Hello all, I recently did a compression test on my 1981 Stage 1 V8 (Carb 3.5). This came about as I was chasing down a tappety/lifter like rattle when warm and the car backfires cold or warm. I was dumbfounded when Cylinders No8 and No2 returned values of flat 0 (psi) and 25 respectively. All other cylinders returned values that barely differed from 110 to 115. Oil down the cylinder DIDN'T affect the results in 8 & 2. I removed spark leads from 8 and 2 and backfires stopped. I'm beginning to suspect valves as I pulled rocker shafts to have a look at wear and tear on rods and shafts etc and they all seem within margin.. The strange thing was that when I loosened off the rockers to let valves stay closed on springs I still returned same results. Anyone able to shed some light as to whether I need to pull a head or if I've got bigger issues...? I find it hard to believe I've dropped piston rings...
3.5 valve & compression issue?
- Thread starter Tin_Man
- Start date
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i bet you've got worn cams, once they start to go they're like butter
What mileage has the engine & its current camshaft done?
80k-ish is often suggested as the useful life of the cam & followers.
And what's the compression ratio? I'm guessing 8.13:1 - you'll find it stamped on a flat next to the dipstick tube - but if it's had different pistons fitted that could change things.
Your compression figures for the rest of the cylinders are very low & it would be worth doing the oil test on the rest of the cylinders.
If it is 8.13:1 you should be looking at figures of 135 minimum according to a 1977 LR factory manual I have for that engine in a carb'd 3.5 RRClassic
If you do decide to do the cam & followers I'd suggest you also change the timing chain & its gears.
Worth pulling the heads & checking the valves & seats & bore wear.
You'll likely find the rocker arm internal oilways (oil from the shaft flows through this oilway to the top of the pushrod) are well clogged (several of mine were blocked solid) & the rocker shaft will have a lot of crap inside it.
Been there done that!
80k-ish is often suggested as the useful life of the cam & followers.
And what's the compression ratio? I'm guessing 8.13:1 - you'll find it stamped on a flat next to the dipstick tube - but if it's had different pistons fitted that could change things.
Your compression figures for the rest of the cylinders are very low & it would be worth doing the oil test on the rest of the cylinders.
If it is 8.13:1 you should be looking at figures of 135 minimum according to a 1977 LR factory manual I have for that engine in a carb'd 3.5 RRClassic
If you do decide to do the cam & followers I'd suggest you also change the timing chain & its gears.
Worth pulling the heads & checking the valves & seats & bore wear.
You'll likely find the rocker arm internal oilways (oil from the shaft flows through this oilway to the top of the pushrod) are well clogged (several of mine were blocked solid) & the rocker shaft will have a lot of crap inside it.
Been there done that!
Last edited:
jwrrc
Well-Known Member
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- 619
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- Warwickshire, UK
This is a bloody brilliant reply. Having a proper "I love Landyzone" moment over here
Well as it turns out it was all of these problems worn cam, valves carboned up soo bad they no longer seated properly, rocker oilways clogged, piston rings well worn, cyl 8 had gouges over 3mm deep and piston was swaying in bore.. but hey, the crank was fine as I just had it machined and there was already a brand new timing chain, I managed to pick up a fresh block for 300 bucks. Hooray
