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Exhaust Popping

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SherpaPilot

SherpaPilot

Jedi Hopeful
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Looking for a little advice on my 72 TR6. Due to the usual Stromberg annual float failures, I did a partial rebuild on both carbs including new floats and valves. After the reinstall, the car ran ok, but while driving and whenever the throttle is closed such as slowing to a stop, I get a popping exhaust noise. I played with the timing but it had no effect. The floats were set as per the maintenance manual of 17mm. I'm not sure if this is electrical or fuel related. Does anyone have any ideas or experience like this? Thanks for any help.
Don, Austin, TX
 
Probably running a bit 'fat' (rich). Rather than fuss with timing, raise the jets by half a flat at a time and drive it... or drop float level a millimeter.

Personally, the "popping" is a way of life. A bit fat is better'n lean. :wink:

<span style="font-style: italic">EDIT: LOWER the NEEDLES 1/16~1/8 turn at a time... I didn't read Stromberg. Ron's subsequent post thumped me into reality. Sorry.</span> :shocked:
 
Make sure you have the tool that holds the assembly in place as you are turning the needle with the allen wrench. If not you may tear the diaphragm. It might be a good idea to pull the diaphragm and piston out and examine the diaphragm closely under a good light, to rule out any tears. That way you can also make sure that the needle adjustment is still attached. There is only about 2 1/2 turns on the needle and then it will separate. So you can keep turning the allen, but no response...

This way, with the holder out for inspection. You can rule that possibility out.

HTH
 
That popping on deceleration is typical of what happens if your Throttle Bypass Valve (TBV) is not opening. Turn the TBV adjustment screw 1 (one) Clockwise turn at a time until the popping stops.
Better yet, replace the TBV diaphram and see if a fresh flexible diaphram clears the popping up.
Replacing the TBV diaphram which is attached to and surrounds the actual brass valve should be part of a carb refurbishing. Problem is the diaphram is not included in the rebuild kits. It's Moss # 365-745.
BPNorthwest usually has them at a good price.
 
Thanks to all for your help. I'll start chasing the problem in the morning. Yes, I have the correct adjustment tool. I have never touched the TBV before, so this is new territory for me but sounds not too complex. I'll ket you know how I make out.
 
SP, if the TBV diaphram has become stiff and brittle with age, it may not respond to adjusting.
A fresh flexible TBV diaphram will allow the valve to respond to the intake manifold vacuum during deceleration, whereas a stifff one will not
 
Following the directions in my maintenance manual, I warmed the engine and ran the vacuum test. According to the Bentley instructions, if the rpm increase was to 1300 after removing vacuum, the bypass valve is working correctly. My test was right on target. I removed both pistons and visually checked the newly installed parts; didn't see any binding or out of place things. Both needles were leveled and re-adjusted two turns counter-clockwise (leaning). The engine fired right up but the unusual exhaust popping was still present. My bypass valves do not have any sort of external adjustment as pictured in the manuals. I called Moss and ordered up two new bypass diaphragm kits. In retrospect, this engine fits the profile for having a bad bypass valve in spite of passing the vacuum test. I have frequent erratic idle problems and now the exhaust seems to be telling me something. Guess I'll know if this fix will work in a few days after my UPS guy shows up. Thanks again for your help.
 
I used to have the same "popping on over-run" problem with the ZS carbs on my 73 TR6.
It was running in a very lean condition caused by a combination of incorrect mixture needle adjustment plus low fuel levels in the float chambers; I don't need to tell you who messed all that up in the first place! When I fixed the lean condition the problem went away.
Check the colour of the plugs after driving for a while at proper temperature to guage where your mixture is set currently and adjust from there, don't forget to re-balance the carbs.
 
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