145 - 130 - 155 - 160 warm compression test

[Jim Lee]

Hi all,
I am just getting back into full time TR3'ing.
1959 TR3a with recently rebuilt head.
Wondering if it is time to replace the rings.

Just did a warmed up compression test so now I have:

Cold - Warm - Difference

1 - 110 - 145 +35rel
2 - 130 - 130 0
3 - 125 - 155 +30
4 - 125 - 160 +35

#2 cylinder seems the strangest to me with no difference.

Are the +30's considered normal? I am going to do a leak down test as soon as I can get a homegrown tester together.

Another question on a totally different subject. The engine runs great but I have what feels to me like a large dead spot on initial acceleration. It is really bad when dead cold, like to stall out if I am not very careful, but is still there after it is fully warmed up. It is most noticeable when slowing down to make a 90 degree turn either way. I have to make sure that I keep the revs up or it is a while before I can accelerate out of a turn. I don't expect it to respond like a fuel injected car but I know that it has done better than what it is doing now.

Does this have to do with the SU damper oil doing its thing? There was definitely some in them as I could feel the resistance screwing the top in. It might have been a little bit low but not much. Have not given it a test ride since I topped it up. The lighter the oil the quicker the carb response right? I have Marvel Mystery Oil in there now but thinking about putting motor oil in there to see if it makes a difference. I am wondering if the Marvel Mystery oil is too light so that the carb is getting too much air and fuel at the same time. It feels like it has to 'catch its breath' when I get on the pedal too quickly

Could this also be a problem with the timing? Other than this dead spot the engine seems to run really well. Nothing close to a backfire or lugging. I am also wondering if I have the dreaded throttle spindle wear? I can actually bring the rpms down low enough that the car will stop running by standing at the engine and pushing up the linkage without too much force. It appears to have another fraction of an inch that it will sometimes go back to and if I am driving and the car is not warmed up it will stall. I have sprayed carb cleaner on both side of both carbs and not heard any difference in idle so I am kind of puzzled on this. It will be idling fine at about 11 or 1200 rpm and then all of a sudden I will come to a stop and the idle will drop way low...low enough to shut the engine down. I am also wondering if my floats needs adjusting as the shut downs usually happen when I am braking hard and/or pointing down hill. Is there a rule of thumb for where the floats should be?

Thanks to all and happy motoring.

Jim Lee
_________________________
1959 TR3A TS50550L S50715E

 

[AltaKnight]

Did you try a "wet" test? put a little motor oil in each cylinder and retake the compression test.
I assume you had the throttle wedged full open and the choke off during the initial tests?
The pressures should increase with the wet test if, they don't then that's a sign of problems not connected to rings, ie valve leaks, head gasket leaks etc
Try spraying some carb cleaner round the throttle shafts to check for leaks, the engine will change speed if there is one.

 

[Geo Hahn]

...I assume you had the throttle wedged full open and the choke off during the initial tests?...

...and the SU pistons propped in an upright and locked position (at least that's how I do it).

I would also double check the valve clearances.

 

[TruCraft]

Hi all,
I am just getting back into full time TR3'ing.
1959 TR3a with recently rebuilt head.
Wondering if it is time to replace the rings.

Just did a warmed up compression test so now I have:

Cold - Warm - Difference

1 - 110 - 145 +35rel
2 - 130 - 130 0
3 - 125 - 155 +30
4 - 125 - 160 +35


Another question on a totally different subject. The engine runs great but I have what feels to me like a large dead spot on initial acceleration. It is really bad when dead cold, like to stall out if I am not very careful, but is still there after it is fully warmed up. It is most noticeable when slowing down to make a 90 degree turn either way. I have to make sure that I keep the revs up or it is a while before I can accelerate out of a turn. I don't expect it to respond like a fuel injected car but I know that it has done better than what it is doing now.

Does this have to do with the SU damper oil doing its thing?
Could this also be a problem with the timing?

I am also wondering if I have the dreaded throttle spindle wear? I can actually bring the rpms down low enough that the car will stop running by standing at the engine and pushing up the linkage without too much force. It appears to have another fraction of an inch.

My opinions, no expert.
Sounds like your car is running pretty good.

*Start with a basic tune up, plugs, wires cap, timing.....
Something like a cracked dist cap or bad wire can mess everything up. A simple check is a late night, lights off, visual check for sparks around the cap and wires.

*Compression readings are good for a weekend driver.

*I use 20w-50 engine oil in the damper, mystery oil is way to thin and will cause a rich condition off idle.

* Vacuum leaks around the shafts will make tuning a nightmare. Carbs need consistant vacuum. Re-build for sure, and adjust float levels.
Being a machinist I also installed O-rings between the carb & throttle shafts.

Remember, the thing you think is fine, or was fine, probably is the problem now!
Go back to basics, all the little things add up.
My 2 cents.
Lyle

 

[TR3driver]

*I use 20w-50 engine oil in the damper, mystery oil is way to thin and will cause a rich condition off idle.

Me too, and I agree the MMO is way too thin. But thin oil causes a lean bog on acceleration (very similar to what Jim describes), not rich.

I did some experiments years ago and decided that straight 20 weight was optimum for my engine. But the 20W50 works fine, and is more convenient since that's what I use in the crankcase as well.

Starting from cold (meaning 50-60F), I leave the choke on for the first few miles, reducing it gradually to keep the engine from chugging (a sign the mixture is WAY too rich) but still responsive to the throttle. Takes maybe 2-3 miles before I'm down to just fast idle; longer in colder weather.

