Knock knock...

**Homestar** · 11th October 2013, 01:16 PM

Tought I'd start a different thread for this rather than post on my 101 thread. The 3.5 V8 in my 101 has a an issue with engine knocking under heavy loads - has had since I bought it. It is usually not been an issue as I don't often push it that hard, but yesterday it became a big problem for me.

Heading back down the coast yesterday in quite warm conditions - 30 plus degrees, towing a trailer through the hills. Now, I haven't owned it long enough to do many KM in the heat, so this was really a first, but the knocking was so bad under load that I could barely use half throttle in any gear. This meant that at times I was only doing 30kmh going up some of the longer steeper hills.

Engine temp would also climb quickly and it was this that was guiding how hard I pushed the throttle. I never let it get above 95 and as soon as I crested the hills, the temp would return to 86 very quickly. It doesn't overheat sitting in traffic - I had some long waits at roadworks in the same heat and the engine actually cools down to about 82 sitting there idling, so I think the cooling system is fine.

Would I be right in assuming the knocking also increases the combusion temperatures, and hence makes the engine heat up quicker when this is occurring?

Once I got into Vic again, I hit the cool change and it went back to being ok again, but this is obviously something I need to sort out.

So, from what I gather, mixture and timing are the two things most likely to cause this? Now I can check the timing when I get home, and assuming that is not the cause, how do I check and if necessary adjust the mixtures on the twin carbs?

Oh, and as I've known about this issue, I have been pouring 95 or 98 into the tank to try and minimise this, but in the heat it was to no avail.

So, any advise greatly appreciated. I plan on doing more land distance stuff in it, so this is one of the first things I need to fix.

Hopefully I haven't done any serious damage to the engine, but I'll pull the plugs when I get home and stick the borascope in each pot to check the pistons are still in good shape.

Cheers - Gav.

**101RRS** · 11th October 2013, 01:20 PM

Yes sounds like you are pinging - you did the right thing increasing the Octane of fuel but if is still an issue should be one of three things - as you have highlighted - mixture too lean, timing too far advanced (try going back to the spec 3 degrees BTDC and see what happens) or there is carbon in the combustion chambers.

Sounds like a good tune up should fix the issue.

Garry

**weeds** · 11th October 2013, 01:20 PM

who's there

**cooperrat** · 11th October 2013, 01:38 PM

I knew that would happen tee hee

**Mick_Marsh** · 11th October 2013, 01:38 PM

Originally Posted by garrycol

Yes sounds like you are pinging - you did the right thing increasing the Octane of fuel but if is still an issue should be one of three things - as you have highlighted - mixture too lean, timing too far advanced (try going back to the spec 3 degrees BTDC and see what happens) or there is carbon in the combustion chambers.

Sounds like a good tune up should fix the issue.

Garry

Your timing may be a little different as yours is a 12V model.
Could it be any issies with the diaphragm you changed?

**Homestar** · 11th October 2013, 01:41 PM

Originally Posted by Mick_Marsh

Your timing may be a little different as yours is a 12V model.
Could it be any issies with the diaphragm you changed?

It was doing it before I changed the vac advance diaphragm too. Changing that has reduced the idle speed as there is no vacuum leak now, but I still have the pinging.

**Homestar** · 11th October 2013, 01:44 PM

Originally Posted by garrycol

Yes sounds like you are pinging - you did the right thing increasing the Octane of fuel but if is still an issue should be one of three things - as you have highlighted - mixture too lean, timing too far advanced (try going back to the spec 3 degrees BTDC and see what happens) or there is carbon in the combustion chambers.

Sounds like a good tune up should fix the issue.

Garry

That's what I'm hoping for Garry.

. I need some specific info on the carbs before I can proceed though. I can balance them as I have the gauges to do this, but I have no idea how to check and change the mixtures in these puppies though.

Anyone got a manual on them - I can't find the info I need.

I'll check for carbon build up when I get the borascope out.

. I want to make sure I haven't started melting any of the pistons...

**Hoges** · 11th October 2013, 02:00 PM

Couple of thoughts:

Firstly, the carbs may be OK!

If the engine has been subjected to relatively short runs in cold(er) conditions then it's probable that it has been running rich (assuming it has automatic choke?) hence the carbon buildup that Garry is pointing to. This becomes evident under heavy engine loads.. the carbon begins to "glow" and forms a constant ignition point...

Second thought is that the vacuum advance on the distributor is not functioning properly and is sticking in the advance position from when you have a high vacuum on overrun... so when you give it more throttle for the hills, the vacuum reduces but the distributor plate isn't sliding back far enough and allowing the timing to retard sufficiently... may pay to have a sticky inside the dist. and check it is adequately lubed.

There are several engine treatment products which can remove carbon buildup ... despite some negative press, if used regularly in fuel they will clean crud off the valves and piston tops... I've had success pouring some into the cylinder through the spark plug holes and allowing it to dissolve some of the rubbish then mixing the rest into the fuel.

Guess it's a bit like an aircraft engine...use it or lose it! Make it work hard fairly regularly rather than occasional short runs...

cheers

EDIT: what sort of carburettors are they?

Edit 2: if they're Stromberd CD175s ? try http://www.bmcno.org/tech%20tips/zen...ech%20tips.htm

**Homestar** · 11th October 2013, 02:27 PM

Hi Hoges, thanks or that. They are the Strombergs.

Manual choke only on this puppy and it does some decent runs. I have done 3,000KM in it in the last 2 weeks.

Previous owner also ran it hard, using it regularly for touring with a tandem trailer in tow. Think FNQ and the Simpson, but I will check inside with my borascope when I get home. I will post the results next week when known.

I went right over the Dizzy before I left on this trip as the vac advance had died. When I replaced this, I checked everything on it - it is a Bosch Scorcher Electronic item that is only a few years old. Everything was lubed properly and all working - both the vac and mechanical advance. Then Engine has only done 47,000KM from new

**Eevo** · 11th October 2013, 02:44 PM

Originally Posted by Hoges

Guess it's a bit like an aircraft engine...use it or lose it! Make it work hard fairly regularly rather than occasional short runs...

one of our aircraft has exhibiting similar symptoms. turn off the engine and the prop keeps going, now thats scary.

my money is also on carbon build up.

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