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Back firing under load

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Jim Lee

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I am trying to get my 1959 TR3A back on the road where it belongs and I need some advice. It idles very well at 800 rpm. I could leave it there all day idling. I can rev it up sitting in the garage without a load on it but I can also make it backfire badly through both the exhaust and the carb by punching the gas while sitting there. This is after it has warmed up. Which leads me to my first question. I just put a recored radiator in and an 8 blade fan. The temperature gauge used to go up to its 6 o'clock position and stay there once it warmed up. Now it only goes to the mark between the 90 and 185 mark. Which I estimate to be about 132 degrees if the mark actually corresponds to half way between the two marks on either side. I am pretty confident that the temp gauge is working correctly mostly because it used to go right to the 185/6 o'clock mark and stay there. 50 degrees cooler is a big difference I would think and I am wondering if the engine is just not warming up though that is kind of hard to imagine after running it for half an hour.
Another puzzling sign to me is that it does not appear that my vacuum advance is working. Using a timing gun I see little if any difference between having the vacuum pipe attached to the distributor versus not attached. My experience has been that the mark on the crank pulley dances
around much more with the vacuum connected. As far as I can
tell it looks exactly the same. Another very obvious symptom is that every cylinder is running very, very rich. Both times I looked at the plugs they were caked with soot which cleaned of easily but proved pointless as as soon as I replaced them they would cake right up again.
It ran as well as a 47 year old car can run before I took it off the road to recore the radiator, replace the motor mounts and the fan...change all the fluids etc.
I am wondering if what I have described could be symptoms of
the vacuum advance not working either because of the weights in the distributor not doing their thing or the vacuum line being plugged up. That is an awfully small opening on the distributor side of the vacuum pipe. It is definitely connected well to the manifold. I have checked and made sure that the carb dampers are filled. I had great hopes that they were just low and it was leaning way out on acceleration. The plugs tell a different story than that.
One last distinct symptom is that when I gathered the courage to go up and down my street, after idling and experimenting with the timing, it bogged really bad under the slightest load. Is it possible that all this could be attributed to a vacuum advance not working?

All thoughts and ideas, catcalls and strongly worded diatribes most welcome.

Thanks very much,
Jim Lee
 
I had the exact same problem with my TR4A (same motor as your TR3A for all intents). After much hunting I was able to solve the problem and it was that the engine was not getting hot enough. In my case it was due to a poorly installed thermostat. I was amazed a what a difference it made just getting the engine warmer.

I too had plugs that looked rich but the engine seemed to run lean on heavy acceleration (I confirmed this with the colourtune plugs). If you set the carbs per the manual with an engine that is cold you will get the symptoms you describe (plug look and backfire). I know because I had the same issue nagging me for a almost a full driving season.

Look to see what you have done that prevents your engine form getting up to 185. You may have too much bypass around the thermostat, or you may have the good rad and a stuck open thermostat or even the heater core cooling too much.
 
The first thing I would check is the oil level in the dampers. Superficially, it sounds like you have a weak fuel mixture upon acceleration.
 
A lean mixture will cause backfiring. Now is it too lean because your engine isn't hot enuff?
 
Adrio wrote in part:
"Look to see what you have done that prevents your engine form getting up to 185"

I am thinking about what changes were made to the car since it was running great and that is the first, and only so far,
that comes to mind. Before it was heating up to what I assume is the normal operating temperature of 185 degrees as
it has always climbed up there with only a few instances a few degrees lower and a few degrees higher after shut down.
I work in tech support and have learned the hard way many times to look at what has changed since 'it' was running (fine in this case). 50+ degrees is a major difference in the operating temperature. That is the most obvious change I can think of comparing before and after scenarios. I have not plugged up the bypass hose as I have heard recommended elsewhere to help in cooling. I probably have a more efficient radiator as it is totally recored. I do have the heater valve open and I will definitely close that but I have had it open in the past without it making the engine run 50 degrees cooler. I will also check the thermostat as soon as I can to make sure that is operating correctly. It is not terribly cold here in North Carolina in fact it was unseasonably warm today hitting 80 degrees I think. I also agree that it feels like a lean condition when I accelerate under load or not. It is as if I am running out of gas but as soon as I let off the accelerator it goes back to being a fine running engine. Which is why the plugs showing as so rich puzzles me. I also find it hard to believe that the distributor advance not working would have this radical and effect on its driveability but then again I have never had an advance stop working. First thing I did was check and make sure that my carb dampers were full and could feel that
resistance as I put the plunger back in.

So far I am thinking I am in the strange position of having a TR that runs too cool. I hope I find something awry with the thermostat but that would be quite a coincidence that it decided to go at the exact same time that I replaced the radiator and added a much more efficient fan.

