998 na 1275 head power lost
Turbo Mini Forums

Last week i was driving the 998 with 1275 head. ( block has pockets and im running it like this for 2 years now)

Was doing 130 kmh on the high way but all the sudden it started to have a power lost combined with what i could cal a noise like something was rotating out of balance. ( hard to discribe)
Pedal to the metal the mini would no longer do more then 110 kmh

Came home and saw that the sparkplugs wore all black... wore all nice and brown for months on a row before.

Yet ignition wasn't twisted and the dizzy cap looks clean.
Carbs haven't gone out of tune
And compression still is 13to1 ( as a tought i eighter bended a valve or i broke the head joint)

First and second the power lost isn't noticed and the engine still revs like before if you puch the trotle in neutral.

Went in with an endoscope. Exept dirty pistons there isn't all that much strange to see in there.

What i did see come to think of.. the engine was running slightly hotter then before. But cant say how long it was. Checked the gauge once my mind sprung to head joint.
Not extremely hot and temperature could be pulled down fast by using the heater inside.

What i can think of now is a faulty coil or faulty fuel pump.
But how can i test it ( only have acces to basic tools) + am i looking right or should i pull the head anyway to see if it isnt there?

All valve adjusters were still fix to.

you can do anything if you set your mind to it...
i rather blow it up winning than keep it together losing.

finish date set for project 1... march 2018

O compression wast tested cold because i didn't wanted to run it hot before i find the problem.

you can do anything if you set your mind to it...
i rather blow it up winning than keep it together losing.

finish date set for project 1... march 2018

First thoughts
Sounds like a binding exhaust valves especially. If you have manganze valve guides, you may have bent a valve the guides will need reaming.

Change the coil first though.

Any one else?

My first question would be what do the afrs do?

One of my old 1275 heads suffered terrible valve bounce, it would rev through it in lower gears but hit a wall in 3rd and 4th just a thought

A head refresh and it was loverly untill the threw the main bearings lol

Edited by Brett on 25th Feb, 2016.

Yes i moved to the darkside

Instagram @jdm_brett

Unfortunately i do not have an afr gauge in this one.

So i tested this. Spark is white and jumps a 8 mm gap with easy.
Compression hot is 185 lbs per square inch.
Fuel pump gives me about 650 cc per minute. ( that seems low to me since my 1100 cc chopper pump is suposed to give 950cc a minute )

Really in doubt to take of the head or not to check.

If a valve is bent shouldn't it give bad compression on that cilinder?

What also still bothers me is 1 the sparkplugs were nice and brown for over 1500 km or so and all the sudden all 4 of them are black and powder looking.

And 2 it came without a warning. Just 1 second doing nice 130 and still able to pull other second 110 top.

you can do anything if you set your mind to it...
i rather blow it up winning than keep it together losing.

finish date set for project 1... march 2018

The head has its original guides. Can i test for a binding valve?

you can do anything if you set your mind to it...
i rather blow it up winning than keep it together losing.

finish date set for project 1... march 2018

Springs wore within norms ( cant tell how much) when i placed the head. ( still thats 10000 km ago and 4 years past)

you can do anything if you set your mind to it...
i rather blow it up winning than keep it together losing.

finish date set for project 1... march 2018

so just back from the garage stripped the carbs ( top ) and seen the jets wore almost fully down ( it are zenith twin carbs) so wanted to turn them back up ( adjusters sit on the underside of the fuel pot) and one of the 2 adjusters had a drop of fuel on them ( from winding down to far) i remember that i winded them down so far because otherwise when i corner hard the engine stalls for a second or 2. it might have something to do with it ?

looked at that build up again and it actually is better then i thought. still can read the marks top and A on the piston top so..

you can do anything if you set your mind to it...
i rather blow it up winning than keep it together losing.

finish date set for project 1... march 2018