Vapor lock

[JeffMoore64]

Gents,

Took my TC out for a spin after letting her warm up for about 10 - 15 minutes. Got about 1 mile from home and car died. Turn her off and back on and noticed the fuel pump as just ticking away of course it wouldn't start. After about 45 minutes and a short tow home the ticking stopped and she started right up. Can only assume it was vapor lock.

Question; what is the best way to prevent this from happening, and when it does what is the fastest way to "unlock" the carbs?

TIA

Jeff

[Steve Simmons]

In my experience, it shouldn't happen while you're moving. The only time it should possibly happen under normal circumstances is sitting motionless for extended periods of time in very hot weather, or a few minutes after parking when very hot. I've driven many times in temperatures over 110F without issue, as have several friends. That is, until I shut it down. In weather like that it's a good idea to open the carb / manifold side of the bonnet when you park unless you plan to stay there a while, in which case it will cool on its own with the bonnet closed.

Make sure there are no leaks on the intake side of the fuel pump, and that the fuel cap is venting properly! If it happens again, try cooling the fuel pump and see if it begins to pump again. The old trick is to slice a grapefruit in half and stick it on the pump. I'm sure there are better ways though.

[JeffMoore64]

Thanks for the comments Steve,

No leaks on either side of fuel system, I did open the gas cap to vent when I was having the issue so that could not have been it, I didn't feel the fuel pump to see if it was hot so that is a possibility. However when the car wouldn't start, presumably from a no fuel situation, when I turn the key off and back on the fuel pump was just ticking away like it either could pull no fuel (tank is half full/empty). So maybe vapor lock is the wrong thing to think. Anyway going to go drive it around this afternoon and see where I get stuck. Hopefully giving it a rest will get me home!!

Cheers,

Jeff

[Duncan M]

Jeff, 2X everything Steve says. Remember that clogs in the inlet side of the pump can burn up your fuel pump; same reason advice for any additional fuel filter when using the SU pump is to always put such filter after the pump-- before the carbs. That said, there is a course mesh screen at the fuel outlet, inside the tank. Rust flakes etc from inside the tank can build up over that mesh, and often advice for problems like yours is to use compressed air in the main line- back to the tank- to break up the accumulation and allow free flow; a temporary/diagnostic solution. If the mesh screen breaks, of course the flakes can then go on to clog up the main fuel line to the pump. There is also a screen filter built into the SU pump, and can be got to by removing a hex plug type fitting.

Often with true "vapor lock" the engine can be started by giving some choke, just until it fires up.

Pics to give a general idea, actually TF outlet?

screen.jpg (8.46 KiB) Viewed 6801 times

[JeffMoore64]

Thanks Duncan,

Couple points for discussion:

The fuel screen in my tank is missing, which is why I've installed an inline fuel filter back near the tank. That filter is clear so I can do a visual, it is clean as a whistle. I just recently lined my tank with POR15 so that filter better stay clean!!

I'd not thought I'd need a filter after the pump if I had a good filter before it. I can see the need if there was only a screen prior to the pump.

As I was the one that put the inline filter in, I probably should check the screen in the pump. I do know the pump was replace sometime in the past as I have the old one in a box of parts that was passed along to me.

I did take the car out for two errands today totaling 4 miles and a stop at each place. Car ran fine. So still a bit puzzled.

Jeff

[Duncan M]

Then maybe it is something after the pump. Each carb assembly has a simple screen filter (and wound wire spring thing) to strain fuel as it comes out through the fuel lines into the float bowls. That could explain the ticking pump that kept ticking, as it tried to fill up the bowl, past a partly clogged shut screen.

[Steve Simmons]

It's possible the pump itself has a problem. You could put a spare in and see what happens. Returning to the air leak possibility, even the tiniest leak can cause problems so even if there is no fuel coming out, air could still be getting in.

[Richard Michell]

I would disconnect the fuel hose coming out of the pump, immediately downstream of the right-angle fitting on the pump outlet, and attach a length of clear plastic pipe to that fitting. Hold it reasonably vertical, reach over the screen and turn on the ignition. You will see (and hear) almost immediately if the pump is pumping. If it is, turn off before you fill the pipe, put a finger over its upper end, disconnect the other end and return the contents to the tank. You may have gathered I have done this a few times.

[JeffMoore64]

I didn't know that each carb also has a screen, more items to check now.

So every time time I go to start the car, I turn the key and wait until the pump stops ticking (4 - 5 seconds) until I try starting it. Is this normal or does this point to a possible air leak in the system? I just figured the fuel siphoned back to the tank a bit. Maybe not. I don't have a spare working pump and would rather not invest in one if I don't have to.

[Richard Michell]

Jeff I had waited for someone more expert to answer your question but here goes. To me, a few seconds of ticking as you describe is quite normal for a car that has stood a little while, provided it stops! It is almost the signature of these cars (and others with SU pumps and carbs). I don't believe it is an indication of an air leak. It is just that some fuel is lost from the carb float bowls while standing - via evaporation out the vents and, for some of us, via a fuel leak around the jet assembly. The pump fills the bowls again when you turn the key.

Are the lower portions of your carburetors dry after a short run, particularly at the jet adjusting nuts?

If you have 4 - 5 seconds of ticking after just a short run and a short stop then there is some other problem. It should be only one or two ticks in that circumstance.

[JeffMoore64]

Can you send me a diagram or photo of where the adjusting nuts are? I've spent no time working on them so that is a new area for me.

[Richard Michell]

Sorry, can't send a photo at the moment. The usual leak shows up as wet fuel at the lowest part of the carbs where the choke levers attach (not the fuel bowls). These levers attach to the lower ends of the jets - the slim metal cylinders with yokes on the bottom end, where the choke lever attaches via a pin. Move your eye up the jet and you will see a biggish nut with a coarse spring above it. That is the adjusting nut. A common leak is down around the jet as it passes through the carb body, fuel appearing on the nut below and in that general area. It is usually readily visible and easy to touch to confirm.

To locate the area pull the choke out then in and watch for the jets to move down (out) then up (in). There is one on each carb.

[Chris C]

If you search for SU Carb adjustment you will find plenty of info and diagrams:

http://www.mgexp.com/article/su-tune.html

As previously suggested disconnect the feed to the carbs and check that the pump is delivering fuel.