Technical Rough running at medium speed

[smart51]

Today I went out in the car to get greeted by the entire car shaking and rattling alarmingly when the engine revs got to about 3000. The car started and idled well and ran well at 2000 RPM. At 3000 it is severely down on power too. I turned around and limped home in 4th at 30 MPH.

For context, a few days ago, I got a timing strobe and set the timing. The car was running well afterwards, hitting 65 on the bypass. I found out that I wasn't getting full throttle with the pedal to the floor, so I pulled the pedal back to 'straighten' it. Today was meant to be a test drive to prove it had worked.

The points have been gapped, the carb cleaned and the float level set. It was all good.

Where is a good place to start looking for a 3000 RPM shaking problem?

 

[Goldnrust]

Is that rattling at full throttle only? Or does it happen at all throttle openings?

If your timing is still advanced beyond standard, it could be knocking/detonating now you’ve got full throttle.

 

[smart51]

Is that rattling at full throttle only? Or does it happen at all throttle openings?

If your timing is still advanced beyond standard, it could be knocking/detonating now you’ve got full throttle.

All throttle openings. Timing is standard and set with a strobe. I'd given it some throttle with the strobe and it did advance, though this was a couple od days before I went out in it.

 

[smart51]

Another day another adjustment. I checked the points and a 0.58mm feeler gauge could fit between, but a 0.60 wouldn't fit. I've adjusted that so a 0.48mm feeler is loose, a 0.50 has some friction and a couple of larger sizes are very tight with a lot of friction. With the timing adjusted to suit I went for another run.

OK, so some improvement. It runs better at low speed better than it has ever done. It sings very sweetly at 35MPH in top = 2450 RPM with a 500R gearbox. It also sounds better than it ever has a 60 MPH / 4200 RPM. Whereas is used to sound slightly strained at that speed, it now sounds very happy. it won't go over 62, though, so it is down on power.

However. The whole car resonates and rattles like hell between 2800 and 3350 RPM (30MPH in 3rd and 45 MPH in 4th). Any good ideas what to check next?

 

[smart51]

Someone suggested I check the engine mounts. The gearbox mounts look new and the engine mount is fine. They also suggested I check the fan. The fan looks good, though the fins are not equally spaced. Is this normal?

I had a thought about centrifugal advance. It does advance when I rev the engine, but could the advance be wrong some how? It seems odd to me that the engine runs well at speeds above max advance and speeds below it. But in the zone around 3000 RPM, it is off some how causing vibration. What do you think?

 

[Ant Knox]

Does the vibration happen when revving the engine stationary? If it does, then perhaps set the speed on the hand throttle whilst in neutral and get out and inspect; you may just be able to pinpoint where it is coming from.

 

[smart51]

Does the vibration happen when revving the engine stationary? If it does, then perhaps set the speed on the hand throttle whilst in neutral and get out and inspect; you may just be able to pinpoint where it is coming from.

The vibration is at a minimum when running with just a tiny bit of throttle. An open throttle (wide or half) or a closed throttle on overrun gives lots of vibration.

 

[fiat500]

Could the impeller be loose on the shaft or the mounting boss have a worn keyway?
Another possibility would be to have worn rubber on the driveshaft coupling(s).

 

[smart51]

Could the impeller be loose on the shaft or the mounting boss have a worn keyway?
Another possibility would be to have worn rubber on the driveshaft coupling(s).

The fan seems to be fastened securely. I doubt drive shafts as it happens in all 4 gears.

 

[Goldnrust]

Given that it started the day that you adjusted the throttle cable, did you lean on anything in the engine bay? Could the fan be catching the housing?

 

[fiat500]

The fan seems to be fastened securely. I doubt drive shafts as it happens in all 4 gears.

If it shakes the whole car and if it's not the mountings, the fan or the driveshafts, it's got to be engine, gearbox or clutch....something that revolves.
So, an internal gearbox fault, a loose clutch cover, damaged friction-plate or splines, loose or damaged flywheel or internal engine faults.

