Nick Posted April 12, 2010 Report Share Posted April 12, 2010 (edited) Hi guys, My 1973 TR6PI had an engine rebuild approx 12 months ago with all new pistons and rings... all was fine in initially and was run in carefully for 1500 miles. Mid-summer last year it developed a what felt like a misfire. Fuel system was checked and adjusted by Mike Pumford and I have checked/replaced/adjusted all ignition parts as necessary. The car idles ok and performs well under hard acceleration but judders badly when cruising at light engine load. All shafts and joints are tight underneath. Two possible culprits... the car has apparently been fitted with a fairly "hot" cam... and compression tests show pots 3 to 6 all very close to the 180 mark but with No1 at 195/200 and No2 at 160/165 (increases to 175 with "wet" test). The car burns no oil so to me it looks like a worn bore/compression ring issue which would be disappointing to say the least given the recent rebuild. Before I start dismantling things anyone got a suggestion as to the most likely culprit and/or know of anything else which might cause the anomaly in the compressions? Edited April 14, 2010 by Nick Quote Link to post Share on other sites
AndrewP Posted April 12, 2010 Report Share Posted April 12, 2010 Could be one or more of the following (least its what I can think of quickly) Cracked head/valve seat Faulty spring (causing the valve to not close correctly) seized piston compression rings Id be surprised if it was a piston ring problem, but I guess it can happen. Did you do the rebuild yourself? Ive seen piston rings fitted the wrong way which caused similar compression issues after a rebuild, but from memory it was immediately evident when the engine was started and run for the first time. Were there compression figured taken straight after the rebuild as sort of a baseline to go from? Either way unfortunately, it sounds like a head off job at minimum. One question though, firstly (as it was the same question asked when I performed a compression test) were all the tests performed under the same conditions (throttle wide open, count the number of cranks etc). Someone else on here can maybe elaborate, but it may cause some differences, but maybe not ~40PSI in the case of your #1 and #2! Anyway..just food for thought. Quote Link to post Share on other sites
Nick Posted April 12, 2010 Author Report Share Posted April 12, 2010 (edited) Edited April 14, 2010 by Nick Quote Link to post Share on other sites
Nick Posted April 12, 2010 Author Report Share Posted April 12, 2010 Thanks Andrew, The rebuild was done profesionally not by me but unfortunately I have no initial compression readings to compare. I did wonder about the wrong way up rings issue and have also read that the alignmemt of ring gaps can create a problem? It was running a bit rough at the beginning but this was put down to bedding in and the fact that the PI system was quite badly setup. All compression tests were performed in the same way and have been repeated three times over the last month... same results (broadly speaking) each time. Seems weird that No1 is high and No2 is low... could this mean anything... ie could one fault of some description cause the anomaly in both readings? Quote Link to post Share on other sites
rpurchon Posted April 12, 2010 Report Share Posted April 12, 2010 exact symptoms to mine. turned out 1 lobe worn away on camshaft . take the rocker off.start it up and it will be obvious if one or more valves not opening fully. richard Quote Link to post Share on other sites
Nick Posted April 12, 2010 Author Report Share Posted April 12, 2010 (edited) oops Edited April 14, 2010 by Nick Quote Link to post Share on other sites
Nick Posted April 12, 2010 Author Report Share Posted April 12, 2010 Thanks Richard... Will try that. Quote Link to post Share on other sites
88V8 Posted April 12, 2010 Report Share Posted April 12, 2010 exact symptoms to mine. turned out 1 lobe worn away on camshaft . take the rocker off.start it up and it will be obvious if one or more valves not opening fully. Interesting thought. If the camshaft was soft, or the followers were soft, or the camshaft not run in properly for the first 20 minutes of the engine's life, could be. In which case a lot of that worn-off metal will now be in the white-metal bearings, busily chewing away at the crank. Surprised though, that it would produce misfiring under light load. The Rover V8 is famous for flattened lobes, that just results in power loss and reluctance to pull the revs. How is it below 1500 rpm, does it pick up cleanly on a very light throttle? If the rings were fitted with inline gaps that would produce a lot of blow-by into the crankcase. Such an elementary error though, shouldn't happen with a pro engine build. Ivor Quote Link to post Share on other sites
