TR2/3/3A backfiring tr3

[Jim Lee]

Hi all,

I finally got my distributor together and my TR3 is idling better than ever. Nice and steady.
Now my problem is backfiring on acceleration under load. I have tried changing the timing and
have gone from about 4 degrees btdc to maybe 12. It does not seem to be related to the timing
as I could not tell any difference. It will not stop running but under load I get some really irritationg
backfiring. I have checked my plugs and they look ok to me.

Thanks very much,
Jim Lee

 

[poolboy]

Your carb mixture might be too lean for the demand.

 

[Jim Lee]

So I could rotate the mixture nut at the bottom of the carb one flat either clockwise or CCW. I will remember when I look at my nut as they were originally one flat
leaner. When I first pulled the plugs the first two plugs looked too rich so I turned both of them one flat leaner. I forgot about that. I will return them
to where they were originally and see what happens. This IS the only way to adjust mixture right Poolboy?

Thanks very much,
Jim Lee

 

[poolboy]

I don't know if one flat is going to be enough if it's so lean it's causing a backfire..
What oil are you using in the dampers..sometimes a thicker oil will give you better results during the acceleration process.
This might provide some help:
https://www.youtube.com/watch?v=NtBHJVhARPI

And
https://www.youtube.com/watch?v=60Bj_2cZQnc

 

[TR3driver]

I don't know if one flat is going to be enough if it's so lean it's causing a backfire..

I agree. Try going a full turn and see what difference that makes. If it pulls better but idles too rich, you may have worn mixture needles and/or jets.

Normally the mixture nuts are the only mixture adjustment required on the early SUs. But it is possible that someone has replaced the springs or needles, both of which also affect mixture (and are used to adapt the carbs to modified or different engines).

You're still using points, right?

Another test worth doing IMO is to use an ohmmeter (or DMM on ohms scale) to check the "end to end" resistance of each spark plug wire. It's possible for them to fail internally and still look fine. And since it takes a lot more voltage to fire a plug under full load than at idle, the problem may only show up under load.

As a last resort, it is worth trying a new cap & rotor. Again they can sometimes fail and cause your problem, while looking perfectly fine.

Random anecdote:
On the way home from VTR 2000, a local club member's car went from running badly to not running at all. There were 5 or 6 of us all trying to diagnose the problem on the side of the freeway. Eventually we discovered that there was spark from the coil, but none at all from the distributor. Rotor was brand new (from the show) and looked the part; yet it was shorting the spark to ground. Even after we knew it had to be the problem, none of us could see any flaw at all except for one tiny spot that _might_ have been just a little bit duller than the rest. After cobbling a replacement from a cut-down TR6 rotor (go figure, none of us had a spare 4 cylinder rotor even though most were driving 4 banger TRs), the engine went back to idling OK. But I thought there had to be a reason for the rotor failure, so I checked the plug wire resistance and sure enough, one of them was bad. Cut the plug boot off (it had fallen apart during examination), stripped the wire back and attached it directly to the nut on the spark plug. It was still running great when we got to LA (although of course Doug replaced the wires & rotor once he got home).

 

[pdplot]

When you say backfiring, do you mean through the carbs or an exhaust backfire (like when you shut off your ignition and then turn it back on). A sticking valve could cause a miss too.

 

[Jim Lee]

I believe it is through the carbs.

 

[pdplot]

Possible causes for a carb backfire :mixture too lean; valve problem, e.g sticking intake valve, possible ignition problem - check point gap, ignition wires, etc. A compression test might show a problem or a vacuum gauge (I'm no expert on that - some of the other guys on this board could give you more help in using a vacuum gauge). Rest assured sooner or later you will solve the problem. Part of the fun of owning these cars is the fact that you can actually fix them yourself, unlike today's electronic nightmares. I have one sure test. I lift the hood, look down and if I can't see the ground, I slam the hood back down.

 

[TimK1955tr2]

I had a similar problem a while back, really loud backfiring through exhaust, sounded like a gunshot. I had changed the points before the backfiring started and found that I didn't install the little fiber washer under the one pin the points swivel on. I think the spark would fire at random times...

 

[Jim Lee]

It is definitely backfiring through the carbs. I have a video that should show this. I discovered that I could do it right in the garage without even having
to put it under load on the street. With the engine up to temperature all I have to do is pump the throttle and you will hear this big hesitation as if it could not get enough air or gas I am not sure. I am hoping you can hear it well in this audio. At first I thought it was a timing issue. I don't think it is because I change the timing alot, dynamically using the strobe light, and it makes no difference. I also put the mixture nuts back to where they were to begin with. The #1 plug looks perfect, #2 a little rich, #3 and #4 very rich. Also I do have a Petronix ignitor instead of points.

You should be able to hear the gasping the engine is doing and then finally at the end you can actually see the backfire through the right hand carb.
For some reason you cannot hear it.

Any hints, tips or observations most welcome.

Thanks very much,
Jim Lee

 

[Jim Lee]

This is not such a great picture but it shows the plugs 1 through 4 from left to right. #1 looks good to me and then they get
rich looking to my eyes.

