Misfire & Low Compression & Rich Mixture?
Discussion
I am using a Triumph 13/60 engine in a kit car that I am building. Over the course of the build I have been running the engine every few weeks on tickover in the garage and seemed to run smotthly.
Now that I am getting the car ready for its first MOT I have started to notice a number of engine issues:
a) Once warm the engine misfires when I rev it. This misfire does not seem to be specific to one cylinder. I have replaced the distributor and this has had no impact at all.
b) The engine has a Stromberg CD150 carb which has been professionally rebuilt. However cold the weather it does not require any choke at all to get the engine started. This seems strange?
c) The compression on all 4 cylinders is low (110psi where it should be nearer 150psi) and putting some oil in the cylinder before the test makes very little (eg 2psi) difference.
My feeling is that a) might be the coil but it has also been suggested to me that 'it sounds "like some cylinders not firing and pumping combustible mixture straight into a hot exhaust manifold, sometimes known as "eight stroking". But b) suggests to me that the mixture is far too rich but "eight stroking" is caused by a weak mixture (or spark?). c) suggests problems with the valves because I have checked the clearances a couple of times.
So I think the head needs to come off. But would I be better-off sourcing a replacement (used) engine rather than spending money on refurbishing the head of this engine?
Now that I am getting the car ready for its first MOT I have started to notice a number of engine issues:
a) Once warm the engine misfires when I rev it. This misfire does not seem to be specific to one cylinder. I have replaced the distributor and this has had no impact at all.
b) The engine has a Stromberg CD150 carb which has been professionally rebuilt. However cold the weather it does not require any choke at all to get the engine started. This seems strange?
c) The compression on all 4 cylinders is low (110psi where it should be nearer 150psi) and putting some oil in the cylinder before the test makes very little (eg 2psi) difference.
My feeling is that a) might be the coil but it has also been suggested to me that 'it sounds "like some cylinders not firing and pumping combustible mixture straight into a hot exhaust manifold, sometimes known as "eight stroking". But b) suggests to me that the mixture is far too rich but "eight stroking" is caused by a weak mixture (or spark?). c) suggests problems with the valves because I have checked the clearances a couple of times.
So I think the head needs to come off. But would I be better-off sourcing a replacement (used) engine rather than spending money on refurbishing the head of this engine?
From my memory of the Triumphs I owned they all needed choke to start when cold.
In your shoes I`d look at the carb settings.
First make sure the dashpot has the correct amount/spec of oil in it and is free to move and the diaphragm isn't damaged.
When the piston is lifted by the lifting pin it should rise easily and drop back down with a satisfying clonk. If it doesn't you might need to remove the dashpot cover and investigate. The needle might need recentering in the jet.
Strombergs had a piston lifting pin which when pushed up at hot idle should, when the carb was correctly adjusted, have the effect of raising the revs slightly instantly followed by rough running caused by over fueling.
IIRC the mixture was adjusted by a nut on the jet at the base of the carb.
A basic setting was found by winding the nut up as far as it would go and then backing it off 2/3 turns.
You can then make small adjustments until you get the required response from pushing the lifting pin.
Once you`ve done that set the tickover by adjusting the screw on the throttle spindle linkage.
Probly 800 rpm would be in the ball park.
Its been a while since I did this and is all from memory. HTH.
In your shoes I`d look at the carb settings.
First make sure the dashpot has the correct amount/spec of oil in it and is free to move and the diaphragm isn't damaged.
When the piston is lifted by the lifting pin it should rise easily and drop back down with a satisfying clonk. If it doesn't you might need to remove the dashpot cover and investigate. The needle might need recentering in the jet.
Strombergs had a piston lifting pin which when pushed up at hot idle should, when the carb was correctly adjusted, have the effect of raising the revs slightly instantly followed by rough running caused by over fueling.
IIRC the mixture was adjusted by a nut on the jet at the base of the carb.
A basic setting was found by winding the nut up as far as it would go and then backing it off 2/3 turns.
You can then make small adjustments until you get the required response from pushing the lifting pin.
Once you`ve done that set the tickover by adjusting the screw on the throttle spindle linkage.
Probly 800 rpm would be in the ball park.
Its been a while since I did this and is all from memory. HTH.
Edited by Hammer67 on Sunday 22 December 17:18
Thanks guys,
I do not have any history on this engine. It came with my donor car about 6 years ago. I thought that I checked the compression at that time and it was all good. But, over the last 6 years I have only cleaned/painted the engine, checked/replaced its anciliiaries and fitted the new distributor with electronic ignition. I have only had it running in the garage occasionally over the last 6 years but I am 100% sure that the misfire has recently started.
I am thinking that the compression issue needs to be tackled first as it it not going to be 'cured' by fuel or ignition adjustments? In fact, wondering whether low compression would cause the carb to operate incorrectly and inefficient ignition?
I do not have any history on this engine. It came with my donor car about 6 years ago. I thought that I checked the compression at that time and it was all good. But, over the last 6 years I have only cleaned/painted the engine, checked/replaced its anciliiaries and fitted the new distributor with electronic ignition. I have only had it running in the garage occasionally over the last 6 years but I am 100% sure that the misfire has recently started.
I am thinking that the compression issue needs to be tackled first as it it not going to be 'cured' by fuel or ignition adjustments? In fact, wondering whether low compression would cause the carb to operate incorrectly and inefficient ignition?
I wouldn't worry about the compression at the moment especially as all cylinders give a similar reading. Are you losing oil, probably not? Is the oil in the sump clean, probably is?
A low compression should find it easier to run as it needs less energy to compress the air/fuel in the cylinder. I think your issues are elsewhere.
A low compression should find it easier to run as it needs less energy to compress the air/fuel in the cylinder. I think your issues are elsewhere.
Back out on the car this afternoon - first chance since the Christmas break. No signs of petrol in the oil.
Consciuos of advice that 'I think your issues are elsewhere.' I decided to ignore the compression issue, and before purchasing any new components, replace the female connectors on the end of the wires that go to the coil. Ran the engine and the same misfire started occuring. Suddenly noticed a spark refelcted in the shiny vacuum unit which is attached to the distributor. First guess was that I have a crack in the distributor cap but, given I didnt have a spare to hand, I decided to elimenate the Dysnamo first. Unplugged the two female connectors from the Dynamo, ran the engine and misfire has disappeared. Engine sounds sweet under both low and high revs - albeit I am just running under no load in the garage.
I had given the Dynamo a full test before putting it onto the engine a few months ago. So my first thought is that there must be something wrong with the wiring or the Control Box. But can anyone suggest how the Dynamo could be the root cause for a misfire under load? Could this be the cause of the weak spark leading to the "eight stroking" suggested earlier?
Consciuos of advice that 'I think your issues are elsewhere.' I decided to ignore the compression issue, and before purchasing any new components, replace the female connectors on the end of the wires that go to the coil. Ran the engine and the same misfire started occuring. Suddenly noticed a spark refelcted in the shiny vacuum unit which is attached to the distributor. First guess was that I have a crack in the distributor cap but, given I didnt have a spare to hand, I decided to elimenate the Dysnamo first. Unplugged the two female connectors from the Dynamo, ran the engine and misfire has disappeared. Engine sounds sweet under both low and high revs - albeit I am just running under no load in the garage.
I had given the Dynamo a full test before putting it onto the engine a few months ago. So my first thought is that there must be something wrong with the wiring or the Control Box. But can anyone suggest how the Dynamo could be the root cause for a misfire under load? Could this be the cause of the weak spark leading to the "eight stroking" suggested earlier?
Edited by cbjroms on Wednesday 1st January 16:34
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