V8 engine trouble - any ideas?

[malcolm]

This doesn't have much to do with Renault 4s but maybe you guys can think of something I can't.

The problem is with my MGB V8. The engine starts and runs really sweetly with excellent power, idle etc. It's great until a couple of miles (long after the choke has gone in). Then it misses badly - not firing for a few turns and then picking up again. It seems temperature related - if I leave the car for a few hours it will normally be fine again. The problem seems to be getting worse.

I've tried plugging an ignition strobe in (one of the inductance ones that goes around the plug leads). Sure enough, periods of not firing correspond with no flashing of the strobe. That made me think it was ignition. It's a Mallory distributor. I've tried a spare coil, new leads, new cap, rotor arm, points, low tension wires, condensor, vac advance. I even tried an earth wire on the ignition base plate in case the earth was poor.

The carb has fuel in the float chambers. I did wonder whether it was flooding, but the car seemed just as bad with the fuel pump disconnected which ought to rule that one out. The symptoms are so similar to fuel starvation that I'm feeling fuel could be something to do with it. I wondered about vaporisation but the problem seems to happen before everything gets too hot, and it's not a town issue. It's a Weber 500 4 choke carb. I've had the top off and blown out the jets - it's all clean and filtered so I don't think it's dirt.

The engine itself is nearly new and has compression, good valves etc so I don't think the problem is mechanical. Besides, when it runs it runs perfectly.

I'm completely baffled. Has anyone had a problem just like fuel starvation that has turned out to be something else. I think the clue is it happens when the engine is warm, but not too hot. Any ideas would be much appreciated.

:clementi:

 

[Steve Hennessey]

Hello Malcolm,

There are just 3 things I can think of. Firstly, I think the coil on this is a ballast resistor type 6 volt. Dodgy ballast resistor maybe?

Going along the thoughts of fuel starvation, is there an inline filter in the fuel pipe that looks OK, but is blocked. Or, is there a gauze filter in the tank on the end of the pick up on the sender unit that may be blocked with silt.

Also, on a lot of Webers, there is a small gauze filter inside the top of the carb body where the fuel goes in to the float bowl.

It has got to be one of these.

Steve

 

[malcolm]

The problem has hopefully become a whole lot easier to fix - the car broke down completely yesterday and I had to resort to a breakdown truck. Of course they insist on sending a bloke along to try and fix it first. His comments - "it's got fuel, it's got sparks - what more does it want"

Thanks for the ideas Steve. I've just tried a non-ballasted coil with a seperate voltage feed. Still no luck. I've got good fuel flow into the float chambers. I'm going to give the carb a really thorough clean up, then pull the dizzy off for another check and set the ignition up off an auxillary battery. My current feeling is the spark is a bit weak - maybe an engine earth. Any other ideas much appreciated.

:yellowre: <---Reliable car: much better than V8 MGs.

 

[malcolm]

Ahhhh - I think figured it out. It's running incredibly rich. Just like having the choke fully out. That would explain why it stops working when it starts to get hot - it fouls the plugs and the sparks can't get across.

I wonder if the fuel pump is running at too high a pressure and is overcoming the float chamber valves. There certainly seems to be fuel where there shouldn't be fuel, although I fitted a manual switch to the pump and the engine seemed to run better with the pump connected. Maybe it's the jet specs. I'm going to order a regulator and check the jet specs. Seems like it's starting to fit in with today's set of problems (mucky plugs and petrol smell from exhaust).

Fingers crossed....

Edit: The floats weren't set up correctly causing the float chamber to overflow into the carburettor. That's sorted now. Also the jets seem correct.

And still the Bl**dy thing won't start. Even after plug cleaning. I'ts got to be ignition (once more). I've found in the past that tricky problems are normally a combination of 2 or more different problems - that makes them hard to diagnose and fix. I'll maybe take a break from it and come back later with renewed energy.

 

[malcolm]

I think I might have figured it out. What if there was a really small crack in the (heated) inlet manifold. A crack between the water passages and the intake passages. When the car was cold - no problem - no water would pass through the crack. When the car gets hot the water is pressurised and starts squirting water through the crack - which puts the fire in the cylinders out.

I noticed a puddle of water under the manifold (and under the car). Hmmm. I'm going to do a pressure check.

Edit: No that's not it. I've ordered a whole load of new ignition bits. Maybe it's simultaneous failure of 8 spark plugs. Yep - that's got to be it.

 

[malcolm]

Whoopeeee - it's fixed.

Turns out the spark plugs were of the surpressed variety, and the ignition leads weren't the best quality. Everything seemed to be working, but the spark just wasn't enough to overcome the torrent of fuel the carb is capable of delivering - particularly when everything warmed up a little.

While I was figuring things out I wrote a whole lot more about the problem - see V8 engine trouble on VORD.net