Vapour lock - more likely flooding due to malfunctioning float chamber vent valve
I have read the thread with interest. I had the same problem and it turned out to be either a badly adjusted, or sticking, float chamber ventilation valve. the owners club magazine published a letter from me on this in April or May (I think) 2010. It was also included on the forum, but seems to have been lost in the change to the new system (or maybe I just can't find it). At the risk of cluttering up this thread, I am going to attempt to add it here:
I have been very interested to see the topic of poor starting of a hot engine rear its head again in the February and March 2010 issues of the magazine. The first, which is, incidentally, a repeat of a previous contribution from the former Kingpin column, suggests that the problem is due to vapour lock and solutions include pouring cold water over the carburettors (quite impractical) and re-routing the fuel pipes. Although modern fuels may vaporise at a lower temperature than fuels in use when the car was new, this is hardly likely to be the problem in England even in summer. Before I got to the bottom of the problem, I tried re-routing the pipes, including insulating them with heat reflecting tubing from Demon Tweeks, but to no avail. The second suggests that the problem can be cured by adjusting the temperature compensators. Both suggestions are for the most part misleading. Well here it the bottom line in my experience:
(a) Float-chamber ventilation valves. The major reason almost certainly lies with the float-chamber ventilation valves, either because they are stuck or not properly adjusted, usually the latter. In either case the vapour that accumulates above the fuel in the float chamber cannot escape and so forces fuel out of the chamber up the jet tubes thus flooding the inlet manifold. Instructions on how to adjust the valves are in the Service Training Notes (see Triumph Service Training Notes 1973 Emission Control Systems, pp 28-29 at: http://www.mv.com/ipusers/last/emissions/booklet.pdf).
Sticking can be alleviated by putting a little WD40 down the vent port which is the tube on the carburettor that leads off to the air filter. It should also be noted that it is wise to have functioning, properly adjusted, bypass valves, whose primary purpose is to prevent the mixture going weak on engine overrun (see Service Training Notes, pp 49-50)
(b) Temperature compensators. In terms of poor starting when hot, adjusting the temperature compensators is a bit of a red herring, because the decrease in the richness of the mixture on going from a compensator fully closed to one fully open is small and is unlikely to have any significant effect on the starting. They should, of course, be properly set up and matched one to the other to achieve similar richness of the mixture at hot on both carburettors, (see The Vintage Triumph Register - The Art of Temperature Compensator Adjustment http://www.vtr.org/maintain/temp-compensators.shtml). Anyway, on cold, the cone should seat properly, but in some remakes does not. Use good originals.
(c) Needle valves. It is of course worth checking that the needle valves in the float chambers are not letting by. Use good ones from Burlen fuels with viton tips, not cheap remakes or ones with ball valves. And also be sure that the floats are not punctured and thus fail to float.
There is a thread on this topic with references on the Forum at:
May I conclude by saying how much I like the return to dealing with technical issues in the magazine. It makes it, once again, a pleasure to pick up and browse and the format and standard of production are excellent.
David Bowyer
I have read the thread with interest. I had the same problem and it turned out to be either a badly adjusted, or sticking, float chamber ventilation valve. the owners club magazine published a letter from me on this in April or May (I think) 2010. It was also included on the forum, but seems to have been lost in the change to the new system (or maybe I just can't find it). At the risk of cluttering up this thread, I am going to attempt to add it here:
I have been very interested to see the topic of poor starting of a hot engine rear its head again in the February and March 2010 issues of the magazine. The first, which is, incidentally, a repeat of a previous contribution from the former Kingpin column, suggests that the problem is due to vapour lock and solutions include pouring cold water over the carburettors (quite impractical) and re-routing the fuel pipes. Although modern fuels may vaporise at a lower temperature than fuels in use when the car was new, this is hardly likely to be the problem in England even in summer. Before I got to the bottom of the problem, I tried re-routing the pipes, including insulating them with heat reflecting tubing from Demon Tweeks, but to no avail. The second suggests that the problem can be cured by adjusting the temperature compensators. Both suggestions are for the most part misleading. Well here it the bottom line in my experience:
(a) Float-chamber ventilation valves. The major reason almost certainly lies with the float-chamber ventilation valves, either because they are stuck or not properly adjusted, usually the latter. In either case the vapour that accumulates above the fuel in the float chamber cannot escape and so forces fuel out of the chamber up the jet tubes thus flooding the inlet manifold. Instructions on how to adjust the valves are in the Service Training Notes (see Triumph Service Training Notes 1973 Emission Control Systems, pp 28-29 at: http://www.mv.com/ipusers/last/emissions/booklet.pdf).
Sticking can be alleviated by putting a little WD40 down the vent port which is the tube on the carburettor that leads off to the air filter. It should also be noted that it is wise to have functioning, properly adjusted, bypass valves, whose primary purpose is to prevent the mixture going weak on engine overrun (see Service Training Notes, pp 49-50)
(b) Temperature compensators. In terms of poor starting when hot, adjusting the temperature compensators is a bit of a red herring, because the decrease in the richness of the mixture on going from a compensator fully closed to one fully open is small and is unlikely to have any significant effect on the starting. They should, of course, be properly set up and matched one to the other to achieve similar richness of the mixture at hot on both carburettors, (see The Vintage Triumph Register - The Art of Temperature Compensator Adjustment http://www.vtr.org/maintain/temp-compensators.shtml). Anyway, on cold, the cone should seat properly, but in some remakes does not. Use good originals.
(c) Needle valves. It is of course worth checking that the needle valves in the float chambers are not letting by. Use good ones from Burlen fuels with viton tips, not cheap remakes or ones with ball valves. And also be sure that the floats are not punctured and thus fail to float.
There is a thread on this topic with references on the Forum at:
May I conclude by saying how much I like the return to dealing with technical issues in the magazine. It makes it, once again, a pleasure to pick up and browse and the format and standard of production are excellent.
David Bowyer
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