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TR2/3/3A Pre-Ignition

frankfast

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Just got a TR3A. When I shut off the ignition switch the engine rumbles a bit before it stops. I set the contacts and timed the ignition to 4 degrees before TDC but it still does it. What may be the cause?
 

TR3driver

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Lots of possible causes. How is your idle rpm? Have you tried higher octane fuel? Do you have an electric fan fitted? Is the engine stock? Does it burn oil?

One easy workaround is to leave it in gear when you shut it off. With one foot firmly on the brake, let the clutch out just after you turn the key off. With a little practice, it becomes automatic and so smooth that you won't know if it wanted to run on or not. I drove for a lot of years that way.
 
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frankfast

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Idle is too low. I put in high octane. No electric fan. I'm not sure if the engine is stock since it was "rebuilt". I haven't had it long enough to check oil consumption.
I've read that the engine performs better at 8 degrees before TDC than 4 degrees. Is that true?
 

TR3driver

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Worth a try. On mine, I tweak up the timing until I can just force it to knock (by lugging in high gear with the engine hot) and then back off a couple of degrees.
 

doc50

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Carbon buildup? That would create a glowplug effect. Maybe get a scope to look in the cylinders.
 

Sarastro

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What you're describing is dieseling, which has been a problem on and off with LBCs forever. As doc50 describes, it's often caused by carbon buildup, which is caused by incorrect carb or timing adjustment, oil burning, and probably other things I'm not thinking of. You need to be sure everything is adjusted right. A fresh tune-up is a good idea if you haven't done that yet. Then, take it for a long drive, where it has plenty of time at full temperature.

If that doesn't help, the simplest thing is the stall-it-with-the-clutch solution.
 
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frankfast

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I pulled the plugs and the rear two are black and the front two are a dark brown. Would this indicate that the rear carb is running too rich? I have a shop manual which describes tuning the carbs which I intend to do within the next couple of days.
 

shoopal

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An old Welsh mechanic gave me this tip many years ago and it really works to decarbonize an engine. With air cleaners off, engine RPM around 1500, fine spray a small steady stream of water into each carburetor separately for up to 2 minutes. Not longer as this can blow a hole in the top of the piston.
After this treatment, make sure the mixture is set correctly (at idle, lift the piston slightly, engine should not increase) and the throttle shaft bushings are tight (a spritz of oil around the bushes should not change idle speed) P.40 maintenance manual is instructive.
 

Geo Hahn

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...Would this indicate that the rear carb is running too rich?...

It would suggest it.

You are on the right track to look at the plugs as an indicator of mixture - you're hoping for a cocoa brown as illustrated in many manuals and online.

Go slow with the adjustments - I make a paint mark on the mixture nut and keep written notes so I know what I have done... and can put it back to where it was if I manage to make things worse.
 
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frankfast

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In previous post I mentioned that the motor didn't shut down completely when the ignition was switched off. This was referred to as dieseling probably caused by carbon build up. When I removed the the spark plugs the rear two were black so I decided to tune the carbs according to the procedure in the workshop manual. When trying to adjust the jet adjusting nut, the nut was stripped in the locknut making the adjustment impossible. The cork washer was also split. This probably caused the mixture to be too rich and fouled the plugs. I've ordered the proper parts from Moss. I'm hoping this solves the problem of the car dieseling.
 

TR3driver

Great Pumpkin - R.I.P
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The part with the male threads is only held by friction. Often when they turn like that, someone has turned the mixture nut all the way lean and jammed the threads.
The usual reason for that is because the carb is running rich at idle. There are other possible causes of course, but wear in the jet and/or needle seems to be both common and perplexing. The wear is caused in turn by driving with the jet not properly centered.

Assuming you do find that the nut is jammed rather than stripped, and there is no other obvious reason for it to be idling rich (like the upper jet gland is missing or the float valve is leaking); I would replace the jet and needle. Even a little bit of wear can cause strange problems, like overheating at speed.

And of course center the jet carefully afterwards, preferably with the nut set all the way lean (but not enough to jam the threads if course). The piston must land with a distinct click.

On my carbs, there is enough play in the dome-body joint to affect the centering. So I gently turn the domes clockwise while tightening the screws, to get a repeatable position.
 
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frankfast

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According to the manual, the jet adjustment nut should be turned all the way in and then backed out 2 1/2 turns. That worked with the front carb but the adjustment nut on the rear carb kept turning. When I removed the assembly, I couldn't separate the adjustment nut from the locking nut. Either the male adjustment nut is stripped or the female locking nut is stripped. I ordered both and the spring from Moss.
According to the manual, this adjustment must be made before the throttle stop screws are adjusted to synchronize the carbs. Then the jet adjustment nuts are turned in unison to maintain a proper idle speed.
After I finish this procedure and there is still a problem with the rear cylinders running rich I'll have to consider changing the needle and jet.
 
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