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Another engine problem to diagnose.
smart51 - 9/8/23 at 07:28 PM

Short story: I went out today and the whole car started vibrating madly when the engine got to 3000 RPM. The engine wouldn't pull through that speed. It started and idled well and would drive at 2000 RPM. The car ran well when I was last in it.

Back story: I've been fixing up my 1975 Fiat 500. I've gapped the points, set the timing with a strobe. I've cleaned and checked the carb and set the float height. The engine ran much better after that, but I noticed the throttle was only opening to 70% before the pedal hit the metal. So I bent the pedal back in to shape to give the full range of travel and went out for a test drive. That's when the massive vibrations happened as set out.

The car has a 594cc air cooled twin with carb and distributor. There are no engine electronics. What might cause the engine to run that badly at 3000 when it runs fine at 2000?

adithorp - 9/8/23 at 10:25 PM

Hard to say but sounds ignition. Heres a rough/quick checklist.

Points/contacts.
Did you clean contacts before fitting? Oil on new contacts will cause bad spark.
Did you only check gap with feelers when new? Gaping used points with feelers will cause bad spark.
Did you check the dwell angle? More accurate than gapping.
Did you lube the cam with a dab of grease? If not it'll wear and alter the gap.

Condenser
What colour are the contacts? Any blueing or burning and you need Condenser as well as points.

Distributor
Any play in the shaft? That'll cause the gap to vary. Setting by dwell will help overcome that as its dynamic.
Is the wiring in there good/secure.
Are the bob/centrifugal weights free and springing back?

Are the rotor and cap and leads good? Rotors can cause issues. Missing centre electrode in cap is often (and oddly) missed. Are the leads as original? ie. Copper or carbon core?

Are the coil connections good? Nothing loose.

Is the engine earth good?

smart51 - 10/8/23 at 05:12 PM

First off, I'll answer the questions.

The points were fitted by the dealer who sold me the car. I've done about 300 miles on them. They look like new, shiny and silver.
There is no "distributor" part to the distributor, just points. The bob weights move freely and return quickly under spring pressure.
I haven't checked the dwell but I did grease the cams.
The condenser is new and the earth seems good.

Today I checked the points gap and it was a fraction wide. It is now spot on. I reset the timing using a strobe. The result is mixed.

The car runs better at 2000 to 2500 RPM than it has since I bought it. Very sweetly actually. From about 2800 to 3350, the entire car resonates like hell. From 3500 it runs well again and at 60 MPH /4200 RPM it sounds better than it ever has. Again really nice. It won't go over 62 MPH (GPS on a level road) which suggests it is down on power; it should do about 68.

I've reached the limit of my knowledge on engines here. What would cause vibration like that at quite a narrow rev range?

Mr Whippy - 10/8/23 at 06:22 PM

Could be the cooling fan maybe a blades missing, or the engine mounts are shot, ifirc they are funny design. Hard to tell over the internet.

smart51 - 11/8/23 at 11:29 AM

quote:
Originally posted by Mr Whippy
Could be the cooling fan maybe a blades missing, or the engine mounts are shot, ifirc they are funny design. Hard to tell over the internet.
The fan looks fine, its bearings are good. The gearbox mounts look new, which is a shame, an easy fix like this would have been nice. The rear mount is a bit odd, a spring and rubber bushes. That all looks good too.

It's strange that it is so speed specific. Could the bob weights be moving at the wrong time? So without centrifugal advance it runs fine and at speeds above max advance it runs fine, but at speeds where it's getting to max advance (3000 RPM) it is somehow off?

Mr Whippy - 11/8/23 at 11:57 AM

Could just be a resonance issue with a large engine mass to car ratio. With my car the top speed is more limited at how everything is vibrating (and becomes unpleasant i.e around 45mph) which seems to change day to day and that's down to the tiny hard engine mounts and a small but massive engine. If I put my foot on the clutch and idle, it all goes away.

