Club Triumph

My TR6 is popping/back firing under load after running for a while.  Garage have measured high hydrocarbon on 1-3 cyls and suspect this is cause (2000 compared to 500 on 4-6 cyls).  Compression on all cyls (engine recon some years ago but only 3k miles done) is even and good. Oil pressure great.  Any clues anyone?  Don't want to start a witch hunt/engine strip down if its likely to be something else.  Carbs are recon set HS6 SU and appear to be OK.  Also garage found a score mark on no 1 cyl, not sure if this is an issue since compression OK.

Mixture too rich especially as it is running on carbs and affecting cylinders 1 - 3. How were the garage so sure? So presumably one carb set too rich. Could also be oil perhaps a sticky piston ring from not much use but if compressions are okay that may be a blue fish.

The high hydrocarbons is the RESULT of the misfire, not the cause.  The hydrocarbons measured are the unburnt fuel.  Do you know whatthe CO reading was at the same time?  Low CO with high hydrocarbons is either an ignition misfire or lean misfire, high CO with high hydrocarbons suggests very rich.

Presumably they are able to measure emissions from 1 - 3 separately due to twin exhausts?  As you are running on carbs, problem with 1-3 indicates issues with the front carb.  Popping and spitting back under load suggests going lean - maybe a sticking needle valve or restricted flow to that carb?  If that carb is fed second from the fuel pump then it is also possible that not enough flow is being delivered by the pump.

Nick

Thanks for ideas - Yes - HS6 SU carbs -  and yes twin exhausts.  I replaced float valves with gross jets (ball type) to stop flooding, since old valves kept sticking, randomly.  As far as I know full supply to carbs is Ok, but will check front carb now which is fed first - unless garage got wrong exhaust/cyl numbers...
CO was even and "OK" but don't remember figures.  Garage tried switching needles and that didn't make difference.  Will at least concentrate on 1 carb now or 3 cyls (leads, lugs, etc....) or flow to that carb.

As Nick says the high HC reading is the result, not the cause. This, as he also says points to a mixture problem with the front carb. Whether too rich or too lean you will get high HC (I would suggest that a HC reading of 500ppm on the rear carb is too high also - you would expect around 400 at idle dropping to below 250 at 3000rpm if all is well) so the CO reading needs to be known to ascertain whether it's too rich or too lean for sure.

You could just try the old fashioned way of finding out too rich/too lean i.e.  richen it up and see what happens/lean it off and see what happens, but it sounds more likely to be too little fuel, though not deffo. If it is too weak don't discount the possibility of an inlet leak on the front carb. This could be a split breather, badly worn throttle spindle (less likely), duff gasket on carb body to manifold or even manifold to head gasket, esp if the toggles holding the manifold to the head aren't located correctly (I've had this one a lot as they're tricky to fit and see).

Good Luck

Thanks - should know more later today when I get an update report from the garage.

Well garage got hydrocarbons down to 600 by mixture and timing and tappets adjustment.  Still popping when hot and under load though.  They are trying new leads, plugs, have tried a different coil.  When I get it back I have a gt6 dizzy to try.....  Now also worried about the fact I reused the existing short inlet manifold, maybe will try a long reach one since carb float chamber is too close to steering column, and I believe the long reach one offers more power.  Plugs were BP6E (?) should I try ES?