General TR Power Issue on Acceleration

[KVH]

I have two TR4As. Of late, and as some of you have noticed, I've been trying to get my "custom" one running as good as my standard road car. The custom one has a lightened flywheel, a special camshaft, Silverstones, a surrey top and an LSD Nissan differential. It's a cool car, but it just won't run like my other one.

After a few good rides with my new Pertronix (that so far I really like), I'm back to a puzzling symptom I'd been expecting might change. At around 3300 rpms I'm losing power in second and third gear. It's as if the engine is being choked off. I suspected bad timing, but that's not the issue, nor is it the fuel pump or carbs as best I can tell.

I don't know enough about mechanics to say whether I've got a compression or valve problem, but I can certainly do a compression test.

I do have another idea what might be going on here.

Could it be that my clutch is badly slipping? I've not heard of that being common with Triumphs and I do sense a loss of "power" at the higher rpms (in both second and third gear) which seems more consistent with something "engine" related. I know to some of you it should be obvious if my clutch is the problem, but I've just not experienced that before. And I'm not the one who added that lightened flywheel and new clutch 8 years ago.

One thing is clear: I could drive for hours and never notice any issue, especially in overdrive on the highway, if I weren't pressing for performance in the higher rpm ranges in second and third gear.

I'm actually feeling sluggishness in first gear as well, but not the noticeable "power choke" that I get in second and third gear at the higher rpm range. In other words, there's definitely an rpm component to my problem.

Maybe I should've run a compression test before posting this topic. There's a novel thought.

Anyway, my lack of expertise is evident so I'll await some good advice.

Thanks all.

 

[TR3driver]

If the clutch was slipping, you'd hear the engine rev up and see the tach jump. Usually you won't feel a power loss until the engine revs go so high that it isn't making much torque any more. So I doubt that is the problem.

From your description, my first guess would be a fuel delivery problem. Possibly a restricted line or a clogged filter. Paper element filters in particular can absorb water and close up until they won't pass enough fuel for high rpm operation. Old soft lines can fail internally and be partially blocked by a piece of rubber, or hard lines can fill with FOD. One test you can do is to try sneaking up on the higher rpm with a light throttle. If it goes higher before you feel a power loss, that would tend to indicate a fuel problem.

Or, run down to Horrible Freight and pick up a fuel pressure gauge. https://www.harborfreight.com/fuel-pump-and-vacuum-tester-93547.html Tee it into the fuel line right at the front carb (or better yet between the carbs), and run the line out where you can prop the gauge under a wiper while you take a test drive. If I am right, you'll see the pressure drop to well under 1 psi at about 3000 rpm.

Offhand, I can't think of anything else that would manifest in 2nd & 3rd, but not in 1st. Weak ignition, floating valves or an air restriction could certainly cause rpm and load related problems, but they would show up in any gear.

Low compression usually shows up more at lower rpm, rather than higher. At higher rpm, there is less time for the compression to leak away. I drove a TR3A for about 6 months that was so badly worn, it couldn't blow more than 70 psi on any cylinder (and the worst was only about 25). It actually ran OK from about 3K to 4.5K, even though it could barely pull the car at 2K. (Previous owner had apparently never planned to keep the car, and had driven it until it wouldn't run any more then abandoned it. His sister sold it to me, on the condition that I drag it out of his mother's yard.)

 

[bnw]

"His sister sold it to me, on the condition that I drag it out of his mother's yard"
I had an Austin Healey 100/4 given to me back in 1972. It was being used as a dumpster and was over flowing with garbage.

 

[NutmegCT]

And if all else fails, check continuity on all the plug wires, and swap out the coil.

Just my two cents.

Tom

 

[KVH]

Today, I'll do two things, one at a time. Replace my fuel filter. Replace my coil. Report soon. thx

Randall, just fyi, I am sluggish in first gear, too. It's just not nearly as pronounced. In second and third it's like I'm just being choked off, hitting a power ceiling, like a modern car's automatic rpm safety cutoff.

 

[poolboy]

What kind of carburetors are on the engine ?

 

[KVH]

Su hs6

 

[Simon TR4a]

Some aspects of the symptoms you describe are confusing, and I suggest you get back to first principles to diagnose your problem.
From your description I think ignition is less likely than fuel- a misfire should be audible and should feel like a shuddering or jerking in the car.
I also understood the problem only appears under acceleration, not cruising at a steady speed, even at the same rpm. It may be hard to notice in first gear as you spend so little time in first and the low gearing means the car accelerates quite quickly even if not running very well. Did you try accelerating for a few seconds in 4th gear (not overdrive or you are speeding!)? The same problem should be evident as the engine should do the same thing in any gear.

The first thing to check is mixture; if the mixture is leaning out around peak torque rpm, which I think is 3,000rpm with a stock cam and likely a little higher on your cam, this would cause a dip in the power curve. As Randall suggested, this could be caused by a fuel pressure problem, either clogged filter, blocked line or bad pump,though in my limited experience there is usually enough fuel in the float chambers to allow several seconds of hard acceleration. Does the problem appear almost immediately or only after several seconds of acceleration? Try checking the plugs for signs of incorrect mixture. This will only be a general indication of how the car is running. Check plug gaps while you have them out- it too wide ignition could be ok at steady speed but poor when accelerating.
Next you might try putting in two new plugs, one in either of the first two cylinders, the other in either of the second two, so you are checking both carbs, then drive the car at the speed that causes the problem, then push in the clutch and turn off the ignition while pulling off the road. (This is called a "plug cut".) Now take out the fresh plugs and read them for mixture to see if you are lean or rich in the situation that is causing the problem. If they look different at least one is wrong, maybe both! Check what needles are in the carbs, they should be at least slightly richer than stock needles because of the cam. Check the setting of the jets, i.e. how many flats are they turned down.