Other than checking to make sure that the thermostat is not stuck open is there anything else I could do to get my temperature up? I never thought I would be asking that question on this car.

Thanks all.

Jim Lee
 
You can check to see if the vacuum advance is roughly working by removing the distributor cap, sucking on the vacuum line going to the distributor(I know, crazy - but works). You should see the points plate turning. Secondly, if you can easily suck air through the same hose, you know your vacuum diaphram is bad.
Try troubleshooting a little more. Do an "idiot check". Like, make sure the vacuum lives are hooked up, make sure there are no vacuum leaks around the intake manifold, make sure all the parts of your carburettor are free(I recently had my bypass valve stick - car basically would not run.), floats moving, make sure the slides in your carb are not sticking.


Adam H.
____________________________________________________________
1972 Triumph Spitfire.
 
I was going to suggest the same as Adam H. that you check the advance plate operation by sucking on the tube connected to it. It could also be that the bob weights have started to hang a bit. It has been a little humid around here lately and if you haven't oiled the components below the advance plate they could be gummed up a bit.

I also thought your condition sounded like lean burn. Checking for oil in the dashpots makes sense as does checking the operation and level of the float valve.

However, whatever you do, check things one at a time and don't change 50 things at once. Work through this methodically starting with what you know is wrong (like that really low engine temperature)... one step at a time.
 
Check to make sure the inlet valves in the carb. bowls are not sticking shut and lowering the float level in one or both carbs.
 
I hate to stick my neck out and say this, but in that engine it has been my experience that the vacum advance does not do much. When I was having the same problem I took a vacum pump and checked my advance and found that the diaphragm was leaking. So I changed it. It made no difference. Once I got the thing all fixed (by solving the thermostat problem) I put the old advance unit back on just to see what difference it would make and it made no difference.
 
How long has the car been sitting.......

take a gas sample from the float bowl or line.....yellow? smell like old paint thinner.?

can you check the fuel pressure?
 
So far I am thinking I am in the strange position of having a TR that runs too cool. I hope I find something awry with the thermostat but that would be quite a coincidence that it decided to go at the exact same time that I replaced the radiator and added a much more efficient fan.

Jim, maybe the thermostat was not closing before you did the cooling system repair, but the engine was not running too cool because the old radiator and fan were not working as well as the new ones do.
 
I don't mean to be argumentative, but I don't think it's possible for a car to run "too cool".

Anyone can start their car, ice cold, and with the choke on, drive away right away.

The manifold, carb, and most of the engine are still as cold as the ground they sit on, and yet there is no backfiring.

I would be hesitant to think your problem is cooling related.


Adam H.
____________________________________________________________
1972 Triumph Spitfire.
 
Discovered a few things this evening. The thermostat is working fine. I forgot that I replaced it a few hundred miles
ago and did the pot of hot water test. Works exactly like the
new one I got today. Also the vacuum advance does not work. I sucked on it with my Mighty Mac and absolutely nothing happening. Is it possible to rebuild or replace that diaphragm (sp?). I peaked inside it and it looks pretty corroded and nothing is moving or moveable. I have another temperature gauge that I am going to use to make sure my 47 year old one is working fine. I am pretty confident it is but I want to make sure. I have got the apron off in probably one tenth of the time that it took me a week ago.
Haven't scratched anything yet. I am going to remove the radiator and replace my tr6/tr250 8 blade fan with the original and see what I can see. A friend who knows alot more about cars than I do said that he was not surprised it was running terrible under any kind of load if the temperature was really that low (about 132 I estimate). Also was not surprised that the plugs were getting caked up
immediately as at that low a temperature the gas was probably barely vaporizing. I still cannot think what can be keeping the temp so low other than the fan I replaced.

I'll keep you all posted.
Thanks very much,
Jim Lee
 
[ QUOTE ]
I don't mean to be argumentative,

[/ QUOTE ]Nor do I, but... [ QUOTE ]
I don't think it's possible for a car to run "too cool".

[/ QUOTE ]I'm afraid it is quite possible! Why else would they run better as they "warm up"? /ubbthreads/images/graemlins/laugh.gif
 
You may have stumbled on the problem: vac advance does not work and that causes backfires. I suggest you take the dizzy cap off, remove the rotor button and check to see if the big screw in the middle of the shaft end (the part the rotor button slips over) has been serviced with oil. That is where TR6 dizzy's require oiling from time to time. If the moving parts are frozen there, lube the shaft and work the parts that move until they are free. If the shaft has been regularly oiled, and the moving parts are free, disreguard this email. /ubbthreads/images/graemlins/smirk.gif
 
[ QUOTE ]
I don't mean to be argumentative, but I don't think it's possible for a car to run "too cool".

