TR2/3/3A this little miss in the engine

[sp53]

So I have this little miss in the engine that I have been trying to figure out. I have put in new plugs, points, and condenser. Tried 3 coils and 2 condensers and 2 fuel pumps and plenty of fuel comes out of the tank; plus I have moved the timing up and down a little without any change in the stumble and have quality fuel in the tank from more than one gas station.

The miss or more like a stumble like too much fuel gets in or maybe not enough happens when I wide open throttle early on in 2 and 3 gear; I have not really tried 4th gear. I can punch it in first gear with no problem; I guess because the gear is so low; moreover, there is no ignition backfire. I can ease into the gas pedal and prevent the stumble in 2 and 3rd, but when I floor it early on in 2 and 3 it stumbles, but it will bust out of the stumble if I stay with it. The fuel mixture seems correct. I tried it a little lean and a little rich but no real difference.

My thoughts are worn out needles and jets or even the car is not totally up to operating temperature because of our cooler climate. I have a new radiator and tropical fan, a 160 sleeved thermostat and the temp is gauge is about a 1/8 before 185, so the car runs on the cool side, but that has never been a problem before and I have had that set up for years.

Before I get a new master SU kit or change the thermostat, I thought I would get some other opinions because I have lost my mind somewhere on this one. I cannot remember the last time I rebuilt the carbs and have been driving this car since 1971, so I know the car, but I am missing something.

 

[TR3driver]

What do you have in the dashpots? Is there enough of it?

Do the carb pistons move freely with the plunger removed? Land with a click?

What about wires and distributor cap?

 

[Joe Schlosser]

Randal beat me to it
It sounds like the needle is not moving properly. They do need to move freely
What oil is in the dash pot?? If too thin you may go lean for a little and cause your "stumble"
The rate of movement of the needle when punching the throttle is like an accelerator pump in other carbs.

 

[CJD]

+1 to dashpots. Is this the newly restored car?

 

[sp53]

The car is my older driver and the pistons drop fine and I have enough 20W oil in the dash pots. The wirers are new, but the cap is an older brown Lucas cap. I will change the oil in the dash pots and try a new aftermarket cap; I have one somewhere. Maybe try some ATF in the dash pots; I heard that was supposed to work well. I have been a little sick/ill so work has kinda slowed down on the project car being an old carpenter can really pull you down sometimes, but I am starting to feel like putting some time into the project. I painted the bumper brackets last week. Thanks you guys I will report back on the suggestions and hopefully we will get without spending too much.

 

[TR3driver]

If you are getting sputtering on acceleration, you want heavier oil in the dashpots, not lighter. 30 weight is specified for temps between 40F and 70F (40 weight above 70F).

 

[CJD]

Bummer! Hope you get back to speed quickly.

Oh, one thing, if it is a dashpot problem, adding a little choke will help reduce or eliminate the sputter. If it is ignition, the choke will likely not help it, and may even make it worse.

 

[charleyf]

I agree with Randall. Use heavier oil, not ATF in the dash pots. Another way to slow down the piston rise is to replace the springs inside the dash pots. But work with the heavier oil first. You can even try mixing some oil like 30 weight and 80 wt. to thicken it up for your test.

 

[TomMull]

For what it's worth, I've used the same oil in the dashpots as in the engine for many years and miles without issues. Also note that 30 wt engine oils is approximately equal to 80 wt. gear oil in viscosity.
https://www.acculube.com/lubricant-viscosity.html
Tom

 

[sp53]

Hey Randall could I use a 10-40 oil or does the oil have to be a straight 40W; Tom I find that interesting about the 80W. I never would have figured that. Is the thickness of the 80 for cooling or something?

 

[TomMull]

I'm no expert but the viscosity scales for gear oil and engine oil are different, so you can't compare one with the other. They also react somewhat differently to heat but I don't have any details.
For what it's worth, one of our British Car Club members has a 1939 MGTA which has inscribed on the SU dashpot screws "Use Bicycle Oil". I wonder what the viscosity of that is.
Tom

 

[charleyf]

For what it's worth, I've used the same oil in the dashpots as in the engine for many years and miles without issues. Also note that 30 wt engine oils is approximately equal to 80 wt. gear oil in viscosity.
https://www.acculube.com/lubricant-viscosity.html
Tom

Yes Tom, but carbs are like us and get old and get worn down. So when it quits working like it used to it is time to change something. Or if you can find an actual fault correct that. I had the same stumble or miss in one of my cars and determined that low oil in the dash pots was the reason. I had to replace the oil often .
But another car that we were trying to install a new set of SU HS6's we went through a whole list of possible cures to a stumble problem.

 

[TomMull]

Sorry, I didn't mean to say I didn't add oil or that it didn't stumble when it needed dashpot oil, just that I used the same oil in the carbs as I put in the engine. It also usually seemed to be adequate to top off the dashpots at oil change intervals.
Tom

 

[TR3driver]

That's what the owners manual says, to use the same single weight oil in dashpots as specified for the engine.

Multi weight oils are basically specified at 0C and 100C. So 10W40 acts like 10W at freezing and 40 weight at boiling. In between those two extremes, it acts somewhere in the middle.

Since the carbs hopefully don't get that hot, 10W40 is probably roughly equivalent to 30 weight.

 

[sp53]

Well I figured it out. . I had removed the domes and cleaned everything and filled them 10- 40 and still had the problem. One of the brass plugs to fill dash pots had stripped out and the plug would pop up when the piston raised. I finally got a little more aggressive with the trouble shooting and revved the engine with the hood up and noticed the brass plug would come up with the RPM. The plug and not the dome were stripped and plug would come up and fall right back down in the hole like nothing happened. I have seen this problem before with stripped domes but those times the filler plug would stay up some or would not go back down.

 

[CJD]

Very interesting cause! I'll add that check to my tune-up.

 

[TR3driver]

Gentle reminder : in spite of that big hex plug, the dashpots don't need to be much more than finger tight. Just tight enough yo keep from vibrating loose.

Later on, they switched to plastic plugs without flats, just lobes to tighten with your fingers.

 

[sp53]

I agree Randall that is how I missed it after I check the oil because I just snugged it with my fingers, and if I remember correctly I used to put some of that red goo on the threads of that carb because the threads were getting weak. Fortunately, I was able to purchase a nice cash of carbs off Evilbay ,so now I have plenty of good parts. It was actually amazing how nice these 6 carbs I got are for 100.00; the picture they used was poor and I kinda took chance and got lucky