If the problem is not yet solved, move on to the ignition. One at a time remove the plugs, reattach the plug wires and place the plug so the metal part you put the plug wrench on below the insulator is grounded to the cylinder head and you can observe the plug gap. Start the engine (it is running on three cylinders) and check the spark. Remember the saying, white is right, blue will do, red you"re dead. If spark seems good you need to check timing, no point having a good spark at the wrong time! Does the car idle well? If so static timing is probably good but advance curve may be off due to the wrong springs in the distributor, wear in the shaft, though with a Pertronix installed this should not be a problem, or if you have vacuum advance a leaky hose. (Depending on the cam, your idle may be a bit rough anyway, if you have more information this might be useful.

If you do not have the time or tools or confidence to do all this stuff a mechanic can do a tuneup, this would give you some idea what would be involved and what the cost would be, but obviously it is interesting and useful to try to do these things ourselves.

All the above just my opinions, others may have better advice, good luck!

 

[charleyf]

This may seem to simplistic but have you checked to make sure your linkage is opening up the carb butterflies completely? Remove the air cleaners and lift the piston so you can see the butterfly in the back of the carb throat. Have somebody depress the accelerator pedal all of the way down. The butterfly should move from vertical (closed) to horizontal (open). If it does not make it all of the way open you need to adjust to allow it to open.
Charley

 

[NutmegCT]

I think this problem goes back to:

https://www.britishcarforum.com/bcf...tions-Distributors-Idling-Turn-Signal-Flasher

Anyway, it's one reason I suggested the culprit might be coil and/or plug wires.

Hang in there!
Tom

 

[KVH]

I changed out the fuel filter. No difference. I'm on the carbs now. I actually believe the butterflies might not be opening fully as suggested. I have some strange linkage play. I'll also check mixture and balance. And check out the plugs. Failing all that, I'll swap the coil out--but using the Moss diagnostic test it wouldn't seem it's bad. I'll report more. Question though: If the full play of the throttle shafts while depressing the accelerator foot pedal is about 70 degrees on the 360 dial, does that seem normal? One thing is clear--doing it by hand moves them further, maybe another 20 degrees to a full 90.

 

[KVH]

I just swapped out my coil, but I'm pessimistic it'll make a difference. I'll report after a late dinner. I've seriously, thoroughly checked everything else. But I've still got power loss at high rpms.

It's not the compression. I've got 160 in all 4 cylinders.

If it's not the coil, it's got to be fuel. Those butterflies aren't opening fully. I'm not sure what to adjust there, but I can see they're not flat with the air-flow. To me they look 75 percent open, but who knows. I "Por 15'd" my tank a year ago. Maybe I've got something blocking the fuel but I doubt it because the flow at the pump was good--the input to it. Anyway, coil report to follow.

 

[charleyf]

Your adjustments for the accelerator are found in two locations. The first one is to check the pedal and make sure that you can push it all of the way to the floor. Look at this don't just feel it go down. There is an adjustment bolt in the firewall that allows you to "restrict" the pedal going all of the way down to the actual floor. Also sometimes the carpet or padding gets in the way and the pedal is stopped by these before even getting to the bolt. The second place is on the linkage prior to the carbs.
On one of my TR4's the pedal had been bent previously and it was hitting on the right side before it got to the firewall or the bolt.
Charley

 

[KVH]

Re: Solved: Power Issue on Acceleration

It seems I may be the source of my own problems. After a new fuel pump installed, a switch to Pertronix ignition, a new fuel filter, a new coil and readjustments of carb mixture of other antics, my loss of power issue during acceleration seems to be solved.

The prior owner added a draft tube to the crankcase of my TR4A engine. Not sure why. I recall him saying he liked that better. Thinking I was being a purest, I added a PCV valve to the valve cover. I have a vented valve cap.

Well, I removed my PCV valve and I have no "choking-off" and loss of power in the higher rpms anymore.

Whether the problem was a defective PCV (it was bought new from Moss, so I doubt it) or a spring too strong or too weak, I just don't know. What I do know is that was the crux of the problem. Maybe with the draft tube it creates too much ventilation or causes some condition that counters the PCV valve I don't know.

I'm going to check it, and I may add a different spring, but I'm more inclined to do as the prior owner did, and just leave it off. I suppose I can't rule out that at peak acceleration my PVC hoses were collapsing inward. I did use special oil hose and attached a shape spring to guard against collapsing of side walls, but I've learned not to rule anything out. And, maybe the PCV is just defective and is sticking in the closed position. I suppose I'll have to read up more about the theory of the PCV.

All said, now that my power is better, and high rpm performance seems normal, I'm experiencing a little "missing." Rare, but it's there. That has me worried it's my Pertronix distributor. We'll see.

It's all in the life of a Triumph owner. What thrills British Leyland gave us all.