 

[smart51]

Given that it started the day that you adjusted the throttle cable, did you lean on anything in the engine bay? Could the fan be catching the housing?

I took the air filter housing off this afternoon to inspect the fan. Apart from the blades of the fan not being evenly spaced, everything seemed good. It isn't loose or binding anywhere.

 

[smart51]

If it shakes the whole car and if it's not the mountings, the fan or the driveshafts, it's got to be engine, gearbox or clutch....something that revolves.
So, an internal gearbox fault, a loose clutch cover, damaged friction-plate or splines, loose or damaged flywheel or internal engine faults.

This is what is starting to worry me. It isn't gearbox related, it is very specifically engine speed related. Either it is fuelling / spark related causing the engine to run rough at a very specific speed, or something is loose somewhere that is resonating at a specific speed.

It could be that the bodyshell resonates at 50Hz and that it just so happens that the car is amplifying a smaller engine problem.

 

[smart51]

Elsewhere, people suggested it may be leaning out on the transition from idle to main jet. So, 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). Checked the tinware for tightness. No change.

I'm going off the idea that it is a problem with the transition holes because the problem 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 is turning enough to drive the wheels. Making barely enough power to turn itself is somehow just fine.

At this point, I think it needs to go to a specialist. Can anyone recommend someone in the Birmingham area who knows how to work on these?

 

[smart51]

I've just done a compression test on the engine when cold. 80 PSI on the front cylinder and 90 on the rear. It's a bit low, but is is a problem?

Edited to say I've read the book and done the test again with the engine warm. 100 PSI in the front cylinder and 102 in the rear. Much more satisfactory.

Edit again to say I forgot to do it at wide open throttle. 100 PSI is with the throttle closed.

 

[smart51]

I've decided to take a methodical approach to define the problem and draw up a list of tests to track it down. Can you look through and suggest other things to inspect please?

The problem

* The car vibrates wildly when the engine revs are between 2800 and 3350 RPM in all gears and in neutral, at all throttle openings except just off idle.
* Vibrations can be felt through the seat. Pushing against the passenger's door reduces some of the vibrations. Rattles can be heard in the dash, doors and from the rear of the car.
* The car vibrated a little at this engine speed when I got it a couple of months ago, just not very much. Since then, the car has been serviced. The points gapped, the timing and the valve clearances set. The tunnel "furniture" have been removed and reset. The door trims removed and reset.
* The problem started after a few minutes of driving at 60 MPH.
* The timing was set to ~20° BTDC when I bought it (set 10° before the wrong mark on the crank case). I have adjusted it by a few degrees either way and road tested it. It runs very sweetly at speeds lower or higher than the problem range. It ran very badly when set 10° before the correct timing mark.

Problem is engine related, not road speed related - happens in all gears and in neutral. It goes away when the clutch is dipped.
Problem is RPM specific - 2800 - 3350
An engine problem must cause vibrations that make the body resonate

It could be that the engine is vibrating more in the bad RPM range
It could be that the engine is fine and that the body has shaken loose enough to rattle with a good engine.

Test table

cause	part	test	result
something is loose
	crank	remove the sump and inspect	-
	cam	full engine dissasembly	-
	big ends	remove the sump and inspect	-
	pistons	remove the head and inspect	-
	flywheel	remove the cover and inspect	-
	clutch	remove the cover and inspect	-
	fan / tinwork	inspect	All OK
	engine / box mounts	inspect	All OK
	exhaust	inspect	All OK
something is imbalanced
	crank	remove the sump and inspect	-
	flywheel	remove the cover and inspect	-
	clutch	remove the cover and inspect	-
	oil filter	remove the cover and inspect	-
	fan	remove the tinwork and inspect	runs smoothly at all speeds
fuelling, air or spark
	fan / air filter / induction	inspect	All OK
	fuel supply	wouldn't work at high revs but not at medium revs
	carburettor	strip and clean	stripped and cleaned - except the inaccessible progression tube
	plugs	inspect	All OK
	leads and coil	not likely to be rev specific	works at other speeds
	distributor	inspect	weights move freely. Contacts open and close right.
	condenser	check points contact colour	grey not blue
general checks
	compression	do a compression test	100/102 (throttle closed)
	valve clearances	remove cover and measure	one was slightly off
	points gap	measure	0.50mm gauge has some friction. 0.53mm gauge feels very tight
	spark timing	measure	pully mark is in line with the 10°BTDC line when the strobe is set to 10°

