TR2/3/3A Engine rough on acceleration

[mmgwhite]

My 59 TR3 runs fairly smooth at idle. Mixture and carb balance seem correct.
When accelerating the engine runs "rough". When backing off the accelerator it smooths out even at high rpm
There is an accelerator position "sweet spot" where it runs rough then smooth then rough then smooth.
below that position it cruises along just fine. Can obtain redline without roughness by accelerating slowly.
lugging down going up a hill it is pretty bad and seems to lose power too.
Appreciate any ideas to get me started in the sleuthing process.
Matison

 

[trrdster2000]

Check the advance curve. You may have broken springs and stuck plates. Put a timing light on and check that the mark moves up and down or around in this case and not jumping all over the place. It should remain steady along with the RPM's.

 

[Brinkerhoff]

Vacuum advance unit hooked up wrong maybe.

 

[mmgwhite]

Thanks
I forgot to mention this car has the Pertronix ignition mod.
I put the light on it and it seems at idle the mark on the pulley to be about 3/16 inch ccw from the pointer. With opening the throttle the mark moves steadily and evenly ccw depending on RPM.
I disconnected the vacuum line from the distributor and it made zero difference in the vacuum advance with throttle. Still moved the same. How can that be??
Matison

 

[Geo Hahn]

I think vacuum advance has the biggest effect at high RPM & light throttle - like when you're cruising down the highway. Difficult to create that with a static car. If you want to test the vacuum advance you put a vacuum pump on it (or maybe just suck on a hose if your lungs are strong - it is not very many pounds).

The timing position you describe sounds about right for static timing but may be too retarded at idle since centrifugal advance comes on very soon on these distributors.

In any case, the most interesting number may be what advance you have at high RPM - say, 3000 RPM -- where most of the effect of the centrifugal advance is in play.

 

[TR3driver]

Sounds more like a fuel delivery problem, to me. Wrong timing can cause loss of power, or knock, but usually not rough running. And it takes a lot more fuel to accelerate or pull a hill than to cruise on level ground.

I'd be looking for a clogged fuel filter, or line, or a bad fuel pump (possibly a valve that came loose inside the pump).

 

[Elva164]

When you lift and the engine smooths out, is there any additional power? If so, that would suggest a lean condition to me.

 

[mmgwhite]

Thanks for all
I thus have a few more things to check.
Will also try multiple runs with enriching mixture a half flat at a time to see if it smooths out.
My mech fuel pump was replaced with an electric pump by the previous owner. I guess I better get back under the car and see if there
is an in-line filter that could be the culprit.
Matison

 

[glemon]

I don't think your vacuum advance is working, I can see a marked difference in the advance with it connected on my car, and others I have owned, the easiest way to test it is to suck on the advance tube, if it is leaking you will know it, it should hold the suction you create, the diaphragms do wear out eventually.

 

[mmgwhite]

Thanks. I will find some small hose and try that too.
Matison

 

[mmgwhite]

test result:
found some small hose and approp sized hose clamp. Seems to hold vacuum for about 30 sec then begins to bleed down over next 30 sec.
Is that enough of a leak to cause problems??
can one change the diaphragm alone or do I need a whole new distributor?
Matison

 

[trrdster2000]

Matison, Couple of places sale the vacuum unit, not cheap. Used is not a good option, they are as old as yours. Then again you might have a buddy that will let you try his/hers.

Wayne

 

[poolboy]

test result:
found some small hose and approp sized hose clamp. Seems to hold vacuum for about 30 sec then begins to bleed down over next 30 sec.
Is that enough of a leak to cause problems??
can one change the diaphragm alone or do I need a whole new distributor?
Matison

George in post 5 gave a good explanation of when the vacuum advance comes into play...Disabled or defective from whatever doesn't fit the "rough on acceleration" scenario....but the fact that you found the advance module unable to hold a vacuum does suggest a vacuum leak.
That in itself as slow as it is, doesn't seem like it would have much affect.
But why don't you tightly cap or plug the nipple that is generating the suction (not the one on the distributor) and take a test drive. See if the vacuum leak or the lack of a functioning vacuum advance has any affect on your situation ?

 

[sp53]

I used an electric fuel pump with a pressure reducing valve in line. The set up gave me the same problems you are experiencing. I was basically running out of fuel when I climbed a hill or wanted power quickly. I went back to a stock mechanical pump.

When I still had the electric pump, I played around with the regulator valve and was able to get more fuel delivered, but at the time I was dealing with carbon build up on the plugs also and was frustrated. In the end, I went with a NOS distributor and mechanical fuel pump and got the results I wanted.

 

[Brinkerhoff]

You do have your vacuum source on the carb and not the manifold ? Some people ( nobody on this list I'm sure) are pulling manifold vacuum which is not what you want.

 

[CJD]

In my experience the vacuum advance will make a car "ping" or run weak...but I wouldn't call it "rough". So, I am doubtful that the advance unit is causing a rough engine.

Spark can make an engine run rough...but normally loss of spark for any reason will result in the occasional back fire out of the tail pipe. You haven't mentioned that, so I also doubt you have an ignition issue.

You are left with fuel. That's where I would concentrate my time.

 

[TR3driver]

test result:
found some small hose and approp sized hose clamp. Seems to hold vacuum for about 30 sec then begins to bleed down over next 30 sec.
Is that enough of a leak to cause problems??
can one change the diaphragm alone or do I need a whole new distributor?
Matison

The vacuum module can be replaced as a unit, or rebuilt. I have seen articles on how to replace the diaphragm yourself, but it's a lot of work and the result is ugly (after grinding away the fold of metal that holds it together, you install a ring of sheet metal screws with the points exposed).

But leaking down over 30 seconds is not enough to affect anything, let alone cause rough running. I would definitely work on the rough running first.

If you want to double-check that the vacuum unit is not the problem, just disconnect and cap the line as suggested above. Full power and idle will not be affected at all (since the vacuum advance does nothing under those conditions). You might possibly be able to notice that throttle response is not as crisp at part throttle, but most people won't even notice that.

 

[sp53]

Some other things to check are the coupler that connects the cabs together. If that is lose only one carb is working. Just look at the linkage idle screw when you move the throttle by hand; they should both move together.

The other one is not enough fuel is getting into the float bowls because of incorrect setting at the float or a partial plug at the needle and seat. There I pull the tops off the bowls and see if the fuel is the same in each.

 

[NutmegCT]

When I drove across the USA in my ol' Mercedes four cylinder a year ago, I discovered the old fuel pump + clogged fuel filter prevented the carb bowl from filling, when driving under load (up hill) with open throttle in high gear. Same symptoms you described in your first post.

The engine was sucking lots of fuel, and the slow revs meant the fuel pump wasn't moving fast enough to re-fill the bowl. I removed some crud from the fuel filter, and the problem disappeared. Probably would have also disappeared if I'd driven up hill in 3rd instead of high.

Just my two cents.
Tom

 

[mmgwhite]

OK, fixed it.
Turned out to be lean mixture all along.
There is an art to setting the mixture that I am only just starting to appreciate.
I assumed lean as suggested above and enriched a flat at a time. Test drive each time. Now runs great. Smooth even with acceleration or lugging down
Thanks to all.. I love this forum. So many knowledgeable people willing to share.
Matison