I have a 1600 Xflow in my kit car. I've been chasing problems tuning it over the last year and recently fitted a new cam. It was running a
little better each time i made an adjustment until this weekend when it began missfiring a lot, just about managed to creep it home. I did a
compression test and had around 80psi in all cylinders (engine hot, throttle bodes disconnected, plugs out) which seems low. I added a little
oil down the plug hole and retested and saw well over 100, maybe 120+ psi. I did a leakdown test with the pistons at TDC and had next to no leakage,
<10%, though i could hear a little through the sump.
So my assumption is that this means rings/bores? When i last had the engine out i did pour a little oil down the bores to see if it leaked and after a
couple of days it hadn't dropped at all. And the very low leak down results (at TDC at least). Currently have +0.030 pistons in.
So question is do i need to get the cylinders bored? Or could new rings on existing pistons do the trick... how to i tell? If a bore is needed would
+0.060 be the best bet to leave some future options?
Alittle more background. Most of my tuning has been to try and get rid of popping through intake unless i run rich and overrun/backfiring in the
exhaust on decel. I've read these can be linked to compression due to poor combustion.
Start with simple stuff, what are the rocker gaps? If you’ve put a new cam in it may have settled and gaps are tight.
100psi to 120psi sounds low but I don’t know the history of the engine or what the expected psi should be.
How many miles has the engine done?
I have done rings only on Pinto engines but they are usually in spec low mileage rebuilds, you would need to measure bores and pistons to see what the
What pistons are in the engine 1600 or 1300/1100 to increase compression in a 1600 people put pistons in from the smaller engines as bores are the
same but stroke is shorter so pistons have a higher deck height.
I eat to survive
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I breath to pi55 my ex wife off (and now my ex partner)
Ive rebuilt many xflows. and there cant be an easier 4 stroke engine to work on!
Problems that would give a low reading
Valve stem guides worn,
Valve/rocker gap to tight
Worn or scored bores
Piston rings failure, ie cracked due to and an overheated engine (ive had this problem)
Engine needs a rebore
Unlikely as its all four cylinders, but a bad head gasket,
If it was my engine, I would take the head off, turn it on its side so the exhaust or inlet ports are facing upwards
and fill all four ports with petrol/diesel, if the levels drop quickly, you have bad valve seating, ive had heads that have leaked
the whole lot in 2 or 3 minutes, but after lapping them in, can last several hours
If the valves need lapping in, it would be prudent to have new guides fitted first
Now level all the pistons so they are at the same height, and put 500 mm of oil into each of the bores
again, ive had engines that have drained the oil into the sump quickly, and also, virtually not at all,
If the drain is quickly, its a rebore, and pistons, although just new rings would be a sort of bodge, but might not sort the problem
Also, your 1600 engine is actually a 1630 now
Thats was probably spelt wrong, or had some grammer, that the "grammer police have to have a moan at
Thanks for the suggestions, here is some more info & answers...
- I double checked my valve clearances. All ok, only very minor adjustment. I had already checked them and the head bolts a couple of hundred miles
after putting the cam in.
- When i had the eingine out to replace the cam (few months ago) i did a leak down pressure test with compressed air. I did have a little passing two
of the intake valves. So while i was changing the springs i also lapped the valves in to stop those leaks. I've not put petrol in the head to
check it but from the leakdown i don't think it's terrible.
- I did put oil in the cylinder bores with them all equal. I didn't notice the level drop significantly after a couple of days.
- Pistons are '1600' not '1300' (i think Cortina ones 18684), +0.030 9-1 compression ratio
- I'm getting spark at all plugs and injectors spray fine too.
- Enging has always had to run a little rich. And has surrered from some overrun and intake pops since i got it.
So i can't see a smoking gun in all this. But the low 80 psi compression does seem lower than folks online suggest.
A good point about the valve timing, what timing gears/ chain did you use? Simplex can and do stretch . Did you use the timing marks on the gears to
set valve timing? If so I would suggest timing using a DTI and timing wheel setting the valve timing to the cam manufacturers specifications
I have a vernier and single chain for valve/cam timing. I did get the timing it spot on to spec using a gauge & timing wheel when i changed cam.
Chain is fairly new so suprised if it had stretched but is a chance the vernier has become loose and slipped. Worth checking while engine still in
The diagnostic specialists are into in-cylinder compression testing for diagnosing this type of issue with it theyy can spot bad valve timing or a
pugged exhaust very quickly A setup for doing it combining a £60 oscilloscope and a cheap eBay 200psi pressure sensor.
I just checked my valve timing three times and it looks good, within 1 degree. Running out of easy fix options now. The big increase in compression
with a little oil is making me think it's bottom end.
I've check the timing and it's spot on. Yes have a leakdown tester and showed less than 10% leakage but i could hear it passing the
pistons (when sump off). One thought is that at the top of their stroke they're ok but further down the bore are passing?