View Full Version : TR6 74 1/2 tr6 idle and vacuum

06-02-2012, 04:07 PM
I've reached the point that I need some help. The motor starts cold/no choke/ and idles at 850. Vacuum is 14. After a good warm up the idle and vacuum begin to drop and it runs rough until it stalls. I have adjusted the carbs and checked for leaks without success. I am thinking about going to different carbs. This is a recent rebuild so I'm only learning the hows and whys. Any thoughts?

06-02-2012, 04:37 PM
I'm not much help, but it sounds like it is running rich for some reason. It shouldn't start easily with no choke.

I assume you have the carbs set full lean and have verified that the "enrichment valves" (aka chokes) are all the way off and aren't leaking. On a 74 I believe the cable has to actually push the lever to close the valve.

Might be worth checking that the mixture adjustment actually moved the needle. The adjustment screw is only held down by a spring clip, and I have heard of the clip moving upwards if the needle carrier is tight in the piston. Sometimes the hex inside the screw strips and you can't tell that the screw is not turning with the wrench.

Since you say it's a fresh rebuild, I would also double-check the valve timing. Having it off by a tooth can make the engine run very rich. Valve lash will also affect mixture (although to a lesser extent).

Fuel pressure on the high side can sometimes show up worse as the engine warms up. Hot fuel is less dense, so there is less force available from the floats to hold the float valve closed against the fuel pressure.

Float bowl vents all hooked up correctly? Can't think of anything else offhand.

06-02-2012, 09:39 PM
With the air cleaner housing removed so you can see inside the carb's mouth, is there gasoline puddled up when the engine stalls ?

06-03-2012, 01:06 AM
The carbs sat for over 10 years so I rebuilt them prior to installation. I've rebuilt a lot of carbs but these are the first Strombergs so I'm learning something new. I've checked timing, valves, vacuum leaks, etc. and found nothing wrong. Tried adjusting them three times without success. After the post I isolated the carbs and slightly cracked each throttle at idle. Front one dropped the vacuum to 0 ??? Rear dropped it only a couple of psi. So, somethings up. Any ideas cause I'm getting close to swapping them out for Webers. Those I understand and can tune.

06-03-2012, 05:46 AM
you might check and see if the manifold is tight to the head and there is no leak between the carbs and the intake. If you go to webers, you will no doubt get a lot of yeas and nays on here

I run triple 40DCOE and it takes some time to get them right, the setup you get from anyone will be most likely close but plan on still playing with the jetting and plan on doing a lot of reading to see how they work and I would look for a set on ebay, you will never know what a previous owner has done to them


06-03-2012, 08:09 AM
You say you measured the vacuum, Where did you measure it ?
If the throttle disc are opening and the air valve moves freely, then mechanically the carbs are functioning. Fuel delivery would be the next thing to explore that's why I asked about the puddle of gas in a previous post. Sometimes the float chamber needle valve is responsible for a lot of fuel delivery problems.
Hard to beat this guide:

AND THEN there's the way you might have handled the plumbing associated with the carbs.. Cap the wrong nipple and you've got problems.

06-03-2012, 08:45 AM
You say you measured the vacuum, Where did you measure it ?

That was my thought too. Sounds like you are on the vacuum retard port, which only works with the throttle closed.

06-03-2012, 08:45 AM
Larry - what specific carbs do you have on her, as strange things happen over time. Are they fixed or adjustable needles?

06-03-2012, 02:41 PM
DUH! Thanks poolboy and tr3driver. Forgot about retard so I went to the manifold and much better on the vacuum reading. Still drops rpm and stumbles after 10 minute warm up. I pulled the vacuum off the Run-on Control Valve and it immediately went to a high idle. Plugged the line and back down. Is the valve supposed to open or closed when running?

06-03-2012, 03:21 PM
Here's the description from the Triumph Emission publication of 1973

"'Run-On' Control Valve
Due to the rigorous U.S.A. Federal Laws governing the exhaust emission and the use of specified fuels, current production engines have a tendency to 'run-on' after the ignition is switched off.
To combat this condition a run-on control valve is fitted to the bottom of the charcoal canister and actuated through the ignition switch.

A solenoid moves a valve which seals off the air inlet to the bottom of the charcoal canister.
At the same time a passage is opened in the valve body to allow manifold vacuum to act through the canister, vent valve hose and drilling to the carburetter, causing a depression in the float-chamber.

The vacuum created equals the vacuum in the constant depression area of the carburetter, thus preventing fuel from rising and entering the air orifice and therefore stopping the engine.

As the oil pressure drops, an oil pressure sensitive switch breaks the electrical circuit and de-energizes the solenoid.

The engine is then ready again for operation"
https://www.mv.com/ipusers/last/emissions/page_52.html <span style="text-decoration: underline"> </span>

06-03-2012, 08:50 PM
Yup. You've either got a bum valve, or it is getting power when it shouldn't. It normally runs from a special contact on the ignition switch, that supplies power only when the ignition is off. Then it gets grounded through a special contact on the oil pressure switch, that is closed only when the engine has oil pressure.

