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TR4/4A TR4A backfire - help please

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Adrio

Jedi Knight
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I have been fighting with my TR4A for a while now. I had thought it was a carburation issue but now I am stummped. The engine is newly rebuilt (800 miles) and I have done a compression check and have about 150 on all four.

How it behaves is that it backfires out the exhaust when the engine is run up past 3000 RPM for more then 4 or 5 seconds or when i try and accelerate hard. Any guessas as to what is going on.

At first I had thought it was going lean there, but now I am wondering if the ignition and spark (or lack thereof) could have this effect?
 
My first instinct is that you've got it too rich.

How do your plugs look? Sooty & damp?

What weight oil in the dashpots?
 
Agree with Geo, backfiring is usually caused by incompletely burned mixture burning in the exhaust manifold.
Most often caused by too much fuel, but I suppose big timing changes caused by something like a loose distributor could lead to the same result, or a spark plug not firing consistently.
Simon.
 
The plugs look sooty. But I have been playing with mixture to try and solve the problem and nothing makes it go away. I have the jets all the way up and it still happens. When the idle mixture is correct (as per the book, lift the little things) it still does this. It tends to idle just fine

As to the dash pot oil, I am using heavy oil. But that is because I changed from thin oil to try and solve this problem.

Given what you say about unburned gas exploding in the manifold then a poor ignition system could result in the same thing. Is this correct?? I have a new coil, cap, wires, points, condensor.

I may swap out all the ignition parts from my TR3 (including the whole dizzy, plugs wires and coil and see it that makes a difference.

I am stumped and getting frustratied. It is cold and time for me to put the cars to bed for winter, but I can't even drive it strorage this way as it will not make the trip.
 
An update. I am fairly sure the thing is running rich as indicated by the black soot covering I put on the front of my wifes car in about a minute which was parked behind me (I could no longer read the license plate). The jets are all the way up and it seems to idle OK.

The ignition system is fine, as I swapped in the one from my (previously) perfect running TR3A (I better be able to get that back together without a hitch)

I am stummped and at the end of my rope.
 
My first guess and I hope I am wrong, is a lobe off your new cam. Pull the valve cover and check the lifts of the exhaust valves......bummer thing to happen.....Did you check the lifters for rockwell hardness??? lots of cam lube??? carefull cam breakin???
MD(mad dog)
 
In fact tonight I took the valve cover off and checked the valve lash. Some where a bit tight (maybe .080) but they are all set correct now and it did not make a difference.

I did not replace the cam on the rebuild as the old one looked fine. I used lots of assembly loob when I rebuilt the engine. I did a compression test yesterday and they are all about 145-150.

I did notice that the pistons in the carbs do not move up much when I open the throttle. Maybe only about 3/16 of an inch.
 
I would think it's a rich mixture, but I'll throw out a few more ideas:

a) make sure it's "backfiring" you're experiencing, and not missing which could mean plug wire problems, arcing, etc. I had that issue with my 4A when the rear boot on a plug was too close to the fuel line of all things;

b) if it's truly backfiring, be sure of the following: i) the timing is correct and not retarded excessively; ii) vacuum lines are properly in place; and iii) the carb jets are centered with the pin dropping freely when the jet is adjusted up to its hight point--maybe most important;

c) try adjusting and balancing air intake with the air filters off

My experience is that backfiring is pretty difficult to see--you have to be really rich. I try starting at eight flats up, and I prefer a "slightly" rich mixture to a lean one.

Good luck. Sorry I can't narrow it further.
 
Another area you might check is the distributor. These do accumulate dirt and lose their lubrication over time. My TR6 would not idle properly but ran well at speed. It was not getting the proper vaccuum advance. Try cleaning and using a light lubricant. Carb cleaner would work well with a general purpose oil.
 