Anyone can start their car, ice cold, and with the choke on, drive away right away.

The manifold, carb, and most of the engine are still as cold as the ground they sit on, and yet there is no backfiring.

[/ QUOTE ]

That is true but only with the chock on. I am sure if someone with a car running too cold were willing to pull the chock just before each acceleration there would be no backfire (that is what I did to prevent backfire before I solved the problem with the thermostat).

Back to your problem, it does sound like you may have an advance problem as well. It might be agrivated if the advance mechanism does not rotate on the plate in the dizzy. The TR3 and TR4 engine is timed staticaly and the mechanical advance kicks in at extremly low RPM. So if your mechanical advance is frozen that would have the double effect of holding the timing at 4 degrees for all ranges as well as preventing the vacum advance from pulling the timing as well.

When you used the vacum pump did the vacum pressure hold or did it fade right away. In other words is there a hole in the diaphragm? If no hole I would start to look for the plate allowing to move.
 
No more backfiring - But why?

The Latest:
New winter (190 degree) thermo that has been tested in our own
kitchens. New temp gauge. Put back the original fan. Disconnected and plugged vacuum advance pipe because that diagphram is definitely done. No resistance at all when I blow or suck on it. Idles at 800 after some warm up like unto a rock now without what was definitely a vacuum leak at the open diagphram. NO MORE BACKFIRING!!!!!!!! But I have no idea why.

My working theory right now is that when I blanketed my plugs with the layer of soot, however I managed that (maybe by running so cold with the new fan and radiator?) It put the plugs in such a bad way that ignition was very much a hit or miss thing. When it missed it backfired, either through the exhaust or burped in the intake. Had a frightening and frustrating experience in my garage last night when, with the new/old orginal fan reinstalled, I could only run on the starting fluid I sprayed directly into
the carbs. Spray, start, run for about half a minute, then stopped like running out of gas...which I guess it was. Spark/ignition is obviously working great...has to be fuel not getting to the carbs right? Disconnected the fuel line and as soon as I turned on my little Facet electric pump fuel came pouring out. Checked both float bowls. Full up exactly as they should be. Removed float bowls from carb. Flows freely from float to carb. Needle centered and able to push up with the normal amount of resistance provided by my Marvel Mystery Oil in the dampers. Everything ok. Finally remembered that I had an extra new set of plugs. What the heck, it cannot get any worse after spending most of the day getting the old fan back in. New plugs, started right up and ran like the good old days. I do not know the physics involved but those plugs must have only been able to do their thing when starting fluid was sprayed directly in.
The raw fuel coming from the float bowl wasn't doing it. Which is kind of confusing to me because I have run with plugs that looked just as sooty as those I removed. Anyway
that made me feel alot better now that it was running. Had to reseal the thermostat housing so that was it for yesterday. Today, thermo housing fine, no leaks. Did the 'only running with the starter spray twice'. Third time started up and stayed up. Idles great. Wondered what happened to my headlights pulling out of my garage? Ok, no apron..no headlight. Ran up and down the street without lights. Seems to run fine but its hard to tell as it is black as ink out and I would feel kind of silly explaining this to my neighbors..especially one driving down the street. Temperature on my brand new gauge barely touched 160 F. It's about 45 degrees out and my apron is off but I am still wondering how it could be so low. In any case NO BACKFIRES tonight. Did timing after it warmed up and set it to what I estimate is 8 degrees. Idles great.

Last thing I don't get. Promise. I have a 190 degree thermo
in there and I know there was water flowing to my radiator.
How can that be when my thermo gauge, with the sender right before the thermostat is showing 160 tops? More like 150-155?

If I can manage it I will get up early enough to drive around without lights tomorrow morning and see how high I can get the temperature with some real load on it. Or more probably I'll be lazy and put the apron back on after I get home from work tomorrow so I can drive around after work and
not have to wonder about the wisdom of driving an all black
car around on a moon less night sans lights.

Thanks to all you out there with the patience to read all this lamentation. I love and hate that car with the balance
falling on the love side.

Thanks,
Jim Lee
 
Re: No more backfiring - But why?

[ QUOTE ]
The Latest:

I do not know the physics involved but those plugs must have only been able to do their thing when starting fluid was sprayed directly in. Thanks,
Jim Lee

[/ QUOTE ]
Starting fluid has such a low octane rating that it will ignite on heat of compression without any spark, just like a diesel. Not too good since there is no control of the firing/timing point. Sometimes an advantage if there is weak spark or fouled plugs.
D
 
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