Last time I posted, I had managed to get the secondary throttle on my carby working but still wasn't happy that it was giving full power. I was fairly sure the secondary diaphragm had a hole in it so I bought a new one and installed it today. (should have asked the price first - it was $170).

Well, the power is certainly there now, accompanied by a lovely burble at full throttle even with the air cleaner on. However I still have serious problems related to the vacuum system.

The vacuum line to the secondary diaphragm was connected to an outlet towards the base of the carby on the same side. When I take the line off, I can't feel any vacuum on that outlet at all. Connected this way, the car runs smoothly but the secondary throttle does not operate.

The manual shows the vacuum line to the secondary diaphragm coming from one of the outlets on the thermal valve, so, using a tee piece, I tapped into one of the lines from there. At idle, I can feel good vacuum on this line and when I connect it to the secondary diaphragm, the secondary throttle works and I get good power.

The problem is that the idle then jumps up to about 1300 - 1500 rpm. If I cut off the vacuum to the secondary by squeezing the vacuum line, the idle drops back to normal. (previously it was idling smoothly at 600 - 700 rpm) If I then release the pressure on the line, the idle stays normal until I hit the throtlle again.

In practice, I have to constantly ride the brakes to keep the speed down. I tried reducing the idle speed with the idle speed adjustment knob but then it stalls often. It is no longer smooth and the exhaust smell indicates it is running very rich. A couple of times, when I switched off the engine, it kept running on (dieseling). I figure that the secondaries are still open at idle.

So, the question is, how do I fix this? Should I mess around with reducers or something to get just the right amount of vacuum or is there some other way of adjusting it.

Thanks in advance for any clues!
Coldamus

There should not be any vaccum going to the secondary at idle, what you have done is causing the secondary throttle to open at idle.It relies on vaccum being generated in the carby during acceleration and it also depends on the load on the engine as well.

There should not be any vaccum going to the secondary at idle, what you have done is causing the secondary throttle to open at idle.It relies on vaccum being generated in the carby during acceleration and it also depends on the load on the engine as well.

Yes, I suspected it was supposed to work like that. Does that mean that the vacuum outlet on the carby that it was originally connected to should only provide vacuum during acceleration? If so, I think it might be blocked. I may have to take the carby off and clean everything.

disable the thermal valve. my secondaries work 100% of the time.

I would dismantle the carb or at least take it off and spray some carby cleaner and clear out the lines. Some after market secondaries dont work as well as i had one on my l300 and they wouldnt open up untill 5000rpm.

This evening I put the vacuum lines back where they originally were. I checked the outlet at the base of the carby (from which the vacuum hose goes to the secondary diaphragm). It seems to have a one-way valve because, from the outside, I can suck through it but not blow (at least while the throttle is in the idle position).

This is opposite of what I expected. You'd think that would mean the carby could only blow but not suck the secondary diaphragm.

Anyway, when I put everything back on, the idle speed was easily adjusted back to 700 rpm and the engine runs smoothly and cleanly again. The new diaphragm definitely made a difference because previously in this configuration, the secondary throttle did not work at all. Now it does, although not until higher in the rev range (starting at say 3000 - 3500 rpm) than with the vacuum line going direct from the thermal valve to the secondary diaphragm (say 2000 rpm up).

There is plenty of power but not as much as with the vacuum line direct. Still, it is much better like this than running horribly rich. I was worried about damaging the engine with the dieseling and the raw fuel down the cylinder bores.

Thanks for your help everybody. It is not quite perfect but getting close.