T25 only running on 3

[andygriffiths]

Hi,
I'm after a few pointers on getting it running on all 4

It's a 1.9 watercooled petrol.

I've been rebuilding British bikes for 30+ years so know the basics, but will admit to being a pleb when it comes to 4 wheels.

I have checked the usual plugs leads etc, and done a compression test with all being roughly the same give or take 5 psi (135 ish from memory)

This has only started happening since going to a local garage to get the heater sorted and the tick-over adjusted, as it was high when warm (I am guessing was an air leak)

Anyway the garage now say it has dropped a valve and needs a replacement engine, which I am suspicious about due to the compression test being OK

All pointers would be mucho appreciated.

Thanks

Andy

[937carrera]

You have good reason not to immediately accept the verdic the garage if you have done the compression test and obtained those results.

First step would be to have the engine up to temperature and then pull the plug leads off one by one.

If the engine speed stays the same or only drops marginally on one cylinder, you know which cylinder is off. Try that and report back

[andygriffiths]

Thanks for that,

I already know which one is not running as the exhaust is cold.

Its the passenger side at the back.

Thanks

Andy

[937carrera]

What engine is it - help us to help you by putting the details in your signature

Have you got a good spark on that cylinder and made sure it isn't a duff plug ?

E D I T - I see you may have already done that.

So you have spark, fuel (assuming it's carb not FI) and compression under compression test, but maybe not when running. I would be taking the valve cover off and making sure you don't have a broken valve spring

[kevtherev]

Thanks for that,

I already know which one is not running as the exhaust is cold.

Its the passenger side at the back.

Thanks

Andy

So as a biker I would have checked for a spark.
#3 & #4 are subject to the brake booster vacuum hose
If this is compromised there will be a misfire.
As its so particular this is an electrical fault

[andygriffiths]

Thanks Kev,

Yes, tested, and swapped the plugs, and leads, , kept the same on the disti.

Am I right in assuming the brake booster vacuum hose is the one going into the manifold on the left?

Thanks

Andy

[kevtherev]

Thanks Kev,

Yes, tested, and swapped the plugs, and leads, , kept the same on the disti.

Am I right in assuming the brake booster vacuum hose is the one going into the manifold on the left?

Thanks

Andy

Yes.
If it leaks air in it will be specific to 3&4

[937carrera]

As the misfire is on 4 only, then it can't be the brake servo pipe.

To affect a single cylinder then possibilities include

1. Electrical to that cylinder, either spark to the plug or the plug itself - but plugs / leads have been swapped and test out OK
2. Fuel to that cylinder - if it is injected and there is an electrical problem on the injector, fuel supply failure or an injector failure. The OP hasn't provided more details on the engine, I'm assuming it is a single carb which doesn't differentially affect one cylinder but we dont know.
3. A compression problem. Compression test shows ~ 135 psi which should be absolutely fine, but that is only at cranking speed. I suggested a broken valve spring which could mean valve bounce at normal engine running speeds.

I would expect dropped valve, burnt valve etc to show low compression.

[ajsimmo]

Carrera, I just said almost exactly that on the phone to him last night. I suggested a broken rocker arm, or failed lifter - something preventing a valve opening on no.4, most likely inlet valve. It would still show compression, but not fire. I also suggested setting valve preloads as a way of checking
a) what they're currently set to
b) if the valve opens on tightening, or if the lifter is soft and compresses.
And yes, it's carb. So if we have compression, spark and fuel it would fire. So if it's fuel that's missing, it can only be a valve staying closed (as staying open would prevent compression, and inlet open would also upset running of other cylinders).

Sent from my ONEPLUS A3003 using Tapatalk

[937carrera]

Thanks for putting the post up Andrew, you never know if the discussion has gone offline or to PM's

Good to know he's in sound hands

[andygriffiths]

I found the problem, can you see it ?



Hopefully it hasn't trashed the rockershaft bearing as it not been run for long.

[937carrera]

Difficult to see in the picture, but I assume one of the rocker shaft clips has come loose and allowed the shaft to slide, meaning one of the valves (exhaust on number 4) wasn't opening ?

[Oldiebut goodie]

Any better? Certainly looks out of place to my old eyes.

[andygriffiths]

Nither, the nut came off holding the rocker shaft down, fortunatley it was just sitting in the bottom, if it was a vertical engine it would have been a disaster.

[Oldiebut goodie]

That is the disadvantage of being a diesel guy - I assumed that was the end of a bolt outside the head.
The other must just be the angle of dangle of the pic.