MondeoGhiaX Posted October 26, 2009 Report Share Posted October 26, 2009 hi. is anyone on here able to help me interpret the results of a compression test i have just done? 1275cc A-series engine - 4 cylinder - 9.75:1 CR handbook compression figures: 185 - 210 lb/sq in. Cyl no. 4 3 2 1 190 161 161 168 200 170 170 177 (wet test) only cylinder 4 appears to have a satisfactory reading. i am not sure that an all round increase in pressure of 9-10 lb on the wet test would be classed as a significant difference? some info: 1. the cylinder head was re-furbished and converted to unleaded a few 1,000 miles ago, and all valve clearances checked and re-checked. 2. there is no apparent loss of coolant, or mixing of oil and coolant. i really need to know where to go from here, before i commit to a full (expensive) engine re-build or waste time and money replacing the head gasket for no good reason. people have mentioned a "leak down" test. would it be worth having one done? would your typical garage even know what a leak down test is, and have equipment to do it? thanks in advance. ps, i can give much more info about the car and the performance problems i have with it, if it helps. Quote Link to comment Share on other sites More sharing options...
boost mad Posted October 26, 2009 Report Share Posted October 26, 2009 What made you want to do a compression test in the first place ? What problems are you having? Also, how are you doing the test ? how many times are you turning the engine over to get the reading Quote Link to comment Share on other sites More sharing options...
CIH Posted October 26, 2009 Report Share Posted October 26, 2009 I'm not so hot with engines but I'm assuming the engine hasn't performed as expected since the top end work ? Quote Link to comment Share on other sites More sharing options...
MondeoGhiaX Posted October 26, 2009 Author Report Share Posted October 26, 2009 ive been trying to fix a bad idle thats been present since ive owned the car. after replacing just about everything possible over the last 15 years (apart from the engine itself), ive decided to start the diagnostic process again from scratch. hence the compression test. i did the test as per the instructions, and cranked the engine on the starter for between 5 to 10 seconds. to be fair, ive always thought the car pulls quite well, and is fine at high cruising speed. but it often misfires at light throttle/low speed. and whenever i stop at a junction etc, it hardly ever idles at the same speed. sometimes the revs drop to 700rpm or less, and it starts to splutter, other times it fast idles at about 1,000rpm. its completely random, and it isnt anything obvious like the throttle linkage. i dont think the cylinder head job has made things any better or worse to be honest, but i want to eliminate a fault with the head before i look elsewhere. thanks. Quote Link to comment Share on other sites More sharing options...
boost mad Posted October 27, 2009 Report Share Posted October 27, 2009 my first port of call would be with the ignition system. I can admit to not knowing a thing about the A-sieres. Is it on points system? If the issue was cause by a heavy breathing engine ( i.e blow by via the rings or guides ) then the combustion air would be going through thge breather system. On an engine that age, no doubt the breather is vent to atmosphere. try and locate it and, if you can thry and gauge the amount of air that is coming out as obviously this will change when the idling issue occurs again if it is down to blow-by is it carbed ? have you stripped it down or replaced it ? the floats maybe sticking or the jets blocked Quote Link to comment Share on other sites More sharing options...
Tango Posted October 27, 2009 Report Share Posted October 27, 2009 my first port of call would be with the ignition system. I can admit to not knowing a thing about the A-sieres. Is it on points system? If the issue was cause by a heavy breathing engine ( i.e blow by via the rings or guides ) then the combustion air would be going through thge breather system. On an engine that age, no doubt the breather is vent to atmosphere. try and locate it and, if you can thry and gauge the amount of air that is coming out as obviously this will change when the idling issue occurs again if it is down to blow-by is it carbed ? have you stripped it down or replaced it ? the floats maybe sticking or the jets blocked The 1275 'A' series had a solid block, unlike the smaller capacity 'A' series that had removeable tappet covers with a breather tube. I guess the carburettor system is SU, but is it a single 1.25", 1.5" or twin 1.25"? If it's running on twins they're quite difficult to synchronise, but either of the singles shouldn't be a problem. Presuming you've checked the obvious like the oil in the dashpot that dampens the throttle response (if it's low the piston carrying the needle valve can fluctuate and cause the bad idling you describe and variation in idle r.p.m). The carburettor itself only has the needle and jet that can wear to any degree, and this can also cause the idle fluctuation you are experiencing. The points shouldn't be a problem, it's either runnig or not as far as they're concerned. The weakness in the ignition system is the distributor cap, so make sure it isn't cracked. Often the cap suffered from a hairline crack that is very difficult to see, but provides a track for the HT to short out on the metal body of the distributor, which could also cause intermittment misfire, but more likely on higher rpm, not idling. The worst scenario is that you have a leak between number 3 and number 2 cylinders, caused by the head being incorrectly torqued or either the head or cylinder block not being completely flat, but my first guess would be the carburettor. Quote Link to comment Share on other sites More sharing options...
SMARTLY Posted October 28, 2009 Report Share Posted October 28, 2009 Surely there are two problems here? One is the idling and the suggestions of carburation or ignition are first things to get checked, do you know a garage with old fashioned ttuning machines that would identify the problem. The second problem is the compression and a difference like that in my opinion needs investigating. Again that is something the old fashion ( Crypton / Krpton ? ) type tune up machines should identify the problem fairly easily. It's not as simple as the modern one plug and look at the computer there are lots of cables to connect but in the end it runs through a cycle and gives a reliable diagnostic. Quote Link to comment Share on other sites More sharing options...
MondeoGhiaX Posted October 28, 2009 Author Report Share Posted October 28, 2009 firstly, thank you for your ideas and suggestions. after speaking to a few people about this, it seems my compression test readings are not quite as bad as i first thought, and not necessarily indicative of a fault. yes, it appears that 3 cylinders are reading lower than ideal, and one cylinder is higher than the others, but probably not the cause of the misfire. i am going to re-check the valve clearances again, just in case they are a bit tight. the car does have twin su carbs, and they are notoriously difficult to tune/synchronise. but i have replaced the carbs, along with the distributor (with electronic ignition), and just about every other ancilliary component of the engine over the years. but every time i think i have cured the problem, the mystery misfire returns again! i suppose it wouldnt hurt to change the dizzy cap, plugs and leads again now, as these are no longer "new". to be honest, i have gone round and round in circles trying to solve this problem. i will have one last try at doing a complete tune-up from scratch. if this doesnt work then i think i will be forced to seek professional help.. Quote Link to comment Share on other sites More sharing options...
MondeoGhiaX Posted October 28, 2009 Author Report Share Posted October 28, 2009 sorry smartly, i spent so long typing my post that i missed yours. thanks for the suggestions. Quote Link to comment Share on other sites More sharing options...
SMARTLY Posted October 28, 2009 Report Share Posted October 28, 2009 One tip for looking for ignition problems is to start the car in a darkened garage ( black out any window and shut door partially briefly ) with the bonnet open then look for spark jumps obviously don't put your hands in without turning the lights on or using a torch. I've used this in the distant past to diagnose a leaking HT cable. Another is to pull off the HT leads one at a time with an insulted pair of pliers ( or specific tool ) and ensure the engine note drops on each cylinder, if it doesn't you have found where to look. Also look at each plug in turn after a run and make sure they are all the same colour. I used to do my own mechanics when I had early 60's Fords and BMC cars when I was a young and it was all I could afford. I've still got the "colourtune", "timing light", point setting tool and spark checking device somewhere !! Quote Link to comment Share on other sites More sharing options...
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