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TR6 76 TR6 Can't Maintain Idle

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Don_Houston

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I converted to electronic ignition, set timing, adjusted valve clearance, rebuilt, tuned and sync'd carbs. Starts great, runs great. After about 30 minutes of driving then coming to stop light, will not hold idle which starts dropping off to stall. What would you check next? Many thanks!
 
Don - did you have this same problem before converting to electronic ignition?

Tom
 
What do you have connected to the float bowl vent lines? Does leaving them disconnected make a difference?
 
This, that I colored red, is the nipple for the float chamber line that Randall asked about.
 
The problem was worse before I changed to the electronic ignition in that the car would not start after shutting it down after a 30 minute drive (seems like a flooded situation). After sitting overnight the engine would start right up. The float bowl vent lines are connected and run to the carbon canister. I will try disconnecting to see if the problem persists. All of the original emissions are installed except for the air injection system which has sealed plugs in the manifold. A mechanic that I am working with said his next step would be to check for vacuum leaks. Any other troubleshooting suggestions? Many Thanks!
 
A vacuum leak is a definite possibility. But I would still check the bowl vents first. The canister is supposed to be replaced every 50k miles but no one ever does so; and if it gets clogged up it can kill the engine at idle.
 
If there is a next time, reach around and flip open the gas tank fill cap and see if the engine recovers.
 
The car has 24k miles but sat in a barn for 30 years, so no telling if the carbon canister has issues. I will let you know how it goes. Thanks!
 
Yes, new fuel filter. Fuel tank was professionally refurbished and coated, new fuel lines all the way and new fuel pump. Pump pressure was checked, ok. I rebuilt the carbs myself (first time to attempt something like this), the only internal adjustment I did was to check the float height.
 
Just throwing this out there. Where is the timing set at?
 
Current step I am taking is to disable the vacuum retard as the theory is that the carb is not adjusting properly when the vacuum retard kicks in? Does this make any sense? What part of the carb would be malfunctioning so that it does not adjust to the vacuum retard? I will let you know how the test run goes, but I appreciate your thoughts. Many thanks!
 
The retard doesn't exactly kick in at idle. If it's working, as long as the rpms are below 1100 rpms when the engine is idling, the retarding is going on.
If you disconnect the vacuum line to the distributor's vacuum retard module AND seal or cap the vacuum source which is a 1/4" nipple on the bottom of the front carb and the idle speed increases, then that is an indication that the retard was in effect and working.
Countless people leave it that way and bring their idle speed back down to 850-900 rpms, with equal counterclockwise rotation of the carbs' throttle stop screws.
But that's the only thing I can think of that someone might have described as "the carb not adjusting properly when the vacuum retard kicks in".
 
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I agree, the carb doesn't "adjust to the retard". But it can't hurt to disable the retard and see what happens. Normally, it should bring the idle rpm up pretty high, like 1200-1500 rpm and you may find that the throttle stop screw won't bring it back down all the way. If so, check the linkage to the float vent valve, it may need to be adjusted to let the throttle close far enough.

Maybe I missed it; did you try disconnecting the lines from the idle bowl vents? They have to be open to the atmosphere but normally that is through the carbon in the carbon canister (so the carbon can absorb the fuel vapors). If the canister is clogged (they are supposed to be replaced every 48,000 miles but no one ever does), it can apply pressure/vacuum to the idle vent port and mess up the mixture at idle. A stuck, clogged or miswired anti-runon valve can do the same thing.

Might also be worth pulling off the line to the EGR valve as a test, make sure it isn't getting vacuum at idle. If the EGR passes gas at idle, it will kill the engine. (In fact, that's how they used to test them during a smog check, pull a vacuum on the valve and see if the engine dies.)
 
Since this is all beyond my skill level (I'm learning), I have a great team helping me, in addition to the great forum team here. Here is where I'm at for now, I have not taken it out for my own test run yet. -The timing was set to 10 degrees BTDC with the vacuumretard disconnected.

-With the vacuum retard connected, timing SHOULD fall backto 4 degrees ATDC, a movement of 14 degrees, when everything is functioningproperly. Idle speed should be set to about 850 RPM with the timing atthis point.

-On my car, the vacuum retard is inconsistent in terms ofhow much it moves the timing. It usually ends up between 3 & 5degrees BTDC, a movement of only 5 to 8 degrees. We set the idle speed to850 RPM with the timing in this range. This is where we came up short. We did not verify where the timing ended up with the vacuum retardconnected. We only verified that the capsule held vacuum, moved thedistributor's breaker plate, & that it caused timing to move. Had weverified where the timing actually ended up with the vacuum retard connected,we would have realized a potential problem despite the fact that it ran fineduring our test drives.

-The reason that the car was failing to idle is that, undersome conditions, the vacuum retard would move timing more than it did when weinitially tuned the car. This is what happened when the car stalled &failed to hold idle after the first freeway run.

-On the second diagnosis, the car would idle sometimes,but then the idle speed would gradually fall until the car stalled. Wenoticed that, as the idle speed was falling, so too was the timing. Thisis where we picked up on the inconsistent performance of the vacuum retard.

-The reason I stated that it was a potential carb issue isthat the vacuum retard is only receiving 5"Hg of vacuum at idle. Although I did not find a spec for manifold vacuum, this sounds low tome, & IMO, the vacuum retard is not receiving sufficient vacuum to performconsistently. Other than an internal engine problem, which I don'tbelieve you have, a vacuum leak, which I could not find, I could not think ofanother reason why manifold vacuum would be that low.

-The only other possibility that I can think of is a problemwith the distributor's internals preventing the vacuum retard from moving thebreaker plate consistently. However, the centrifugal advance moves fine.

-Because the car is not subject to emissions testing, weelected to disconnect the vacuum retard, plug the fitting on the carb, &set the idle to 850 RPM with the timing at 10 degrees BTDC. With thissetup, the car performs well & idles consistently under all conditions.

-If anyone has any further input, I'd appreciate it.
 
If your engine is stock and in decent shape, and if the ignition timing AT IDLE is really and truly in it's sweet spot, the manifold vacuum AND the vacuum at the nipple on the bottom of the front carb should be in the 17 to 20 in-Hg range...you can deduct 1 in-Hg for every 1000 ft above sea level.
It takes at least 15 in-Hg at idle for the retard to be maximun.
According to Bentley with only 5 in-Hg of vacuum the maximun retard would be between 3.5 and 4 degrees.
 
Sounds like you've nailed it, Don. The retard port is pretty simple, but it is rather small. About the only things that could go wrong is if the port is clogged with debris (maybe old fuel varnish); or if somehow the throttle plate is not moving far enough to expose the port to manifold vacuum. (The port is positioned such that the edge of the throttle plate is on the intake side of the port with the throttle closed, and on the manifold side with the throttle open.)
But I endorse your solution, just cap the port & leave the retard disconnected.
With modern thinking, the retard actually increases emissions anyway. (CO2 is now regarded as a pollutant.)
 
Don_Houston said:
-Because the car is not subject to emissions testing, we elected to disconnect the vacuum retard, plug the fitting on the carb, &set the idle to 850 RPM with the timing at 10 degrees BTDC. With thissetup, the car performs well & idles consistently under all conditions.
Click to expand...

Exactamente.
 
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