Buick straight 8 running rough, help plz

[Blomqvistjs]

I bought a buick special 1939 stock engine. 4liter and stock single carb

 

It idles fine, revs fine

But under load its rough as a bad missfire or maby not firing on all cylinder.

 

Checked distributor for play,

Vaccum adv hasnt rusted shut.

Point looks good.

Swapped coil didnt help,

Tried do disable resistor.

Spark plugs and wires new last year by prev owner

 

Can it be fuel problem?

How do i set ignitiontiming on this car?

Does anyone have a manual for the carb?

[Mark Shaw]

Try timing with a vacuum gage:  http://automotivemileposts.com/garage/v2n8.html

 

 

[Guest]

A compression test could be in order.  If the car had been running slightly lean or tappets too tight valves on 1 and 8 could be burnt. The vacuum gauge that Mark Shaw mentioned would also tell you if there were burnt valves.

[plymouthcranbrook]

My 52 Plymouth will Idle fine always but at time will seem to miss little while under light load. i think it must be a carburetor  issue. Maybe that is your problem as well.  Especially with today's ethanol fuels

[Frank DuVal]

On 11/1/2017 at 3:58 AM, Blomqvistjs said:

Tried do disable resistor.

 

What resistor?

[Blomqvistjs]

Ballast resistor since its 12v the coil get 12v at startup but when running it gets lower voltage

[Bhigdog]

On 11/1/2017 at 3:58 AM, Blomqvistjs said:

Spark plugs and wires new last year by prev owner

 

Miss fire under load? The first thing I'd check is the previous owners  ignition work............................Bob

[Spinneyhill]

Mechanical (and vacuum?) advance working properly?

[Graham Man]

You could have broken or week advance spring/s in the distributor or a stuck or sticky weight/s.  For how easy it is, I just pull the ignition plate and clean/lube my mechanical advance from time to time, much better acceleration.

[fordrodsteven]

I once had a car misfire when under a load and finally found the problem to be a loose screw that held down the condenser inside the distributor.

[Frank DuVal]

Hmmm, 1939 Buick. 12 volts. That's why I asked, not a stock system, so wouldn't be on my list of things to check.

 

You can temporarily jump around the resistor. Points WILL wear faster (turn blue) but you will be fine for a quick run to see if there is a change of roughness.

 

Have you tried pulling one spark plug wire at a time out of the distributor cap and see if every cylinder disabled drops by the same RPM? Note, only do this on stock points/coil systems. It will spike and damage electronic ignition systems.

[Blomqvistjs]

As said, not ballast resistor ot coil, vac adv is moving but dont know if its movibg as it should, didnt try to pull wires cause it runs fine when not undet load, strange thing is that it worked fine the first day when driving the car home for 250-300miles 

[1939_Buick]

30 minutes ago, Blomqvistjs said:

As said, not ballast resistor ot coil, vac adv is moving but dont know if its movibg as it should, didnt try to pull wires cause it runs fine when not undet load, strange thing is that it worked fine the first day when driving the car home for 250-300miles 

As above have you cleaned out  the complete fuel system from tank to carb?

[Blomqvistjs]

Tank have been dropped by me, fuellines and electric pump by prev owner.

Pumpsize cant be to small for such a little carb

[Ben Bruce aka First Born]

Does it have the Morain flat round filter just ahead of the carb?  The element in these can get plugged with varnish from the gas.  Also some carbs have a screen inside the gas line inlet.

 

  Ben

[Spinneyhill]

7 hours ago, Blomqvistjs said:

fuellines and electric pump by prev owner.

 

What is the fuel pressure from the non-standard electric pump?

 

It is not the little carb that causes fuel burn, it is the big flat head engine guzzling it. So is the pump able to supply the amount of fuel needed for that engine at the pressure required? The carb. is just the spoon feeding it.

[37_Roadmaster_C]

With a nonstock electrical system you need to look closely at everything as it has been futzed with by someone that has unknown knowledge and experience. Ok enough for my rant on nonstock systems...  One thing that has not been mentioned is carb calibration.  What I am thinking is that you said you relocated the car 250-300 miles away from its previous home.  Is the new home at a significantly different altitude?  I have seen carbs that require different jetting if moved from a low or high altitude to the opposite.  Just a thought.....

[Blomqvistjs]

Not an altitude ptoblem, i think.

finland is pretty flat.

 

Where it was bought was 232meters (760feet) over sealevel and im now at sealevel

 

On the trip home we also drove at sealevel for 1hour and it was fine.

 

 

 

 

[Blomqvistjs]

It runs really rough when driving on highway, but when stepping on it it gets a little better, sometimes runs fine. (Most often it only runs a little less rough)

But with constant speed underload

It isnt fun at all

[Larry Schramm]

Here is a job aid from GM that I use to help diagnose engine problems using a vacuum gauge. 

[37_Roadmaster_C]

I think the altitude difference is to small to be a problem.  I like the idea of watching the vacuum while the problem is happening. It is fairly easy to hook up a vacuum guage with a long piece of vacuum tubing so it can be used while driving. This could tell you a lot about what is going on.

[jenz38]

Your Choke flapper are fully open if Engine is warm ? ...if not , its possible that its will suck to closed position under load.

Also i would try an testride without aircleaner...maybe the old full metal ones are clogged with dirt.

Edited November 6, 2017 by jenz38 (see edit history)

[Blomqvistjs]

Still no idea, under load it starts to missfire, swapped coil abd wires, tried swapping the resistor.

At idle and when reving it runs fine.

As soon as i try to drive and it gets under load it wont fire on all cylinders

[Ben Bruce aka First Born]

I just pulled plugs for a compression check. Found one [#1] closed!  No wonder it was rough. Not too noticeable at idle/high speed.   Second time this has happened.  Different engine, different cyl.  Perhaps pull the plugs and see, then do the compression check and vacuum test recommended.

 

  There is a reason! Not a spirit thing.

 

  Ben

[Terry Stock]

Your description matches a problem I had once.  The carburettor & heat spacer (there is a better name) weren't secured to the manifold tightly enough.  Air got in through the tiny gap.  Ran smooth at a steady speed, but to accelerate up a gentle climb or to leave town for the open road it was a really rough ride.