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TR6 1973 TR6 carb/ TBV

sundown

Jedi Hopeful
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Took a ride last night the TR is running pretty good except when I let off the gas it starts to pop from the exhaust the timing is spot on via a vaccum gauge.Could it be the carb mixture or do I need to mess around with the TBV?
 

TR3driver

Great Pumpkin - R.I.P
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Either way will work, I believe. The TBV was essentially to suppress the popping with leaner mixtures. (Well, actually to suppress the misfiring under overrun conditions, but that's what causes the popping).
 

titanic

Jedi Warrior
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I might be wrong, but I thought the purpose of the TBV was to limit the vaccum on overrun to reduce emissions. SUs of the same era used a less elegant approach-a spring loaded valve in the throttle disc.
Berry
 
Last edited:

poolboy

Yoda
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It's name Throttle Bypass Valve pretty much describes it's function
When the manifold vacuum exceeded approx. 22 in-Hg the valve is supposed to open and allow whatever mixture is in the carb at the time to bypass the closed or nearly closed throttle disc and enter the combustion chamber.
That addition combined with the limitation of manifold vacuum provides a better mixture so that the combustion is more likely to take place within the combustion chamber instead of waiting until to explode as it enters the exhaust system with more available oxygen. I imagine this cleaner combustion is desirable as far as emissions go as well.
The diaphragm that surrounds and seals the valve, doesn't seem to last all that long. When it's flexibility is lost, the valve may not be able to respond to the manifold vacuum it senses.
The valve doesn't open and you sometimes get that popping
I have mine disabled with a gasket sealing both the inlet and outlet, but I still don't get that popping when I down shift...sometimes if the engine is really hot..a gurgling sound right at first during substantial engine breaking conditions.
 
OP
sundown

sundown

Jedi Hopeful
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Thanks, poolboy I am going to disable mine as well. I do not like that pop pop sound. Would a too lean carb cause that also?
 

poolboy

Yoda
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Yes I believe too lean a mixture will contribute to the popping..Just keep track of your adjustments and do a little experimenting.
But incase I was misunderstood, disabling the TBV may increase the chances of the popping continuing or increasing.
You might also consider disabling the Vacuum retard by removing the vacuum tube from the bottom of the rear carb and capping the same nipple.
That same high manifold vacuum that activates the TBV, will also activate the Vacuum Retard..
The retarded timing doesn't exactly provide ideal conditions for combustion, either, as evidence by the engine picking up idle speed when the retard is disabled and the timing allowed to advance.
You'll have to adjust the idle speed down, though..if the retard was working properly in the first place.
By doing a little experimenting, I think you'll quiet things down.
 
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