The engine in my XK120, which was professionally rebuilt less than 10,000 miles ago, is running very uneven with low power.
With two exceptions, every indicator seems normal: good spark at all plugs, clean plugs, clean carb with proper fuel levels and equal air flow, air/fuel mix carefully adjusted, good cylinder compression and more.
The two exceptions are dramatically abnormal:
manifold vacuum is very low: only about 10 inch Hg at around 2500 rpm.
the temperature, measured with infrared thermometer, is very low on the three rear exhaust headers: only around 100F versus about 250F on front three headers.
My interpretation of this information is that there is probably an air leak in the rear section of the intake manifold/head gasket (I have checked on other sources of air leaks such as the carb throttle bearings and found none.)
However, before I undertake the considerable effort to remove the carbs and the intake manifold, I would like to know if any of you have another possible diagnosis of my engine problem.
Has it been running like this from first start since the rebuild, or has it gotten progressively worse?
Rather than a leak on the manifold gasket, could it be a leak on a vacuum assist line to the brakes? - Try clamping/ sealing the hose to the booster.
You could try a squirt bottle of soapy water to see if it temporarily seals any leak around the gasket - Wont fix the problem but is useful in diagnosis.
Jon,
Thanks for the suggestion but my XK120 does not have vacuum assisted brakes. I have, however, checked for leaks around the vacuum advance on the distributor. The strange exhaust temperatures makes me think it is a leak which only affects rear cylinders.
Re the intake gasket, I have tried spraying soapy water, quick starting fluid and WD40 around the edge of the gasket but this doesn’t seem to affect the engine performance. So that is one data point - but not a conclusive one - which makes me doubt my intake manifold gasket theory.
Phillip
From your description and being a XK120, you most likely have a carb problem. Mixture is to lean from the low temperatures to the point it is not firing. Either the float level is to low or needle/jet mismatch or just way out of adjustment. Vacuum advance and timing will effect all of the cylinders equally. Is the temp measured at idle only? Try holding the engine revs at 1500 and see if the rear cylinders start firing. (temperature wise).
The only things different about the front and rear cylinders are the carbs
and the exhaust (assuming you have duals). The rear carb could be not
working or the exhaust could be plugged. I lean towards a malfunctioning
carb. Spray that starting fluid into the rear carb while it’s running and
see if the engine starts to run on all six.
Mike Eck
New Jersey, USA www.jaguarclock.com
'51 XK120 OTS, '62 3.8 MK2 MOD, '72 SIII E-Type 2+2
Mike and Dick,
I have done as you suggested and raised fuel level in rear carb and adjusted mixture so idle speed is max and air flow balanced and lift fin results suggest right mix. Also tried suggestion squirting starter fluid in rear carb and that slows rather than speeds engine.
Test drive now shows good results: smooth idle, good, smooth acceleration, good power, all well.
But exhaust temperature on rear headers is still very low compared to front and vacuum still too low. Why is that?
Phillip
Hello Phillip, a couple of questions come to mind from reading your initial post.
“Air/fuel mix carefully adjusted” could mean different things to different people. Can you say a quantitative measure of the air/fuel ratio and how it was done or can you describe what “carefully adjusted” means?
Exhaust header temperatures of 100F and 250F both are hundreds of degrees below typical temperatures in most conditions of engine-running in steady-state measurements. Also, beware that infrared temperature thermometers work well only so far as the user understands blackbody radiation and the thermometer adjustment needed for different materials. How they are pointed, how far away from the material, and material properties all matter on getting a decent infrared quantitative measurement. These temperatures you report could warrant describing how you got these numbers.
Sound like you are making decent progress on the problem, good continuing luck!
so if you only had posted the above…then no issue and you’d be out for a nice drive !! If it ain’t broke don’t fix it. But,…seriously “good power” is relative…so still maybe the carb piston freed up…or jet etc had some clog and the spray loosened it up, cleaned it…but still carb tune may be a bit off. Not sure I’d trust the temps taken as others have noted. Road test for power on a hill…and after a nice run…with almost no idle time on coming to your garage, shut down and check the plugs for lean, rich or good. The vacuum mystery remains.
To restate my situation now: the engine is running well but I still measure big temperature difference between front and back exhaust headers (using both infrared thermometer and by hand - I can put my hand on exhaust headers when engine is running) and intake vacuum, which is very low. These are significant anomalies which I cannot explain. Can you?
just guesses now…which I am loathe do do…but seems it is at that point: Mike Eck asked if you had true dual exhaust…all the way separate? Your test with starting fluid spray at the intake manifold areas produced no change in running, right…so that eliminated a leak there. (fire extinguisher always on hand…and stand back !!) The heat difference in exhaust manifolds is common in marine engines due to water cooling blockages and flapper valves …I have no idea how that might apply…and does not explain the low vac. A clogged exhaust might affect both in some way…just guessing tho, and should reduce power, hang light paper or cloth at exhaust exit, each side if dual…if single an uneven pulsing may be detectable. Are there any air bubbles escaping in your radiator water? Is exhaust whitish? Any difference in what the spark plugs look like after a run?
Nick
I’m just thinking out loud here (well I’m actually typing in silence). Phil says that “- the temperature, measured with infrared thermometer, is very low on the three rear exhaust headers: only around 100F versus about 250F on front three headers.” The only way I can see you having such low temperatures on an exhaust, is that there is no ignition in those cylinders - eg no explosion = no heat.
Does it run any rougher if you remove the ignition leads to these rear cylinders? If there is no ignition on these rear cylinders then removing the leads shouldn’t have any effect.
On a 4 cylinder bike , if its running on 3 , you touch the header pipes to see what one is the coldest , then the first step take a plug out to see if you have a spark !
You say you can put your hand on a exhaust headers when the engine is running , that don’t sound right , can anyone else do that ?
With no combustion at all you would expect that just the heat of compression would raise the exhaust gas temperature to the neighborhood of 118F (20c in, 47.5c out, 9:1 c.r.)
but…oddly, Phillip says the engine is “running well”.
??? nearly all things I can think of, or mentioned here would cause missing, or at least loss of power, I was leaning toward exhaust restriction, may not miss but certainly would be a loss of power.
Nick
sorry to be negative but this makes no sense. you cannot have a running engine where you can place your hands on the exhaust manifolds. ASSUMING they are not insulated and that you are not wearing welders gauntlets!
What if the fuel was shut off completely at the rear carb, such as with blockage in the float bowl or something? Would an engine run on just the 3 front cylinders? Would that explain how the rear exhaust manifold could be less hot than the front?