Reduction in RPM while idling

rafrover

Member
Hi guys, another weird symptom has came up. Engine will stall after driving for 20mins or so while idling. As you can imagine it's not quite ideal as it happens at traffic light stops and other junctions and it is dangerous. It cruises well and drives well but after a while and I think at a certain engine temperature, I can feel the engine idling speed reducing and the Tachometer confirms it. So usually I get about 600rpm when gear is engaged. But that will go slowly below 500 and slightly lower before it cuts off completely. I often have to pull the choke or put it into P, as at P idling is at about 800-900rpm, while I am at a traffic stop just to ensure it doesn't die.

Any idea why? And any remedy to suggest?
 
Re: Reducing RPM while idling

Those symptoms are normally the result of the engine running weak and improving things by pulling the choke out a bit would tend to confirm that.
 
Re: Reducing RPM while idling

harveyp6 said:
Those symptoms are normally the result of the engine running weak and improving things by pulling the choke out a bit would tend to confirm that.

Thanks Harvey. What's the best way to remedy that?
 
Plus 1 on that however if that is not the cure I also had similar symptoms as this which were related to fuel vaporization.
 
Hi Raf,

The symptoms that you describe could be attibuted to a very weak idle mixture, which can be corrected by richening said idle mixtrue. Another possibility is that the seals have dried out resulting in a weak mixture, even if the actual needle setting was set correctly.

When were the carburettors last rebuilt? When you look closely at the seals around the butterly shafts, can you seen noticable cracks?

Ron.
 
Hi Ron, this very helpful. Thx u. Carbs were rebuilt back in June this year when I bought the car. Al did mention to turn some screws to enrich it but I am trying to find out exactly which screw. I was very worried the engine needs a rebuilt or something but this is good news though. Why do you think this is happening only now? It always idle well before. Which screw should I turn? Is it D?
 

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By the way, there seems to be emission control and non emission control. Mine is a 10.5CR. But how do I know if it's an EM?
 
Hi Raf,

The mixture adjustment screw, certainly on the HIF6 is on the front side. I have never worked on HS6 carburettors, so can't be as much help here as I would like to be.

Does seem odd if they were rebuilt just this year to have gone off so quickly :?

The emission control carbs appeared on both those for the N.Z and for the Australian markets. Poppet valves were fitted into the butterflies, this was not common practice for the U.K market cars, so it is my understanding.

Ron.
 
Hi Raf,

On the mixture adjustment, I have had a look in one of my workshop manuals, and the adjusters appear to be on the underside of each carburettor. I have to say, the HS6 looks a real pain in the proverbial when compared to the HIF6.

However, having read again your original post, I just need some clarification. The loss of idle speed only occurs at operating temps and only after 20 mins or so, is that correct? If that is indeed the case, then as Graeme surmised, I would be looking at a fueling problem, i.e vapourisation. Pulling the choke out when this occurs can sometimes make a difference, but it won't correct the cause of the problem.

I take it that your engine has a mechanical fuel pump installed?

Ron.
 
Hi Ron, yes thats exactly right. The loss in rpm is only after driving 20mins. If I am on D it's obviously worse and the engine can just stall so I often pull the choke or put it to P to get higher idling speed. the fuel line and reserve tap has been done up properly by Al. So power while driving is good and I am using an electrical pump. The pump very often can be noisy though.

Back to vaporisation, will adjusting the mixture or set idling speed higher solve the issue for good or is this a sign of something to come?
 
SydneyRoverP6B said:
Hi Raf,

The mixture adjustment screw, certainly on the HIF6 is on the front side. I have never worked on HS6 carburettors, so can't be as much help here as I would like to be.

Does seem odd if they were rebuilt just this year to have gone off so quickly :?

The emission control carbs appeared on both those for the N.Z and for the Australian markets. Poppet valves were fitted into the butterflies, this was not common practice for the U.K market cars, so it is my understanding.

Ron.

I think i have found the mixture adjustment nut, thanks to the Haynes manual that was gifted by Al. My Rover was as I was told sent to Singapore for British ambassador in the 70s. Later it was sent back to UK and got worked on before it was shipped to NZ. Cant say it is an Emission control version but I think it was made in UK.
 
Just had a read on vaporisation in another thread. I recently started using BP 98 exclusively, last two top ups. Not too sure if this has anything to do with the issue.
 
Hi Raf,

No, adjusting the mixture won't prevent vapourisation from occuring, nor will raising the idle speed. Whereabouts is your electic pump fitted? Is it down underneath fairly close to the tank or within the engine bay? What brand is the pump? Facet?

Using 98 fuel, provided it contains no ethanol, (only specific 91 octane contains ethanol at 10% in Australia, don't know about N.Z) should make it better over lower octane fuels, as the vapour point is at a higher temp in Summer grade fuel.

Ron.
 
Thx Ron, really appreciate ur help. The pump is mounted very close to the windscreen washer bottle. From AA NZ site, BP 98 contains no Ethanol. Pump is HKT Corp HEP02A. So should I be using 95 and try that first?
 
No Worries Raf, always glad to help whenever I am able to :wink:

Now I don't know whether the HKT pump is a puller of a pusher. The type will influence the location. As yours is within the engine bay, we need to do some tests to ascertain whether the pump might need relocating.

Ideally what we do is to disconnect the fuel line after the pump, then with the help of an assistant, switch on the pump and note the fuel delivery, catching the fuel in a suitable container. Switch off after a few seconds. Do the test again, this time when the problem is occuring and note the fuel delivery. My money is on there being a noticable difference in volume, the second test showing rather less.

This will show that the pump needs to be relocated, ideally as close to the outlet of the tank as practicable.

Ron.
 
Ok that makes sense. I had the pump above the carb previously, near the wing at the top of the wheel and recently had it relocated below the carb near the washer bottle and that might have caused it to behave in this way. I will definitely try to put it back where it was and report back soon. Thanks Ron. This is a big help. Feel alive now. Haha
 
Had a look around and there's no technical details on this pump. It looks like the sort which is usually a pusher though. This could lead to vaporisation. The other possibility is that it's actually overfuelling. If it's an injection pump the fuel pressure would be too high. The pump itself is approx $15 and is straight out of china.

If your current pump is under the bonnet, and you don't want to change the wiring, the HUCO pump is known as a good solution and is designed as a puller pump.
 
Another thing you could try is to T off the line at the carbs and run a small bore line back to the tank from there.
This will allow the fuel to circulate and minimise vapourisation under the bonnet. It all depends on whether the pump
is volumetrically up to the task, though, as it will involve more flow required to feed the bypass AND the carbs.
 
rockdemon said:
Had a look around and there's no technical details on this pump. It looks like the sort which is usually a pusher though. This could lead to vaporisation. The other possibility is that it's actually overfuelling. If it's an injection pump the fuel pressure would be too high. The pump itself is approx $15 and is straight out of china.

If your current pump is under the bonnet, and you don't want to change the wiring, the HUCO pump is known as a good solution and is designed as a puller pump.

Hey mate, thx for that. I had a look at Huco. And saw this on a local auction site and it says it eliminates vapour locks. Any thoughts? Cheaper too. :)

https://trademe.co.nz/983698719
 
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