TR4/4A Stumble off Idle with Hot Engine on Hot Days

[Geo Hahn]

I have probably posted this problem before but as it recurs every time hot weather returns and nothing I've tried resolves it, I'll have another go.

Once the engine is well warmed up I get a coughing stumble coming off idle after sitting through a long stop light. More info:

Happens in hot weather (mid-90s or so). All other aspects of running seem fine - plugs and the usual methods all suggest the mixture is about right. I have to use the choke whenever the engine is cold even on warm days - so that also suggests to me the mixture is not far off.

I am using premium gas (Shell) & 20W50 oil. The engine never overheats.

I installed a homemade heat shield under the ZS carbs - does not seem to help.

I have tried both lighter and heavier oil in the carbs (everything from MMO to 20W50) and still get the stumble.

If I goose the throttle a bit (like a teenager) before starting to move I can more or less prevent the problem.

Sure seems like the intake is just loading up with fuel while I sit and has to clear its throat before normal running can resume.

The one thing I can think of that I have not tried is adjusting the float level in the carbs. These are ZS so the float sits right below the carb body. It is possible that one or both floats are partially sunk - if I go to the trouble of pulling the carbs apart I will probably go with solid floats.

What am I missing? Would misadjusted (or partially sunk) floats account for this (I would expect other problems from that but have none)?

It is really just an annoyance but still I feel I should be able to make it better. Thanks for any ideas.

 

[poolboy]

You could be running a bit too rich.
And about that heat shield...If it's just a sheet of metal, it probably isn't as effective as it would be if there was a layer (or two) of heat insulation..
That's what it took to make my ARE 'heat shield' effective.
This:
https://www.jegs.com/i/JEGS-Perform.../10002/-1?parentProductId=1318562#moreDetails

 

[TR3driver]

Another possibility might be a float valve that doesn't quite seal properly. I had one on the Stag that acted kind of like that. Something you might try is letting it idle in the driveway and see if you start getting black smoke out the exhaust.

Are the intake and exhaust manifolds bolted together as original? Seems like that would have to prevent any puddling, especially in hot weather. Mine are deliberately disconnected, but I notice the effects mostly when the engine is cold.

 

[trrdster2000]

Geo, A possibility is in the linkage and one carb wants to start before the other. Not as prone on the TR's as it was on the MG's but have seen it happen when just a little tweak was done after synchronizing and tightening everything up. Also weak spring on the shafts. The early ones were heavier than the later or replacement ones and did a little twist before reacting. So if you have one of each, there would be a slight stall.

Just one of the things that has driven me nuts over the last 60 years of messing with the LBC.

Wayne

 

[sp53]

I would try a different coil and maybe condenser after that.