Petrol odours and carb performance

[mickthefitter]

Do all our old cars smell of petrol inside when they are running? Or is it exclusively ALL the old cars I buy?

My twin-carb auto has always smelled of fuel. It is less bad now than it was, because of all the things it had wrong with it when I bought it. Originally it had a weeping fuel pump, damp float chamber gaskets, wasn't burning fuel properly because of the incorrect distributor advance curve, a blocked tail pipe, and my inability to set the carbs properly because of both of those two factors and more. Now it's got 'rebuilt' carburettors (more like exchanged), fuel-tight gaskets, new piping, the right advance curve, and a functional exhaust pipe. It still smells of petrol though inside the car when on choke and for a prolonged period during the warm-up. Once properly hot, as far as I can tell, the smell goes away (unlike on my fuel injected Escort where I'm now waiting for an opportunity to replace all the injector seals, even though there are no visible leaks on THAT!)

When I was a young driver I cannot remember ANY of the carburettored cars back then smelling of fuel. Cortina, Viva, Marina....BMWs, fwd VWs and the last carburettored car I owned as a 'modern' was a 1991 Toyota Corolla bought in 1995. Every single classic car I've bought since 2003 has either smelled of fuel or carbon monoxide, or both. Obviously fuel is the lesser of two evils there.

Also I want to ask, if anyone knows the answer, is it normal for a twin carb car to 'fluff' when revved unless fully warmed up? Because that is another thing my Marina has always done, both before and after the pre-purchase defects. When it's started on full choke, basically it revs cleanly. But whether being driven or being warmed at rest, as I push in the choke the initial response when pressing the accelerator is a sort of 'flat spot' band where it splutters and fluffs before clearing its throat and revving properly. I get this at every slow down and stop (if being driven) until the engine is properly hot and those carbs have warmed up too, when it seems to work perfectly fine. I'd say also that's when the fuel smells begin to disappear.

So like I say, it had this flat spot when it was running sick, and it's now supposedly running healthily, This twin-carb auto has always had this characteristic. Ac far as I'm concerned my old 1975 single carb 1.8 automatic Marina wasn't like it. The only other twin-carb cars I've owned had six cylinders, a 2.5 litre Triumph and a Jaguar 2.4. The Triumph smelled of petrol a bit but otherwise the engine was it's best asset. I could push the choke in really quickly on that car and it would run fine, and I've read similar accounts like that of driving Triumphs. The Jag was another story, but that was a nightmare for a number of reasons.

So if anyone can please educate me on how a twin-carb 1.8 is supposed to behave, whether in a Marina or an MGB, I'd appreciate it. And also if people live with an accept a bit of a petrol smell in 40-50 year old cars, or if, like I say, it's just the ones I buy.

Mick.

[Kilroy]

I have found that a number of SU carbs I have owned over the years have exhibited the symptoms you describe.
I put it down to wear on the internal parts, and it seems to be a function of the "damper".
When the choke is on there is plenty of fuel for any situation, but without choke the piston will often rise a bit too far too soon, causing a lean mixture situation. I would call it a "hesitation".
Using a thicker oil in the dashpot will often address this, and I play around until it does...

[mickthefitter]

Thanks for your input Kilroy. The carbs, of course were 'rebuilt' (very much in inverted commas) in 2017 before I knew all the issues afflicting my car, like the blocked up tail pipe and wrong springs in the distributor, so are supposed to be unworn, but I was never completely happy with the service I got from the guy who rebuilt them. But it's useful that you are telling me the mixture is too lean when this happens. I wonder what difference the carburettor castings and manifold getting heat into them makes, to the mixture, when this hesitation characteristic disappears? The problem with having an automatic car is that I can't rev it up to clear the carbs before pulling off at a junction or roundabout...going from foot-brake to throttle when I want to pull off during the warm-up period means I just have to get through the hesitation spot until the car picks up, which is what I find most different about this twin carb model compared what I can remember about the single carb 1.8 automatic I had in the 1980s. There's 20-50 engine oil in the dashpots at the moment - I guess I could try gear oil!

[locost_bryan]

SU Dashpot Oil (125ml), Part No: BDP125S

We sell two different types of oil for this purpose which are still made to the original SU factory specifications.

The first is SU Damper Oil, which is a straight 20 grade and comes in a handy 125ml bottle. This oil is for use where the carburetter is fitted with a damper inside the piston and can be used on all SU, Zenith and CD carburetters.

SU Damper Oil (125ml), Part No: BDR125S

The second type is SU Dashpot Oil, which is for use in earlier, pre-war, SU carburetters where no damper is used. In the case of carburetters fitted with dampers the oil should be filled to a level just below the top of the hollow piston rod, so that when the damper is fitted it does not displace the oil any higher than the top of the piston.

SU Dashpot Oil (125ml), Part No: BDP125S

Calver Special Tuning - Techinal Info - SU Carbs Dashpot oil

SU Dashpot oil - manufactured for the specific application of SUs in STANDARD applications. That is on engines that are completely standard as the manufacturer produced them. It is a little on the thin side, but the whole engine tuning data for each engine is based on using this oil. Used as such it works very well, especially where economy is the main goal and the car is driven very sympathetically.

ATF - 'automatic transmission fluid'. A very good substitute where the genuine SU dashpot oil isn't available for use in STANDARD spec engines.

20W fully synthetic engine oil - not to be confused with 20W50 engine oil of any sort. Just a 'straight weight' oil. I use this on practically anything where any modifications at all change the spec away from original. This includes any induction/exhaust changes no matter how small. Largely because the engine is likely to see more 'aggravated' use. Active use of the throttle pedal requires a more stabilizing effect on the dashpot piston along with slightly more resilience to give the desired pick-up. The main benefit of the synthetic type is its consistency over a very broad operating temperature range.

10W/40 engine oil - gives very similar results to the 20W fully synthetic oil as defined above. It is certainly easier and cheaper to get as most modern automotive engines use this oil. Semi-synthetic is best, although fully synthetic is as good but rare in the price/'what you have in the garage' stakes. Standard mineral oil is OK, but gives poorer performance until the engine warms up unlike the synthetic oils as outlined above.

[david painter]

I agree with Bryan I have used 10w40 oil in my dashpots for many years always worked very well
Dave