stebec Posted November 21, 2023 Report Share Posted November 21, 2023 I’m looking at doing a compression test on my spitfire 1500, it says I have to isolate my fuel pump, how do I isolate the pump? steve Link to comment Share on other sites More sharing options...
johny Posted November 21, 2023 Report Share Posted November 21, 2023 It must be refering to electrical pumps as theres no practical way of isolating a mechanical one. If youre going to spin the engine on the ignition switch it is a good idea to disable your spark by disconnecting the negative wire(s) at the coil... Link to comment Share on other sites More sharing options...
Pete Lewis Posted November 21, 2023 Report Share Posted November 21, 2023 if its a mechanical pump i have never heard or needed to isolate any pump you just need a good battery state all spark plugs out fit the gauge , open the throttles and crank over to get the highest reading and the jobs done where does the fuel pump idea come from ...who's instructions ???? Pete Link to comment Share on other sites More sharing options...
stebec Posted November 21, 2023 Author Report Share Posted November 21, 2023 19 minutes ago, johny said: It must be refering to electrical pumps as theres no practical way of isolating a mechanical one. If youre going to spin the engine on the ignition switch it is a good idea to disable your spark by disconnecting the negative wire(s) at the coil... Thanks Johny Link to comment Share on other sites More sharing options...
stebec Posted November 21, 2023 Author Report Share Posted November 21, 2023 5 minutes ago, Pete Lewis said: if its a mechanical pump i have never heard or needed to isolate any pump you just need a good battery state all spark plugs out fit the gauge , open the throttles and crank over to get the highest reading and the jobs done where does the fuel pump idea come from ...who's instructions ???? Pete Hi Pete, I read the instructions wrong, I’m knew to the classic car scene so it’s a big learning curve, how do I keep the throttles open? steve Link to comment Share on other sites More sharing options...
Pete Lewis Posted November 21, 2023 Report Share Posted November 21, 2023 best to rent a foot or a brick !!! they need to be open to get air in easily.or at the idle setting will reduce the result Pete Link to comment Share on other sites More sharing options...
daverclasper Posted November 21, 2023 Report Share Posted November 21, 2023 Don't know if I've imagined this?, is it better to do it when engines at operating temperature?. Link to comment Share on other sites More sharing options...
Pete Lewis Posted November 21, 2023 Report Share Posted November 21, 2023 a hot engine does spin over better and yes can give a better result than a cold one but often the compression fault determines where it runs or not to get it heated , what you are looking for it the range of results being around within 5% or each other pete Link to comment Share on other sites More sharing options...
PeteH Posted November 22, 2023 Report Share Posted November 22, 2023 Repeat the test, with a "spoonful" of oil in each cylinder, it also gives a (better) indication of ring condition. Or so the pundits used to say? Pete Link to comment Share on other sites More sharing options...
Mjit Posted November 22, 2023 Report Share Posted November 22, 2023 (edited) If you get good compression without oil you don't need to bother retesting with oil. If you get poor compression without, retest with oil. The oil 'sinks' so helps the piston rings seal so if you get good/better compression with oil your poor compression's probably down to worn rings/boars. Oil doesn't 'levitate' though so has no impact on the valves so if you still have poor compression with oil it's probably down to worn valves. Or of course it could be both! Edited November 22, 2023 by Mjit Link to comment Share on other sites More sharing options...
Wagger Posted November 22, 2023 Report Share Posted November 22, 2023 OK, if you spend a long time cranking a six pot with the throttles open and fuel pump enabled, then you may fill the exhaust with a load of ready to pop mixture next time you start it. I put a lovely circular smut on the garage door doing just that. The mechanical pump can be disabled if it has a priming lever. Just tie it up and it cannot pump fuel. Probably totally unnecessary on a four pot. Link to comment Share on other sites More sharing options...
Pete Lewis Posted November 22, 2023 Report Share Posted November 22, 2023 with all the spark plugs out there wont be much inlet vacuum to suck any fuel in from the carbs only on the cyl you are testing Pete 1 Link to comment Share on other sites More sharing options...
Wagger Posted November 22, 2023 Report Share Posted November 22, 2023 I did mine with the plugs in, one at a time in case I was disturbed and muddled it all up. It was many years ago. Link to comment Share on other sites More sharing options...
stebec Posted November 23, 2023 Author Report Share Posted November 23, 2023 These are the results of the compression test Number 1 was 150 Number 2 was 160 Number 3 was143 Number 4 was 155 are these Ok or could there be a problem Steve Link to comment Share on other sites More sharing options...
Josef Posted November 23, 2023 Report Share Posted November 23, 2023 Well your highest vs lowest readings are 10% separated, as Pete said above you’re looking for a max of roughly 5%. Do you get consistent readings though? If you’re not familiar with the process I’d do three readings per cylinder. If you get consistent readings per cylinder you can be confident of the reproducibility of your method and therefore confident that you’re seeing a real difference between cylinders. Link to comment Share on other sites More sharing options...
johny Posted November 23, 2023 Report Share Posted November 23, 2023 Then as said previously repeat with a dessert spoonful of engine oil in each bore... Link to comment Share on other sites More sharing options...
JohnD Posted November 23, 2023 Report Share Posted November 23, 2023 (edited) Your No.3 is 8% lower than the mean for the others, so yes, could be a problem. My limit is 5%. But if you haven't done it before, practice again , and confirm. As said three or four complete tests. The spoonful of oil will tell you if the low pressure is a leaky valve. The oil will raise the pressure on all cylinders, by improving the ring seal. It won't affect a leaky valve, where the pressure will not rise. John Edited November 24, 2023 by JohnD 1 Link to comment Share on other sites More sharing options...
Pete Lewis Posted November 24, 2023 Report Share Posted November 24, 2023 and always check your tappet clearances before any comp test , small errors will produce a variation in your result Pete Link to comment Share on other sites More sharing options...
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