daverclasper Posted June 6 Report Share Posted June 6 Hi. Sounds a bit like a Morris 1000, though maybe not as pronounced. Any ideas please? Link to comment Share on other sites More sharing options...
johny Posted June 6 Report Share Posted June 6 Might be worth doing a compression test Dave to see if the valves are all sealing well. Also check for air leaks round the manifold. Whats your ignition timing set to? Link to comment Share on other sites More sharing options...
micklewis Posted June 6 Report Share Posted June 6 ignition slightly retarded will do this as will running it on 92 octane fuel when set to run on 99. nothing wrong with a burble on over run . turn the radio up and enjoy the drive Link to comment Share on other sites More sharing options...
johny Posted June 7 Report Share Posted June 7 Yes I wonder if, as discussed on here, the addition of ethanol in fuel has worsened the problem by making combustion take longer so its still happening when the exhaust valves open.... Link to comment Share on other sites More sharing options...
daverclasper Posted June 7 Author Report Share Posted June 7 Mine always runs on Tesco 99 RON, didn't think there was ethanol in this?, in my case? Link to comment Share on other sites More sharing options...
johny Posted June 7 Report Share Posted June 7 I read it as having up to 5% ethanol and indeed this would be a good way to achieve the higher octane rating (ethanol increases octane)... Link to comment Share on other sites More sharing options...
PeteH Posted June 8 Report Share Posted June 8 There was a suggestion some time back that the addition of ethanol in some 99ron was actually regional? and dependent on where the Supply depot was? A tanker driver once told me that ethanol was actually added as the tanker was being filled, something to do with ethanol increasing moisture content when held in the actual fuel storage tanks?. Pete Link to comment Share on other sites More sharing options...
cliff.b Posted June 8 Report Share Posted June 8 My Spitfire started doing this and I found it was the vacuum advance mechanism had seized so on the over run, it was more retarded than it would normally be. Replacing the vac unit fixed it. I would expect generally retarded ignition timing would have the same effect. 1 Link to comment Share on other sites More sharing options...
Gary Flinn Posted June 10 Report Share Posted June 10 To add to this debate on Ethanol in Fuel, a memo was circulation in Tr Circles with regards to the effects of Ethanol on Metering Units for the PI System, its dated 2016 so no idea if it is still relevant to Esso Synergy Supreme Plus petrol or the others mentioned nowadays, memo below:- "Esso Synergy Supreme Plus unleaded petrol does not contain ethanol if bought from a service station in our region (Derbyshire). To elaborate further, not all Esso filling stations sell their premium unleaded., and not all Esso Synergy Supreme Plus unleaded petrol is free of ethanol - fuel purchased in Devon, Cornwall, Teeside and Scotland being a case in point. For anyone who has thoughts that all premium unleaded fuels from any of the major manufacturers does not contain ethanol, I can confirm that, at the very least, Shell and Texaco premium unleaded fuels does contain up to 5% ethanol. Tesco seems to be a mixed bag – some stations do, some don’t, and I am waiting for a reply from BP. In general some, but far from all, pumps in filling stations indicate the ethanol content of the petrol dispensed. Furthermore, it is possible, but not as yet confirmed, that Total Unleaded (all grades) sold within our area does not contain ethanol!" Regards Gary Link to comment Share on other sites More sharing options...
johny Posted June 10 Report Share Posted June 10 I dont think its even worth asking Gary as they all 'reserve the right to change with no prior notice'! If it has an E number it could contain ethanol up to that percentage... 1 Link to comment Share on other sites More sharing options...
johny Posted June 10 Report Share Posted June 10 On 08/06/2024 at 10:33, cliff.b said: My Spitfire started doing this and I found it was the vacuum advance mechanism had seized so on the over run, it was more retarded than it would normally be. Replacing the vac unit fixed it. Ive had a bit of think about this and want to clarify the vac mechanism. As I understand it the purpose is to improve fuel consumption when cruising on partial throttle by advancing the timing and then when the throttle is opened or closed this advance is lost and the timing returns to that purely determined by the dizzy centrifugal weights acting on engine speed. The vac unit cant retard more than this and in fact on some versions of the six engine it wasnt even deemed necessary to fit them.... Link to comment Share on other sites More sharing options...
