Car is running rough on highway...but great on street.

[Steve W]

1968 CdV. 472 motor.  Idles great in driveway...revs smoothly and quickly. On the street runs great all day...unless I hit the highway and try speeds over about 60 mph. Then it starts to miss, like a cough, runs rough...slow down below 60 or so and it runs fine.
I have replaced points, condensor, cap, rotor, fuel filter. Wires and plugs look great. Adjusted the carb. Timing and dwell at factory specs. Vacuum a pretty steady 12-15" Car is daily driver, gas is fresh.
So what can it be?  It runs great in the garage and on the street! No probs at all. Its only when I try to get up to, and maintain, speeds of 60 mph and up.

[The Tassie Devil(le)]

Could possibly be a slight blockage at the fuel tank pickup, causing a lack of fuel getting to the carby.

One way of checking a fuel pressure problem at high delivery needs is to install a pressure gauge, and see if there is any fluctuations when lots of the stuff is required.

BUT, don't put a liquid gauge in the passenger compartment in case the line fractures.

Bruce.

[Renato]

You need to set up the AC pump. I think it can not transport enough fuel. You can test it easily. Park the car and start a fast acceleration.   

[TJ Hopland]

This is an interesting problem.  What ever condition you are hitting over 60 you would think you would at least briefly encounter at lower speeds around town.  Have you tried manually shifting to 2nd or 1st around town to simulate highway engine speeds at lower road speeds?  Not sure what this would prove or disprove though. 

I dont know what the mechanism was to disable the secondaries in 68 but my other thought would be to keep the secondaries from opening for a test to see if perhaps at those speeds its into the secondaries when it should not be?

Sounds like you have quite a few tools, have you tried running the vacuum advance through its range at idle?  Does the dwell hold steady through all the rpm ranges?

[R Schroeder]

Some other things to check would be a worn distributor shaft bushing.
Centrifugal advance weights. Are they free,and springs intact.
Clean air filter.
Heat riser stuck.
Leaky vacuum lines.
Could be the fuel pump ,as mentioned too.
I would do the same as T.J. mentioned and get the rpm's up ,and hold steady just to see if it did it that way. Around 2500 should do it.
You also could do this in the drive with air cleaner off ,and see if the choke is being pulled shut at high rpm's.
Carburetor ever been rebuilt ?

[Richardonly]

Sounds like a lack of fuel as mentioned earlier.

[Dave Shepherd]

Remember the sock on the fuel line in the tank, common issue and symptoms.

[R Schroeder]

Does this year have a vented gas cap ?
Roy

[Raymond919]

I had a '68 Caddy convertible which did something similar. I knew little about engines and Cadillacs at that time (37 years ago). I eventually found the steel line carrying fuel had a kink in it. It was slight and hard to see. I replaced that section of line and the problem was solved. I hope yours is as easy.
Ray Schuman

[TJ Hopland]

The kinked fuel line jogged something in my brain,  wasn't there something unique about the 68's fuel pump and fuel filter setup?  Seems like it was a one year thing and getting hard to find parts for?      Wondering if perhaps someone has altered the system and its perhaps hoses or filters that were added that could be causing a flow issue?   I have done patches with a bend in the hose where it eventually kinks itself.  Some cases I ended up using marine hose which has a much thicker wall. 

Also am reminded that most people never think to check the hoses at the back by the tank.  Not sure about 68 but in later years you could replace them without dropping the tank.

[Steve W]

TJ... Yeah, the 68 fuel filter is a cannister inline between the fuel pump and the carb.
I was lucky enough to find a NOS fuel filter on ebay. I checked the steel lines and replaced the rubber lines with new.
I can rev the heck out of it in park in the driveway...no prob. Holds a high idle just fine...which tells me the carb is getting supplied with fuel alright....right?

Carb was re-built by previous owner's mechanic about 5 years ago or so.

Roy...the 68's have a non-vented gas cap.  How do I check for a "worn distributor shaft bushing"?

Thanks for the ideas, guys! Keep 'em coming...

An I know its hard to diagnose over the internet, but I'm just looking for advice! I just don't want to get into the expense of replacing part after part after part to chase this thing, if I can help it!

