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TR4 Intermittent misfire


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Not posted anything technical for a while but this one should get some heads scratching.  Had to pull out of the RBRR on Saturday evening due to a misfire problem.  The car goes fine through the gears and revs up to 5k with no hint of a misfire but when on a motorway in overdrive top at about 3500 rpm (so 80 ish) it starts to misfire and lose power.  Pretty much consistently- does somtimes pull for a while at 3500 but then a misfire sets it.  Car slows to 60 ish or less.  However, once youve left the motorway, generally it clears and you can cane it down a B road without a hint of an issue.  

I am running with an Accuspark distributor which has proved faultless for many miles.  However, swapped that by the roadside for a brand new one of the same make (new leads to coil supplied with it) and the problem re - occurred in exactly the same way. Changed coil and plug leads.  Had carbs apart and cleaned and blown out float chambers etc.  Fuel pump reconditoned by the chap recommended on here a few years ago but may be culprit?  Changed the inline fuel filter and there were what seemed like bits of fuel hose floating around -I wondered if high fuel demand meant these blocked the filter but lower flow meant these bits weren't a problem - however given that swapping it didnt change things, that theory was blown.

I feel its a fuelling issue but not really sure.

Any thoughts or observations most welcome.

Bob

 

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Hi Bob, is the tank vent working, if it's in cap try running with cap open.

Chris

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Are your fuel lines Ethanol  resistant material or are they starting to break down....black small lumps could be fuelpipe/ line degrading.

Edited by iain
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The vent is in the cap and seems fine but I'll give running with cap open a go.  

I agree with comment re fuel lines, new kit on order regardless of what has caused this as all looking a bit tired and bits in filter are a worry.

By the way, does anyone know if Dave Davies is still recondiotioning pumps  - ive got a spare to recon so may as well do that now if it comes to swapping the pump.  I used him before but cant find his details.

Forgot to say using  HS6 carbs that I bought fully reconned may  be 4 years ago but I wondered whether a sticking piston could be at fault  - fine when flying up the revs but perhaps just sticking when accelerating steadily from 3000 to 3500 in o/d top?

 

Bob

 

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  • 2 weeks later...

Well just to close this one off.  Thanks for all the suggestions.   Turned out to be fuel lines partially blocked -all renewed. Tank emptied and swilled out.  Very clear once renewed how much extra fuel flow there was.  Clearly pump could suck enough through to keep the float chambers full enough in normal driving but sustained high spead driving it ran short.   Useful though in that runs far better after everything else checked, cleaned  and tightened!  

Bob

 

 

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If fuel line was partially blocked, it suggests that your tank was shedding small particles into the fuel.

Have you given your tank the sloshing treatment to prevent this happening again?  (See Section K3 of the Technicalities CD). I sloshed my tank in 1997 and the problem has not recurred.

Ian Cornish

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Tank looks pretty good (from what I can see) and swilling with fresh fuel didn't produce any bits and pieces but may do sloshing treatment as well in time.  On my car, there is not a brass union to the tank (as shown by the manual) rather a rubber fuel hose and I think it was the degrading of this small section that caused the issue.  All renewed with ethanol resistant hose and copper now right the way to the carbs. 

Bob

 

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I too have a misfire on a TR4a.After changing to electric fuel pump/ new Dizzy Doc coil(now fitted to wheel arch, old one was red hot)changed plugs to the ones Roger recommended.

It still misfire's after a short run. After looking at the float bowls the rear was 1/4 full and the front 3/4 full. Ran the pump with the rear disconnected and pipe into jar, full flow of petrol,

bloody strange!. Every thing is new or rebuilt and has run with out missing a beat for 1500 miles.So onto the rolling road Wednesday and hopefully it will manifest itself there and they will find it at Turner Tuning in Apsley, Hemel Hempstead.

Fingers crossed.

Regards Harry

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1 hour ago, harrytr5 said:

After looking at the float bowls the rear was 1/4 full and the front 3/4 full.

 

Are your floats leak-free? I had a pinhole in one of the floats a few months ago.

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The solder used on original SU floats can be eaten by the ethanol in the fuel, with the result that the float gradually fills with fuel and sinks.  Burlen can supply floats made of entirely different material, and these are unaffected by the ethanol.

I know nothing about the floats in the Stromberg carburettors used on (some?) TR4As.

Ian Cornish

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Found it. Underneath the needle jet to the float was a minute piece of plastic film its diameter no bigger than 2-3 mm acting as a disc sometimes blocking fuel sometimes not.Believe it was left in there after machining and then assembled (new carbs).

Howard put me onto this after discovering swarf which was doing the same thing.

Good to talk.

Regards Harry

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