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68 Bentley T engine rattle under load.

4138 Views 107 Replies 12 Participants Last post by  M635uk
Hi all, I have been restoring my T1 5139 for the last 6 months and have completed the interior etc, however I have an annoying engine rattle under load which I cannot diagnose. L believe engine mileage may be 143k.

It feels like it is a collapsing hydraulic tappet, but all nice and quiet at idle or steady load or decellerating. I have had all the tappets out, tested them with the proper RR leak down tester and found 3 that would leak down very quickly at 2 seconds, 5 seconds and 7 seconds. Minimum should be 20 seconds maximum 90seconds. So the 3 weak ones were replaced with known good ones, put it all back together and the same rattle persists.

The noise is engine speed related, feels like it’s coming from the engine gallery area and is more like a hollow rattle than a knock. The noise increases if the load on the engine increases. It’s not the hydraulic brake pumps by the way. Those have been removed and the braking system has been converted to a conventional dual master cylinder and remote servos.

Would anyone have some ideas of where to look next? Weak valve springs? Cam looked good, no lobe wear.
Any assistance appreciated.
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Should be easier to locate with a stethoscope or similar .
Hi, thanks, but I can’t use a stethoscope when it’s under load I.e driving down the road and at idle it’s all quiet. To be more precise the noise is only there when car is accelerating. No noise when reving the engine.
If you can find a shop or friend with "chassis ears" you may be in luck. They are very helpful
Yes, I could do with an old boy, take him up the road. It does it instantly you accelerate and it’s a very distinctive rattle. I will do a vacuum test as well to see if that indicates anything. Also might run it on 7 cylinders and see if that pinpoints anything.
I would bet the timing is off causing valve chatter upon acceleration. Check your timing before you do anything BTW chassis ears are a series of microphones and can locate these issues.
had pinking before in a different engine, more tinny noise than what I have here. Definitely sounds mechanical to me. But I will check timing thanks.
How about big-end knock.
Maybe oil pump strainer is clogged, just a wild guess.
Check timing at 2000 RPM maximum advance should be no more than 25°.
Smoke test for possible vacuum leaks.
Check mixture strength, if carbs set too lean pre-ignition will result.
Check compression, heavy carbon buildup in combustion chamber will cause pre-ignition, more likely in early 9:1 engines.
Test drives each time with different plug lead off spark plug and plug wire grounded. Plug wire must be grounded out to prevent flashover in cap or rotor causing damage. This may pinpoint which specific cylinder is causing the issue. Do this by removing plug leads from cap one at a time and have a short well grounded high tension wire near the cap that you can plug into cap socket that plug lead is pulled from.
Those tests should help pinpoint the fault.
Isolating cylinders one at a time on test drives should help determine if it is a gudgeon pin bushing or connecting rod bearing. Both of those would give a lower pitch sound than pre-ignition pinking would.
Wow, some great responses here, overwhelmed by the input and and also the speed of replies. Great that so many ideas have come in. I will take a methodical approach. Ignition seems plausible seeing as its load related. Hoping its not gudgeon pin bushing…I will keep the updates coming. Many thanks.
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Ok time for an update. I checked the ignition timing at 1200 rpm, it was bang on 15 deg so set correctly.
I then moved onto the carbs and progressively richened both up, engine seemed to run sweeter but still this annoying rattle every time I went for a quick drive. Both carb jets are now about 4mm below the bridge of the carb. So I don’t believe it is pinking or I have timing issues or running lean.
it is definitely a hammering noise, quite a hollow sound, rattle like and engine speed related but only under load.
Any further thoughts?
Also carried out a vacuum test, hopefully the attached photo shows it. Rapid Flickering around 21in Hg. Max 22 min 20.
Gauge Measuring instrument Font Gas Circle

compression test results on all cylinders shows around 120 psi mark except for cylinder 6, that one is down at 60 psi. So I carried out a leak down test on that cylinder, it’s leaking into the inlet manifold at TDC for that cylinder. So a leaky inlet valve I may have. Not sure if that going to account for the rattle though.
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Might try to see if I can identify if it’s related to a certain cylinder by grounding each plug wire in turn. Starting with cylinder 6…
Pull the rockerarm/valve cover off the suspected side. Check the springs on the rocker arm shaft. When broken, they can cause clatter with the rocker arm nearest the spring.
Ok a further update now.
Swapped out a spare set of rocker shafts and rockers I had which were in very good condition. Still the rattle so it’s not weak/broken springs on the rocker shaft. Also isolated each cylinder plug lead one by one - rattle is still there so not able to identify it To a particular cylinder.

However I did manage to take a quick video of it. Link below at the bottom.
It sounds a little alarming but that’s probably my phone. The two rattles you can hear are from two quick accelerations I did. With this problem still being load related could this be a gearbox issue? Oil level in gearbox is correct.
As ever any further comments/ thoughts welcomed. Thanks

Bentley rattle
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