Chronic misfire
Discussion
My vintage car (1925) engine has developed a very bad misfire. Syptoms are slight misfire which got worse over course of 20 odd miles. Eventually could'nt drive car and called R.A.C. who confirmed all electrics are OK. Fuel delivery is OK also, All things that are first tests like change plugs, clean contact breakers and gap, replace H.T. lead all done and still no improvement to running.
I found that one plug was wet and now suspect that a valve may have stuck, the engine still sounds OK except the misfire of course. If its a stuck valve whats the best way forward for me to reolve the problem, I hasten to add that my mechanic skills are limited. Thanks.
I found that one plug was wet and now suspect that a valve may have stuck, the engine still sounds OK except the misfire of course. If its a stuck valve whats the best way forward for me to reolve the problem, I hasten to add that my mechanic skills are limited. Thanks.
Compression test first. If a marked difference is found on one cylinder then it's time to start stripping down.
A vintage/veteran car is not the ideal place to start learning motor mechanicing as most problems will require quite a high degree of innovative problem solving. Your best bet is to find a local enthusiast and learn from them.
Steve
A vintage/veteran car is not the ideal place to start learning motor mechanicing as most problems will require quite a high degree of innovative problem solving. Your best bet is to find a local enthusiast and learn from them.
Steve
Thanks Steve, I have now done an compression test and found that all four cyliders are 5.66 and 6.00
so I think I can rule out valve or piston trouble.
The only electrical parts I have'nt replaced are rotor arm and contact breakers. So I guess thats my next step. I usually book straight into garage at this point but want to learn as I go now.
so I think I can rule out valve or piston trouble.
The only electrical parts I have'nt replaced are rotor arm and contact breakers. So I guess thats my next step. I usually book straight into garage at this point but want to learn as I go now.
Your RAC man was the nearest thing to an expert so what did he think was wrong?
Have to assume he checked all the normal things like sparks and fuel so your probably looking at something like slipped timing although the comment you made that one plug was well oiled would still point at a one cylinder specific problem.
Pity your not closer so I could have a play. Being old I like old engines.
Steve
Have to assume he checked all the normal things like sparks and fuel so your probably looking at something like slipped timing although the comment you made that one plug was well oiled would still point at a one cylinder specific problem.
Pity your not closer so I could have a play. Being old I like old engines.
Steve
Thanks for suggestions guys, I tried a different coil and it made no difference at all and RAC man confirmed electrics OK to the spark plugs. Fitted all new plugs as matter of course and no difference. Had my carb' jets out and all clean fuel side OK. I ran engine for a few minutes and found that all 4 spark plugs were getting hot as they should, so I am begining to think it could be timing slipped somehow. I have gone as far as I can now and booked car into my excellent local independant garage. (they have done excellent work for me in past on vintage)Being a top garage they booked until 24th June.
Really disapointed that I can't fix it but thanks again for suggestions.
Really disapointed that I can't fix it but thanks again for suggestions.
Do'esnt appear to be a condensor fitted, but I agree it is exactly the symptoms of a condensor falure. It is a 6v coil conversion wired through the old magneto. This setup has been fine during my 3 years ownership. Now find the misfire is straight from cold engine. Run the engine for few min's and found all 4 plugs get hotish so all firing up? Starting to look more like timing slipped? Can't get the old girl into garage until 24th but will post up the answer for curious minded.
Investigated Magneto conversion and it appears an internal condensor is very likely to be retained. Could this be the item that is causing the engine to run on what appears to be three cylinders? or if the condensor is faulty would the engine refuse to run at all?
Edited by crankedup on Wednesday 3rd June 11:28
Investigated Magneto conversion and it appears an internal condensor is very likely to be retained. Could this be the item that is causing the engine to run on what appears to be three cylinders? or if the condensor is faulty would the engine refuse to run at all?
Edited by crankedup on Wednesday 3rd June 19:27
Bro' had a look at the engine yesterday and checked / adjusted everything/ coil, leads, points, plugs, fuel delivery, carb' and got the thing to run consistently but only on three cylinders of the four. Kept spitting back thru carb'. Said earlier car booked in but any other suggestions welcome as to whats wrong.
Pumaracing said:
Check the valve clearances.
Thanks for input, have checked valve clearances and all OK. Had rocker cover off and everything looks as it should except one pushrod has an slight bend. Found this by twisting it by hand and then watching it as engine runs. It is only a little and may have been like it for years, I don't know.crankedup said:
Had rocker cover off and everything looks as it should except one pushrod has an slight bend.
