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No 6 cylinder appears to be running weaker


daverclasper
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Hi.  Vitesse Mk1 2 Litre.  This is according to plug colour and seems to be getting more so in the past couple of months.

Have tried a manifold leak test around no6 area with brake cleaner, though not conclusive (maybe a tiny leak not showing and also I have always had  an uneven idle, so maybe difficult to tell a slight change).

Any suggestions please, or could it be another issue?.

Any help, great, thanks.

PS, No difference in how it runs that I can tell, except occasional split second hesitation after say slowing down for a roundabout and then giving it some gas.

Dave

Edited by daverclasper
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2 hours ago, Pete Lewis said:

I would suggest a compression test , you may have a valve showing leakage

This is one possibility.  Also worth trying the compression test with the throttle shut.  If 6 gave a higher reading in this situation then it could indicate a manifold air leak.

Years ago I also had a cleaner plug on no 6.  Initially I thought leaner (though normally 2 & 5 are the leanest running), then I realised it was actually cleaner rather than leaner and wondered about headgasket issues.  This was supported by slow water loss, but not by cooling system pressurisation with the engine running or by compression test results.

While starting the dismantling process to examine the head gasket I found that there was a little water in the inlet runner for no. 6 and realised that there was a water path to it from the manifold heating jacket.  Swapping the inlet manifold solved the problem and the head stayed on.

Nick

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Water inJection was a old idea, I made a coil of brake pipe round the downpipe of my 53  mk5 minx to squirt steam into the carb as a unknowingly idea from a youngster, the pipe furred up and blew a hole  are well the best laid idea's  was  doomed

But Nicks  clues are well worth a careful  look in the manifold area.

Pete 

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Dave, I found that an electronic ignition conversion engineered out one of the variables, the good old points. My idle was transformed to even and consistent and gave me confidence in that setup so as then be able to look elsewhere. Try electronic, even if you go back to points, you’ll learn.

Also, the pipe-in-the-mouth-of-the-carb to set the flow can be surprisingly accurate, set the balance too. If you’re local to Chelmsley Wood/South Birmingham, I can lend you a proper, graduated air-flow meter and a timing light. Let me know. 

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Oops. Forgot. No 6 can run hotter due to compromised cooling! The waterways in the block/head get full of crud at the rear first. If you remove the block drain plug and nothing happens, it’s blocked. I ended up removing my head and cleaning out the block from above with a wire and multiple fills with water. 

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Thanks a lot for offer of loan of carb balancer Adrian. Not local to Brum though. Would be interested in trying that as I set mine by ear with a hose pipe to ear, though maybe not as accurate.

Did have E.I on for a bit, but couldn't notice a difference. My dizzy shaft play does not seem too bad?.

My block drain is very free flowing, maybe a sign that the rest is clean, though not proof I imagine.

Cheers, Dave 

 

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Hi. Well finally found time to do a comp test. Found it bit difficult to get a seal (rubber bung/push in type, maybe rubber hard, as a vintage one) though managed it with the dry test. All 6 cylinders between 190 and 200.

Second test with throttle closed showed a difference of 0, or + / - 1 difference from first readings (I guess they should all have been -, as less air)?.

Wet test not accurate, as could't get the gauge to seal properly and my helper tan out of time. 1 and 2 cylinders/230 and 220 higher. Did try no 6 ( the one that's running weak, showed 5 higher, but this test was leaking worse and ran out of time at this point.

So, not sure what to make of that, except looks like losing comp from the cylinders/pistons on at least some of them.

Cheers, Dave   

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Dave,

If the dry test was consistent across the block then the wet test is irrelevant.   It's only to check if a bore that's down is due to piston ring leak, when the pressure will rise (it alwyas does leak or not) or valve leak when the oil won't help.  And if there was only a minimal difference with the throttle closed (Nice one, Nick!) then no manifold leak.   BUt the other way to chek that is to spray fuel (well,  perhaps A fuel, brake cleaner will do or WD40) at the manifold gasket.  If there is a leak then the added fuel may cause a brief blib in engine rate or note.

May be worth looking inside your manifold.     It has to split the flow from on crab to three cylinders that are pulsing away, and a fault in the inside wall might cause that to go off ('m cluthcing a straws here!)

John

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Thanks John. I was sort of chuffed with those tests across the bores, as not done this test ever before, and my car does use a bit off oil . 

So, maybe not compression/valve intake related. Will think about air induction related with the manifold and Nicks possible coolent issue into manifold.  

I have done the spraying around no 6 intake manifold and didn't notice a difference, though my idle is irregular at around 700 revs and maybe hard to show a difference, could raise to maybe around a 1000 when it smooths out and test, do you think?.

The car isn't driven hard, so maybe if it is a bit more fuel weak on one cylinder, maybe not the end of the world (or even my Triumph) ?.

Cheers, Dave

 

Edited by daverclasper
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Hi. Just wondering, as I'm assuming those those comp reading are pointing to valves/cylinders being ok and  therefore intake manifold small leak still possible, would it be worth nipping the manifold nuts up slightly for no 6 with a torque wrench. I did undo one side nut to fit bracket for the breather valve as car didn't have one a while back and tightened it by feel, on the cautious side (as it looks like those bracket things could bend), as couldn't find torque wrench at time.  Even if it improved for a short time only, may tell me something?.

Dave 

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