7 replies to this topic

[Ginge620]

So just rebuild my 998 and got around 250 miles. Within those miles i have noticed that the engine runs far better than before in every way (Up to 4000rpm due to running in there is more power especially low revs say 1500 - 3000 rpm the car just seems to take off quicker, revs climb faster, also much better throttle response i always had like a 1-2 sec delay where as now it more or less instant, no noticeable flat spots between 1000 - 4000 rpm. Note this i only according to the Butt dyno

 

Now the problem, Any gear, under acceleration half throttle or more there is a knocking noise that starts around 2800 rpm, then goes away around 3000 rpm. Its not pinking because i heard that when adjusting ignition timing and got that to go away after retarding the dizzy. Only tuned by my self adjusting on feel and the colour of the spark plugs. I do want to get is rolling road tune but i want to get some miles on the engine and probably switch to an Aldon yellow dizzy for the more suitable advance curve.

 

I am going to look into this further checking things around the engine not necessarily the engines internals could be manifold knocking on something but it does sound internal, bit like the knock/rattle on diesel engines.

Also going to check compression because i want to compare the results anyway to how then engine was before the rebuild

 

Info about engine is in my signature also im running 99 Octane fuel and the 12g202 head had 40 thou skimmed off and some light modification the the combustion chambers and ports according the "yellow engine bible".

 

I might also be able to get an audio file uploaded later depending if my phone picks the sound up.

 

Just thought i would make a topic so anyone with ideas can suggest them.

 

Thanks

Edited by Ginge620, 18 August 2015 - 12:37 AM.

[Spider]

It's very hard to diagnose noises and sounds from behind a keyboard, but I'll just say, don't discount that it could be pinging. At the RPMs where you are experiencing this, would be around-ish peak torque, where it is most likely to ping.

 

Depending on your Cam, Dynamic CR, Fuel and the dissy currently fitted, it could need considerable retardation.

 

Not saying this is the problem, only don't discount it.

Edited by Moke Spider, 18 August 2015 - 12:19 AM.

[Ginge620]

It's very hard to diagnose noises and sounds from behind a keyboard, but I'll just say, don't discount that it could be pinging. At the RPMs where you are experiencing this, would be around-ish peak torque, where it is most likely to ping.

 

Depending on your Cam, Dynamic CR, Fuel and the dissy currently fitted, it could need considerable retardation.

 

Not saying this is the problem, only don't discount it.

Ok, so i forgot to mention, i have tried setting the ignition timing with a strobe @1500rpm with vac pipe off and sealed, all i could managed was 8 degrees because the vac thing hit the oil pressure switch/line. I tried it at 8 degrees but all i got was poor/delayed throttle response and less power to the point where pulling out from a T junction becomes a problem.

I understand its not easy to do this behind keyboards but anything helps. Also its the standard/original dizzy and a Kent 266 cam.

 

When i heard the pinging due to too much advance it sound more distant/quieter than this knock i get atm but what confuses me and like you say not to count out pinking is that the knock does go away when i come off power. Hopefully ill have more info after work.

Edited by Ginge620, 18 August 2015 - 12:37 AM.

[Spider]

Is the sound there on steady part throttle at those revs?  Or does it only become audible on acceleration?  I understand that maybe hard to determine given that it's a fresh engine.

 

How old is the fuel?

 

I've had fuel that was old (or something sold as fuel ) that on the slightest opening of the throttle, not even what you'd call load, the engine pinged it's head off (literally for my poor mate).

 

Could it be a mount problem, where the engine is tilting on acceleration and so the exhaust (or something else) is contacting another part?

 

Exhaust resonance?  Loose Baffle in the muffler (though this would be towards the back of the car)?

[sledgehammer]

+1 all the above - pinging / pinking , gets louder under load , quieter off load , any run on ?

 

does it do it at standstill or just under load ?

 

what is the oil pressure like , were the ends good ? head re-torqued ? 

 

oil getting to the rocker gear , rocker condition , valve lash re-set ? (in the past had a push rod bend slightly pushing rocker over when reved)

 

it really needs a vid

[Cooperman]

It does sound like 'pinking'. Try retarding it several degrees and drive it. It will hen be much too retarded and will get hot, but just do it once as a test. if you are pinking and continue to do so it can break the rings and score the bores - then another re-bore & pistons will be needed.

[gazza82]

You might want to check the dizzie is correctly aligned or rotate it and compensate by moving the dizzie drive to get more 'swing' so you can time it properly. One tooth on the drive/cam makes quite a difference.

[Ginge620]

GOOD NEWS. Ive fixed it. So when you have newly rebuild engine and something starts knocking you always assume the worst. It turns out it was the damn air filter. I had foam behind it which is what caused it to be a dull sounding knock. 

I refitted the stock air filter previously had a cone style mini sport filter, and no longer any knocking. I went out for an hour drive and never heard it. I have to say too its nice not having the massive induction noise all the time.

 

Thanks for the help