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6 cyl Bristol cars Type 400 to 406 - restoration, repair, maintenance etc

401 Tune Up

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Old 15-10-19, 02:14 PM
Join Date: Jul 2019
Location: Westward Ho, Devon
Posts: 22
Default 401 Tune Up

I have a 401 which I have only owned for a few months and it came fitted with a 123 distributor but also the original Lucas in a box of parts. She has been at Dave Baskervilles in Barnstaple for the last week or so as I have known him for years and he is the only outfit with a rolling road in these parts. Anyway it seems my 401 had a couple of sticky tappets, carbs needing a tweak but he tells me the 123 had an intermittent fault. She was certainly misbehaving by way of occasional snatching etc. I am due to pick the car up tomorrow so will know more then. My 401 has a 403 head but not sure re the cam (401 or 403) but I know that Dave will play around with the advance etc to get the best out of her (I hope). I will let you know the results in a day or so. We are in North Devon by the way.

I can now update you on the 401 tune up. Dave Baskerville balanced the carbs adjusted the tappets (some were at zero clearance) and detected an intermittent fault on the 123 distributor so replaced it with the original Lucas unit with a new coil and capacitor. He then adjusted the static and dynamic timing to get the best results on the rolling road. I have to say as I drove away today the car felt transformed. Smooth idle and power delivery, and more of it as well. I was always suspicious of the electronic 123 so am very happy to be back in mechanical land. I will now try and scan the dyno report but if this does not work she gave 86bhp at 4300rpm at the flywheel. The torque was 107 ft lbs at 3975, so quite a cammy engine. It certainly comes to life above 2500 on the open road. My next job is to sort the clutch judder so I will be attempting to remove the gearbox myself. Mmmm.
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File Type: pdf bristol 401 power curve.pdf (573.5 KB, 22 views)
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Old 15-10-19, 09:25 PM
Join Date: Jul 2008
Location: Sydney, Australia
Posts: 386

Ralph M ,
Thanks for your interesting post .

I dyno tuned my 400 on a rolling road about 15 years ago with similar results .
It's engine at the time was 85A block/crank/rods , 8.5 pistons , 403 cam , 100 head , 34ICH Weber carburettors and Mallory twin point (adjustable) distributor .
It achieved 71bhp at 4600rpm (at the rear wheels) . The operator thought it would be around 95 -100bhp at the flywheel. He was amazed by how much ignition advance was needed to achieve maximum bhp at various revs through the range . From memory the advance curve was
Idle/Static 12BTDC
3000rpm 36
5000rpm 45 or 47

Could you please advise the advance curve that your engine is now set on .

Distributors on a Bristol engine need to be properly earthed.
If one has the advance/retard mechanism operational it is best that the distributor body has an earth lead to the engine, as a smear of oil can give a bad earth . I have found that once the distributor advance is set, the distributor should be locked into position giving a positive earth as it is then not necessary to be adjusted.

Do you know what advance curve was used in the 123 distributor.

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