Lumpy idle with backfire

[CJG]

Following a ca. 400-mile trip to Germany (cruising speeds 70-85 mph) which went without a hitch, my car has developed a lumpy, uneven idle, together with regular backfires (frequently from the exhaust, esp. at idle and on the overrun, occasionally from the carbs at speed, when changing down).

 

My suspicion is that there is a problem with the carb setup or the plug leads (plugs are all new NGKs, electronic ignition module has been fitted). Oil consumption is not excessive and the engine revs cleanly.

 

Any ideas?

 

Regards,

 

 

CJG

[Don Elliott]

During your trip home, did you almost empty the fuel tank? If the fuel level gets very low, sediment can get drawn into the fuel pipe. I had a similar problem with my 1958 TR3A (SU carbs and mechanical fuel pump) and solved it FINALLY (after two weeks) by half filling the tank with fuel and then putting my air compressor hose to the fuel line and I drove all the sediment back into the tank. Mine is running fine since then.

Edited April 11, 2010 by Don Elliott

[graeme]

CJG,

 

My suggestion is a burnt exhaust valve, possibly caused by high speed driving with a weak mixture. Try a compression test, should give a clue.

 

Cheers

 

Graeme

[CJG]

Thanks for the advice.

 

In case of a burnt valve, wouldn't I have noticed the overheating on the temperature gauge (it didn't move above 90-95 deg.). The car is fitted with a Kenlowe auxiliary fan.

 

I did, however, nearly run the tank dry.

 

Regards,

 

CJG

[88V8]

In case of a burnt valve, wouldn't I have noticed the overheating on the temperature gauge (it didn't move above 90-95 deg

No. Unfortunately, a weak mixture doesn't show on the gauge. Most of the heat in the cooling system arises from friction.

If it's burnt an exhaust, it will pop on the overrun, not sure I'd expect spitting back. As Graeme said, a compression test will show it up.

 

However, if you don't have an unleaded head, that period of high-speed cruising might have been the final straw in ongoing valve recession. Worth checking all the clearances.

 

Hopefully it will be as Don said, **** in the carbs.

 

Ivor

[ianc]

Whilst i agree that dirt in the fuel &/or burnt valve(s) are the most likely culprits, it is worth mentioning that leaks in the exhaust manifold can cause a considerable (and embarrassing) amount of popping and banging on the over-run.

Ian Cornish

[67_gt6]

Whilst i agree that dirt in the fuel &/or burnt valve(s) are the most likely culprits, it is worth mentioning that leaks in the exhaust manifold can cause a considerable (and embarrassing) amount of popping and banging on the over-run.

Ian Cornish

 

 

..as can breaking the tail pipe off the box on a speed bump. may not be noticeable at first glance

[Geoff359]

If the car has Stromberg Carbs, check the diaphragms!

[Badfrog]

If the car has Stromberg Carbs, .........

 

Clean the petrol line.

Check compressions and plugs.....

Dump the Strombergs.......

 

 

Badfrog, been there, done that, got no T-shirt......

[Chris59]

About valves recession, I have took off the head of a TR4 last week, measuring 1,5 mm recession per exhaust valve : time for hardened seat inserts I think

 

Chris.

[North London Mike]

Had a similar problem on a recent tour of Somerset. Car ran like a dream and three days in started to miss on one and then two cylinders (which is noticible on a four )

 

After checking spark and fuel both OK, drilled down to the next level and found out that the front two cylinders were out by pulling the plugs one by one. So, front carb then! Stripped it by the road side, nothing..............finally got to the in line filter and found that it had collapsed, starving the front carb and causing the miss under load. Charged the filter from stock, 100% fix. Real bugger to find though, 45 minutes

 

Moral of the story, keep looking.

[trfour]

Find the problem yet? Check those plug wires, boots & cap/rotor while your at it. Hopefully the ignition module is working properly. Ditto for the coil. Check your fuel filter, check the flow to your carbs. Sounds like an ignition/fuel issue. Good luck!

[CJG]

Find the problem yet? Check those plug wires, boots & cap/rotor while your at it. Hopefully the ignition module is working properly. Ditto for the coil. Check your fuel filter, check the flow to your carbs. Sounds like an ignition/fuel issue. Good luck!

 

 

 

Thanks, yes - I think I managed to sort it last night. Firstly, the rotor arm was fit only for the bin - a new one made a big difference. I also changed the plugs and discovered, quite by chance, that the capacitor wasn't connected up properly - the lead to the distributor cap was just resting (!) on the contact (sometimes).

 

We haven't been for a test run yet, but she ran pretty smoothly in the garage. If the problem returns, I guess it's the air filters next (the last lot of plugs was quite sooty).

 

Regards,

 

CJG