Vacuum Leaks

[Burdekin]

I've just rebuilt my carb and in the process of getting the mixture correct. The isolating plate between the carb and manifold is cracked so I have ordered a new one. The carb is a stromberg 175 and I've replaced the butterfly/shaft and seal, I haven't changed the shaft bush. I will test the carb for vacuum leaks once running.

Idle is a bit rough before I rebuilt the carb and I've not really made any attempt at tracing possible vacuum leaks so I want to check over it all to make sure there are none. What's the best aerosol spray to use to check for leaks with the engine running? I've ordered all new hoses, do they need to be on tight with a clip holding them on or okay just to be pushed on? I just had engine work done so there should be a new gasket on the manifold. Would a leak in the servo cause a problem with idle? Anything else that could be an area for a vacuum leak that I should check?

[142 Guy]

Three options:
-water
-propane torch
- carb cleaner or starting fluid

Water is the safest; but, perhaps less effective. You squirt the water around the suspect areas and listen / watch for the water being sucked in. The water may temporarily reduce the air in flow at the leak site resulting in a change in operation.

With a hand held propane torch, open up the valve and then run the tip of the torch along the gaskets and other suspect locations. At the leak location the propane will get sucked into the engine and the propane will cause the mixture to go from lean to normal / not so lean which should result in a change in engine operation.

The carb cleaner / starting fluid method is the same as the propane except you are spraying a combustible liquid instead of gas. Use a straw on the spray can so that you are not spraying combustible liquid all over the place.

On my B20E I was chasing a persistent manifold vacuum leak for a long time. I eventually hooked up a shop vacuum cleaner in reverse to pressurize the manifold and did a soap bubble test to find the leak. Easy to do because the D jet throttle body was a perfect match for a 2" vacuum hose. Not so easy with a carb, particularly if you have two. Pressurizing the intake would cause the float chamber to pressurize which could be a bad thing.

[Burdekin]

Replaced the cracked carb isolation block and also noticed the brake booster connection nipple was loose and don’t think it will stay tight so needs replacing. Re-did the mixture and idle speed and she is purring at idle now. Will weld a boss on the exhaust for my wideband sensor and will see how things look out on the road. Also ordered a vac gauge to see how that looks and will see if I can use it to set idle and then compare to what the wideband says.

[142 Guy]

That rather looks like a 140 which has strayed into the Amazon forum; but, never mind.

I have never used a wideband on a carb equipped engine. From my personal experience with killing wide bands on the B20E on my 140, I offer up the following comments if you are going to leave the sensor in service permanently as opposed to just for tuning (This is not advice since that implies a higher degree of expertise).

- Do not energize the heater on the wideband before the engine is running. If the sensor comes up to full operating temperature and is hit by condensation from the engine starting up the thermal shock can fracture the sensing element.
- Do not allow the sensor to sit in the running engine exhaust stream without the sensor being energized.
- Fuel mixtures that are really rich can shorten the sensor life by contamination. Get your fuel mixtures 'close' before using the sensor to do final sorting. If the engine burns oil the oil residue will lead to an early death for the sensor.
- The best position for the sensor on the exhaust pipe is at 12:00 o'clock so that moisture running down the exhaust has the lowest chance of entering the sensor. Based upon my 140 experience, that is likely going to b a bit of a problem because of body interference if you locate the sensor where the dual down pipe joins the exhaust system. I have a header and my sensor is in the tail piece adapter and I had a problem just getting the sensor up to the 10:00 / 2:00 o'clock position. You may have to show some creativity in positioning the sensor - closer to the engine is better just because of minimizing the potential for condensation hitting the sensor.

Most wideband retrofits on fuel injected cars run the power supply off of the fuel pump relay. Except for the prime cycle and cranking the fuel pump relay does not run unless the engine is running which is a close match to how the sensor should be operated. You don't have that luxury. As an alternative, you could pick up a tach signal off of the ignition coil and use a one board controller which would monitor the RPM and energize a control relay for the wideband controller once the RPM exceeded 300 RPM (typical transition level from cranking to running)

[Army]

Originally Posted by 142 Guy

...the Amazon forum;...

Goodness me!

I sincerley hope that will be corrected and the full credit to the other types will be acknowledged!

[Burdekin]

Originally Posted by Army

Goodness me!

I sincerley hope that will be corrected and the full credit to the other types will be acknowledged!

I've been rumbled. 🤣