Is this on the cylinder that is misfiring? It may be significantly reducing valve opening and either causing high levels of egr (exhaust) or may be restricting the intake significantly, reducing the charge reaching that cylinder reducing combustion stability (intake).You're missing something very basic here I suspect. AIUI the engine is not working properly on one cylinder and one only and that has been consistent from the start of this misfire. If so it can't be a general problem with any thing that feeds every cylinder like the fuel system or any part of the ignition system upstream of the dissy cap and that one last HT lead and spark plug.
The problem must be unique to that one cylinder and possibly only manifesting fully when the engine is hot. Things that spring to mind are a burnt, bent or cracked valve, cracked head, broken valve spring, cam lobe problem, gasket issue etc.
Do another compression test, preferably with the engine hot and pay particular attention to the rate of compression rise per engine revolution. The suspect cylinder might not rise at the same rate per rev as the others even if it reaches a similar final value. Ideally a leakdown test with compressed air would be a better test. However if it's a valve spring or similar problem it might only be manifesting symptoms at running speed not cranking speed. Check the valve springs very carefully and measure the valve lift on each valve with a dial guage to make sure a cam lobe hasn't worn off.
On a rolling road dyno you'd be able to measure the power contribution of each cylinder by isolating each in turn as the engine runs and that would point definitively to where the problem lay but I suspect we have enough info anyway. Stripdown time methinks.
The problem must be unique to that one cylinder and possibly only manifesting fully when the engine is hot. Things that spring to mind are a burnt, bent or cracked valve, cracked head, broken valve spring, cam lobe problem, gasket issue etc.
Do another compression test, preferably with the engine hot and pay particular attention to the rate of compression rise per engine revolution. The suspect cylinder might not rise at the same rate per rev as the others even if it reaches a similar final value. Ideally a leakdown test with compressed air would be a better test. However if it's a valve spring or similar problem it might only be manifesting symptoms at running speed not cranking speed. Check the valve springs very carefully and measure the valve lift on each valve with a dial guage to make sure a cam lobe hasn't worn off.
On a rolling road dyno you'd be able to measure the power contribution of each cylinder by isolating each in turn as the engine runs and that would point definitively to where the problem lay but I suspect we have enough info anyway. Stripdown time methinks.
Many thanks for all the valuble input suggestions, your references to valve / cylinder head type issue are what I fear. This work is beyond my limited experience but whilst I might well spot the fault I need experienced eyes to spot any other issues lurking. The car is booked in at my local well respected engineering garage. Thanks again, I will write up diagnosis for your interest.
Also I should have mentioned that on the overun the engine had been popping back thru the exhaust, this had been occuring long before the current problem manifested itself. The fuel/air mixture is set by carb' jets and this carb' was fully overhauled two years back (2000 miles)and was just fine, then exhaust popping started last Summer. (car not used in winter)
Also I should have mentioned that on the overun the engine had been popping back thru the exhaust, this had been occuring long before the current problem manifested itself. The fuel/air mixture is set by carb' jets and this carb' was fully overhauled two years back (2000 miles)and was just fine, then exhaust popping started last Summer. (car not used in winter)
Edited by crankedup on Wednesday 10th June 14:11
This reminds me of a problem I had with my E-type. I could not get it to run on all 6 cylinders. I could vary which cylinders were not firing by altering the points gap. After much head scratching the problem was the shaft driving the rotor arm in the distributor was bent and according to the points gap one set would either not open or not close. As I said that caused much head scratching and foul language
Popping back through the exhaust on the overrun is a symptom of a leak or crack in the exhaust manifold or initial section of the exhaust system. If this is in the manifold on just one cylinder and a big enough crack it could indeed upset the running enough to cause a misfire. Exhaust manifold leaks screw the fuel mixture to buggery. Sometimes enough to prevent proper firing.
You don't give us the whole info from the start you ain't gonna get the right help!
As for stripping the engine down, historic engines are pretty basic and if you want to start learning it's as good a place as any. I can help from photos if you get stuck or want advice on a specific issue. Despite being retired I might even do the necessary machining if sufficiently tempted.
Dave Baker
Puma Race Engines
You don't give us the whole info from the start you ain't gonna get the right help!
As for stripping the engine down, historic engines are pretty basic and if you want to start learning it's as good a place as any. I can help from photos if you get stuck or want advice on a specific issue. Despite being retired I might even do the necessary machining if sufficiently tempted.
Dave Baker
Puma Race Engines
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