Adrio....
I would have thought it was an ignition timing problem except for your comment that the air valve pistons are only moving 3/16".
They should fully open when you push them up into their bores.
If the 3/16" movement is all you can move them with your finger then they must be binding somehow and that will definitely lead to poor mixture control and likely cause all kinds of backfiring.
If that's the case then you will need to dismantle them to find and fix the problem.
The other common cause of rich mixture that can't be adjusted out with the jet adjustment is when the fuel level is too high in the float bowl.
One other thought, did you remove the timing chain in the rebuild? Could it be that you have assembled it one sprocket tooth out?
 
Hi Adrio,
I understand the problem to be as follows:
"How it behaves is that it backfires out the exhaust when the engine is run up past 3000 RPM for more then 4 or 5 seconds or when i try and accelerate hard. Any guessas as to what is going on."

A combustible mixture is somehow getting into the exhaust system while the engine is under load.

A very lean mixture would burn so slowly that it would be still burning as the exhaust valve opened. This is not likely to happen when the engine is under power. Retarded ignition timing could do the same thing but again, not likely under power. This would likely show up as an extremely hot (glowing) exhaust manifold. A lean mixture will usually be accompanied by spit back through the carbs.

A very rich mixture would be too rich to burn in the exhaust system unless it was supplied with extra air via an exhaust system leak & even then only under deceleration. An extremely rich mixture will still burn in the combustion chamber without missfire & power only down a little when the engine is under load.

It would appear that a chemically burnable (correct) mixture is being intermittently passed into the exhaust system & ignited by the hot exhaust gasses. The most likely cause is intermittent ignition. I know you replaced the ignition but see no mention of replacing the spark plugs & or narrowing the gaps. Low supply voltage to the coil would cause ignition to breakdown under load.

Other less likely possibilities are screwed up cam timing, a sticking exhaust valve, or cross firing in the distributor cap or plug wires.

"I did notice that the pistons in the carbs do not move up much when I open the throttle. Maybe only about 3/16 of an inch."

This would be normal if you are observing the carbs while the engine is NOT under load.

You have my sympathy, such problems can be very puzzling.
D
 
Dave and others,

Thanks for your replies. They have me thinking. One comment you make about the low voltage has me thinking. I was thinking about ignition being the problem so I did swap out all the ignition parts from my TR3 (that is the entire distributor, the cap wires, plugs, and coil). I even "hot wired" in that I ran a test lead directly from the battery to the coil input. Still the same.

Now the voltage low comment though is getting me. I may try swapping out a battery as well. I thought that with a generator the car can be run without a battery and still run just fine. Could it be that my battery is presenting such a low impedance that the output voltage of the charging system drops when I accelerate? The reason I am grasping at the voltage thing is that in the process of trouble shooting this my voltage regulator stopped working (I was able to get it back and set right after I cleaned the contacts on it). But none the less it has me wondering. how do you think a weak spark would show up in the way the carbs react when setting the idle mixture

The other confusing thing is that I can't get it lean enough at idle either. At idle with the jets all the way up the exhaust is black and covers anything behind it with soot.

I did a compression check and it is at about 150 for all. If I had put the timming chain on a tooth off woult that not show up on the compression check?

As to the pot only going up a bit. That is only with the engine running. If I lift them with my finger they go all the way to the top and fall back down as they are suppose to.
 
What kind of fuel pump do you have on the tr4? If it is electric and pumping at 10+psi it could pump gas right through the seats and give you this problem.

George
 
I have the stock original fuel pump in there. Never touched that I know of. I am however thinking that the peoblem may be a float needle leaking fuel. When I set the float level in all this it seems to seal OK, but I wonder with the engine running if the seal is giving way.
 
[ QUOTE ]
Adrio, Got it Fixed yet?

[/ QUOTE ]
The short answer is "no". The long one is I worked on it over the weekend a bit and was able to get it running well enough to drive it to storage (all but the last 20 feet) and that is where it sits.

However, from the puttering, I was able to determine with 90% confidence that the problem is ignition. The car only has 800 miles since the rebuild and all the ignition parts were new then. I had swapped in the TR3 ignition, but I think I just had bad luck in that the plugs from the TR3 ended up being bad.

Over the winter I will order all new ignition parts and try again in April.
 
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