I'd suspect that something on you engine is perhaps not very well balanced, again maybe the fan, maybe the flywheel. Without removing them and checking the balance perhaps you could move the resonance by altering the spring rate of the gearbox mount? I don't think it's anything to do with the points as out of adjustment or worn they just cause poor performance or rubbish starting. The fact that you can even get to 60mph kind of rules that out... oh how I dream of such reckless speeds

pewe - 11/8/23 at 02:39 PM

Simple suggestion - have you cleaned the points by passing sandpaper between the contacts then blowing them out?
Sometimes its the easiest things that are missed.

Dingz - 11/8/23 at 09:10 PM

Could it be fuel starvation can you try running it on a jury rigged tank?

pigeondave - 12/8/23 at 02:17 AM

Could it be a bit of dirt in the carbs?
If you're usyit more maybe somethings come loose and is blocking the crossover from idle to main jets.
Or maybe the progression holes are a little blocked up?

I know I had the wrong carbs on the zetec for a bit, I had 3 progression holes, needed the 5 hole version. Wouldn't pull between the ranges you say, either side was good though.

smart51 - 12/8/23 at 10:42 AM

I've stripped and cleaned the carb, making sure to poke wires into the transition holes. I sanded the base of the carb flat (it wasn't far off). No change. I've even tried running the idle mixture a bit rich to see if that eases the transition from idle to main jet. Nope. I'm now officially stumped.

The latest staw I'm clinging to is that there is nothing wrong with the engine, and that every body panel on the car is now loose and vibrating at the same time.

adithorp - 12/8/23 at 11:32 AM

Is it engine related or could it be running gear? I took it from your first post it was engine but from your last, maybe not.

Is it speed or rev' linked? If its speed it could be as simple as a badly balance (rear) wheel. I say rear as that shakes the car rather than the more familiar feeling of a front thst shakes the steering. Its less noticeable in a larger car but in a small 500...?

smart51 - 12/8/23 at 11:56 AM

quote:
Originally posted by adithorp
Is it engine related or could it be running gear? I took it from your first post it was engine but from your last, maybe not.

Is it speed or rev' linked? If its speed it could be as simple as a badly balance (rear) wheel. I say rear as that shakes the car rather than the more familiar feeling of a front thst shakes the steering. Its less noticeable in a larger car but in a small 500...?


It happens in all 4 gears and in neutral, all at the same range of engine speed so at different road speeds in each gear. It is at its minimum when there is just a tiny bit of throttle - just enough so that it is neither driving the car, nor being driven by the car. At part or full throttle, or with the throttle closed and either slowing down through the rev range or going down hill, it is worse.

I'm going off the idea that it is a problem with the transition holes because it happens both with the throttle closed and with it wide open.

Something to do with the engine must be vibrating and setting off the rest of the car. It is only in a narrow RPM range and only if the engine is being turned by the wheels or turning enough to drive the wheels. Making barely enough power to turn itself is somehow just fine.

Mr Whippy - 12/8/23 at 01:23 PM

Glue your car together better it sounds like it's falling apart

rusty nuts - 13/8/23 at 07:28 AM

As the fault seems to be rev related it may be the crank is out of balance? From memory the Fiat air cooled engines have a centrifugal oil filter in the crank pulley, if that has been neglected then a build up of contamination in the filter/pulley could possibly be the cause? It certainly wouldn’t do any harm to clean it especially if it hasn’t been done lately , you may have a gasket or “0” ring to change while your in there?

smart51 - 13/8/23 at 03:37 PM

The distributor on the car doesn't have a rotor. It has points and a twin coil so it sparks both plugs at once. Neat.

I set the static timing with a cheap strobe off Amazon. It showed that the timing starts to advance if you blip blip the throttle. But it shows the timing g going back to 10 degrees at higher revs. I'd put it down to the cheap strobe not liking high advance angles on a 2 cylinder engine.

Today I had the bright idea of running the engine with the distributor cap removed and shining the strobe on the insides. The return spring for the weight stays in the same place so using the strobe seems like a valid test. Off idle, the spring moves a bit but goes no further as the revs advance. It doesn't seem to stretch at all. Do you think I'm right in diagnosing that as the timing not advancing properly?

