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Classic Engines, Modern Fuel

Comments on Topic: pump ticking fast

Submitted by Anonymous
25-Jul-2020

HI,

My fuel pump is ticking verfy fast and irregularily when hot while it is extremely quiet when cold, once per 40 seconds. I guess its is due to fuel boiling somewhere. But I can't explain this observation.

Where is the bubble ? In the pump body, downstream, upstream ? Can you explain this phenomenom ?

Laurent.

Submitted by The Author
25-Jul-2020

Laurent,

I went out in my TC a few weeks ago and got stuck in a traffic jam on a hot day. Like you my pump started to tick quickly and irregularly.

This is amost certainly caused by vapour bubbles either in the pump itsself or the fuel lines running from the pump to the float chambers. Petrol vapour is compressible. With the needle jets on the carburettor float bowls closed, all the pump can do is compress the vapour which "re-expands" when the diaphragm relaxes. As vapour is released into the float chambers and fresh petrol enters the system, the pump will stop clicking until that petrol starts to boil. Hence the irregular clicking.

On the TC the petrol hose from the pump to the carburettors runs through some very hot areas. Across the top of the bulkhead to the rear carb, then over the top of the exhaust manifold to the front carb. Almost certainly the vapour forms in these pipe runs when the fuel flow rate is low.

However, the good news. This is possible a good thing to happen. The Hot Restart Problem is caused by petrol boling in the jet of the carburettor. If it boils BEFORE the float chamber, the vapour is released as the petrol enters the float chamber leaving only the higher boiling point components to enter the jet. These are less likely to biol in the jet and cause problems.

Hope this helps.

Paul

Submitted by Anonymous
27-Jul-2020

Thanks Paul. Interesting rational. But still details that I don't catch in your explanation.

As the diaphragm releases, the bubble cannot re-expand toward the pump because of the check valve so it expands toward the needle jet as the bowl empties ?

Also I don't think the bubble can release from the hose because it is blocked in the highest point of the hose. Carbs are rather low points. So this bubble remains until it cools and condenses. Maybe with fresh fuel entering the hose. It can also release with high flow in the hose, hence not idle.

Well, complex phenomenom but I think we have it. You might know this video :

https://www.youtube.com/watch?v=cadNfSNi_Oc

Laurent.

Submitted by The Author
28-Jul-2020

Laurent,

Thanks for the comment and link to the video.

Although it looks like the bubbles on the video get "trapped", I do not think this is the case. The fuel hoses are quite small diameter and if the fuel is still flowing it will "push" the vapour through. In my case the engine was still running fine which suggests that fuel was getting to the carbs OK.

Thank you for your point about the check valve on the pump. I think you are correct in what you say.
Thinking about the irregular ticking, yes, it is caused by vapour bubbles in the fuel hoses, but not in the way I originally suggested.

During normal operation, the pump will deliver fuel at a constant rate and constant pressure until it empties. Then it will “click” and refill with another charge of fuel.

When the needle valve is closed the fuel line between the pump and float chambers is “sealed”. Should the petrol boil, it will increase the pressure in this part of the system, stalling the fuel pump. However, when the vapour “bubble” is released into the float chamber, the pressure will suddenly drop. On the video, these vapour “bubbles” appear to be quite large and will vent from the needle valve very quickly. The fuel pump will suddenly need to deliver a large amount of fuel. Hence the rapid clicking until the fuel lines have refilled.

Paul

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