Weber carburetor help

[Clay Perrine]

All,
I have a 1970 factory 914-6 with the original Webers on it. It has been running great up until recently, when it started to misfire on one cylinder.

I started troubleshooting and found that #5 is dead. The first thing I did was pull the spark plug and replace it. No results.

I swapped the #5 and #6 plug wires, the miss stayed on #5.

I ran a compression test on the engine, all of them show approximately 155 psi.

I put the plug back in, and fired it back up. I then took a gas torch and slowly fed acetylene into the stack on #5. The idle sped up and smoothed out.

So I pulled all the jets in the carb and cleaned them. I pulled the top and checked the air bleeds, and the aux venturi for any blockages. Every passage tests to be clear. Even the accelerator pump works correctly.

Frankly, I am stumped on this one. Can anyone think of something I missed?

Thanks in Advance!

[Mixed76]

Are you sure you have spark? Test #5 with a spare plug against ground while running?

Acetylene is pretty reactive might not be the best confirmation of spark/ lack of fuel.

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[Clay Perrine]

I just went out in the garage and double checked it. I hooked the plug wire to a new spark plug and put it on top of the motor. I am getting a good spark.

[Jim2]

Seems like your acetylene test verifies you have spark but lacking fuel.

Is the misfire only at idle (idle circuit) or under load?

I’m not a carb guru, just trying to help narrow it down.

[930cabman]

Originally Posted by Jim2

Seems like your acetylene test verifies you have spark but lacking fuel.

Is the misfire only at idle (idle circuit) or under load?

I’m not a carb guru, just trying to help narrow it down.

Exactly. If #5 is not firing at idle only or not firing under load is a big question

[Dpmulvan]

Originally Posted by Clay Perrine

All,
I have a 1970 factory 914-6 with the original Webers on it. It has been running great up until recently, when it started to misfire on one cylinder.

I started troubleshooting and found that #5 is dead. The first thing I did was pull the spark plug and replace it. No results.

I swapped the #5 and #6 plug wires, the miss stayed on #5.

I ran a compression test on the engine, all of them show approximately 155 psi.

I put the plug back in, and fired it back up. I then took a gas torch and slowly fed acetylene into the stack on #5. The idle sped up and smoothed out.

So I pulled all the jets in the carb and cleaned them. I pulled the top and checked the air bleeds, and the aux venturi for any blockages. Every passage tests to be clear. Even the accelerator pump works correctly.

Frankly, I am stumped on this one. Can anyone think of something I missed?

Thanks in Advance!

Do you try cleaning the hidden gallery between bottom of emulsion tube and idle jet?

[Clay Perrine]

Originally Posted by Dpmulvan

Do you try cleaning the hidden gallery between bottom of emulsion tube and idle jet?

I got the email you sent to me with the info on the hidden gallery. I am very reluctant to go that deep into the carb. I am going to seek professional help.

Thanks everyone.