So the solenoid should be active only when the key is off and the engine still has oil pressure.

06-04-2012, 09:42 AM
I don't know if the 74 1/2 has a water temp controlled valve that when hot allows vacuum to the distributor or when cold does not allow vacuum to the distributor (idle speed will be lower w/o vacuum because the static timing is retarded). My '74 TR6 has this system in order to heat the motor up a little faster (retarded timing will make the motor hotter). I mention it as an area to explore.

06-04-2012, 10:09 AM
I don't know if the 74 1/2 has a water temp controlled valve that when hot allows vacuum to the distributor or when cold does not allow vacuum to the distributor (idle speed will be lower w/o vacuum because the static timing is retarded). My '74 TR6 has this system in order to heat the motor up a little faster (retarded timing will make the motor hotter).

It's the other way around. The valve blocks vacuum to the vacuum retard unit when the cooling system starts to overheat. Blocking the vacuum will increase the idle rpm, because the timing is not retarded. The high idle will hopefully pull more air and cool the motor off.

When properly installed, of course.

06-04-2012, 10:18 AM
Actually you got that backwards,vettedog. It's called a Thermostatic Vacuum Switch and when the coolant reaches about 200*F it shuts off the vacuum to the retard. That causes the timing to advance, engine to pick up speed (if the engine is idling), coolant circulates faster and the crank driven fan increases air flow.
You can read about it here. Scroll down to
"Frame 43"

06-04-2012, 07:56 PM
Thanks for sticking with me on this. I checked the run on valve and it's not working. I now also understand how it works. Since the motor does not diesel it seems not to be an issue (correct me if I'm wrong). Also, since it only opens for a short time at shut down, I doubt that it has any effect on my problem of idle drop and rough running after warm up.
When I got the car, most all smog was missing (along with most other external motor parts) so I eliminated everything. Only the lines to the canister and breather ports remain. The motor had been bored and I believe an upgraded cam installed.
The motor fees strong, I just cannot seem to get the carbs adjusted to optimize power and a smooth idle.

06-04-2012, 08:06 PM
So does that mean no vacuum line to the distributor?

06-04-2012, 08:18 PM
A lot of times it's just an ignition component or the timing itself. Do you have the timing spot on and the mixture adjusted to compliment the timing ?

Regarding the plumbing, do you recognize the nipple that I have colored red ?
What's their status ? Is there a hose attached to them ? Is it capped or plugged ?

When the engine is idling can you blow air (by mouth) into the red nipples ?

06-05-2012, 02:37 AM
I originally static timed to 4 deg BTDC and vacuum to dist. Made adjustments to carbs per book. Exhaust was popping on deceleration so I timed it 10 deg BTDC and plugged vac to dist. Adjusted carbs again. This got rid of the popping.
Hoses on red nipple go to canister. Have not tried blowing on the nipples (is this a trick?). Other hoses on carbs are from nipples on top/back to line from valve cover to canister and nipple on bottom/back of front carb to dist vacuum (plugged). Balances fine but can't get it smooth with mixture adjustments.

06-05-2012, 07:16 AM
The nipple painted red in the above photo is the float bowl vent at idle. Blowing into it just confirms that the vent valve is open (which it should be with the throttle closed).

You might try temporarily disconnecting both of the "red nipple" lines and see what effect that has on your dying problem. If it changes, your carbon canister may be clogged inside. Early ones could be serviced, but I think by 74 they were welded together.

06-05-2012, 08:28 AM
What Randall said, plus the spring loaded plunger that opens and closes that vent sometimes gets gummed up or otherwise keeps the vent from opening at idle or at rest.
You can just blow thru a hose or straw into the red nipple. Just be sure that it's not stopped up when the carb is at rest.
If it's clear and you still have problems with adjustment..make sure your needle has not come un threaded from the adjuster. If you can make more than 2-1/2 Clockwise turns with the Allen wrench..that's what happened.
If that chacks out I'd go fully CW, then 1 Counterclockwise turn as a starting point.
Most of the carbs on stock engines I've fooled with have a sweet spot somewhere between 1 and 1-1/2 CCW turns from fully CW. That is if everything else is in good tune and no sticky valves to confound things.

06-05-2012, 08:18 PM
Ok, think I may have it. I must have been reading your mind as I pulled the top checked the needles and readjusted from the full lean. Set them about 3/4 to 1 turn and they were pretty close. Took it for a test drive and had no problem with idle. A little of the exhaust popping is back so I may be a little lean still. Took it up to 4500 rpm and it felt strong but power was starting to fall off. Back to the garage and it still idled well. I found another run on valve in my spare parts so I will swap that out. I'm going to leave it for now but may still go to different carbs.
Thank you for your help.

06-05-2012, 10:51 PM
This is the throttle bypass valve diaphram. If anything is wrong with the TBV it's usually because this diaphram has lost it's flexibility. It's about a 12-13 dollar part and may be responsible for the popping on deceleration.