smart51
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| posted on 27/7/23 at 04:01 PM |
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Can you help me diagnose engine rough running please?
My classic Fiat 500 has been running well but suddenly wasn't the other day. Out on a run, it started to feel a bit lumpy and hesitant. It was
noticeably down on power. It seemed to run OK for a quarter of a second then hesitate, then run for a quarter of a second, then hesitate. When it
warmed up it would quietly die while idling. Every time I waited for a gap to turn right. Extra throttle or choke wouldn't keep it alive. It
would fire right up again though.
For reference, it is a 594cc air cooled twin. It has a twin coil, wasted spark ignition from a later Fiat 126 which has points but no
"distributor".
I've done a few checks. The spark plugs are brown, suggesting rich running. The carburettor float was set a quite high which would cause rich
running. The carb jets were clear and clean. The valve clearances were a bit tight, which causes problems when the engine gets hot. The points,
condenser and coil are brand new.
I've adjusted the float and clearances and done a test drive. The engine is severely down on power at low revs and a bit rough at running speed.
It still dies at idle when warm but now a bit of throttle or choke keeps it alive. I've set the throttle stop screw slightly more open. Now it
idles for a few seconds, almost dies and recovers over and over in a cycle.
What do you think it could be? What should I try next?
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gremlin1234
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| posted on 27/7/23 at 04:15 PM |
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check the fuel filter
also (less likely but can occur) check if there is a vacuum in the tank.
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rusty nuts
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| posted on 27/7/23 at 06:44 PM |
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From memory, it has been best part of 50 years since I worked Ona 500 with running problems but at the front and the rear of the engine there is a
bolt with a hole drilled through it . When the engine is running is air being pumped out of either of the bolts? If so you have a head gasket problem.
Tight valve clearances will cause problems as will air leaks on the induction side and incorrect ignition timing . It might be worth checking the
carb jets aren’t blocked?. Wouldn’t be the first time new points or condenser have been faulty . Not familiar with the distributor you have but are
the points correctly adjusted and have you tested for correct ignition advance?. Have you tried pulling the plug leads off with the engine running to
identify which if any cylinder is or isn’t firing?
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smart51
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| posted on 27/7/23 at 08:02 PM |
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I've checked the main jet and idle jet, both are clean. The only fuel filter is the one in the carb inlet - that looks OK. Running with the
fuel filler open makes no difference.
I've tried adjusting the idle mixture. It seems to make no difference at all.
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coyoteboy
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| posted on 28/7/23 at 07:21 AM |
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Brown or grey plugs is normal running.
You have a problem idling which is improved with throttle or choke.
Throttle will squirt additional fuel. Choke will reduce air (increasing fuel).
Jets are clean. Filters are clean. Spark is good.
Sounds like an air leak post-carb, or a compression problem?
[Edited on 28/7/2023 by coyoteboy]
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Mr Whippy
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| posted on 28/7/23 at 08:13 AM |
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If it was rich to that extent it would be chugging like on choke, the plugs would be sooty and black smoke would be coming out the exhaust. It sounds
like it is running lean as you need to add more fuel using the choke.
That you only have a filter in the carb and this is a very old car does open up the risk that crap is falling off the tank and is in the fuel.
I'd definitely be pulling apart the fuel pump too as it could simply have a valve jammed and not flowing enough and the float level is too low.
Most pumps also have a gauze filter which can become blocked and lead to fuel starvation. I'd recommend you add a large easy to clean filter
between the tank & the pump. If the inside of the tank is becoming rusty it would be wise to have it steam cleaned and coated.
I'd love a Fiat 126, they look tremendous fun 
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smart51
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| posted on 28/7/23 at 02:38 PM |
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Turns out it was the points. While they were new, the dealer that fitted them didn't gap them correctly. The car now runs better than when I
got it.
There's just one niggle left. If you drop the clutch while the engine still has a few revs on it, it drops so quickly it sometimes just dies.
If you wait until the engine is almost idling, or if you use the throttle to slow the drop in revs, it idles just fine. What is a good way to tune
that out?
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cliftyhanger
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| posted on 28/7/23 at 03:44 PM |
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increase teh idle speed a smidge.
revs tends to fall below the idle speed before increasing a little. If idle is too low it can stall. Play with the timing too. No experience with a
500, but plenty with 60s/70s cars. Timing is best set by "ear". Keep advancing until it pinks under load, then back off until it stops
pinking. Of course, that affects idle speed.
[Edited on 28/7/23 by cliftyhanger]
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smart51
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| posted on 28/7/23 at 05:32 PM |
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quote:
Originally posted by Mr WhippyI'd love a Fiat 126, they look tremendous fun
I set out with the idea of getting a 126.
They are an improved version of the 500. More interior space, bigger engine, synchromesh. A bit more safely. But looks that were "modernised
in the 70s". It was the latter that put me off. I went with the 500 because it was the original, because it looks nicer, and because I got one
from last few years where fitted the bigger engine from the 126. Best of both worlds.
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Mr Whippy
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| posted on 28/7/23 at 07:30 PM |
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quote:
Originally posted by smart51
quote:
Originally posted by Mr WhippyI'd love a Fiat 126, they look tremendous fun
I set out with the idea of getting a 126.
They are an improved version of the 500. More interior space, bigger engine, synchromesh. A bit more safely. But looks that were "modernised
in the 70s". It was the latter that put me off. I went with the 500 because it was the original, because it looks nicer, and because I got one
from last few years where fitted the bigger engine from the 126. Best of both worlds.
Glad you got it running better, there's not much to go wrong on these old cars. I'd still fit a better fuel filter. Make sure to you rust
proof it well. I grew up seeing a little 126 going past my primary school driven by a tall woman. Always though it was really funny.
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rusty nuts
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| posted on 1/8/23 at 03:47 PM |
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Unfortunately a lot of modern so called “technicians “ haven’t a clue when it comes to things like points or valve clearances
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adithorp
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| posted on 1/8/23 at 05:59 PM |
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quote:
Originally posted by rusty nuts
Unfortunately a lot of modern so called “technicians “ haven’t a clue when it comes to things like points or valve clearances
It's so long since I fitted points it'd
take a while to get the gap right now. I could do it virtually by eye back then and a weeks holiday was enough to mess with that skill.
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