Developed a Misfire
Turbo Mini Forums

Just wanted to get a second opinion on my conclusions...

This is my 998 turbo with megasquirt controlled ignition and HIF carb.

I have developed a misfire which is like hitting a brick wall. The problem seems load related.
I can drive it normal in 1st and 2nd generally once I hit 3rd it misfires and I can't gain pace. Boost is fine in 1st and 2nd. If i sustain 2nd it will start to misfire. 4th also misfires.

I have tried the following:
Changed spark plugs
changed ignition leads
changed coil pack
performed megasquirt log - all looks ok
changed fuel pump
blown out fuel filter
stripped carb and cleaned float filter
checked dashpot
checked tappets

i got a compression test done tonight and have
75 psi
50 psi
50 psi
50 psi

it was 75 across all 4 when i last did one.
(not sure how good the gauge is so absolute values may be a load of bollocks but it is repeatable on the same cylinder)

I can only assume after all this that either the head gasket is gone between 2-3 and 3-4, the rings are knackered or the valves/ seats are knackered.

Anyone suggest anything else before I give in and have to take it apart?

Thanks
Dan

1275 N/A Sprite, 998 T2 Turbo Mayfair
1275 EFi Turbo

That compression is way low, what's the C/R?

9:1

1275 N/A Sprite, 998 T2 Turbo Mayfair
1275 EFi Turbo

i would expect 170-180 ish psi on a 9:1 cr depends on cam and rocker clearance but 50psi is whay off

Edited by Brett on 2nd Aug, 2012.

Yes i moved to the darkside

Instagram @jdm_brett

can you email me the log, i'll take a look, but yeah those figures look well low, and if its just the gauge, its odd three have droped by a 3rd.

Was that done at wide open throttle?

Have you tried doing the tests with a drop of oil down the bores?

Edited by matty on 2nd Aug, 2012.

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1/4mile in 13.2sec @ 111 terminal on 15psi

My fresh built 1293 did that exact thing last week, checked everything and had similar results on the compression test, took the head off and it was a blown gasket ! My fault for not re torqueing the head down after wearing it in.

So I thought I'd report back about this.

Hadn't had any time until now to look at it so have been driving around with revs below 3000 to stop it misfiring.


Today I checked tappets and set a bit of a larger gap (0.33 rather than 0.3 mm) and bought some decoking fluid that you put in you fuel tank.
Didn't help.
Noticed that my exhaust was black with carbon so it seemed rich but the misfire was into the carb which suggests lean. So I replaced the air pressure pipe on the fuel regulator. To start this didn't help either but gave it a bit of a rag in first gear (mainly in rage of not being able to figure out the problem) and a plume of smoke came out the exhaust and it works fine now.
So I'm guessing a small leak in the pie causing rich off boost and lean on boost due to not moving the diaphragm. Perhaps also tappets were a bit tight so now it has blown out all the crap and is getting enough fuel.

Compression is low and you can feel it blowing past the rings but the car now works as it used to.

1275 N/A Sprite, 998 T2 Turbo Mayfair
1275 EFi Turbo

Just out of interest, in what way can you feel it blowing past your rings?..

Main Build
http://www.turbominis.co.uk/forums/index.php?p=vt&tid=489133

Upgrade to Turbo
http://www.turbominis.co.uk/forums/index.p...tid=448367&fr=0

there is a lot of blow out of the breather pipes (more so than there used to be). Perhaps "feel" is the wrong term to use.

1275 N/A Sprite, 998 T2 Turbo Mayfair
1275 EFi Turbo