Mr Whippy - 13/8/23 at 06:01 PM

Not sure why you think a lack of advance results in vibration? in my experience it just means less power. But it will happen to both cylinders the same amount so still balanced power wise. If there is vibration it generally means something mechanical and spinning is out of balance. Even loosing complete ignition in a cylinder doesn't produce much vibration. I'm still going on something like the fan or flywheel/clutch is out. Perhaps someone changed the clutch but didn't line up the marks properly as they are usually balanced as a pair in the factory.

adithorp - 13/8/23 at 06:17 PM

quote:
Originally posted by Mr Whippy
...Perhaps someone changed the clutch but didn't line up the marks properly as they are usually balanced as a pair in the factory.


Do what?

smart51 - 13/8/23 at 09:11 PM

quote:
Originally posted by Mr Whippy
Not sure why you think a lack of advance results in vibration? in my experience it just means less power. But it will happen to both cylinders the same amount so still balanced power wise. If there is vibration it generally means something mechanical and spinning is out of balance. Even loosing complete ignition in a cylinder doesn't produce much vibration. I'm still going on something like the fan or flywheel/clutch is out. Perhaps someone changed the clutch but didn't line up the marks properly as they are usually balanced as a pair in the factory.


I take your point. Something changed this week so I guess I've been looking at things I've tampered with - Carb and distributor.

Yesterday, I removed the top of tinwork and ran the engine. The fan runs true with the engine doing 3000 RPM. It is not loose, there's no play and there is no damage. Given that the fan is bolted to the generator, the same goes for that. For rotating mass, that leaves the crank / flywheel / clutch and the centrifugal oil filter, plus I suppose the camshaft and oil pump. It has to be the kind of loose that had no effect at low or high speed, but resonates in a 500 RPM band around 3000.

[Edited on 13-8-2023 by smart51]

Schrodinger - 14/8/23 at 09:02 AM

It could be something like a bulge in one of your tyres causing a major imbalance ?

Mr Whippy - 15/8/23 at 06:20 AM

quote:
Originally posted by adithorp
quote:
Originally posted by Mr Whippy
...Perhaps someone changed the clutch but didn't line up the marks properly as they are usually balanced as a pair in the factory.


Do what?


Usually from the factory the flywheel and the clutch housing are balanced as an assembly and there is often a stamp or mark to allow you to put it back on in the same orientation, it's good practice to mark it anyway before removing. Obviously once you replace the pressure plate that all goes out the window but hopefully the replacement is balanced enough but chances are it will now be a bit out. This was probably more of an issue in the old days when tolerances were not as good as today.

smart51 - 15/8/23 at 01:20 PM

I've found my old compression tester. I haven't used it since we moved house. No 1 cylinder is kind of OK. 90 PSI dry, 105 with a couple of drips of oil. No 2 cylinder is 80 PSI wet and dry. Low compression engines like these should make 100 in tip top condition.

Edit to say, I've read the bookend repeated the test with a warm engine. 100 and 102 PSI. Much more satisfactory.

[Edited on 15-8-2023 by smart51]

gremlin1234 - 15/8/23 at 03:13 PM

quote:
Originally posted by smart51
I'd put it down to the cheap strobe not liking high advance angles on a 2 cylinder engine.

the strobe light only flashes with the spark, it has no concept of 'advance angles' or the number of cylinders.

smart51 - 15/8/23 at 03:24 PM

quote:
Originally posted by gremlin1234
quote:
Originally posted by smart51
I'd put it down to the cheap strobe not liking high advance angles on a 2 cylinder engine.

the strobe light only flashes with the spark, it has no concept of 'advance angles' or the number of cylinders.
mine has an LED display andx4 buttons. One switches from 2 cly to 4. One switches from showing RPM to showing advance. The other two adjust the advance. My guess is that the advance predicts when the next spark will come and flashes the light just before it.

adithorp - 16/8/23 at 06:51 PM

The advance setting retards the strobe flash so you can bring the marks back to TDC (useful when the engine only has a TDC mark or you font know how many degrees the marks indicate) and the advance figure then tells you how advanced the spark is.

The 2/4 is for 2or4 stroke though 2 also works with wasted spark systems where the HT lead its connected to sparks every tdc.

Have you managed to check the dwell angle yet? A decent multimeter should be able to do it.


[Edited on 16-8-2023 by adithorp]