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jarnol123 Member
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I have a 1975 Jensen Healey with Dellorto 45Es. I recently installed a pertronix flamethrower coil and ignitor, new cables, dizzy cao, rotor etc. Spark is fine. Fuel flow to the carbs is good but when I try and start it's extremely difficult and when the engine does fire I have to keep pumping the accelerator to keep it going but then it just dies. I've been reading a lot on the dellortos and other people's posts and I think I'm probably running rich as the car was being driven at 5000' altitude where I bought it a few years ago. How do I tell what jets I have installed? Is it possible they are too rich and I'm not getting enough air? The spark plugs are generally black (wet) after I go through the cranking process. Any ideas?? |
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Greg Fletcher Administrator 
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This problem has nothing to do with altitude. Black, wet plugs do not bode well. That indicates that fuel is not being burned in the combustion chamber as it should. There could be several causes. If this were my car, the first thing I would do is a compression check on each cylinder as this will tell you immediately what's going on. If compression is down considerably or even nil in at least one cylinder, that points to worn valves, rings or both. |
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jarnol123 Member
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I've never done a compression test before..could you tell me how to do it on my non-running Jensen? Thanks |
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Dan (Florida) Member
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Was this car running when you bought it? If so, something has happend in the meantime. It may be several things like bad gas. stuck carb floats, distributor timing off. In your part of the world there should be no end of car nuts that could help you with these checks without spending a fortune. I wouldn't trust a 907 to an inexperienced mechanic. There are sites including this one that provide information on jet size / settings etc. Look in the carburation forum on this site and read everything . Then google Dellorto dlha 45 for more reading. You cant own one of these cars without starting a learning process that goes on until you sell. My car has been on the back patio since October with the engine out for a bad rear main oil seal . I'm waiting for new bearings to come in now. |
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jarnol123 Member
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Dan - Thanks for the info... Yes the car was running before but i do know that the compression was low. Problem is I had that test done at a local garage while the car was running. I wonder if there is a way to do a test at home with my non-running jensen to see what the compression is now and whether it is consistent across all cylinders. Fom what i remember the compression was low 100's with the cylinders within 15-20% of each other. I'm hoping that it may just be the valves but of course you don't know until you take the head off. |
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JodyKerr Member 
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http://autorepair.about.com/od/enginetroubleshootin1/ss/compress_test.htm The only issue is that the car should be warm. But, seriously, before you start messing around with the carbs you should be 100% certain that it's not anything else. Most of the time the issue is not the carbs, but something else in the ignition/fuel delivery system. Too many people assume it's the carburettors without enough reasonable evidence. From there the carbs get fiddled with/disassembled until the only resolution at that point is to take it in to a shop. #1 - ensure that the ignition system is working correctly and the timing is right. #2 - Ensure the fuel pump works correctly and the lines are clean. #3 - use a local fuel supply to the carbs and try to start the car that way. |
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jarnol123 Member
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Ok i follow how to do the compression test but of course I can't get my engine warm because it won't start. If I do what i was doing before, ie crank it over, pump the accelerator so that it almost starts, would that be sufficient? ie at least get it a bit warm? |
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Greg Fletcher Administrator
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The engine should be warm, otherwise you'll most likely get inaccurate readings. If the engine is cold, you can still check for a dead cylinder, since that would be dead either way. A manual compression test is done by removing a spark plug and screwing in a compression gauge in it's place. The engine is not running of course in this scenario. Typically, one will disconnect the coil (since we can't have a running engine), insert your compression tester gauge, open throttle to full open position, have someone crank engine 3-5 seconds. Repeat for each cylinder. If your engine were warm, you would expect readings within 10% of each cylinder and 140 to 160 pounds in each cylinder. |
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