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6 cyl Bristol cars Type 400 to Type 406

B 400 - backfire

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Old 05-06-19, 04:34 PM
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Default B 400 - backfire

During measuring the voltage at 3000 rpm all the carburettors produced backfire. I'd heard that backfiring while driving before but not as many as during that test. Any hints?
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Old 05-06-19, 09:51 PM
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Bela,
Have you balanced the carbies accurately?
Are the idle mixtured adjusted?
Is the ignition timing correct?
Have you replaced points and condenser?
Does the engine run smoothly at idle ?
Are all the jets clear and not blocked?
Do the have the sparkplug leads connected as per the firing order?

These are just a few of many things to check.
Geoff
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Old 06-06-19, 05:10 AM
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Hello Geoff,

I'd hoped the difference between running idle and under load would give an idea.

I guess the smoothness during idle is not that what could be expected from 6 cylinders. But the heart of the car is an old engine.

The car got a 468 GBP service before I bought the car. Since that service the car ran 260 mls. NOS contact set, rotor arm, ignition condensor, new spark plugs (NGK C6HSA) and piece of H.T. lead are on the bill. A new NGK coil is also mentioned.

The carburettors have been inspected and ignition timing was checked.

Idle mixture srews, throttle butterflies synchronisation and throttle linkage had been set to correct idle speed.

Setting the valve clearance was also part of labour which all has been done by a motor engineer.



It seems I should go the steps again as far as I'm able but I'm sure my skills are much lower than that of the motor engineer. I hope I will get in contact with some one who really knows. Most mecanics are searching the connection for a diagnostic tool.

On the other hand the specialist inspected the Enot one shot lubricator but there was a lot of oil along the chassis. No words in his report. It is not the oil line going out of the Enot pump and I still investigate to find the failure.

I'm supported by ear phones so any sound is difficult for me - e.g. I did not remark that only one horn is working and it was my son in law who made me sensible for the backfire during driving the car. Getting old does not have benefits allways ;-)

Last edited by Bela; 06-06-19 at 05:17 AM.
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Old 09-09-19, 07:49 PM
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Adjusting correct valve clearance and changing 2 high resistance HT lead plugs made a big difference. No backfire into the carburettors and better performance at all. Tomorrow a longer drive will show the facts.
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Old 11-09-19, 07:05 AM
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The ride shows better performance at higher revs and only one backfire for 1.5 hours. Near 3000 revolutions the engine is more powerful than before.

But stop and go in town is not as good as driving in the country. Idling is not quiet and I had to notice insufficient power at low speeds.

I will replace plugs, leads and distributor cap.

What is the best range for ignition with modern fuel? At the moment it is 5 (crankshaft) static.
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Old 13-09-19, 07:26 AM
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Bela,
I suggest you try to achieve the following depending on condition of your Lucas Distributor . (At 70 years old it is doubtful it is working correctly .
Even new they were not top quality items .)

The advance curve I have used on many different Bristol engines is

Static 12 deg BTDC
3000 rpm 36
5000 rpm 43

Is your TDC mark accurate ?

I have an new old stock original Lucas cap if you are interested, but recommend you check if the distributor bob weight system and springs are working correctly before you spend money on any Lucas Distributor .

What type of plug leads do you have ? I only use wire leads .

Geoff
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Old 13-09-19, 07:59 AM
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Thank you Geoff for the suggestion - I will try that.

The TDC mark is fine and I use copper wire leads.

Thank you for the offer. I'd purchased a distributor cap by REMAX. If it will fail I'll come back.

Winfried
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Old 14-10-19, 05:35 PM
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After changing the distributor cap, all leads with plugs and adjusting timing to 12 BTDC I'm satisfied with the power (but not with the REMAX cap). Only hill climbing in 2nd gear with low speed at 1500-1800 revs and 10 percent incline is a suspiciously lame affair. Is there a way to change into first gear without stopping or is it strictly forbidden?
The REMAX cap doesn't fit the rotor arm. After 100 mls the graphit pin has been milled down. May be the pairing is wrong and I need an other rotor arm.
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