Pete Lewis Posted June 10 Report Share Posted June 10 i have found on the old 25D6 types if the vac springy connector fractures then the contact plate can be free to move around so depends where it decided to sit is what you got . later with a more mechanical vac operating link /or internal spring fail it seems to just sit as staticly set agree it cant retard , Pete 1 Link to comment Share on other sites More sharing options...
Wagger Posted June 11 Report Share Posted June 11 Any leak in the exhaust will cause this. So long as it does not scare horses or the aged, do not worry too much. This seems to be a craze with youngsters right now in their little sport hatchbacks. They are annoying late at night on our bypass. What are fuel companies using to raise octane if it is not ethanol? It cannot be Tetra Ethtyl Lead any more. Is it pure alcohol? Link to comment Share on other sites More sharing options...
johny Posted June 11 Report Share Posted June 11 Yes apparently the companies are having to reduce the octane of the base fuel to maintain the correct rating as the ethanol percentage is being increased... Link to comment Share on other sites More sharing options...
cliff.b Posted June 13 Report Share Posted June 13 On 10/06/2024 at 12:32, johny said: Ive had a bit of think about this and want to clarify the vac mechanism. As I understand it the purpose is to improve fuel consumption when cruising on partial throttle by advancing the timing and then when the throttle is opened or closed this advance is lost and the timing returns to that purely determined by the dizzy centrifugal weights acting on engine speed. The vac unit cant retard more than this and in fact on some versions of the six engine it wasnt even deemed necessary to fit them.... I probably didn't explain my thinking on this very well. Basically, when investigating the "popping" on overrun I found the vac unit was not providing any advance and when I replaced it the popping stopped. I assumed that decelerating on a closed throttle would cause high vacuum and if the vac unit was working this should temporarily advance the timing on the overrun, but as it was not working this advance was not occurring so the timing was more retarded than expected. I know the vac advance can't actually retard the ignition. My theory may be wrong but it was the only thing I could think of to explain why the fix worked. Link to comment Share on other sites More sharing options...
Pete Lewis Posted June 13 Report Share Posted June 13 (edited) the port in the carb throat that developes the vacuum for the dizzy unit is positioned to only work at a light /mid /cruise throttle opening at idle or full throttle its has no vacuum created it on a lucas the digits on the unt spell out when the vac starts ,when it stops and how much advance ie 5-11-7 just an example on delco you dont get those clues Pete Edited June 13 by Pete Lewis Link to comment Share on other sites More sharing options...
Colin Lindsay Posted June 14 Report Share Posted June 14 On 10/06/2024 at 09:58, Gary Flinn said: To add to this debate on Ethanol in Fuel, a memo was circulation in Tr Circles with regards to the effects of Ethanol on Metering Units for the PI System, its dated 2016 so no idea if it is still relevant to Esso Synergy Supreme Plus petrol or the others mentioned nowadays, memo below:- "Esso Synergy Supreme Plus unleaded petrol does not contain ethanol if bought from a service station in our region (Derbyshire). To elaborate further, not all Esso filling stations sell their premium unleaded., and not all Esso Synergy Supreme Plus unleaded petrol is free of ethanol - fuel purchased in Devon, Cornwall, Teeside and Scotland being a case in point. For anyone who has thoughts that all premium unleaded fuels from any of the major manufacturers does not contain ethanol, I can confirm that, at the very least, Shell and Texaco premium unleaded fuels does contain up to 5% ethanol. Tesco seems to be a mixed bag – some stations do, some don’t, and I am waiting for a reply from BP. In general some, but far from all, pumps in filling stations indicate the ethanol content of the petrol dispensed. Furthermore, it is possible, but not as yet confirmed, that Total Unleaded (all grades) sold within our area does not contain ethanol!" Regards Gary I just put £20 in the car a few minutes ago, the pump was marked "Supreme Unleaded" with a sticker claiming "Not a Texaco Product." That garage seems to have stopped selling the 99 RON they used to when it was a 'Solo' brand garage. (Never heard of it either but we get all sorts of cross-border enterprises and to be honest ethanol is the least of the worry) The owner at that time had large American cars and needed the better fuel. It will be a real pity if he's gone as we don't get any of the super Shell or BP fuels over here. Link to comment Share on other sites More sharing options...
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