| Author | Message | ||
     Kevin Lagden Prolific User Username: kevin Post Number: 105 Registered: 7-2009 |
Just had the carbs overhauled and new SU kits installed. The SU Mechanic just told me he used ATF transmission oil in the pots. Anyone had any experience of this? only from what I have read the recomendations vary depending on who you talk to. I previously used a 20/50 engine oil, which I believe to be too thick. | ||
| Brian Vogel Grand Master Username: guyslp Post Number: 1128 Registered: 6-2009 |
ATF (of the Dexron III-type) is one of the most commonly used oils in the carb dashpot. That's what I've been using in mine and you'll find many posts on these forums where others say the same. That being said, there are oils sold specifically for dashpots. SU/Zenith, Penrite, and others sell this using the term "damper oil." You'll notice that the SU/Zenith product is straight SAE 20 weight and the Penrite is ISO 68, which is at the upper end of the SAE 20 range. (See this viscosity cross reference chart) I doubt that a 20WXX multi viscosity oil would be "too thick" given the temperatures at the dashpot and the fact that the stuff sold as damper oil is effectively the equivalent of SAE 20W motor oil. You'll also find that some have been inclined to play with different oil weights as a way to alter damping characteristics. Brian | ||
| Bob UK Unregistered guest Posted From: 94.197.122.89 |
This is a Old motoring chestnut. ATF is fine. 20/50 Will also work. The damper is little affected by oil weight and the piston will work even without oil. The damper is not critical just as long it has oil that is not like syrup. If overffilled the oil will leak out and full power won't be available until the level has corrected itself. The piston won't fully rise until it's pushed the oil out. It doesn't effect the mixture and will only be noticeable at wide throttle and then it will sort itself out within a short distance. On cars where the piston can be fully lifted by air filter removal and a finger used to lift the piston manually excess oil will be immediately pushed out. Unfortunately on RR V8 the horns are in the way. So if one does overfill then removal of the dashpot and push up is required or let it sort itself out. There is a huge amount of vacuum in an engine. I have extensively worked on SU carbs to get them just right and I give little attention to dampers as long as it had oil and wasn't binding then it's good. The correct oil is a on going discussion in many brit iron car forums and opinions vary widely some insist on atf others on 20 /50 3 in 1 oil redex it's endless, any oil between 10 to 50 weight will do. (Message approved by david_gore) | ||
| Paul Yorke Grand Master Username: paul_yorke Post Number: 1336 Registered: 6-2006 |
Hi Kevin, the main purpose of the damper is to give an extra spurt of fuel when you put your foot down suddenly or quickly. Like an acceleration pump on a normal car. In theory it would be nice to get extra enrichment when the engine and intake air is cold and slightly less when everything is hot. Multi grade oil should help this. In practice, when driving I can tell if the cars have no, or very low, oil in them, but not what oil is in them. With a set of rebuilt carbs, now would be a good time to test for yourself to see if it's noticeable and which you prefer. Let us know  | ||
| Kevin Lagden Prolific User Username: kevin Post Number: 106 Registered: 7-2009 |
Many thanks, just paranoia that's all, I'm running a bit rich at the moment so exploring everything. Just had all 8 sparks replaced as the old ones were black with soot. I'm slowly learning how to balance the carbs, and with a limited experience, I'm a little hesitant to have a go | ||
| Mark Aldridge Prolific User Username: mark_aldridge Post Number: 181 Registered: 10-2008 |
5/30 engine oil, gives easy cold starts ( most noticeable on my S1)and I have a 20 litre drum for the Landrover ! | ||
| Geoff Wootton Grand Master Username: dounraey Post Number: 559 Registered: 5-2012 |
Kevin For balancing carbs I find the Uni Syn Carbalancer invaluable. These retail at around US $30. The other aid I quite like is a colortune. I use this as a start point on tickover to get the carbs basically right and then move on to the procedures documented in the workshop manual. Geoff. | ||
| Bob UK Unregistered guest Posted From: 94.197.122.76 |
A quick car park check. A1 and B1 plug leads. One cylinder runs on one carb and the other runs on the other carb. If A1 is disconnected and the rev drop measured vice versa then they should be both the same. This quick check can be used with color tune and other methods. If the piston is lifted an 1/8 of an inch at idle the rpm should rise and then stumble if the engine runs smooth the mixture is rich. If the rpm stumbles or stalls instantly then the mixture is weak. By then checking the rpm drop on disconnected cylinders the exact setting can be dialed in. The idle by pass screws must not be more than 2 turns out. So if they are the main stop needs opening up a bit. 1/2 turn at a time then rev then settle. Once in range of the bypass lock up. If the bypasses are over 2 turns they can whistle. Apart from that it does no damage. The whistle noise can happen on overrun. crow foot spanners. All this is dependent on fuel height in the bowl. Use a twist drill if a rod is unavailable. A slow idle is not a good idea and serves no purpose and of course when the tuning wanders it going to stall which is seriously annoying and eventually it going to do it at the wrong time. IMO I set as fast as I dare bearing in mind the car is an automatic with a super light shift. Plus fuel economy will be worse with the continual stumbling. Carbs like air flow. And the poor starter motor and ring gear. Hence the continual references in the literature to the GEAR BOX CUT OUT thingy. A key position that allows engine to run but locks in park would be good. Interlock it with the hand brake warning lamp. IMO 700 rpm gives a rock solid reliable idle regardless of electrical and other loads. The fan is working nicely and moving a good bit of air thus the aircon will work better and the alternator will happily work everything and trickle charge the battery. 800rpm is getting dangerous because the transmission will bump on engagement and the car will quite easily run along at 5 mph on a level road on idle. At 700 rpm it will creep at about 1 mph. Just right for a graceful take off. Incidentally if one must run into the car in front hit it square on because it spreads the load and if only creeping then much less damage 5 mph and the grill will be smashed up. OR concentrate on driving. (Message approved by david_gore) |
