Fuel & Tuning 2.25l

Hi Folks,
2.25 petrol Series III (1976 with anti-pollution gear) now refuses to idle after some heavy running over some very rough roads. Was idling fine before this. Thought I had cracked it when I found a split in the vacuum advance hose, but fixing this did not cure the problem. Checked the VA by sucking on the hose with the dizzy cap off and observing the movement of the system- seems to be OK. Checked tightness of manifold bolts and security of carby top cover screws etc., but no problems there. Eventually gave up and just advanced the idle screw a little so it doesn't keep stalling in traffic. There was no evidence to suggest that the idle screw had shifted from previous adjustment, and ignition timing was checked before this trip and was fine. Has plenty of power, just won't idle. Points have been replaced with an electronic triggering system. While my "solution" will get me home (Alice-Adelaide) it isn't very satisfying as I figure something must have changed, just can't figure out what.
Any suggestions as to where to look next would be appreciated.

Coop

Rough roads have stirred up the rubbish in the bottom of the tank, and some of it has got through to the carburetter and blocked or partly blocked the slow running jet. Last time it happened to me it was actually the paper filter just before the carburetter was disintegrating and letting little bits of paper into the carburetter.

John

Thanks for that suggestion John. Both fuel tanks are brand new, but that doesn't mean there hasn't been some rubbish in them. Guess I'll have to give the carby a good cleanout when I get home.
Cheers,

Coop

Hi Folks,
We are preparing for another assault on the Simpson- this time its the Madigan line in company with other more modern Landies. (Safety in numbers).
The local tribes ask that we use low-aromatic fuel and to comply we have decided to use Avgas (the other options are difficult to access).
Avgas is 100 octane low-lead fuel. I figure it will help solve any vapour-lock problems if things get very hot, and the higher octane should reduce knocking when the revs get very low (as is likely to be the case nearing the tops of dunes if I've selected the wrong gear).
Wondering what people think about tuning for this fuel? I'm inclined to advance the timing a little more than usual to get the best out of it. I need to get the maximum fuel economy as we will be the only ones running petrol so we need to make sure we have enough.
Cheers

Coop

be careful with how much advance you wind in, and because its got a slightly different density than normal fuel you will need to tune the carby to suit it, too much advance and the wrong mix will lead to piston melty fun.

you can tune it with a clip in o2 meter in the exhaust, when its at operating temperature you want it set just marginally richer than the point where your o2 reading is 0%

Thanks Dave, much appreciated. Just need to find someone with an O2 meter,
Cheers

Coop

Hi Dave,

When you say "tune the carbie" do you mean play around with the jetting to make it suit the avgas all the way through the load/rev range? Because unless that is done then there is still a big risk that it will melt something. Just setting the idle mixture won't be enough. I am assuming it is a Zenith carbie for a 1976 so I'm not sure where you would get hold of alternate jets, not a lot of companies still carry stock of that sort of carbie part for something like that. You also need some understanding of how the carbie works throughout the range and which jets effect which parts. An old fashioned tuner who understands carbies with a dyno would help otherwise you will be pulling the top off that carbie an awful lot :).

Cheers,

TimJ.

Quote:

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Originally Posted by Timj

Hi Dave,

When you say "tune the carbie" do you mean play around with the jetting to make it suit the avgas all the way through the load/rev range? Because unless that is done then there is still a big risk that it will melt something. Just setting the idle mixture won't be enough. I am assuming it is a Zenith carbie for a 1976 so I'm not sure where you would get hold of alternate jets, not a lot of companies still carry stock of that sort of carbie part for something like that. You also need some understanding of how the carbie works throughout the range and which jets effect which parts. An old fashioned tuner who understands carbies with a dyno would help otherwise you will be pulling the top off that carbie an awful lot :).

Cheers,

TimJ.

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yup...

the difference between avgas and stock unleaded isnt that great that it will kill the engine if you do tuning runs, with a clip in o2 meter you should be able to guess it in with maybe 4 runs.

they are a tough little engine.

The 2.25 engine is designed to be fuel tolerant. If you don't touch anything it will run perfectly well on 100LL without issues. You will be able to advance the spark a little to get better performance, but I seriously doubt that messing with the carburettor will either give an improvement worth the effort or damage the engine - although with the higher octane rating it may be possible to run lean enough to cause damage, but only if you do sustained operation at maximum power, something you are unlikely to do in the sort of trip you are contemplating.

John

Thanks again, folks,
"Sustained operations at maximum power" is what we do when on the highway to get somewhere(ie, flat out). However, we won;t be loading avgas until we get to Mt Dare, and from then on it will be relatively slow going, so should have little risk of cooking the engine. There will be short bursts of maximum power to get over dunes, so a few more revs at those times could be useful, but most of the time we will just be pottering along. And yes, the carbie is a Zenith which we recently overhauled.
Cheers

Coop