Cylinder #8 INSANITY

What is the status of the vacuum nipple on the #8 cylinder intake manifold?

On a stock US '78 308, this vacuum nipple supplies a "closed" vacuum system that runs the airbox flap mechanism at engine shut-off -- so "no air flow" should be occurring thru this nipple when the engine is running. As a test (even if you've still got the stock airbox flap+sphere system intact -- which would be a miracle ), just completely block off that vacuum nipple -- if it makes a difference, you need to find out why whatever is hooked to that vacuum nipple is no longer a "closed" system.

 

Yes, the nipple with the red arrow:
Image Unavailable, Please Login

 

During the carb rebuild, did you replace the butterfly shaft bearings with new sealed units?

And you did check that the accelerator pumps are working.....

When you put it all back together, did you tune it using the tried and true method as described on this board and a few other sites, as written up by Carl Jones? The tuning procedure is crucial, as it not only adjusts the mixture (which incidentally is not that important) but the balance between AND WITHIN the carbs. If this isn't right, you'll never get it to run properly.

Carl's article can be found on this website in the tech tips section.

http://www.r-design.net/308/index_e.html

Dave

 

Excellent - sort of. The proper tune/balance procedure would have been an easy fix!

Have you tried running the engine and then spraying some WD40 or similar around the carb to check for bubbling?

I am not familiar with the air-flap thing (mine didn't have that), but this sounds like a good possibility also.

Dave

 

Check for blocked idle jet and/or carb body drillings, by removing all parts and blowing through. It's remotely possible that if someone has replaced the lead drilling plugs at some time, they may have restricted the passage.

Make sure the venturi is installed the correct way around.

Make sure the progression holes are not blocked.

To try to determine if its a high or low speed/load thing.... do a hi-cut...

Drive along at full throttle in 3rd gear for at least 10 seconds... from about 2000rpm to max rpm.... then switch off the ignition, dip the clutch and coast to standstill at the roadside (or use a dyno)...

then remove the suspected cylinder spark plug and check its colour.... if it's still lean it'll be a main jet/venturi high load thing. If not, then you now know for sure that it's a low speed thing only.

The #8 carb is also the last one to receive it's fuel from the pump, and it has the return line attached... so check fuel flow rate, and pressure.

 

Some other ideas:

1. If you disconnect the spark plug wire going to #8 at idle, does the engine tone/rpm change?

2. What is the status of the air bleeds on the 7-8 carb? Is 7 closed and 8 open a little to achieve balance? If balance is achieved with too much bypass air on the 8 cylinder at idle, there will not be enough air flow to adjust the mixture. There is a critical amount of air flow past the throttle plate that is required to draw in the emulsified fuel, sufficient to effect mixture adjustment. Said another way, too little air flow past the idle/progression holes will make mixture adjustment impossible. You can test this by increasing flow in the 8-cyinder a little and see if you are now able to set the mixture.

3. BK and SK flow meters are as good as it gets for individual meters measuring air flow throught the venturi system. At 900-1000 rpm, measurements are close to the noise area. They also do not account for leakage past the throttle shaft bearings, carb and manifold sealing gaskets, and some other sources like the vacuum sphere system. The ultimate setup is to key on manifold vacuum using a manometer, preferably a multi-bank setup.

Bill