Technical Help me diagnose engine trouble

[smart51]

I'm out on my first proper run since buying the car and I'm in Henley in Arden. It's worth a short diversion if you're ever in the area. It has a picturesque half timbered high street with an excellent ice cream shop. The car has been running well but I'm having a bit of trouble today.

The engine feels down on power (I know!) And a bit hesitant. There are brief surges of power and brief lazy moments, changing maybe every quarter of a second. It sometimes dies at idle and doesn't respond to the throttle or choke when it's about to die. At 40 or 45 MPH it runs well enough but not quite as sweetly as usual, but when a hill comes or there's room to speed up, there's no power.

My car is a 500R with a 594cc engine, 28IMB carb and a wasted spark distributor. The coil, points and condenser are new. Where should I start to look for the problem.

 

[smart51]

Back at home now, if anything it was worse on the way back. The engine would die while waiting to turn right. It starts straight away but it doesn't have the power to pull away without a lot of revs.

 

[Goldnrust]

Hopefully the ice cream made up for the car running poorly! I'm yet to stop there, despite driving though Henley regularly myself.

It's hard to diagnose problems like this over the internet, but when you read these pages often you see valve clearances closing up, and condenser failures seem some of the most common issues people suffer with. I had the valve clearances on my engine close up recently, and it sounds quite similar to the symptoms you describe.

Incorrect points gap could also cause similar issues or a blocked jet in the carb. Those two are pretty easy to check too, handily the main jet on these IMB carbs can be accesses without removing the carb from the car.

If those 'easy' fixes don't get you anywhere then a bit of method, rather than just finger in the air, throwing ideas and money at it would be wise.

Check to see if you get a good strong blue spark, if it's a dull orange then it's worth starting with the ignition system. Check the condition of the spark plugs, they can tell you a lot about how much fuel the engine is getting or whether its burning any oil.

 

[smart51]

Investigations so far:

Brown spark plugs, indicating dry fouling and rich mixture. Otherwise look quite new.

Float height set to 4 and 16mm, so fuel level too high, leading to rich mixture.

The main jet is clear. I've not disassembled the carb any further.

Valve clearances were a bit too tight.

Points look like new, which the dealer said they were.

It looks like a combination of being too rich and tight valve clearance.

 

[the hobbler]

Investigations so far:

Brown spark plugs, indicating dry fouling and rich mixture. Otherwise look quite new.

Float height set to 4 and 16mm, so fuel level too high, leading to rich mixture.

The main jet is clear. I've not disassembled the carb any further.

Valve clearances were a bit too tight.

Points look like new, which the dealer said they were.

It looks like a combination of being too rich and tight valve clearance.

Check your slow-running jet (aka, idle jet)--this will have an effect on your both your idle and pick-up from idling. It only needs a tiny bit of muck init to affect it.

 

[smart51]

I checked the slow running jet before reassembling and it is clear too.

The car started and idled well when cold and came off the choke quickly. It is very much down on power at low revs and a little lumpy in the middle. When hot, it still dies at idle, though either a bit of throttle or a bit of choke help.

I wound on a quarter of a turn of the throttle stop screw. This let the engine idle well for a few seconds before almost dying, then recovering for a few seconds then almost dying...

What is a good next step in diagnosis?

 

[Goldnrust]

Have your adjustments helped the high revs / wide-open throttle?

If some choke helps to keep it idling, then you might need to re-adjust the idle mixture now that you've made the other changes.

 

[smart51]

An advantage of disassembling things in a metal pan is you can spot sill mistakes like this. Where should this tiny O ring be in my carb?

 

[fiat500]

An advantage of disassembling things in a metal pan is you can spot sill mistakes like this. Where should this tiny O ring be in my carb?

View attachment 427403

That looks like the "O"-ring that sits tight in a groove on the mixture screw/needle on the carburettor. It looks swollen and damaged, but if there isn't one fitted the adjustment of the carb will be impossible. New ones are a very tight fit.

 

[smart51]

That looks like the "O"-ring that sits tight in a groove on the mixture screw/needle on the carburettor. It looks swollen and damaged, but if there isn't one fitted the adjustment of the carb will be impossible. New ones are a very tight fit.

I've just looked. There is an o ring on the idle mixture screw, in a groove right be the head. It is the same size as the one I've found.

The idle jet has a bigger o ring, which I've replaced with the one in the gasket pack. The only other things I removed were the needle valve and main jet.

The mixture screw doesn't have a o ring at each end does it?

Edit to say I adjusted the idle mixture and took it for a short test drive. Cold idle is a little better, as is driving, though it is not right. When hot it won't idle and won't restart. Sometimes it will catch and instantly die. Sometimes it won't catch.

Also, a few times under trailing throttle, there was a brief whooshing noise from the back. What could that be?

 

[fiat500]

I've just looked. There is an o ring on the idle mixture screw, in a groove right be the head. It is the same size as the one I've found.

The idle jet has a bigger o ring, which I've replaced with the one in the gasket pack. The only other things I removed were the needle valve and main jet.

The mixture screw doesn't have a o ring at each end does it?

Edit to say I adjusted the idle mixture and took it for a short test drive. Cold idle is a little better, as is driving, though it is not right. When hot it won't idle and won't restart. Sometimes it will catch and instantly die. Sometimes it won't catch.

Also, a few times under trailing throttle, there was a brief whooshing noise from the back. What could that be?

Actually, loking at your screwdriver for comparison, that ring is bigger and might be for the valves on the head and is not all that important.
The mixture screw just has the one, tight "O"-ring.
Whooshing noise....no idea on that and can't even speculate. but have you thoroughly checked that there are no gaps anywhere in the intake route to the engine. As a long shot I would check that the two, hollow drilled bolts at each side of the head to block joint don't bear signs of having passed any gases or oil.

 

[smart51]

The o ring in the photo is the same size as the one on the idle mixture screw. I've looked at the weber service kit and it only has 2 o rings, one of each size. One from the idle jet and one from the idle mixture screw. The spare o ring must have come from my carburettor, but may have been lost in there by a previous owner.

Tomorrow, I think I'll look more closely at the distributor.

 

[smart51]

Points! The points needed adjusting. It now runs better than when I got it, especially at lower revs.

Just one small niggle, it I drop the clutch and let the revs fall, it might stall rather than idle. A bit of throttle to slow the drop in revs and it idles just fine. What's a good way of tuning that out?

 

[Goldnrust]

Success! That’s what we like to hear!

If you’ve adjusted the points gap, it will change the ignition timing, so I’d check that if you haven’t already.

Other than that, it’s just a case of juggling the idle stop and idle mixture screw till you get a nice consistent idle, and it shouldn’t ever try to stall when you let the revs fall back to idle.

If you made adjustments with the weak spark, before adjusting the points, you might find they are actually wrong now you have a more powerful spark to burn all the fuel properly.

Should feel like a new car after this thorough tune up!