Twin Dellortos problem

[timnuttuk]

Hi,

 

I'm new to the list, so 'Hi'. Currently building a Tiger Cat Seven.

 

I've just fitted twin Dellortos 40s to my pinto with the original mechanical fuel pump. When idling I notice a constant drip from the auxiliary venturi which then collects in the bottom of the barrel and I guess just runs into the engine. I can just about get it to idle, but it is not smooth. Before I start adjusting the idle screws, I want to cure the dripping. I take it, that it isn't normal...

 

I have replaced the needle valves, and I have set the float height correctly. I'm now at a loss of what to look for next. I don't really understand what causes fuel to be drawn into the venturis at full thottle and not when idling. Any help would be much appreciated!

 

Running without trumpets (they're on order) won't affect things will it? Daft, I know but I'm clutching at straws

 

Thanks

 

 

TimN

2.0l Tiger Cat (in progress)

http://bignuttjob.tripod.com

[Mr Locust]

To start with I always had problems with Dellortos idling on my Triumph engined Locust. The main problem was the stiffness of the mounting on the installation seemed to be critical. Remember they should be 'fast-loose' rather than tightened onto the manifold.

 

Once sorted out, 20-25K miles without significant attention was the order of the day....

 

In the end what worked for that application was;

 

1. use misab bonded seals rather than the double o-ring carriers. They seemed to give a more flexible joint/better seal.

 

2. use thackeray washers rather than the rubber bobbin/washer thingies. The bobbins seemed to go hard after a while and loose their resiliance. I also had problems with poorly plated thackeray washers being brittle so try and get black finish ones.

 

Hope this helps....

 

By the way. Trumpet length was critical on that engine. I got a good improvement in mid range torque with 1 1/4" of spacers behind the 40mm trumpets. Worth a play

 

Ian

 

Green and Silver Roadsport 😬

[oilyhands]

You need a fuel pressure regulator.

 

Oily

[timnuttuk]

Thanks for the replies.

 

I'm getting conflicting opinions on whether I need a pressure regulator or not - anyyone out there successfully running webers/dellortos with mechanical pumps? I turned the engine over with the carb open and fuel is getting shut off when I move the float up by hand, so it doesn't look like the float chamber is being floaded.

 

Anyone know what causes fuel to be drawn through the main jet into the venturi at full throttle, and then what shuts it off during idling?

 

Thanks

 

 

TimN

2.0l Tiger Cat (in progress)

http://bignuttjob.tripod.com

[oilyhands]

Fuel drip from the aux vent is a common problem when fuel pressure is too high, it doesn't mean you have a faulty valve, but simply that the exerted pressure on the valve from the fuel pump is sufficient to overcome the authority of the float/needle valve assembly. Vibration doesnt help.

 

I've have seen this many, many times, a simple FP regulator such as those available from the Carburettor exchange for a few pounds will do the trick, set it to around 3-4 PSI.

 

Airflow is what draws the fuel through the main jet/emulsion tube assembly into the aux vent. At idle there is no draw on the main jest since air velocity is small, at higher throttle opening the air velocity and pressure drop will draw the emulsified fuel through.

 

Oily

[Joe 90]

I don't know anyone who thinks you can get away without a fuel pressure regulator. I would strongly recommend fitting one.

 

SEP field working, not spotted in 103,300 miles. Some photos on webshots, updated 25 Jan

[timnuttuk]

Thanks for the clarification. Regulator it is then!

 

 

 

TimN

2.0l Tiger Cat (in progress)

http://bignuttjob.tripod.com

[Mr Locust]

I used a filter king type with filter bowl and set to 3.5psi on my setup

 

Ian

 

Green and Silver Roadsport 😬