The dying when you pull up on the throttle does sound like something is binding somewhere; plus your idle stop screws are not properly adjusted. Lots of places it could be binding; but the easiest solution might be to turn the stop screws down until they are holding the throttles open instead of the binding.

In my case, several of the joints in the throttle linkage had worn oblong so badly that when I tightened them up (to keep them from popping apart), they would then bind just at the limit of normal travel. In addition, the throttle shaft bores were worn so badly that the throttle plates were dragging on the carb throats and usually not closing all the way. But my problem was that the idle was too high, and I always had to 'blip' the throttle at each stop light to get it down to something reasonable; so these may not be your problems.

 

[Jim Lee]

Hi Randall,

You wrote:
"...but the easiest solution might be to turn the stop screws down until they are holding the throttles open instead of the binding."

I am not clear on that. The stop screws, are you referring to the screw on the right carb that adjusts the idle higher by turning it clockwise? That screw is not touching the body because I had been worried about a high idle not coming down. If there was binding in the linkage would I not be able to see it when moving the throttle linkage manually while standing at the side of the car?

I have done the test where you spray carb cleaner at the throttle shaft bores to see if anything leaks into the intake but did not hear any difference. Also the carbs were professionally rebuilt not too many miles ago so I am fairly confident about them.

I am going to suck out the MMO and put 20-50 weight in the dampers. That is definitely what it feels like. A lean condition that takes a few moments to correct itself. It makes me think of something gasping for air but if the carb is sucking in more air faster than it can handle it it makes sense that it is just the opposite and it is being starved for fuel. I think that at one point I was using transmission fluid...which I think would be thicker than MMO. I cannot remember why I started using MMO to begin with? Maybe someone had a souped up Brit car that could handle all that air?

Thanks very much,
Jim Lee

 

[TR3driver]

The stop screws, are you referring to the screw on the right carb that adjusts the idle higher by turning it clockwise? That screw is not touching the body because I had been worried about a high idle not coming down.

There are a total of 3 idle stop screws; the right (front) carb has two of them, the rear carb has one. One of the front ones is for fast idle, and it should not be touching it's cam with the choke released.

However, both carbs should have stop screws for normal idle, and both of them should be touching the carb body at idle with the choke released. With them turned so they do not touch, the throttle plates should close far enough to kill the engine.

Since that does not seem to be happening most of the time in your case, something else is holding the throttles open.

It shouldn't be hard to track down where the problem is; I would start by loosening the clamp screw where the linkage attached to the front throttle shaft. If that lets the throttles close far enough to kill the engine, then the problem is in the linkage. If not, the problem has to be in the carbs.

Someone else reported just recently that their "rebuilt" carbs did not have the throttle plates centered properly, which would be one potential source of the throttles staying open at idle. But that's kind of hard to tackle, so I would look for other things first. If they do need to be recentered, I suggest having new screws on hand, as the screws are supposed to be staked into place, and unstaking followed by restaking may allow part or all of the screw to fall out and get sucked into the engine.

I've even heard stories of rebuilders installing the throttle plates backwards.

BTW, in my case, I was certain the throttle shaft bores were worn, because I could grab the throttle shaft and move it up and down. After removing the carbs, I could also see that the throttle plates were hitting the bore enough to be held open if the shaft was forced against the bottom of it's bore (simulating the force from the linkage when the throttle was released slowly). After disassembly I tried a new shaft, and it also moved (so worn bores instead of just worn shaft). But there was not a significant vacuum leak on the car, apparently because the shaft was being pulled tight against the engine side of it's bore, and there is almost no vacuum on the intake side of the throttle plate. YMMV

 

[bgbassplyr]

You will also need to 'snyc' the carbs so that at idle, and beyond, both throttle plates are at the same agle-opening. This is done with a 'unisyn' (Moss, etc has them). Loosen the clamp tying the throttle shafts together so that each operates independently. With the engine warm and idleing, place the unisyn over one carb mouth and then the other and compare the air flow readings. Adjust the idle screws so that both carbs read the same. If the idle is too high, back off each screw a little until idle is where you want it, and then resync the carbs. When the idle is where you want it and the carbs a sync'd, retighten the clamp screws.
Nothing to it.

 

[TR3driver]

This is done with a 'unisyn'

Of course, there are lots of other ways too; all of them cheaper than a Unisyn, and just as good or better, IMO.

My favorite is the "SU tool kit" which is available from the usual suspects for about half the price of a UniSyn and includes some other useful tools like a gage for setting float height and a wrench for the mixture nut (when it gets stuck or is too hot to turn by hand).

But you can do an entirely adequate job with just a length of heater hose. Hold one end to your good ear, the other next to the carb throat. Then move to the same position at the other carb. If the hiss is equally loud, they are synchronized. In a pinch, I've even used this method without the hose. Last time I did that (in a restaurant parking lot), the car's owner told me later that it was running better than it ever had!

This is also the method mentioned in the factory workshop manual.

 

[TruCraft]

Randall, your right, I was thinking backwards!
OK, SU carb basics.
If you hit the throttle off idle.
With very thin oil, the piston would rise quickly, allowing lots of air in. The fuel needle also rises, but the fuel can't be siphoned fast enough to keep up.
Causing a lean condition.
Thicker oil would slow piston movement, correcting mixture.
Makes sense.
Lyle

 

[bgbassplyr]

Of course, there are lots of other ways too; all of them cheaper than a Unisyn, and just as good or better, IMO.

Didn't mean to say that unisyn is the only way, just the one I use. Still have my unisyn from 1964.