Sunbeam Alpine cough

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Sunbeam Alpine cough

#1 Post by alpine1592 » Tue Oct 15, 2019 10:52 am

A brain teaser for you!
My car, which I have owned for 40 years, is a 1965 Sunbeam Alpine Series IV, 1592cc engine with a 28/36 Weber carburettor.
About 4 years ago it developed a backfiring cough when accelerating, particularly going uphill. The cough happens when pushing the revs up around 4000rpm in second or third gear and takes the form of a backfire through the carb (flames out the top with air filter removed) which causes a cough and stutter. The problem clears if I slow down and crawl up the hill!

So far I have:
• Replaced the fuel line from tanks to carb.
• Fitted a quality electric fuel pump to replace the mechanical one – including an in-line filter
• Overhauled my current Weber 28/36 at least 3 times and tried a replacement newly reconditioned unit in its place. Both carbs properly set up with recommended jet sizes.
• Fitted new coil, new distributor with electronic ignition, new HT leads, rotor arm and spark plugs.
• Fully reconditioned the cylinder head with new valves, guides and springs. All valve clearances and timing set correctly.
• Replaced the servo (after finding fluid in the drum and some evidence of it getting into the induction system – I thought that was the one!)
• Visited two diagnostic tuners, both of whom have recreated the problem on the rolling road but neither of whom have diagnosed the problem.

The problem still persists so any ideas welcome!

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Re: Sunbeam Alpine cough

#2 Post by harvey » Tue Oct 15, 2019 2:41 pm

Can you replicate the problem by revving the engine to 4000+rpm at a standstill, or does it only happen under load?
Currently over 35 years worth of fixing 35 boxes.
Hoping to reach 65 years worth of fixing 65 boxes.

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Re: Sunbeam Alpine cough

#3 Post by alpine1592 » Wed Oct 16, 2019 12:30 pm

Havent actually tried that so good call Harvey. Will give it a try and come back to you.

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Re: Sunbeam Alpine cough

#4 Post by alpine1592 » Mon Oct 21, 2019 10:33 am

I did a check over the weekend and could not replicate trhe problem with the car at standstill. Revved up to 4500 for 30 seconds or so and no coughing so looks like the problem only happens under load. What doesn't help though is the fact that the problem is intermittent and I have driven a full day uphill and down without the issue occurring. It's a real head scratcher!

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Re: Sunbeam Alpine cough

#5 Post by JPB » Mon Oct 21, 2019 12:16 pm

Is this a decent electronic ignition setup, such as a 123 or is it an Accuspark (or even worse, a S*m*n B*C one)? It's not unknown for failing electronic ignition components to work well enough when there's no great load on the system, but to drop out, work intermittently or fall apart completely when they're asked to do anything vaguely stressful. Many of these cheap systems are built with generic advance curves that cannot hope to work correctly in every single car that uses the same distributor body, where better ones are at least supplied to suit the application other than by physically fitting into the available space.
"Home is where you park it", so the saying goes. That may yet come true.. :oops:

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Re: Sunbeam Alpine cough

#6 Post by suffolkpete » Wed Oct 23, 2019 3:33 pm

You seem to have tried all the usual things. Is there something in the fuel tank that's intermittently partially blocking the outlet so it's not getting enough fuel under load?
1974 Rover 2200 SC
1982 Matra Murena 1.6

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Re: Sunbeam Alpine cough

#7 Post by jpsh120 » Wed May 13, 2020 9:39 am

Is the problem worse when the engine is hot, or does it do it from cold also?
1971 Sunbeam Rapier H120

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