TR6 CP missing on start-up

[Graham S]

Hi all,

I am sure this has been covered many times. I have a 1970 TR6 CP that drives brilliantly and runs smooth at all speeds above idle. However, on start-up it initially fires on only a few cylinders but clears after a minute or so to fire on all 6. It also has a very, to me, rough idle. I have adjusted the pairs of butterfies using an air flow meter which did impove the idle considerably. I did notice that one pair of inlets, cyclinders 1 and 2, gave fairly different flow rated to each other. The head is stage 2 modified and I am unsure which cam it is running. It had the Bosch fuel pump upgrade fitted. Any help would be appreciated.

Graham S

[PodOne]

Similar issues on mine with inconsistent rough idle hot and cold. Traced to wear in the throttle shafts resulting in variable degrees of closure so worth checking. 
 

Might also be due to a couple of the injectors not holding pressure and having to pressurise during the initial start. 
 

Check your HT leads/cap and points are all good before condemning the fuel side.

Andy

[Mk2 Chopper]

1 hour ago, Graham S said:

Hi all,

I am sure this has been covered many times. I have a 1970 TR6 CP that drives brilliantly and runs smooth at all speeds above idle. However, on start-up it initially fires on only a few cylinders but clears after a minute or so to fire on all 6. It also has a very, to me, rough idle. I have adjusted the pairs of butterfies using an air flow meter which did impove the idle considerably. I did notice that one pair of inlets, cyclinders 1 and 2, gave fairly different flow rated to each other. The head is stage 2 modified and I am unsure which cam it is running. It had the Bosch fuel pump upgrade fitted. Any help would be appreciated.

Graham S

You've already done one of the best things to help a rough idle, balancing the throttle bodies.

The other things worth doing are adjusting the valves (tappets), making sure all rubber pipe work is sealed and crack free (not leaking).

Then go through the ignition, plugs, points, condenser, rotor arm, cap and leads, adjust or replace a necessary. Adjust the timing. You may need to set the idle speed again with the bypass valve after doing this. 

Otherwise as has been pointed out there may be wear in the linkage, general engine wear that lowers the compression in one or more cylinders, wear in the Metering unit and so on. 

I'm sure you can get it better though with a good going over the above points. 

Gareth

Edit: I didn't address your initial problem of the missing at start up that clears after a minute or so.

This is pretty typical of a TR6 with P.I. Unless you start it regularly, say every couple of days, it's pretty normal.

It's possible to have hard seals in the injectors that allows fuel to drip out and exacerbate the problem, and again the one way valves at the MU could allow the fuel in the injector lines to drain back.

I tend to start mine and once idling, try and rev it a bit to purge any air, and it usually clears pretty quickly as yours seems to. 

Edited August 12, 2024 by Mk2 Chopper
Additional information

[Steve Morris]

4 hours ago, Graham S said:

Hi all,

I am sure this has been covered many times. I have a 1970 TR6 CP that drives brilliantly and runs smooth at all speeds above idle. However, on start-up it initially fires on only a few cylinders but clears after a minute or so to fire on all 6. It also has a very, to me, rough idle. I have adjusted the pairs of butterfies using an air flow meter which did impove the idle considerably. I did notice that one pair of inlets, cyclinders 1 and 2, gave fairly different flow rated to each other. The head is stage 2 modified and I am unsure which cam it is running. It had the Bosch fuel pump upgrade fitted. Any help would be appreciated.

Graham S

Mine does the same.  

[harrytr5]

2 and 5 tend to go down due to faulty banjo bolts. Moss sells new  ones made by KMI   (Ken Mills Injection). Malcom Jones (then Prestige owner) worked away of dismantling

them to rebuild . Think he took the secret with him. Buy new and problem gone.

Regards Harry

Don't forget those nasty riveted rotor arms. Buy from  a proper source or Dizzy Doc, Martin Jay and have D D stamped on them. lots of fake ones about.

[John L]

When you do a new cold start after a period, hold each injector line in turn and see which one is not pulsing as much as the others, with a weak pulse its usually the little o ring in the injector allowing fuel to run back to the MU, draining the pipe a bit or it it could be the non return valve in the MU doing the same.

Once its running the air will be purged out as you say.

Perhaps some modern injector cleaner may help to allow the needle in the injector to seal better,  dont have proof of this though.

John

[Mike C]

4 hours ago, Steve Morris said:

Mine does the same.  

Mine too. I've accepted it as normal behavior since TR6's largely ceased to be daily drivers. I agree with the idea that once a TR gets air in the injector lines it needs a series of rapid pressure pulses to clear it. I suspect that the long duration pressure pulses, as when the starter is cranking, merely compress and then somewhat release  the air pressure in the lines without letting petrol through.

[Steve 286]

I’m not saying it is but I’ve had cars where the fuel pump Earth isn’t that good by putting an additional earth from  the pump to the chassis may help due to it giving The pump more voltage and inturn more fuel pressure