 

[TR3driver]

Whatever you've got in the dashpots is way too thin. Or else not enough or there is something broken in there. There should be a pause between when you open the throttle and when the piston rises, as it takes time for the air pressure to overcome the resistance of the dashpot plungers moving through the oil.

 

[Jim Lee]

I had Marvel Mystery Oil in the dashpots and today replaced it with 70/89 gear oil. I have used MMO before successfully.
So the carbs are allowing too much air to get in and it is 'choking' with air? They both felt normal to me when I lift the piston
up and it clunks down.

 

[poolboy]

How did they do with gear oil ?

 

[Jim Lee]

That is what is in there now...so not well at all. So you noticed that the pistons were coming up too quickly? Does that mean it is chocking itself off with too much air?

 

[poolboy]

In your video..what I think I heard was an engine that stumbled and missed when you tried to rev it up..I didn't hear a backfire.
While I'm not a big fan of the lift the air valve method of determining the correct mixture I think you need to determine if the mixture is correct.
The spark plugs indicate it is not..
I'm not familar with spark plug recommendations for your car..but make sure you have the correct ones in the correct heat range.
Then see if you can get a smooth idle by playing with the ignition timing..then work on the carb mixture..

 

[MichaelG]

In your video..what I think I heard was an engine that stumbled and missed when you tried to rev it up..I didn't hear a backfire.
While I'm not a big fan of the lift the air valve method of determining the correct mixture I think you need to determine if the mixture is correct.
.

Agree, does sound like stumbling vs. a backfire. Did you rebuild the distributor?...Was the timing set statically then checked dynamically to assure that the advance system (vacuum and mechanical) is operating properly after you worked on it?

 

[Jim Lee]

To me it is stumbling and then periodically backfiring through the carb. At the very end of the video you cannot hear it (which I think is strange because it is like a firecracker going off) but you should be able to see the flash. I will see if I can note the time on the video. What I do not get is that the dynamic timing that I am setting seems to make no difference at all in how it is behaving (badly). I would have thought that changing the timing from what looks to be about 12 degrees to 4 would make a difference. The sound is not very good at all but when I am goosing the accelerator there is definitely something that is not keeping up. When I goose it enough times I get a serious carb backfire. Like I can feel the explosion at the carb. What I can do is punch the throttle and see if I can show you the pistons movement. It sounds like you were saying that they were moving (up) too quickly in the video that you saw.

 

[Jim Lee]

One of many things that confuses me is that my plugs look rich, like they are getting too much fuel, while I believe that backfiring out of a carb
signals a lean condition. Also that after the car is fully warmed up I can stall it out very easily by revving it up too quickly without even any load
being on it. It acts like a cold engine though it is at normal temperature.

I am going to put thicker oil in the dampers but I would think that this would lead to this same condition where something, whether it be air or fuel and
I am thinking fuel now, is not responding as quickly as it should. I am really puzzled by the timing changes not making any difference.

Also, what is "the lift the air valve method of determining the correct mixture"?

Do you all agree that backfiring out of the carb indicates a way too lean condition?
If you look at that video you should be able to see a flash from the area of the right carb.
There was a backfire sound at the same time though for some reason my camera videos audio
did not seem to pick it up.

Thanks very much,
Jim Lee

 

[TR3driver]

One of many things that confuses me is that my plugs look rich, like they are getting too much fuel, while I believe that backfiring out of a carb
signals a lean condition.

The mixture is by no means constant, so yes, this is quite possible. In particular, it is possible for the needles and/or jets to wear (from improper centering or a worn out dome/piston assembly) such that the idle mixture is way rich compared to the cruise mixture. Then since the mixture is normally adjusted at idle, the cruise mixture winds up way lean. In more severe cases, the mixture nut may not have enough range to lean out the idle mixture enough! (My buddy had that problem with his MGA.) Or, the lean cruise mixture may cause persistent overheating (which I now believe is what caused my Dad's TR3A to overheat and eventually swallow a valve).

Also that after the car is fully warmed up I can stall it out very easily by revving it up too quickly without even any load
being on it. It acts like a cold engine though it is at normal temperature.

I am going to put thicker oil in the dampers but I would think that this would lead to this same condition where something, whether it be air or fuel and
I am thinking fuel now, is not responding as quickly as it should.

Well, there is a definite balancing act there. The problem is that fuel has much more inertia than air does, and so it takes longer for the fuel to change speed to exactly match the forces moving it. So when you first open the throttle, the air starts moving faster through the venturi almost instantly, but it takes some time for the fuel to start flowing faster. The force that pulls fuel through the jet is very small. And too rich is far preferable to too lean. (You actually want the mixture to go richer at that point, for better power.)

I am really puzzled by the timing changes not making any difference.

The backfiring is caused because the mixture did not burn (at least not quickly) when the spark happened. So changing when the spark happens isn't going to make any difference.

Also, what is "the lift the air valve method of determining the correct mixture"?

This is the method given in the manuals for setting the mixture (hopefully you can read this image after clicking on it).

Do you all agree that backfiring out of the carb indicates a way too lean condition?

There are other possibilities, but very unlikely IMO. Your video sounds exactly like a "lean bog" (which is when the mixture goes lean as you open the throttle).