88V8 Posted April 12, 2010 Report Share Posted April 12, 2010 (edited) Ivor Edited April 12, 2010 by 88V8 Quote Link to post Share on other sites
88V8 Posted April 12, 2010 Report Share Posted April 12, 2010 (edited) Oops Ivor Edited April 12, 2010 by 88V8 Quote Link to post Share on other sites
Nick Posted April 12, 2010 Author Report Share Posted April 12, 2010 Thanks for the thoughts Ivor... No... would say that it doesn't pick up cleanly on a light throttle... needs a bit of welly but then seems to go really well? Quote Link to post Share on other sites
88V8 Posted April 12, 2010 Report Share Posted April 12, 2010 Thanks for the thoughts Ivor... No... would say that it doesn't pick up cleanly on a light throttle... needs a bit of welly but then seems to go really well? With an airflow meter, check the butterflies at 1200 rpm. Imbalance could be the cause of the misfire. Compressions, hmmm, perhaps a different problem. Was the head retorqued after 500 miles from the rebuild ? Ivor Quote Link to post Share on other sites
Nick Posted April 12, 2010 Author Report Share Posted April 12, 2010 Mike Pumford spent a long time balancing the butterflies etc when he set up the PI system and none of the linkages have been fiddled with since so i reckon that that area should be covered. Interesting point re the retorque though... i know that has NOT been done... another point to check this weekend methinks. Quote Link to post Share on other sites
88V8 Posted April 12, 2010 Report Share Posted April 12, 2010 Mike Pumford spent a long time balancing the butterflies etc when he set up the PI system and none of the linkages have been fiddled with since so i reckon that that area should be covered. Interesting point re the retorque though... i know that has NOT been done... another point to check this weekend methinks. Thinking about this in the bath, if Richard were right and the cam has flattened lobes, that would affect the airflow and could give the same symptoms as unbalanced butterflies. Was the cam replaced when the engine was rebuilt? Slight straw clutching with the head, if the gasket were blown between cylinders, it normally gives low reading in the affected cylinders, not one low one high. But the head should be redone anyway. Ivor Quote Link to post Share on other sites
Guest ntc Posted April 12, 2010 Report Share Posted April 12, 2010 Hi Nick The first thing to check is the valve clearance stone cold,run the engine do your comp test as advised and check again, then come back Quote Link to post Share on other sites
Nick Posted April 13, 2010 Author Report Share Posted April 13, 2010 Ivor, The camshaft was fitted by Racetorations recently (3 years ago - when owned by someone else) but not as part of thir recent rebuild. It was one of their own make and has a quite "hot" specification. If the cam is at fault i can understand the uneven running but would this cause compression issues ie one high and one low?.... maybe more than one problem? Quote Link to post Share on other sites
Nick Posted April 13, 2010 Author Report Share Posted April 13, 2010 Neil, Thanks for your thoughts... I had checked the clearances just before the winter lay-up and all were fine. I'm going to take the rocker top off this weekend and look at the valve movements on pots 1 and 2 as suggested by Richard... will check the clearances again as well. Cheers, Quote Link to post Share on other sites
Guest ntc Posted April 13, 2010 Report Share Posted April 13, 2010 Nick Can you pm me your cam spec/number Quote Link to post Share on other sites
Rob72CP Posted April 13, 2010 Report Share Posted April 13, 2010 (edited) The obvious things have already been suggested so here's a bit of a random one: I'm just completing an engine rebuild and was horified to see the amount of play between the gears on the short shaft that drives the MU and dizzy due to worn pins. I imagine that if yours is in a similar condition it would cause some sort of rough running issues; could be worth a look, particularly if there's no evidence in paperwork of the shaft being replaced or re-pinned. Might also be worth checking the timing and advance mechanism with a strobe to check it's not jumping about due to faulty advance mech just to rule out ignition? Unlikely I know but it's a quick and easy check before you start to strip the head. Other than that, if the MU, TB's and injectors check out my money's on a cam lobe issue. Whilst earlier comments about metal debris are theoretically right, any debris will get washed into the sump and passed through the filter before they go anywhere near the bearings. Don't get me wrong, it's not an ideal situation but if the oil pressure is OK, don't sweat it, I've seen OHC engines with completely wrecked cams run fine for many years after a new cam, spray bar and oil change. Having said that, the compressions don't necessarily look bad enough to cause an issue. Yours are within around 10% of each other, I've had them FAR worse than this and engine ran OK. Rob Edited April 13, 2010 by Rob72CP Quote Link to post Share on other sites
