Forum registration by users has been re-enabled with harder registration questions. Guests should use the Register button top right but if you have difficulty answering one of the spambot countermeasure questions, Email mail @ chalkevalley.info (delete spaces).
If you would like to join the DLOC, please visit the Club's website https://www.dloc.org.uk/
Please don't post someone's email address. Some people don't want to risk it being harvested by spambots and GDPR regulations discourage it.
Always look at "ACTIVE TOPICS" which gives all posts in date & time order if you can't find a post or "Your Posts", as topics are sometimes moved.
Reg. nrs. Please add reg. nr. when posting a photo or anything about a car as this will help searches. Don't add punctuation next to nr. as this negates search.

special sport carbs

Post Reply
davidian
Posts: 32
Joined: Sun Feb 17, 2019 11:32 am
Location: la vinuela spain

special sport carbs

Post by davidian » Fri Jun 07, 2019 11:36 pm

Hi all
Today I did the points and plugs on my car and the plugs were very black and sooty which pointed to a too rich setting. When adjusting the carbs is it best to render one inoperative whilst you adjust the mixture of the other or to do both at once . I followed the instructions from a conquest workshop manual I had kicking about ( which has similar carbs ). but felt that any adjustments I made to one carb had no effect because the engine was running happily on the other carb . Any advice would be welcome.
Regards Davidian

Phillmore
Posts: 847
Joined: Sat Feb 13, 2016 1:25 pm
Location: Worcestershire Herefordshire border

Re: special sport carbs

Post by Phillmore » Sat Jun 08, 2019 8:08 am

Did you check the plugs after a good run at normal engine temperature to ensure that the sooty plugs are not down to the last cold start with choke?

I think it's a case of fine tuning your ears as well as your carbs. I'm no expert but roughly speaking each carb will be supplying three cylinders. The difference in running that weakening the mixture on one carb will make will be very subtle. I'm assuming you have twin SUs? Optimum mixture at tickover should result in the fastest tickover speed. Too rich or too weak and the engine revs will drop. If you have lift pins then raise each one about 1/32". Revs should momentarily rise and then settle back. If it falters it is too weak, if it rises strongly it is too rich. If you don't have lift pins (which just raise the pistons) you can often lift them with a screwdriver once the airbox has been removed.
Andy

1954 Conquest Mk1, 1956 Conquest Mk2, 1957 Conquest Century Mk2, 1955 Austin A90 Westminster

Online
Big Col
Posts: 413
Joined: Tue Feb 16, 2016 12:45 pm
Location: Leeds

Re: special sport carbs

Post by Big Col » Sat Jun 08, 2019 12:04 pm

You will find it much easier to get a hold of a carburettor balancer. They can be bought cheaply from car accessories shops. I have a Gunsun one which works well with SU carbs.
Colin,
I may be slow but I’m rough as well !

Phillmore
Posts: 847
Joined: Sat Feb 13, 2016 1:25 pm
Location: Worcestershire Herefordshire border

Re: special sport carbs

Post by Phillmore » Sat Jun 08, 2019 12:22 pm

Agreed Colin the Gunson Carbalancer is excellent for balancing the air flow but it won't help with balancing the mixture strength. The Gunson Colour tune might help but I never really got on with that. I find it really is a matter of listening carefully for changing engine revs and cross checking with the lift pin.
Andy

1954 Conquest Mk1, 1956 Conquest Mk2, 1957 Conquest Century Mk2, 1955 Austin A90 Westminster

Christopher Storey
Posts: 260
Joined: Wed Mar 16, 2016 6:40 pm
Location: Cheshire

Re: special sport carbs

Post by Christopher Storey » Sat Jun 08, 2019 2:38 pm

Adjusting the mixture to obtain fastest idle will result in slightly weak mixture throughout the rev/load range. The setting needs to be just slightly on the rich side of fastest idle . The correct method is to adjust each carburetter to the point where lifting the piston 1/32 inch = 0.8 mm results in a slight rise in rpm followed by its dropping immediately back to the level before the piston was raised. Note also that it is important to clear the carburetters every 2 minutes or so by running the engine at 1500+ rpm for 15 seconds

Online
Big Col
Posts: 413
Joined: Tue Feb 16, 2016 12:45 pm
Location: Leeds

Re: special sport carbs

Post by Big Col » Sat Jun 08, 2019 11:03 pm

If you can get a hold of the SU tuning manual all well and good. That said try the web site of Burlen, the technical pages are very good for information regarding setting up the carbs.
Colin,
I may be slow but I’m rough as well !

davidian
Posts: 32
Joined: Sun Feb 17, 2019 11:32 am
Location: la vinuela spain

Re: special sport carbs

Post by davidian » Sun Jun 09, 2019 4:28 pm

Hi
Had another go at adjusting the carbs today , rather than working from rich to lean as before I raised the jets up to their highest position, painted one flat on each nut red ( so I could see that both were altered the same) then lowered the nuts a flat at a time until I had satisfactory running , Chris"s suggestion to rev the engine occasionally was a great help as it raises the pistons and allows them to settle naturally into the adjusted jet . One strange thing I found was that in the process of curing the problem I examined and adjusted the float chambers , the floats were floating but if you shook them it seemed there was something inside, so I immersed them in hot water but there were no tell tale bubbles so what on earth is inside them . Many thanks for your help
David

Mark Bullen
Posts: 65
Joined: Mon Feb 15, 2016 1:07 pm
Location: Wiltshire

Re: special sport carbs

Post by Mark Bullen » Sun Jun 09, 2019 5:45 pm

David,

I had a similar problem and found that the float was perforated and drawing-in fuel, causing the floats to intermittently flood the engine. Unfortunately I only found the problem too late and ended up with carburettor over flowing and spraying fuel around inside the engine bay. Potentially very dangerous. I would recommend getting replacement floats, arms and needles as they aren't very expensive and Burlen are quick to deliver. This could be the cause of the engine running very rich, my plugs were quite sooty as well.

davidian
Posts: 32
Joined: Sun Feb 17, 2019 11:32 am
Location: la vinuela spain

Re: special sport carbs

Post by davidian » Sun Jun 09, 2019 7:01 pm

Hi Mark.
I was lucky in that I had a pair of the same carbs I had bought from an auto jumble many moons ago for a fiver and was able to cannibalize them for their floats . I visited lots of web-sights about S U carburetors and there was a piece on an M G forum saying some rebuild kits had seals slightly thicker than original which resulted in the jet being lower than the bridge when in its highest position .I rebuilt mine about three years ago and my jet sits about one sixteenth of an inch below the bridge , could be a contributing factor . The old car seems to be running fine now , I shall put a few miles on her then have another look at the plugs .
David,

Post Reply