Valve spring?...
Discussion
I have problems with an engine. It is not willing to rev above 6000. Above 5500 it developes some noise at the valve area (I think), and drops power. There is a "wall" at 6000. Like a rev limiter.
I have suspected the ignition, but it seems not to respond to changes at the ignition system.
The cam is a Kent 276, and the valve springs are GT double springs. Can they be weak?
I have suspected the ignition, but it seems not to respond to changes at the ignition system.
The cam is a Kent 276, and the valve springs are GT double springs. Can they be weak?
What ignition system do you have?
It's a bit unlikely to be 'valve-bounce' with the double springs, especially at such relatively low revs.
If you have points-type ignition it could be 'points-bounce'. Alternatively it could be that the advance is not advancing far enough at the top end due to a distributor fault.
Is the fuel supply OK, as fuel-starvation at higher flow requirements could cause this?
It's a bit unlikely to be 'valve-bounce' with the double springs, especially at such relatively low revs.
If you have points-type ignition it could be 'points-bounce'. Alternatively it could be that the advance is not advancing far enough at the top end due to a distributor fault.
Is the fuel supply OK, as fuel-starvation at higher flow requirements could cause this?
I have a magnetic conversion in the original 25D distribuor with normal coil.
I have tried putting the points back just to see if..., but no difference it gave. I have also tried replacing the plug caps to see if the resistance have anything to do with the problem, but nothing.
The valve bounce came after the strange sound, which is coming from the head-area. Though it can still be ignition problem.
I will recheck the rev counter, but I can feel that the engine is not at its end. The power curve is still going up there.
I have set the carb with WBO, it looks right. I have a slight weaker area around 2K, but still no missfire, and above this, I have nice strong mixture (0.9-1 Lambda).
The fuel is delivered through a Facet pump, and the pressure is regulated by a Filterking. I have set the regulator to just below the pressure that the needle valve can handle. I think it must be enough.
I have tried putting the points back just to see if..., but no difference it gave. I have also tried replacing the plug caps to see if the resistance have anything to do with the problem, but nothing.
The valve bounce came after the strange sound, which is coming from the head-area. Though it can still be ignition problem.
I will recheck the rev counter, but I can feel that the engine is not at its end. The power curve is still going up there.
I have set the carb with WBO, it looks right. I have a slight weaker area around 2K, but still no missfire, and above this, I have nice strong mixture (0.9-1 Lambda).
The fuel is delivered through a Facet pump, and the pressure is regulated by a Filterking. I have set the regulator to just below the pressure that the needle valve can handle. I think it must be enough.
It's good to go back to basics to try to determine these odd faults.
So, what can prevent the engine from increasing its revs (assuming it's not over absolute max)?
1. Insufficient supply of burnable mixture
2. Inability of valves to hold the compression to point of ignition
3. The ignition system not supplying the right spark at the right time.
The first is fuel supply and/or carburation and that you can probably check.
The second can be valve-bounce (weak springs), broken valve spring(s), slipped cam timing, incorrect valve clearances and rocker(s) seizing on the rocker shaft.
Third can be dizzy not correctly advancing, points-bounce, coil breaking down at high revs (can be quite a ommon fault with modern 'crap' coils), condenser breaking down (if fitted), hairline cracked dizzy cap, ignition leads (usually the one from coil to cap), defective plug(s).
Sorry if I've stated the obvious.
Last week I had a most curious fault. I serviced a Cooper 'S' with an A+ engine and it was running fine. Then I took it to the bodyshop for a re-spray. The paint guy phoned after he had painted it to say it would only run on 1 cylinder. I collected it on my trailer and back in my workshop I cleaned, re-gapped (again) and re-fitted the plugs which I knew had only done about 1000 miles, as I saw them fitted at the rolling road a year before. 3 were wet and one was black (the one cylinder which was running!). Still it would only run on 1. I took off the cap and checked for a spark at the points which was fine. I then fitted another cap with different set of leads. Still no joy. So I just fitted another set of plugs, re-fitted the original cap/leads and it ran fine. I really don't know why 3 new plugs should have 'gone down', but that happened. Minis can be strange things!
So, what can prevent the engine from increasing its revs (assuming it's not over absolute max)?
1. Insufficient supply of burnable mixture
2. Inability of valves to hold the compression to point of ignition
3. The ignition system not supplying the right spark at the right time.
The first is fuel supply and/or carburation and that you can probably check.
The second can be valve-bounce (weak springs), broken valve spring(s), slipped cam timing, incorrect valve clearances and rocker(s) seizing on the rocker shaft.
Third can be dizzy not correctly advancing, points-bounce, coil breaking down at high revs (can be quite a ommon fault with modern 'crap' coils), condenser breaking down (if fitted), hairline cracked dizzy cap, ignition leads (usually the one from coil to cap), defective plug(s).
Sorry if I've stated the obvious.
Last week I had a most curious fault. I serviced a Cooper 'S' with an A+ engine and it was running fine. Then I took it to the bodyshop for a re-spray. The paint guy phoned after he had painted it to say it would only run on 1 cylinder. I collected it on my trailer and back in my workshop I cleaned, re-gapped (again) and re-fitted the plugs which I knew had only done about 1000 miles, as I saw them fitted at the rolling road a year before. 3 were wet and one was black (the one cylinder which was running!). Still it would only run on 1. I took off the cap and checked for a spark at the points which was fine. I then fitted another cap with different set of leads. Still no joy. So I just fitted another set of plugs, re-fitted the original cap/leads and it ran fine. I really don't know why 3 new plugs should have 'gone down', but that happened. Minis can be strange things!
Does it miss when you rev it at standstill or only while you are driving?
If it does miss at exactly the same place whether on light or heavy throttle is won't be fuel.
This leaves ignition, which seems unlikely as you've swapped it over.
Valve bounce, could be, as this will occur at given revs whether under light or heavy load, but again unlikely as double springs installed at the correct height will normally be OK for higher revs than you are achieving.
Incorrect cam timing perhaps if the overlap is in the wrong place you could get a miss at the same RPM regardless of load.
If I were doing this at home, I'd fit a timing light to a plug lead and watch the light as I came up to the miss, if the light became erratic, I'd suspect the ignition, if the light was erratic while attached to the plug lead it could be LT, coil, King lead, cap, rotor, or leads, to help eliminate some of this, I'd put the light onto the coil lead and do the same test, if it's erratic there the problem will be LT, coil or king lead.
If not, I'd be looking for stand off at the intake and suspect a mechanical gremlin.
My guess is a broken valve spring, but I've never been that good a guess work.
Ralph
If it does miss at exactly the same place whether on light or heavy throttle is won't be fuel.
This leaves ignition, which seems unlikely as you've swapped it over.
Valve bounce, could be, as this will occur at given revs whether under light or heavy load, but again unlikely as double springs installed at the correct height will normally be OK for higher revs than you are achieving.
Incorrect cam timing perhaps if the overlap is in the wrong place you could get a miss at the same RPM regardless of load.
If I were doing this at home, I'd fit a timing light to a plug lead and watch the light as I came up to the miss, if the light became erratic, I'd suspect the ignition, if the light was erratic while attached to the plug lead it could be LT, coil, King lead, cap, rotor, or leads, to help eliminate some of this, I'd put the light onto the coil lead and do the same test, if it's erratic there the problem will be LT, coil or king lead.
If not, I'd be looking for stand off at the intake and suspect a mechanical gremlin.
My guess is a broken valve spring, but I've never been that good a guess work.
Ralph
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