MFI Gurus: one stack has little airflow, clean/adjust butterflys?
 

Hi All,

Still trying to get my MFI sorted. My '73 911E starts fine and idles great whether cold or warm. The car revs fine until about 3000 RPM when it starts stumbling and stuttering. I measured the airflow at 3000 RPM, and while the left stack intakes all measured good airflow, the right stack of intakes measures almost no airflow at all.

Adjusting the mixture screws has no effect on the airflow, so I removed the plastic stacks and had a look at the butterflys. One of them looks pretty normal, one is covered in soot, and the third is corroded pretty badly. The action seems OK though, and there are no grooves in the butterfly cylinders.
The engine has close to 150psi compression on all cylinders.

Should I remove the butterfly housing and soak it in Berrymans, and will this possibly help? I know on carburetors that adjusting the mixture screw with no flow effect means you need to rebuild the carbs, but what about MFI?

Thanks for anyhelp or ideas.

Since you cannot improve the air flow by turning the air screws, it sounds to me like at least one of the air channels is carboned up. Remove all the air correction screws (make sure you keep them in the correct order) and spray a good carb cleaner, like Berrymans, into the air passage of the adjusting screw. Wait several minutes and then use a pipe cleaner inserted into the opening to clean the air passages. Spray some more carb cleaner to get any remnants. I know you are only having problems with one cylinder but it's best perform this procedure on all the adjusting screw passages so as to get even results.

When you're done, replace the air correction screws to a fully closed position without forcing them shut. Next open the screws 3 half turns. Run the engine to 3000 RPM using the throttle and check the air intake. Adjust the air correction screws until all six cylinders have the same reading. Let us know if this works.

P.S. If the passage(s) are completely carboned up, you can remove the throttle housings and clean the bore with a drill bit you turn carefully by hand.

Good luck.

Bottlenose, when you say barely any air at all, what number is that on your STE synchrometer? 3 Kg/Hr? Five? It should't be zero, but it shouldn't be ten either, particularly at idle. Did you measure with linkage removed on both sides at idle? You should remove a minimum of both cross links but I would pop off all the linkages for a pure measurement.

Now, since you already have the stacks and the crossbar removed, it's a simple matter to remove the starboard throttle body. This would be a good time to replace the gaskets on the head and the stack, Pelican sells them. Next get yourself one of those telescoping magnets if you don't like bubble gum. Remove the six nuts holding the throttle body to the head and check for carbon buildup or sticking plates. Soak it in B-12 chemtool until all the carbon is gone. Soak it outside and away from open flame, that B-12 is dangerous stuff. Finally put the stack back on, and use the magnet in case you drop any hardware down onto the intake valve. I don' t like bubble gum, that's what I use!

Good luck! Pictures always help show the way for those who will come later.

In case you don't have one, our host sells this synchrometer for $53.80. Very handy!
http://forums.pelicanparts.com/uploa...1144144261.jpg
BTW: I second John's caution about keeping B-12 (or any aerosol carburetor cleaner) away from an open flame. It's very volatile -- even arching spark plug wires can set it off. :o

* This volitility is why carburetor cleaner is so effective in helping to locate a suspected vacuum leak. Spray it on the suspected leak and if there is an actual leak the carb cleaner gets drawn into the engine and acts like propane smoothing out engine idle until it gets burned off and the rough idle returns.

>>Bottlenose, when you say barely any air at all, what number is that on your STE synchrometer? 3 Kg/Hr? Five? It should't be zero, but it shouldn't be ten either, particularly at idle.

I tested at 3000RPM, since that is where the stumbling problem starts. The driver's side stacks each read about halfway up my Unisyn, while each pax side stack barely moves the indicator up at all. At idle, the driver's side stacks read up about 1/4 of the way up and the pax side stacks read nothing. Odd since it seems to idle fine.

I have the older style Unisyn sychrometer recommended by our host in 101 projects. I've since used the one pictured above (a friend had one but he moved away), and it does work much better. Perhaps I should spring for it.

Thanks for all the advice, guys! I'm going to soak the throttle body in Berrymans (I know its dangers), make sure the air passages are clean, and see if that helps before I go further. Meanwhile, any ideas are appreciated!

I added a few new links to the MFI resources that may help. Take a look at this thread Grady/ Sherman(72 Guys) Ok now what?. This thread discusses the problems with air intake caused by carbon buildup, vacuum problems, ignition/distributor issues and other problems that cause rough running. Look especially at the posts from Grady Clay, Warren Hall and John Walker. They (and others) are the real MFI Gurus on this board. Look through the following two posts as well. There are lots of other good resources that might help with this problem:

Ultimate MFI Resources Thread

and here:

MFI Message Board Index