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Exhaust backfire and lack of power after high revs


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Hello - I'm a newbie on the forum so apologies if this is a repeat of anything already posted.

I have a fully restored, rebuilt 74 CR PI that until today has been running really well.

Started it today, fired first time as usual but noticed tickover was lumpy so checked injector hoses for a pulse and number 6 was dead - obviously had air in it so I bled it accordingly - few taps on the jar and I got a good spray cone- I put it down to hot weather and lack of use, although it has been started most weeks.

I took it out for a decent run -I've just got to 3,000 miles so decided to drive it harder than usual today hit 5000rpm a few times and the car was fantastic but then on another hard acceleration the engine coughed, exhaust backfired and I lost power but didn't cut out.

Managed to limp home but any revs over 2000 had backfires and no power. 

Engine ticks over OK and I have good pulses at all the injectors. 

Checked all obvious (to me anyway!) things like vacuum pipe, HT leads, and coil connections etc. 

Running a Prestige Bosch style pump, Metering unit injectors etc. new fuel lines and so on.

Any suggestions of where to start looking would be much appreciated

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95% of the time it is ignition.

In this case I would replace the condensor first. It causes exactly these symptoms and it is cheap and easy to do. I carry a few spare condensors in my 'TR' travel bag for the TR2s (my 6 has electronic ignition.)

If that doesn't sort it the coil is next. I carry a spare coil in the boot so can just swap it over.

Then distributor cap for cracks and the centre lead is OK. Replace the points if fitted. Plug leads then plugs.

Another one is the alternator, sometimes the car will start and run but if the alternator isn't producing the electrons at revs it can cause these symptoms.

It is unlikely to be fuel but if all the above fails start there.

Do it one thing at a time so you can identify what has failed.

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Welcome to the forum and TR-tinkering.

All of the above. If the distributor rotor has a rivet, replace it by a red rotor stamped with DD. It is from DizzyDoctor. The riveted repro’s are a known weak part.
Waldi

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Thanks for all the replies everyone!

I have powerspark  viper ignition coil and electronic ignition fitted - timing OK and distributer tight. Metred the coil today and both readings seem ok but as a start have orderd another one to see if that sorts it.

65 AMP new alternator fitted so would be surprised if thats at fault

I have to find time to pull the plugs out but they are NGK Iridium ones and I will swap the HT coil lead as I have a spare.

 

I will reply when I have found out more

 

Thanks again for replying its much appreciated

 

Cheers David

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8 hours ago, John L said:

Some of the Hall effect electronic modules don't like restive plugs, hope those iridium are not restive.

Also only use silicone HT leads not wire ones.

John

John, I'm interested in this. I put an Accuspark hall effect in my TR2 and the car ran fine for about 4,000 miles. It then refused to rev above about 3500rpm, missing and carrying on.

The supplier sent me a new one free of charge and we overhauled the distributor at the same time. Same problem. I put the points back in and problem solved. 

The car has a generator/dynamo and I believed that this could be the problem. The Accuspark apparently doesn't like the higher voltages that generators charge at.

I didn't change the plugs or leads.

Can you expand on the plugs and leads as an issue with hall effect points?

 

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3 hours ago, John McCormack said:

The Accuspark apparently doesn't like the higher voltages that generators charge at.

???? The voltages are the same unless the regulator is wrongly adjusted. They are electrically noisier though because of the commutator and the contacts in the regulator.

I believe these Hall-effect switches are very sensitive to radio-frequency noise so are rather picky about the HT side of the system. Hence the statements about not using copper leads and resistive plugs, the technical reasons for which are never properly explained by any of the makers.

 

Edited by RobH
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15 minutes ago, RobH said:

???? The voltages are the same unless the regulator is wrongly adjusted. They are electrically noisier though because of the commutator and the contacts in the regulator.

I believe these Hall-effect switches are very sensitive to radio-frequency noise so are rather picky about the HT side of the system. Hence the statements about not using copper leads and resistive plugs, the technical reasons for which are never properly explained by any of the makers.

 

You will find a generator will regularly charge at over 15 volts. Put your multimeter on it and check. I have seen over 16 volts with a low battery and a properly adjusted regulator.

I was told afterwards that the system didn't like more than 14.2 volts.

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12 hours ago, John L said:

Some of the Hall effect electronic modules don't like restive plugs, hope those iridium are not restive.

Also only use silicone HT leads not wire ones.

John

I would have thought it was the other way round - i.e. non resistive ones may cause interference with the electronic points?

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John L thanks for the information - very interesting 

I have silicone leads fitted but I will have to research the spark plugs- I have not heard the term restive before but will later .

I've got plenty to check out now!

I'm using NGK BPR6EIX iridium plugs that have ceramic resistors according to the spec on the NGK website so I would assume they wont be interfering with the distributor 

I have discovered that the HT leads are Not silicone after all  so will start by replacing those

 

 

 

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Basically they have to be the carbon string, not wire, and shouldn't  have resister plug ends either, when I talked to Accuspark, I wanted to fit resister plug ends to stop a bit of radio interference but they advised not to.

Their site is good with a lot of information in the small print.

I have fitted many of their kits to lots of friends cars and my own and not had any problems. hope I haven't jinks it now.

John 

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  • 2 weeks later...

Hello

i have had in the past exactly the same symptoms on a porsche....after quite long search and some electric part replacement, looking further inside the engine showed 2 valve springs broken

...if it can help...

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