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You are here: Experts > Home/Garden > Auto Repair > Triumph Repair > TR6 Backfiring In Carb
Expert: Howard M. Fitzcharles III - 11/1/2009
Question
QUESTION: Greetings,
'75 TR6 started acting up last spring. Noted abrupt high speed misfiring in spring with engine warm and outside air temp cool to cold. Engine generally hesitates on accel when cold. As well, lots of "burble" or popping on decel. The plugs on cylinders 2 & 3 have been showing brilliant white, 1 a little whitish and 4 through 6 proper tan. Comps 135# plus on all cylinders except #4 which is 90#. Wet compression brings it up to 120# indicating a ring issue. Ignition is pertronix electronic with idle set neutral at 2 Deg BTDC @ 850-900 RPM as I am running unleaded 91 octane fuel. Vacuum advance is working as I get 1250-1300 RPM with the vacuum line off the carb. Exhaust system is Monza Pacesetter free flow. Carb cleaner leak test on manifold joints and throttle shaft spindles is unremarkable. 1/4 inch air valve lift tests results in stall. I just rebuilt the carbs and have only the air bypass valve spring left to set (currently fully closed). Both temp compensators are free to move/clean and crack open at nominally 100 deg. I have good fuel level at the filter (which was changed) and a new fuel pump in summer '09. Needles have been set to full rich (full up) but I am still getting reverse flow through the carbs (particularly the front carb). Carbon cannister is still installed but the anit-run on valve is not installed. EGR valve looks to be the original. Haven't tested it (nor am I sure how one does that). It seems to be worse as the engine warms up and as I move the throttle off idle. I know I'm getting good fuel flow as the reverse flow through the carb gives me a nice wet blast on the wheel well. Everything points to a fast burning (ie lean) mixture.... but I'm at a loss. Any things you can point me towards?
Thanks
ANSWER: HI Nelson,
The lifting of a piston in the carb and the engine dies is an indication of a lean condition. The needles adjusted all the way up and still lean is a real problem. Your one dead cylinder is a real problem too and the engine will never run right as long as that cylinder is dead. (100 PSI and below don't even run)
Other than all that, I believe you have an EGR leakage problem. Remove the EGR valve and hold it open by hand and either blast it with a Spark plug cleaner or a sand blaster to try to clean the carbon out of the valve and check that it does not have manifold vacuum applied to it. It should only receive ported vacuum and that is suppose to be through a flame trap.
Howard
---------- FOLLOW-UP ----------
QUESTION: Thanks Howard. I will pull the EGR and try as you suggest. I did some more headscratching and bought a timing light with a digital tach. With that, I have also confirmed I do indeed have some leakage at the spindle seals. I get a drop of 25-40 RPM on the rear and approx 75 RPM on the front carb with the carb test spray test. I used some tape to seal off the shafts temporarily and the problem disappeared. So I will be pulling the carbs to install shaft seals Ididn't come in my overhaul kits unfortunately).
Now to decide how to deal with that one bad cylinder....
Thanks
Answer
Hi Nelson,
That is not normal to have throttle shafts leak so bad as to run out of needle adjustment. I have worked on cars with very badly worn throttle shaft bearing surface and shaft which did effect idle only but never seen one leak enough to use up all of the needle adjustment.
As for the low cylinder you need to run some other tests on it and run the "Wet %26 Dry" test again.
Get one of those fittings that adapts an air hose to the spark plug hole and put that piston at TDC on the compression stroke and put it in 4th gear with the hand brake on hard or someone with the foot brake on and put line pressure in that cylinder and open the throttle and listen in the carbs and got to the tail pipe and listen there for any hissing. (none is acceptable) then open the radiator cap and watch for coolant raising. Then open the oil filler cap and listen to the hiss in the pan. All engines hiss into the pan so note the amount and sound. Then run the same test on a known good cylinder to note the difference. Then make the decision what to do.
Howard
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