TR6 TR6 Hesitation

[Woodmeister]

Quite frustrated here. Have owned 1974 TR6 from new & enjoy doing my own maintenance. But presently have a hesitation (worse when engine hot & mostly occurs under acceleration) problem that I can't solve. Please help.
Had broken my ankle last Summer so couldn't drive the car for 6 months. When trying to use the car again the hesitation problem was there. So for the last several months have gone step by step doing the following with almost no relief.
1) Fuel System to Carburetors: Drained & cleaned fuel tank & purged fuel lines. Replaced fuel filter & fuel pump.
2) Carburetors: Rebuild carbs. Made sure float bowl fuel height is correct. Made sure damper oil is at proper level & pistons operate freely.
3) Ignition: Replaced spark plugs, spark plug wires, coil, & distributor (yes whole distributor unit including points, condenser, rotor, cap). Made sure timing & dwell are set correctly.
4) Vacuum: Made sure all vacuum hoses & connections are tight. Vacuum reads normal (as it has been for last 39 years).

Maybe the carbs are getting too hot & I need to install some type of heat shield (but I never had the problem in past years).
Please help. What am I forgetting?

 

[DanB]

You could try putting a pressure gauge on the fuel line between the pump and the carbs and see if your fuel pump is putting out enough pressure. I'm not sure, but I think it should be somewhere between 5 and 10 psi.

 

[poolboy]

1 1/2 to 2 1/2 psi according to the Manual.
You need to do some more diagnostic test rather than just taking a shot in the dark with possible cures.
I'd start with a Vacuum Gauge and comparing how it reacts to the various scenarios here:
https://sbftech.com/index.php?topic=3020.0
And while you have it hooked up use it to set your engine's ignition timing sweet spot.
https://automotivemileposts.com/garage/v2n8.html

Then, depending on the Vacuum readings, use a compression test to identify which cylinder might be acting up.
Somewhere in the scheme of things the color of the spark plugs needs to be evaluated, but only after sufficient miles to affect the color.

 

[Got_All_4]

could be running lean. have you tried to pull the choke out a little while driving. if it runs better with out hesitation then it's a fuel delivery problem. do you know anyone with a color tune? if your not sure what it is, it a spark plug replacement made out of glass where you can watch the ignition in each cylinder and pin point which cylinder is the problem. Also your able to set the mixture properly by comparing the color of the ignition burn to a color chart.

 

[Woodmeister]

Sorry for my tardy response. Been business traveling & thus away from TR6. Many thanks to DanB, PoolBoy, & Got_All_4 for their advice. Problem turned out to be a played out "Thermostatic Vacuum Valve". This device was used in the vacuum system on 1972 through 1974 TR6 models. This valve senses temperature in the upper radiator hose & varies vacuum accordingly to the vacuum retard unit on the distributor. Moss no longer has this part but I was able to find a used one in a junk yard. So the car is now running well again. If the used valve unit "goes south" then I'm not sure what my next course of action would be. Probably would have to convert the distributor from "vacuum retard" to a "vacuum advance" system. I see that Moss has a part to do this & I live in an area where vehicles are not inspected for operation of pollution controls.
However, I would like to purchase a spare new Thermostatic Vacuum Valve if any of you know where I could purchase one. Thanks.

 

[poolboy]

You may want to read what the people who make that Vacuum Advance unit that Moss and TRF are selling have to say about it and what it does and what it requires.
https://www.britishvacuumunit.com/Ported_Vacuum.html

 

[Woodmeister]

Thanks Poolboy for the very good information.
Although OEM Thermostatic Vacuum Valves seem to be out of stock everywhere, I have found that Rimmer Brothers does offer a substitute unit (part number AEU1056). So I ordered one for the price of 56 British pounds. I'll give it a try once I receive it.

 

[poolboy]

To me the Thermostatic Vacuum Switch (TVS), in fact the whole vacuum retard aspect of the distributor is something that you can live without.
However the 74 engine made use of the plastic "manifold" attached to the TVS for the vacuum to the Throttle Bypass Valve (TBV) via that blue green tube.
That TBV is another thing that many of us live without, seeing that is is prone to causing problems maintaining an idle speed of 850-900 if the diaphram is getting old and brittle.
What I'm beating around the bush about is, if you cap the vacuum nipple on the bottom of your front carb, then adjust your idle to 850-900 you'll do just fine.
As part of that mod, turn the adjustment screws on the throttle bypass valve as much counterclockwise as they go.
You'll have less problems in the long run.

The TBV for the 74 is like a combination of these 2
There is a small brass nipple sticking up like the one on the left, and an external exposed adjustment screw like the one on the right.

 

[Woodmeister]

To me the Thermostatic Vacuum Switch (TVS), in fact the whole vacuum retard aspect of the distributor is something that you can live without.
However the 74 engine made use of the plastic "manifold" attached to the TVS for the vacuum to the Throttle Bypass Valve (TBV) via that blue green tube.
That TBV is another thing that many of us live without, seeing that is is prone to causing problems maintaining an idle speed of 850-900 if the diaphram is getting old and brittle.
What I'm beating around the bush about is, if you cap the vacuum nipple on the bottom of your front carb, then adjust your idle to 850-900 you'll do just fine.
As part of that mod, turn the adjustment screws on the throttle bypass valve as much counterclockwise as they go.
You'll have less problems in the long run.

The TBV for the 74 is like a combination of these 2
There is a small brass nipple sticking up like the one on the left, and an external exposed adjustment screw like the one on the right.

Poolboy:
I just realized I didn't thank you for the very good information. Thank you!!!!!!!

 

[poolboy]

Not a problem woodmeister. I appreciate you taking the time now to do it. Thanks !