Nick Posted April 13, 2010 Author Report Share Posted April 13, 2010 Nick Can you pm me your cam spec/number Neil... Have just done so. Guess a pm is done through your profile? Cheers, Nick Quote Link to post Share on other sites
Nick Posted April 13, 2010 Author Report Share Posted April 13, 2010 Thanks Rob, Not quite sure which shaft you mean but will check in the brown bible. Ignition already strobed and working fine so that's ruled out. Oil pressure is fine so pleased to hear your thoughts regarding debris and bearings etc. Pot 2 compression is actually 35psi down on pot 1 but i suppose within 10% of the others... just a big difference from one of its immediate neighbours? Cheers Quote Link to post Share on other sites
88V8 Posted April 14, 2010 Report Share Posted April 14, 2010 (edited) I had checked the clearances just before the winter lay-up and all were fine. Just a thought... do you know the clearances for this cam? Might not be standard. If it's been set by PO to standard clearances, but should have larger, that may have fried the valves. It also occurs to me that the cylinder with the high compression may be the only one that's still good, and all the others suffering to a degree. Ivor Edited April 14, 2010 by 88V8 Quote Link to post Share on other sites
Guest ntc Posted April 14, 2010 Report Share Posted April 14, 2010 Just a thought... do you know the clearances for this cam? Might not be standard. If it's been set by PO to standard clearances, but should have larger, that may have fried the valves. It also occurs to me that the cylinder with the high compression may be the only one that's still good, and all the others suffering to a degree. Ivor A very good call Ivor I have sent Nick a pm Quote Link to post Share on other sites
Nick Posted April 15, 2010 Author Report Share Posted April 15, 2010 Just a thought... do you know the clearances for this cam? Might not be standard. If it's been set by PO to standard clearances, but should have larger, that may have fried the valves. It also occurs to me that the cylinder with the high compression may be the only one that's still good, and all the others suffering to a degree. Ivor Thanks Ivor, Yes.. i have a spec sheeet for the cam .014 inlet and .016 exhaust... all seems to be fine there although i am going to recheck this weekend. Good point re the cylinders... what is the more normal reading do you know... 180 or 200? I inderstand from Racetorations that my head compression ratio is 10.2:1 if this makes a difference. Cheers Quote Link to post Share on other sites
88V8 Posted April 15, 2010 Report Share Posted April 15, 2010 Yes.. i have a spec sheeet for the cam .014 inlet and .016 exhaust... all seems to be fine there although i am going to recheck this weekend. Good point re the cylinders... what is the more normal reading do you know... 180 or 200? I inderstand from Racetorations that my head compression ratio is 10.2:1 if this makes a difference. So the clearances probably OK. And a good quality cam, in there for three years, shouldn't have a flat lobe. Even if it wasn't treated with cam lube when the engine was being rebuilt 12 months ago. Unless.... the followers were mixed up by the engine builder and had to 'rebed'. Or replaced with new, duff, followers. Cylinder pressure, well if it were just dependent on CR, your 10.2 would give about 150 psi. But it's a function of compression ratio and valve overlap. 180? Could be. 200? Could be. Misfire on a light throttle... the Rover V8 in my previous Landy, the pressure in the cylinders I could get at with a tester varied between 150 psi and 90 psi (ninety, not one hundred and ninety) It was very worn, and they're renowned for flattening their valve train, but it didn't misfire at any time. Sorry, fraid I'm out of new ideas. Better follow all the leads out to the end. Hope it's something simple. Ivor Quote Link to post Share on other sites
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