[bcroe]

My money is on a fuel problem.  But do make sure the OEM timing chain set has been replaced
with ll steel.  Bruce Roe

[TJ Hopland]

If the distributor bushings are worn the dwell reading will not be steady as you rev the engine. 

Revving with no load would not show you a fuel volume problem.   Loaded its going to need more fuel for a longer period of time so that is where restrictions could cause issues.

[The Tassie Devil(le)]

Revving it up in the driveway won't show a fuel problem because the carby fuel bowl will never be sufficiently drained before you have to back off.

The thing about running at 60mph is that the engine is using a lot of fuel, as against just revving it up, and therefore a lack of ability to replenish the carby bowl will lead to the initial symptoms you described.

Bruce.

[Guidematic]

I would go with a fuel issue as well.

I'm not familiar with all the intricacies of the '68, but I always thought the fuel filter was in the outlet in the pump, as it is on my '70.

Bruce is right. A lot more fuel is delivered through the system on the highway than it is around town. However you are not getting into the secondaries on the carb when you are cruising at a steady speed. The secondaries do not begin to open until there is at least 3/4 throttle, and it is a mechanical action. The additional airflow then opens the air valve on the top of the secondaries (what looks like a choke). When the choke is on, there is a mechanical block that prevents the secondaries from opening.

Do a performance check on the pump. Check for restrictions in the lines, and also eliminate the possibility of insufficient fuel delivery within the carb itself. Use a process of elimination starting with the easiest first.

Mike

[Dave Shepherd]

Again, lets not forget the tank pick up screen.

[The Tassie Devil(le)]

You mentioned that you changed the fuel filter, but did you change the sintered bronze filter that is in the carby where the inlet pipe goes in?

These can be neglected, and perform beautifully when new, but slowly decrease in performance as rubbish gets trapped.

Bruce.

[Steve W]

Bruce...no, I did not! I will look at that tomorrow. I didn't know it had a second filter there.
Don't know how to check the tank pick up screen Dave, But I'll look at the shop manual.
Mike, the fuel filter must be different on the 70. The 68 has a big cannister for a fuel filter in-line between the fuel pump and the carb on stainless steel lines.

How much "play", if any,  should there be in the distributor mechanism?
And how do I check my vacuum advance?

OK...so I was inspecting my distributor, cleaned and re-lubed the weights and springs...seems OK...noticed a very slight wiggle-play in the plate that the points bolt to....and just for kicks I re-installed my old, original points again, dribbled a few drops of oil on the accelerator pump stem on the carb ( a "trick" I learned from YouTube...apparently sometimes the seal in the acc pump starts to go bad and dribbling a little oil down the shaft refreshes the seal somewhat temporarily...re-set the dwell and timing and went for a ride. Car just hummed right along at 90 as if there was never anything wrong with it at all!
(However, this time I am simply waiting for the other shoe to drop! I just know it will start that missing/hesitation/rough-running crap again)

I'm thinking about trying to rebuild the carb myself, even tho I've never done one before.
And I'm also thinking about buying a remanufactured distributor and fitting it with a Pertronix system.
Then a new fuel pump...and maybe a new coil.

...sheesh....

[TJ Hopland]

Accelerator pump won't effect a steady cruise.     If the carb has not been modified or abused in its life and the throttle shafts are not worn its not that bad a job to rebuild one.  If its been abused or modified then it takes someone with experience to notice and get things back right.  You can buy the tools to bush the throttle shafts but its pretty expensive if you only have one or two carbs to do.   An example of something that can get you is there is like 3 kits that cover all 35 years worth of models so each kit has several gaskets that have sometimes very slight differences.  What most of us do is match up the old gaskets to the new one.  Problem is if the last guy messed up we end up putting the same wrong one back in. 

Do you have a dwell meter?   Like I mentioned in earlier replies if the distributor bushings are worn the dwell wont read steady especially if you rev the engine. 

When we are talking about points I usually mention that my personal opinion is that 90% of what is available now is complete junk.  I guess this is another case that proves it.  They must have been hanging open at certain speeds.  Standard Blue Streak is the only brand I have found recently that still seems to be decent.