carbs spitting out fuel. why?

[advinnie]

Hi all I have a 1600 twin port beetle with a gene berg 307 cam and 1.25 rocker and dual single barrel carbs. The carbs hve deen balanced with a snail gauge (with the linkage disconnect) and I set the mixture by slowly turning each mixture screw out which I believe riches up the mixture untill the rpm no longer increases and both mixture screws end up being exactly the same 1 3/4 turns out from seated but I have a problem if I accelerate hard from idle the carbs spit fuel out of the top of the carbs. This only happens acceleration from idle (850 rpm) if I accelerate hard from any rpm above 1000rpm the engine runs great and the carbs stop spitting out fuel. Does anyone no if this is normal or what is going on. Many thanks all. Ps I have a SVDA distributor and the fuel pressure after the pump is 3 PSI.

[FJCamper]

Hi Advinnie,

I take it your carbs are not Kadron (which are really Solex) but the single bbl 34mm Solexes?

First, how do you know you actually have fuel blowing out of the carb throat on accelleration? Is this something you can see if you're revving the engine, and the car is parked?

There is a condition called "standoff" that blows fuel vapor (and it is very visible) out of the carb throat. Technically, at whatever RPM you experience this, there is a reverse pressure wave inside the intake manifold that is blowing back harder than the engine can suck it in! Depending on your cam, venturi size, and other details, you can get standoff at high or low RPM's.

I don't know what size venturis you have, but the first thing I'd do is drop a size and see what happens.

FJC

[advinnie]

Hi there. The carbs are empi 34s and you can see spits of fuel coming out of the carbs when you stand next to the engine and rev it plus the sound it makes you can also here when you are in the driving seat as well so im sure it is happening when the engine is under load. Plus the carbs never as far as Im aware never done this when I had the stock cam in it but I cant be sure its only happened since ive installed the gene berg 307 cam. Idont know what size venturis I have and it only happenes when you accelerate from idle not at any other rpm above 1000. I have recently found out that there should be a balance pipe fitted between the two manifolds I dont know why or what its purpose is. Do you know why?

[Phil69]

The balance pipe will even out the induction pulses as it connects both inlet manifolds. Make sure the pipe is a decent internal diameter say 10mm and you should get rid of the stand off.
I run single solex 40/44s on my 1641 single port and had a similar problem till I realised the balance pipe I was running was blocked.
I also modified the top of the carbs to stop fuel from sloping out of the top of float chambers which to my amazment realy made a huge difference to the way they run.
Good luck with sorting the carb issues. Please let us know how you get on.

[advinnie]

How did you mod your carbs to stop fuel sloping out of the float cambers?

[FJCamper]

Hi Advinnie,

Yes, get that balance tube on your carbs -- I was presuming it was in place.

Slosh out of the float bowl is fixed with a small two-inch extension tube inserted into the vent and facing straight up.

FJC

[advinnie]

Them extension tubes came with the empi kit I ordered from the USA I all ready have them fitted. They fit onto the float bowl breather jet ( not sure there exact name) and run 2/3 up inside the air filters. Any way what size pipe and where can I buy it from. And where do I drill the manifolds is it at the top or bottom of the manifolds?

[FJCamper]

Hi Advinnie,

Install the crossover pipe first, by whatever means you must (drilling, etc) to see what effect it has on the reversion.

Be aware that extreme reversion is blowing pressure up through the carb from the intake valve side, and that includes some pressure up the mainjet and into the float bowl.

Good luck,

FJC

[advinnie]

Hi all ive ordered the balance pipe but it may of been to late. I had a engine bay fire today thank god I had a fire bottle to hand. Looking at the burn marks and the fact that the left air filter has total melted on to and into the carb im guessing its a safe bet to say thats where the fire started. I should of known not to take the car out for a drive because yesterday I thought I saw a flame pop out of the left carb but I then thought I must of image it cause I couldn't get it to do it again. Any way car is in the local vw garage now being checked out it may be some time before I report back to you about if the balance pipe has worked or not. flip it three year's building it and this happens.

[FJCamper]

Hi Advinnie,

I am so sorry this happened to you.

Reversion and a carb backfire can cause fires.

FJC

[Phil69]

I will get some pictures to you. PM me you email address and I will send them over.
Or follow this link. http://images.thesamba.com/vw/classifie ... 210320.jpg

[advinnie]

I will get some pictures to you. PM me you email address and I will send them over.
Or follow this link. http://images.thesamba.com/vw/classifie ... 210320.jpg

Thanks for the photo mate

[Steve Arndt]

On my first set of dual carbs years ago I had an intake back fire pop that lit my air filter on fire and almost spread. You aren't the only person it has happened to.

[advinnie]

Thanks mate im still really piss off with it but the engine is back up and running now. The vw garage have an idea of why it may of happened but im not shaw my self, tell me what you lot think. They are saying that the cam im using is designed for a minimum of 1.4 ratio rockers due to the ramp angle on the cam but im using 1.25 ratio rockers which was opening and or cause closing the valve to slowly witch was causing a high pressure return pulse causing exhaust gases to mix with neat fuel and it was only a matter of time until the gases got hot enough to catch the fuel alight and the return pulse to blow it up out of the carb into the air filters and when they had melted away spay all over the engine. Does this sound right to you lot?

[Steve Arndt]

It sounds like they are trying to sell you 1.4 Rockers. It should have less reversion with the reduced lift from 1.25 rockers vs. the designed 1.4s. One of my big engines has an FK10 and 1.25:1 rockers. It works perfect.

You will get it dialed in, it just takes tweaking and tinkering.