Bad fuel?

Philippe Holland

Philippe Holland

Member
Hi,
My recently acquired 3500s is running badly. The previous owner had rebuilt the engine (he's runs a garage specialising in Aston Martins) but here is the low down.
The car starts on choke fine but if I try to accelerate the revs are struggling to rise and then there is a backfire. The car barely ticksover without the choke and again trying to accelerate
without the choke gives the impression that the car is not running on all cylinders and is down on power.

I've done the classic first checks, condenser, points, plugs and leads. The engine is a 10.5:1 and the ignition timing is set at -6 BTDC.
When I changed the plugs, 6 were dry and correct and two were darker and damp.
I've checked the advance which is fine and the vacuum works as it should.
Here is the strange thing, no matter how I adjust the idle screws on their stops, I cannot set a high tickover, hence the need to use the choke.
Adjusting the mixture (HIFs) does very little either.
The car had been sat for 34 years and has only done 15kms since the build. The previous owner said the car was running fine....
The fuel gauge does not seem to work and I recently put in 10 litres of fresh fuel as I thought I was empty.
While running the car for a few minutes last night, the exhaust smell was not the usual "rich choke" but a really foul dirty smell and this morning I've got a headache worthy
of a very rude stag night!
The car has an engine mounted Pacet fuel pump, so I am going to run the car from a jerry can of fresh octane boosted fuel, but I was curious to know if anyone
else has suffered from bad fuel and if the 10.5:1 CR engines require any special adjustments for todays petrol.
Many thanks and best regards,
Philippe
 
Hello Philippe,

Sounds to me as if the problem is largely down to old fuel. Might be a good idea to drain the tank completely and fill with new fuel..98 octane only. Then check the idle mixture by using the lift pins. Provided the engine idle drops ever so slightly when the pin is lifted and then returns essentially to where it was then the idle mixture is will be pretty well spot on. Using the choke as you have to do at present indicates that the mixture as it currently stands is too lean, but that could well be down to old fuel.

Ron.
 
SydneyRoverP6B said:
Then check the idle mixture by using the lift pins. Provided the engine idle drops ever so slightly when the pin is lifted and then returns essentially to where it was then the idle mixture is will be pretty well spot on.
Click to expand...

The idle speed should raise slightly and drop back to where it was previously, if it drops it's too weak. (If it raises slightly and stays there. it's too rich.)
 
Hi.
Sorry to hijack this thread -

Just one quick question - when you raise the lifting pin to observe what the revs do, do you raise it and leave it there (and see what happens), or do you raise it and then lower it (and see what happens).
On my V8, when i raise the lifting pins and keep them there, the engine nearly stalls. When i release the pins, it goes back to normal.
What would be the recommended measurement to lift the pins?
Thank you
Glen.
 
Hello Glen,

When you lift the pin to the full extent of travel and hold it there, that is the process to follow. As your engine nearly stalls when you do this, that indicates that the idle mixture is too lean. The revs need to rise ever so slightly and then essentially return to the where they were prior to lifting the pin. When you then release the pin, the revs return to normal.

Ron.
 
Thank you Ron.
Sorry for the late reply, been away for 2 weeks.
I'll give that a go, as i was lifting the pin and releasing it straight away previously.
Thanks again
:twisted:
 
i wouldve thought that you may need to move the timing closer to tdc on modern unleaded,guy nearby has same engine and on 95 octane it wouldnt take the advance,we dont get 98 herei had something similar when a carb top undid itself,good luck,rich
 
That's interesting. I had similar symptoms on my 1977 P6B, and finally found the RHS carburettor to be flooding.

Turned out to be a three-quarters full of petrol float. Interestingly, the petrol is still in the float! Seems as if it entered via a one-way valve.

The engine, with two new floats and needles and seat assemblies, now idles at a whisker above 500 erpm and pick-up is like a turbine.

Here in Victoria, Australia, I use only BP Ultimate petrol in my two Rovers and Jowett Jupiter and, if it is an emergency situation, I apologise to the cars before pumping in 'strange' petrol!

Hope my experience helps.

Mike A.
 
I've been back working on the car and have got it running much better, but discovered a few things that might help others out.
Running the car on fresh fuel helped change the smell in the garage but little else!
The engine would accelerate well with the choke out, but still backfired and would not tickover with the choke in. No matter how I adjusted the tickover screws the car refused to tickover.
Following the workshop manual I set the carb jets (HIF6) to two and a half turns below the bridge and one jet did not seem to go lower than two turns.
I took the carb off and removed the base cover to discover that the mixture screw that winds into a "v" cutout on the bi-metallic strip that pulls or pushes the jet up and down was at the end of it's adjustment.
A slight rebend of the strip allowed me to gain the extra adjustment.
Funny thing was that when I put the base cover back on, even with the depression for the fuel pickup of the jet in the cover aligned properly, it touched the 90°bend at the bottom of the jet and
pushed the assembly slightly upwards and hence made the adjustment weaker.
As it was close to the two and a half turns starting point I carried on with other checks.
I reset the dwell and set the timing to tdc (98 octane fuel). The car still backfired and ran roughly.
I then twisted the dizzy until the engine ran at it's fastest tickover and backed it off a degree or two. Checking the true TDC with a dial gauge in No.1 cylinder showed that the pointer
was 8-10° off. The car is now at about 4° btdc at tickover and running smoother. It was still struggling to accelerate so being late in the evening and getting tired,
I simply richened the mixture on both carbs to maximum (another turn) and lo and behold it now pulls beautifully and I could adjust the tickover on the screws.
I am a bit surprised that I had to go to the limit of the richness adjustment and that the base cover interfered with the jet pickup, but as the car has not been on the road for over 35 years, I'll
run it for a while and then go back and make any final adjustments.
So the moral of the story is: Don't trust the timing pointer to be accurate and adjust the mixture and air screws gently as it does not take much to go past the "sweet point".

Tonight, I'll attack the waterpump (really noisy bearing) and hopefully in a few days she'll be back on the road...
Philippe
 
G'dday Philippe,

What a revelation!

Back in the late 1950s when I was a young apprentice, on tractors and combine harvesters (they are no different), I learnt the hard way too. Our foreman would not allow us to rely solely on manufacturer timing marks. We always timed engines the proper way. If we knew that the TDC mark was correct, we painted the pointer red for future reference. I once found a MG YB to be 20 degrees retarded. I wondered how it ever staggered out of Abingdon!

For just that reason, I now use at all times, a 360 degrees timing wheel disc, a dial indicator and physically check for actual TDC on an engine I have not visited before.

I will get back to fitting new door mirrors to my P6B.

Regards,

Mike A.
 
I have never used the indicated timing marks as the true reference but always prefer to use "right running beat" . Tuning by ear is the only reliable way to get performance from any engine.

Graeme
 
I was searching for an induction hiss on a Standard Flying 12 using the by ear method, I couldn't find it, but I did discover I had tinnitus :LOL:
 
You must log in or register to reply here.
Back
Top