TR6 1975 TR6 Backfire from Carbs, running Lean - Help!

[MZeraldo]

Hi, I recently purchased a 1975 Triumph TR6, very nice original car. After driving the car around the neighborhood, I have noticed that under acceleration at low rpms (1000-1500rpm) the car often coughs like it wants to stall and will also backfire out through the carbs. Once above 1500rpm the car feels good with no hesitation or backfiring. I am looking for suggestions on what to do next to resolve this issue. Please note that the pollution equipment has been removed from the car and all vacuum lines have been plugged.

These are the things I have done so far;

- removed air cleaner

- removed the spark plugs, and noticed that they are running extremely lean showing a white deposit, signifying mixture is way to lean. I re-gapped to 0.25, cleaned and reinstalled.

- replaced the fuel filter (how much fuel should be in fuel filter when the engine is running?)

- replaced a rear flex fuel line which was leaking

- this car has a petronix electronic ignition system with flamethrower coil, I tried removing the vacuum advance line that goes to the rear carb intake manifold. This seemed to help a little bit, backfiring is not as prominent when this is disconnected.

- adjusted the mixture screws (fine tuning) on the side of each carb, were reset to '0' and were turned out 3 turns. Made little difference.

- the pistons in the carbs move freely up & down, the diaphragms have been replaced and the previous owner says the carbs have been rebuilt. (Professionally or by home mechanic??)

This is what I was going to attempt to try next to eliminate the backfire;

1. Check for adequate gas flow to carbs

2. Synchronize carbs

3. Check the iginition timing (not sure how to do this with an electronic iginition.... is it the same as if it had points & condenser?

4. Check manifold for cracks or leaks using aerosl carb cleaner spray.

5. Reset the jets in the carbs for a richer mixture.

Any suggestions would be greatly appreciated!

 

[NutmegCT]

M - welcome to BCF!

Just wondering, how did the car behave when you drove it before buying it? Did the owner make any comments on engine problems?

BTW, you should open a shop. Your troubleshooting is *impressive*!

Tom

 

[MZeraldo]

I was very gentle with the car when driving it for the first time, & I did notice the odd backfire, but the car runs so well otherwise, it did not concern me.

 

[poolboy]

You do have a couple of misconceptions.
There is no Vacuum Advance..it's a vacuum retard and the vacuum to make it work should come from the bottom of the front carb.
The mixture is adjusted by changing the depth of the needle in the air valve.. There is no adjustability in the jet.
The idle trim screws have virtually no effect on the mixture at any speed other than idle and that is minimal at best.
As far as the rebuild, if it were done correctly, the carbs would not be an issue.
I think I'd want to have a look for vacuum leaks, any place other than the mouth of the carbs that might be sucking air.
If I didn't find any, I'd make sure the air valve diaphragms are installed correctly with both the inner and outer index tabs in place.
Here's some reference material. You'll want some good repair manuals as well.
https://www.buckeyetriumphs.org/technical/technical.htm

 

[dklawson]

This may sound trivial but did you check that the carb dashpots have oil in them?

The TR6 uses different Strombergs than I normally work with. However, screws on the side are probably for air bleeds for emissions. I think that with the carbs you have, mixture is adjusted with a special hex wrench tool inserted down from the top through the dashpot oil (others will have to confirm that).

You could also be experiencing a fuel problem due to a restricted float valve or the floats may be set too low. I know it smooths out after 1500 RPM but if the fuel level in the bowl is too low one of the carbs just may not be able to keep up with demand during acceleration.

You asked about the fuel filter. They do not have to be full (and seldom are full). If you turn the filter so it is fully vertical with the engine running you will be able to removed the trapped air but when the car is parked and the filter is horizontal it will fill with air/fumes again. This is not a problem for the filter.

 

[TR3driver]

Please note that the pollution equipment has been removed from the car and all vacuum lines have been plugged.

I would definitely be reviewing that; since some of the ports must be left open to air or you will have mixture problems. For example, the idle float bowl vents (which normally connect to the carbon canister) must be left open to the air or the idle mixture will be screwed up. You also talk about the vacuum module being connected, even though it is part of the emission equipment, and connected to the wrong place. All of which suggests to me that there may be other mistakes.

- replaced the fuel filter (how much fuel should be in fuel filter when the engine is running?)

As long as the outlet is covered, there is "enough". That air you see is effectively trapped (until the outlet is uncovered) and does no harm. It "should" be there.

- the pistons in the carbs move freely up & down,

Actually, they shouldn't move freely upwards, unless the damper rod has been removed. Sounds like they might be out of oil, which would cause symptoms similar to what you describe. Try topping them up (with ordinary motor oil) and see if that helps.

3. Check the iginition timing (not sure how to do this with an electronic iginition.... is it the same as if it had points & condenser?

The simple answer is "yes", just hang a timing light on it. However, don't use the 4 ATDC timing figure if you have disconnected the vacuum retard or it doesn't work. The initial timing without the retard should be closer to 10 or 12 degrees.

It's also worth reviewing the ignition installation, to see if it is correct. The Pertronix module wants to get full battery voltage but there is a dropping resistor built into the later TR6 wiring harness. A proper installation will have the resistor in series with the coil (or a coil that does not require a resistor), but not with the module.

PS, I see that Doug snuck in while I was typing. If you do get the special tool and try to adjust the mixture down through the center of the dashpot; note that the adjustment has a very limited range, and don't try to force it. The hex socket is in a special brass screw, which strips easily. If you go too far the other way, the needle holder will fall off the screw and go full lean, which will necessitate taking it apart to put the adjustment back together.

 

[MZeraldo]

Thanks for the feedback, it certainly helped clarify many of my questions. I have checked the diaphragms, they have no holes and are seated correctly. I will look into vacuum leaks. The dashpots were full originally, however I currently have drained the oil from the dashpots as I thought they may have used incorrect oil & would like to adjust the needles next. I will refill once I have adjusted them.
Will also check the floats, is there a recommended level they should be set at (17-19mm)?

Thanks,
Mack

 

[poolboy]

You can compromise at 18mm
One more thing from me.. See this nipple that I have highlighted in red.\

Do not cap or plug them.
Either let them stay open to atmosphere like the picture or connect them to a nipple on top of the Carbon Canister like you see in the Emission Control page in the Moss catalog.... either online or paper version.

 

[MZeraldo]

Hi all, just thought I would give you an update.... went out and adjusted the needles using the special tool, and the car runs like brand new! No more backfire/popping. Now just need to synchronize them.

I want to thank everyone for their input, it was greatly appreciated!

 

[dklawson]

Synchronizing is generally done first so each carb is flowing the same amount of air before you adjust the mixture. Once you synchronize them, go back and check the mixture adjustment to make sure it is still correct. Regardless, I'm glad you got the problem sorted.