 

[Goldnrust]

Problem is engine related, not road speed related - happens in all gears and in neutral. It goes away when the clutch is dipped.
Problem is RPM specific - 2800 - 3350

This potentially sheds new light on the issue.

If it shows up in neutral when you free rev the car, but goes away when you press the clutch pedal in, then logically it has to be something that interacts with the clutch.

Does it go away if you lightly press the clutch? Enough to push the release bearing into the clutch and flywheel, but not enough to fully disengage the clutch?

If so I would be focussing my attention on the flywheel, clutch and clutch release bearing.

Though it’s also (less likely and more difficult to fix) a chance it could be to do with the crank thrust or rear main bearing as this is put under pressure when the ouch the clutch.

If it only goes away when you push the clutch to the floor then the difference between a full clutch press and a half, is that the gearbox internals will stop spinning with the full press. So if this is the case, then I’d be looking at a bearing inside the gearbox being worn.

On a separate note, the timing issues are strange, you would not expect the car to run best with its base timing at 20 degrees BTDC. If could be there’s something wrong with the markings on your engine (mix and match of potential parts?) or if the distributer is not advancing properly then you would need to over advance the base timing to reach your full advance further up the rev range. Either way, something to look into, but with the new information about the problem changing with the clutch pressure, I doubt it’s the root cause of the current issue.

 

[smart51]

This potentially sheds new light on the issue.

If it shows up in neutral when you free rev the car, but goes away when you press the clutch pedal in, then logically it has to be something that interacts with the clutch.

Does it go away if you lightly press the clutch? Enough to push the release bearing into the clutch and flywheel, but not enough to fully disengage the clutch?

If so I would be focussing my attention on the flywheel, clutch and clutch release bearing.

Though it’s also (less likely and more difficult to fix) a chance it could be to do with the crank thrust or rear main bearing as this is put under pressure when the ouch the clutch.

If it only goes away when you push the clutch to the floor then the difference between a full clutch press and a half, is that the gearbox internals will stop spinning with the full press. So if this is the case, then I’d be looking at a bearing inside the gearbox being worn.

On a separate note, the timing issues are strange, you would not expect the car to run best with its base timing at 20 degrees BTDC. If could be there’s something wrong with the markings on your engine (mix and match of potential parts?) or if the distributer is not advancing properly then you would need to over advance the base timing to reach your full advance further up the rev range. Either way, something to look into, but with the new information about the problem changing with the clutch pressure, I doubt it’s the root cause of the current issue.

What I'm trying to say is it isn't related to road speed. If you're driving at 30 MPH in 3rd and press the clutch, the engine speed drops, the road speed stays the same but the vibrations go away.

 

[the hobbler]

What I'm trying to say is it isn't related to road speed. If you're driving at 30 MPH in 3rd and press the clutch, the engine speed drops, the road speed stays the same but the vibrations go away.

This is beginning to sound more and more like a transmission problem----damaged release bearing or pressure plate

 

[smart51]

This is beginning to sound more and more like a transmission problem----damaged release bearing or pressure plate

What's your thinking here? I've never worked on a transmission before. If the car is stationary and in neutral, rev the engine to 3k, the car shakes. Drive the car in 1st at 11 MPH, or 2nd at 18 MPH or 3rd at 30 MPH or 4th at 43 MPH, the engine is doing 3k, the car shakes. Do the same road speed with your foot on the clutch, the engine is not doing 3k, the car doesn't shake. What is happening with the release bearing or pressure plate that is causing the vibration?