each carb fuel inlet. It won't get better just with fresh fuel.

[alaric]

Thanks Stresscrax. I've been advised same today at castle coombe. Bother.

Sean.

[mbell]

Ethanol is a solvent, it's known to free any dirt etc in the tank and allow it to flow out the tank with the fuel.

It maybe be the ethanol is letting many years build up of stuff in your tank to be dissolved into the fuel and be deposited in you carbs.

If it continues I'd suggest pulling the tank and getting it cleaned and possibly coated. Along with new fuel filters, carb and line cleaning etc.

[alaric]

Hi. That's exactly the explanation that I was given this morning at Castle Coombe. So I'll be pulling the fuel tank out. As long as its not attacked the fuel line; I can't get to that so easily although I'd like to replace it.

Sean.

[Donels]

Just thinking of your carb flow inbalance. If the cylinder is not always firing due to carb issues, the carb flow will go down as it does not have the exhaust gas flow created depression to suck.

Probably needs Rohan input here……

[mbell]

As long as its not attacked the fuel line;

The standard pipe in the chassis is fine for ethanol fuels. What your running after that is more of a concern, the build up could easily be from that.

[alaric]

Ok good point. I have braided fuel lines fitted by the po.

Sean.

[mbell]

I would be swapping them out before driving the car. Good chance the slime is the ethanol breaking down whatever hose is under the braiding.

Make sure to fit something that is rated for use with ethanol contain fuels.

[RichardHawkins]

Alaric,

Mbell is spot on.

Braided hose looks good but could be covering the wrong tube. Buy tube from a reputable supplier, and it will have the spec printed on the side of the tube. I believe the correct spec is. SAE J30 R9 followed by the diameter you need, and then the pressure rating.

Hope this helps,

Richard Hawkins