TR7 - poor running

[Paul_F]

Help please.

I am recommissioning a 1980 TR7 but I am struggling to get it run properly. Sorry this is a bit long, but trying to preempt the questions and suggestions!

Symptoms:

Starts cleanly, idles OK maybe a little lumpy.

Drives ok at low throttle openings (up to 25%).  Gets worse at higher loads, no power a wide throttle opening and what sounds like a bad misfire.

 

What I have tried:

I have checked spark energy on each cylinder using a tester which shows the spark will jump a very large gap in free air (over 1cm).  This is well into the green scale on the tester.

I have fitted new plugs.

I have checked the timing at idle with a strobe.  I have also checked that if I rev the engine the timing advances, and if I suck on the vacuum advance the timing also advances.

I have tuned the carbs, finally resorting to a colour tune.  Set rich and then leaned off so the combustion is just going blue.

No obvious air leaks on the intake, but difficult to be sure.

Petrol was in plastic Jerry cans, maybe 6 months old.

Previously I have done a compression check and it looked OK

 

Any suggestions welcome, especially if you have experienced similar symptoms!  

Thanks

 

[RobPearce]

It does sound rather like a weak spark. I had very similar on a VW Passat with a generally very tired ignition system, especially when retarded "for unleaded". Presumably you did the test at idle? Does the problem seem worse at higher revs? I had a Dolomite 1850 that effectively imposed a 3500 RPM rev limit because the dwell angle became too short. Idle spark would still show good energy in that case.

It may also be worth richening up a little. The TR7 engine isn't as unhappy with lean as, say, a Vitesse but it's much less happy than a modern car (or even a Sprint). Also try some fresh fuel as the modern stuff has a short shelf life, even in cans.

[Beans]

+ Loosing power at higher revs can also be caused by a partial blockage of the fuel line, leaning the mixture at part throttle and finally starving the engine at higher revs.

[glang]

Possible fuel pump problem? On a mechanical pump if either of its non return valves doesnt seal well some fuel gets pumped through but then as demand increases its not enough. To test this you could try to get up to as high a speed as possible then, when it starts to misfire, lift off the throttle for a while. This should allow the carb reservoirs to refill so that on accelerating again there should be enough fuel to run well for a time...

[Paul_F]

Thanks for the thoughts.  Taking them in turn.

(Hello Rob!). It does feel like spark, I sensed a periodicity to it, but maybe over lean could give a similar effect?  Yes, I did the spark test at idle, so you are right, if the dwell angle were too short it could show up at higher revs as the dwell time obviously shortens.  However the problem seems much more correlated to load rather than rpm.  I should re-check the points gap of course!

The second two suggestions are questioning fuel starvation.  This was my first thought too.  The reason I am less convinced is that the problem occurs more or less as soon as I open the throttle.  I was assuming fuel starvation would take a while (20 seconds at least?) to show up as the fuel becomes depleted in the float chamber.  This may be a wrong assumption of course...please let me know!

I can certainly put some fresh fuel in, strange that it starts on the button if it is duff fuel though.  Is this what you would expect?

Thanks for the thoughts.

[RobPearce]

My old Toledo had a fuel starvation issue - build up of crud on the gauze filter - which caused loss of power at higher load. However, as you assumed, it was not immediate - I could accelerate up Twentypence Road for some distance before it faded out, and it would recover only a short time after lifting off.

If the spark is weak - even if only at higher revs - then it often won't cause a problem at light load. That was the very obvious thing on that old Passat - light load was fine but it would misfire really badly at higher load. It's a lot easier to produce a spark at lower pressure. On a microscopic level, a spark is generated when the electric field accelerates free electrons fast enough to knock other electrons free of the atoms they hit. A stronger electric field obviously help this, but low pressure means the atoms / molecules are further apart so the electrons have more chance to build up speed. Even mains voltage is enough to make a spark jump a 1cm gap if you pump the air down to a near vacuum. At light load, the spark is advanced and the in-cylinder pressure is only a couple of atmospheres at the crucial moment. With the throttle open, ignition retarded, you need to create a spark at five or six atmospheres pressure, maybe more.

[Paul_F]

That is helpful, I will follow the weak spark route and let you know how I get on.

 

[Paul_F]

Hi, a quick update.

I have now fitted  an Accuspark electronic ignition.  Some more fettling required but performance much improved.  No longer has the catastrophic misfire.

Easy job, but removing the distributor is a PITA.

Thanks for the help.

Paul