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PV, 120 (Amazon), 1800 General Forum for the Volvo PV, 120 and 1800 cars |
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Carburettor issueViews : 1803 Replies : 17Users Viewing This Thread : |
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Apr 25th, 2018, 15:37 | #11 |
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Thanks all
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Apr 25th, 2018, 16:04 | #12 |
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Derek;
What "grooves in the piston" are you referring to? I presume you are referring to the ones around the Dashpot perimeter "Airseal" at Red? That area may (emphatically) not be abraded either, but it really wasn't the area I was referring to...I meant the area at what I call the Dashpot and Dome tubes (Green). ...the Red area is certainly a precision fit, but we know there is absolutely no contact here (and it is crucial never to remove any metal from either surface to increase that gap!), and this precision fit is only possible because of the precision (lubricated) fit at the Dashpot and Dome tubes (Green)...and this fit may also never be increased by removing metal with abrasives. Interesting about the circumferential grooves helping with Airseal at red...thanks for that esoteric bit of info and history...(remind me never to engage you in a game of Trivial Pursuit!)... Cheers |
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Apr 25th, 2018, 18:26 | #13 | |
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Quote:
The mk2 colour tune could be used at revs above tickover which was useful for checking mixture as needle lifted. Back in the day we were up for swapping needles to increase performance and it was the subject of much discussion at the local motor club. I moved on to Webber's when I baught fords. I offer advise based on experience, it's up to others if they want to follow it. Twin su on 120s, with one driver they stayed in tune when the owner let others drive home from the pub they quickly went out of tune. He was a mate and I regularly got calls to call around as the car was not going as it should. It's many years ago now that I worked on these cars but lessons learned in youth stay with one, I'm happy to tell how I overcame a problem, it's up to owners if they want to follow my example. If memory serves the precision clearance between piston and pot was about .005 a quick lick with a bit of brasso would be unmeasurable with feeler gauges which was how we did it back in the days before carb cleaners. Paul. |
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Apr 25th, 2018, 23:09 | #14 |
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Hi Simon,
If the pistons are not 'behaving' the same in both carbs, then they are not balanced properly. You're welcome at any mutually convenient time to trundle that car down here to Exeter and I'd be delighted to go through step by step how to get them running exactly the same. You have to start by checking the float heights are the same in both chambers. There are many ways you can do this, some more precise than others. As well, you have to check the piston drops onto the bridge with a good 'clink'. If not, then you have to centralise the jet. Next you HAVE TO disconnect the balancer bar, throttle shaft, whatever you want to call it to make sure the carbs are running idependently from each other for the next few adjustments. Then screw back the choke screws/fast running screws then adjust the idle screws so both carbs are running at the same speed: listen to the same hissing sound, use a Gunson Carbalancer, compare the heights of the pistons - whatever method you want to use, but I recommend using a little hand held air speed gauge (anemometer) available from Amazon for about a tenner. Then adjust the mixture: Colortune, CO probe up the exhaust, whatever method, but I do it by ear listening out for when the mixture is just too strong and it's lumpy, then screwing back in until it runs fast. After doing the mixture, you have to re-check the idle speed hasn't altered followed by adjusting the choke screws back up so that there is a tiny, but matching, gap between them and the cam thingamer for fast running. Once both carbs are running identically, you then BALANCE them. This very non-technical bit gets left out so, SO frequently! You have to set the carb levers to activate at precisely the same time by adjusting the balancer shaft so that the pivots touch the carb levers at the same position. This is really important. It opens the carbs at the same time and closes them at the same time. You can check it but watching the pistons bob up and down when you 'blip' the throttle. If one activates more eratically than the other, the carbs aren't balanced. You can also check fast air speed using the air gauge, but generally, if pistons are acting the same, that's pretty accurate. As I say, jump in the old car and bring it down to Exeter and I'll show you how! PS I don't see a problem with cleaning out the domes etc with Brasso, though never heard of that method before nor see the point of it. However, I would recommend severe caution of going anywhere near the needle or jet with Brasso. In fact, don't even touch the needle with a rag as the brass is so, so soft you can very easily wreck them.
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2006 XC70 D5 Manual 1968 Amazon Estate, B18A + Overdrive 2019 V60 D3 Momentum Pro Manual 1970 Amazon 2-Door 1970 142DL Last edited by swedishandgerman; Apr 25th, 2018 at 23:17. |
Apr 25th, 2018, 23:39 | #15 |
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Thanks for taking the time to put all that down and the offer to help.
I have gone through the tuning routine before, but I didn't disconnect the throttle shaft so I plan to do it again. I actually bought one of the anemometers you mention, on your recommendation in a previous post. I think now that the immediate problem is that I'm unable to balance the carbs as the rear one is being interfered with by a poorly adjusted choke cable but i'll check that at the weekend. Once that's sorted I'll try another re-tune. I know you're probably my nearest 1800 and I should make a visit sometime. Simon |
Apr 27th, 2018, 09:30 | #16 |
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If there's one single guide that I recommend, it's this The SU Carburettor High-Performance Manual by Des Hammill:
https://www.waterstones.com/book/the.../9781787111684 Mummy bought it for my Christmas present! There is a lot of information in there that you'll probably never need, but the tuning sequence is fantastic and makes sense. It's easy! I say, the stuff written in the old Haynes Manuals there is over complicated and misses important stuff out concentrating far too much on what the thing idles like rather than the most important part of balancing the airflow at just off idle.
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2006 XC70 D5 Manual 1968 Amazon Estate, B18A + Overdrive 2019 V60 D3 Momentum Pro Manual 1970 Amazon 2-Door 1970 142DL |
May 2nd, 2018, 14:49 | #17 |
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Just to update the thread, it wasn't the choke cable. The throttle linkage between the two carbs was incorrectly adjusted so it could have been that. After loosening them off and readjusting so they lifted the pistons simultaneously, the problem was solved.
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May 2nd, 2018, 19:36 | #18 |
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It is so, so frequently that at fault!
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