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TR4/4A Stumble Coming Off idle

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Geo Hahn

Geo Hahn

Yoda
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On a TR4 with Strombergs -- I get a stumble coming off idle at a stop light. The engine quickly recovers but it is annoying.

I have checked the mixture by the usual methods (lift-the-pin, Colortune, read the plugs) and it seems fine.

Have tried both thick (20w50) and thin (Marvel Mystery Oil) oil in the carbs -- problem recurs.

The diaphragms look fine (stretched while examining in front of strong light).

Pistons raise fine and drop cleanly.

The problem seems to only occur once the engine is hot. Also, the longer I sit at the light the worse the stumble is when I start to move.

I have not yet examined float level -- could that be it?

The rest of the time the engine performs fine -- but not looking forward to the long hot summer with this going on.

Thanks for any advice.
 
Since it's easy enough to get back to the starting point, enrichen the mixture and try 30 wt motor oil in the damper.
 
This sounds very similar to the problem that I am trying to diagnose with my wedge currently. With mine it's even stranger because the stumble is only happening with one carburetor and not the other. (On the TR7 linkage you can move the front carburetor throttle independantly of the rear one, but the rear one will move the front linkage) It's so bad that when the engine is cold I can stall out the engine completely by stomping on the gas a few times. I went through a compression test, spark plug test, checking the rotor and points, checking the oil in the dashpots (thick 30wt non detergent, and light marvel mystery oil) reaching in and pushing the pistons up and down (they move freely) My next step, after realizing that it's only happening with the rear carburetor and not the front, is to check fuel delivery to that carb, make sure the needle isnt clogged, check all the lines on that carb for vacuum leaks, etc.

Here's a video where the stumble is loud and clear as a loud put put put stutter when I move the throttle on the side of the rear carburetor. I have other audio only videos on my channel where I stall the engine out from a smoothly running engine just by stepping on and off the gas.

https://www.youtube.com/watch?v=rWoSwOJdjgA
 
Try cleaning the ignition rotor. I had a problem last spring where my car, once warmed up/hot would stumble under load. If I didn't catch it the engine would die and it wouldn't start until it cooled for 10-20minutes.

Cleaning off the rotor would fix it for 100 miles or so. Replaced with a quality red rotor and cap with brass ignition contacts.

Bob
 
Sounds more like ignition, especially since you saw no change with some substantial changes in transient mixture (from the dashpot oil).

I'd probably do an ignition tune-up (new plugs, points, condensor, rotor) just for luck, and check resistance of both the point lead and the point plate ground lead while stretching them gently. I've had some really weird symptoms related to those two wires breaking inside the insulation (from being flexed every time you move the throttle). Also do an end-to-end check on the HT coil lead with it connected to the coil & cap; by measuring from a coil side terminal to the center terminal inside the cap. You should find some resistance, but not a whole lot, less than oh, 20K ohms or so.
 
Follow-up -- thus far, a new coil seems to have cleared up the issue.

I have had coil failures in the past but not one that showed up this way.
 
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