SU Troubleshooting. Case Study

[zKars]

Greetings!

I’m in the sharing mood again, so thought I’d share an episode from the series “I have no idea what just happened, but you won’t believe what the solution was”. 

Yesterday, local Z owner wants to drop by for an SU tune on a recently acquired late 71. It’s “running” but is overheating and doesn’t run “Well” overall. Suspect mixture too lean or fuel supply lacking.

So do the usual, no need to bore you with details, but here is the overview, no order intended.

1. Bowl fuel level
2. Fuel pressure
3. air flow sync
4. linkage evaluation.
5. How many turns down for the knozzles,

Nozzles were down 1 turn on the front and 1.5 turns on the rear. Aha. Too simple. Turn them down 2.5 each. 

 As I have decided in the past, got the AFR gauge out finally, stuff it in the tail pipe, 12.5-13 at idle. Much better. That should do it. 

Get the owner to go for a quick around the block test to see if there is a noticeable change in performance. Goes around the block and comes back with big smile. I could here him enjoying the hell out it, the exhaust is not subtle on the car. Says there is 50 more HP and while it was short drive, the engine temps were much lower than before. We are DONE I say.

About this time another friend drops by, so the inevitable conversations ensue and finally I decide the put the air cleaners back on and send the young man on his way.

I do so, he starts the car, and it sounds a bit rough, but off he goes. 

 

[zKars]

Three minutes later he’s back. And the sound of the thing is something quite atrocious!

The exhaust note is distinctly different than when he left. He has this crazy system where the muffler tip is turned down 90 degrees BEHIND the rear valance, which has had the exhaust exit hole filled it. It’s slighty overlappig the underturned lip, so I’m wondering if the exhaust shifted.

The exhaust note is now easily TWICE as loud as before and extremely sharp and staccato is how I describe it. Like a 13:1 race engine! Something isn’t right, so I start scratching my head thinking what did I do or forget to put back? It revs up and runs not back, but there is a hint of misfire in the sound. 

Is it the new rich mixture that’s just burning more distinctly for the first time in a long time, Is it plugs that were burned up from the lean mix that are now misfiring from the new mixture? 

I change the plugs to some fresh BPR6ES and while it’s a bit smoother, it’s still making the intense racket out the rear.

 

 

Edited August 8, 2021 by zKars

[zKars]

Ok, what madness is this? I touched nothing else, what the **** did I disturb?

Ok, what did I do befween the time it ran fine and when it went to crap…. Think….. Well I put the air cleaners on.

Notice I said cleaner”S”. They look more or less like this.

I did not take pictures of the actual situation.

Anyway, These flat pancake air cleaners fit just fine, AND they have a knipple ob the back for a rubber hose to run to the fuel bowl vents. I had faithfully put these back on with the air cleaner bodies of course, good little boy that I am. Little short black vacuum like thick walled hoses.

So on spec, with the car idling madly, roaring away, and not from high RPM, I pull the rear fuel vent hose off the top of the fuel bowl. And then the most magical thing happened.

 

[zKars]

Over about the next 10 seconds, at idle, the engine went from sounding like bad rap concert to the most beautiful symphony orchestra playing a soothing waltz!! The transition was the most astonishing thing I’d ever experienced working on these things. Like I’d laid a hand on it and it’s cancer was cured. I’m sure I heard angels harps too. Well, maybe that was the sound of my breath as I sighed a huge sigh of relief.

So the rear vent hose was kinked closed. It was very short and ran out the back of the plate horizontally, then 90 degree up, then 180 back down to the breather nipple. Kinked right there at the 180.

Edited August 8, 2021 by zKars

[zKars]

I’ve had guys come to me with the fuel supply plumbed into the vent lines, The vent lines T’d and joined to the fuel supply lines, and every other combination of crazy things

It’s not like 4-5-6 were dead, they were alive, BUT the imbalance of richness or lean-ness or power was not making the engine run all that bad, it was just making such an interest exhaust note.

Anyway, file this one away. Either leave your fuel vents open, or connect them to the air cleaner through hose with an actual hole that does all the way through. 

Edited August 8, 2021 by zKars

[SteveJ]

Thanks for sharing that. It's a good thing to add to the memory banks.

That's similar to what I dealt with recently. I tuned a friend's 72, set the floats higher, etc., but the car wouldn't run worth a damn. Another friend messed with the dwell, and the car ran better for a short while. The first friend had me check the car again, and I told him it had to be the ignition. When I finally installed the Pertronix, that car ran like a scalded dog. It was another matter of having fixed everything else, what is left to mess with?