3 Litre inlet manifold

Robert Zannetti · Guest

Yesterday I was working on my 1925 Speed 3 litre and started the engine. My car works on a small fuel pump. When I accelerated the engine, and without sufficient choke, it fired back through the Slopers but I noticed a small lick of flame come from the take off on the top of the manifold that is there for the Autovac. Examining the fitting that was meant to block this opening I noticed a hole 1-2mm dia. How long it has been like this I do not know. Am I correct in thinking that most of the time air would have been going into the manifold and would weaken the mixture? I have now turned up a brass plug that fits into the captive nut and provides an air type fitting. Any thoughts?

John Murch · Posted: Thu Jan 18, 2018 1:24 pm Post subject:

As you suggest any airleaks can have an effect on mixture. But It might be that the blow back cleared the hole that you observed having been previously blocked with goo.

John

Robert Zannetti · Guest

Thanks John and the point you make is a good one. I had been happy with the running of the car except it did seem to take a long time before it would run smoothly without choke and I could then advance the ignition. Some times this would be several miles or up to ten minutes even though the water temperature was up to normal operating temp. When the weather is a little kinder I will take it out to see if there has been any improvement. I will run a colour tune test to check the mixture on each cylinder.

John Murch · Posted: Thu Jan 18, 2018 5:24 pm Post subject:

Your comments re choke usage are Interesting, my car, also on slopers, requires choke to start from cold - But I take off the choke immediately, or after a few seconds.

Chris Card · Guest

Like you, John, from cold I start on choke, but wind it off as soon as the engine fires.

Chris

Robert Craven · Posted: Fri Jan 19, 2018 11:56 am Post subject:

Same here. I wouldn't intentionally run with the choke for several miles or 10 minutes. It would be bound to soot the plugs.

As to the Colourtune, why not first try the test of lifting the piston slightly?

Dan Suskin · Joined: 22 Feb 2011 Posts: 298 Location: Georgia, USA

Same here - start on choke and wind off immediately.

I have never found the Colourtune to be very useful - perhaps my color perception is not good enough. What I do on all my cars now is use an AF meter. On one car the sensor is permanently mounted (out of site completely), but on the others I just shove it into the exhaust about a foot or so and then adjust the carbs as needed.

Chris Card · Guest

Bob - which needles do you have in your Slopers? If you have the BC needle, which is the “weak” one, that may require more choke running. The standard needle is CV.

Chris

Paul Spencer · Posted: Fri Jan 19, 2018 10:10 pm Post subject:

I find I run on choke for a mile or so in cold weather. But then, I don't leave the car idling for long before setting off, and I don't have a thermostat.

Like everyone else I know who has been to a tuner, I switched to K needles. These run much the same as CV at small throttle openings, but richer at wide openings. For me, they improved the performance without affecting fuel consumption. You used to be able to get needle comparisons at mintylamb.co.uk, but this is not currently working while the code gets ported to a new site.

Robert Zannetti · Guest

Many thanks to all of you have contributed, it is rich food for thought. My car, at least prior to blocking the Autovac port, used to backfire and misbehave if I was too hasty with winding off the choke. In many respects this is similar to the performance of my twin cam MGA which also needs choking for a short period. I am envious of you chaps who can wind the choke off as soon as the car has started. I will have a hat with Chris, who lives nearby, and see if I need to change needles if I find the 'problem! remains!

Paul Spencer · Posted: Fri Jan 19, 2018 10:45 pm Post subject:

You may not need to be too envious. I never used to need choke. But then I replaced my worn carb jets and suddenly I did.

Chris Card · Guest

Paul - I briefly tried K needles a few years ago on my 4.5L and the fuel consumption shot up, so I went back to CV. Of course, I may not have set things up properly at that time - the car was fairly new to me.

Chris

Robert Craven · Posted: Sat Jan 20, 2018 9:26 am Post subject:

Robert Craven · Posted: Sat Jan 20, 2018 10:20 am Post subject:

I've now made a table putting BC, CV & K next to each other (I've obviously too much time to waste Smile

). I hope I've copied the figures correctly.
It can be seen BC & CV start off much the same, then CV gets richer, but at the end they match, but perhaps it has to be borne in mind that the flow may still be affected by the dimensions along the length of the needle in the jet and not just the diameter at a particular step in isolation.
The K needle starts off weaker but soon gets distinctly richer.

BC CV K
.099 .099 .100
.095 .094 .095
.091 .090 .0905
.088 .0867 .0865
.085 .084 .083
.0825 .0815 .080
.0803 .0795 .0777
.0785 .0775 .076
.0765 .076 .074
.075 .0745 .0722
.073 .0732 .0705
.072 .072 . 0685
.071 .071 .0667
.070 .070 .065

Robert Zannetti · Guest

Many thanks for the usefull information concerning needles. I have checked mine and I am running on CV needles so that is not likely to be my problem. What did emerge, however, was that I have not been maintaining my carbs as I should. A good clean up of some soot and more oil in the dashpot etc. is the order of the day and I hope for better post choke running!

Dan Suskin · Joined: 22 Feb 2011 Posts: 298 Location: Georgia, USA

That sootiness is probably from running on the choke!

Robert Zannetti · Guest

Sure thing Dan. Things here in the UK are not encouraging for test runs - too much salt on the roads but starting it up after a service of the carbs looks very encouraging with the choke off after about 30 secs.

Ron Warmington · Posted: Sun Jan 28, 2018 1:11 pm Post subject:

Hi chaps... I’m not running slopers of course on my repro Blower. It has the two enormous SUs of the type specified by Birkin, substantially thirstier than those fitted to the Factory 50. What USED to happen was that, at the end of a fast run on a long straight on the track, on full chat, it would start popping and backfiring. Similarly, on long steep downhills, when using the engine as a brake, enormous sheets of flame would explosively exit the fishtail. Pretty daunting for cars following at night! All that was down to the mixture being too weak (needles... and and one of the two electric fuel pumps not working properly). Since we (actually Neil Davies), fitted richer needles and got both fuel pumps working properly, it has run brilliantly, though expensively rich in the mid-range. It no longer suffers from fuel starvation on full chat. We’re right now tinkering to get it right throughout the rev range without detracting from the instant cold-starting (without using the choke at all) and the fantastic power, torque and flexibility. Hope that’s relevant to your dialogue. Ron.
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Ron Warmington

Dan Suskin · Joined: 22 Feb 2011 Posts: 298 Location: Georgia, USA

Explosions on the exhaust side are not a weak mixture. weak mixtures burn slowly and are still burning when the intake valve opens - so they expose through the carb usually

Ron Warmington · Posted: Mon Jan 29, 2018 9:48 am Post subject:

Thanks Dan. I didn’t mention that on startup, I quite frequently used to get a backfire that made its way into the world through the first of the three blow-off valves that the engine has to protect the supercharger. In any event, the problem was sorted about five years ago.
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Ron Warmington

Robert Zannetti · Guest

Dan, many thanks for the succinct explanation of explosions through the carbs. I now have an understanding of what was going on and how the weak mixture, possibly because of the manifold leak, could be compensated by the choke. I am eager to get out there and see if my attentions to the leak and carb. service has resulted in improved running.