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Rob DeScherer
07-08-2004, 11:53 AM
I need some help. I have a 74 TR6, with z-s carbs just rebuilt by Joe Curto. I have removed all of the emissions stuff, except for the carbon canister and its connections. I have securely plugged everything that I removed. There are no leaks on the manifold side of the carbs. And the car still runs lean, no matter how I adjust the carbs.

Then, I removed the hoses that run between the carbs and the carbon canister, plugged the corresponding holes on the carbs, and bingo, the car holds an idle, and the coarbs could actually be adjusted. Although there is a flat spot around 4000 rpm, for the most part it runs properly in the four years that I have owned it.

My question is, why would I have an air leak in the carbon canister system, and why if you do have an air leak there should it affect the running condition? The hose from the carbon canister is on the manifold side of the throttle butterfly, so why should this affect the lean/rich mixture?

I am very confused and frustrated. Thanks for any and all advice.

aeronca65t
07-09-2004, 08:58 AM
It's hard to tell from the info you have given, but these cars mostly have a canister purge valve. If it is faulty, the problem will be similar to a vacuum leak.
I haven't worked on one of these cars in a while, but you state that the canister is hooked to the manifold (this sounds correct). Any leaks in this system will have a large effect on lean/rich condition since it will act as a vacuum leak.
One note: if you have disabled the canister system, you may have also plugged the fuel tank vent (which is vented to this canister). You may want to install a vented cap (or drill a tiny hole in the underside of your existing cap).

Rick O.
07-09-2004, 10:27 AM
[ QUOTE ]
Then, I removed the hoses that run between the carbs and the carbon canister My question is, why would I have an air leak in the carbon canister system, and why if you do have an air leak there should it affect the running condition? The hose from the carbon canister is on the manifold side of the throttle butterfly, so why should this affect the lean/rich mixture?

[/ QUOTE ]

Rob--So you removed the fat hoses that tie into the rockerbox? If so, how are you venting the crankcase? Anyway, check that you have the flow restrictor installed on both the fuel tank vent line and the nipple that the carb vent hoses attach to. Is the canister's solenoid functioning properly (closed when ignition is on)?

Do your carbs also have the smaller ID hoses running to the canister for fuel bowl venting?

The manifold-side carb nipple senses carb depression on the piston side of the butterfly. If you have a leak in that line, it has to lean the mixture. Same would be true if the manifold side depression were sensed.

Hope this helps.

Dave Russell
07-09-2004, 07:52 PM
Hi Rob,
A couple of possibilities.

It's not clear whether you are talking about the large hoses or the small hoses from the carbs. If it's the small hoses continue:

I think your car originally had an anti run on valve. The float bowl venting was controlled by this valve. It's purpose was to effectively apply vacuum to the bowls for a short time after the ignition switch was turned off. This prevented fuel from flowing & stopped run on.

If the bowl vents are now connected to the canister & the canister is also connected to engine vacuum via the large hoses, & this valve is missing or somehow bypassed, this could explain the lean condition.

A quote from here:
https://www.buckeyetriumphs.org/technical/Carbs/CarbsII/CarbsII.htm

"The last part of the breathing apparatus is the electrically operated anti run-on valve introduced on the 73 TR6. Power from the ignition switch in the OFF position is feed through the ON position of the oil pressure switch to the valve, #6 in the sketch; the valve operates when the ignition is turned off and there is still oil pressure. The operated valve closes the external air input to the bottom of the canister via tube #6 and instead connects tube #6 to the intake manifold via tube #7. The depression in the intake manifold sucks air out of the canister and via tube #2 to the float chamber creating a depression in the float chamber. This depression in the float chamber prevents fuel entering the carb via the jet thus preventing run-on or dieseling. The valve releases a few second after the ignition is turned off when the oil pressure drops."

I think reading this whole section will help explain how it is supposed to work. If you have plugged the carbs where the small hoses connected, you don't have any float venting, if you have plugged the carbs where the large hoses connected, you don't have any canister vacuum.
D

Rick O.
07-10-2004, 10:21 AM
[ QUOTE ]
I think your car originally had an anti run on valve. The float bowl venting was controlled by this valve.


[/ QUOTE ]
On later TR6's, the bowl vent is conrolled by a throttle-actuated slide valve on the left side of each carb. [Pre '73 sixes vented the bowls to the intake of each carb at all times.] At idle, the bowl is vented via the vapor recovery system (small hoses going to canister); off-idle, vent is through a hole on the air filter mouning flange. Perhaps Rob has this flange hole covered up by a gasket and is actually experiencing a rich condition due to pressurized bowls.

Rob, check that your carb vent valves are adjusted properly. If unsure, let me know and I'll walk you through the simple procedure.

Rob DeScherer
07-10-2004, 10:24 AM
Thanks for all the help. It would seem to me that I have to check the valve at the bottom of the canister for proper operation, and also the line that runs from the gas tank to the carbon canister. Hopefully, this will solve my problem.