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SIII overheating


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I'm having overheating problems that I can't seem to figure out. I hope you guys can help.

First off, it's a 2.25 petrol, head has been shaved to 8:1 and converted to unleaded. The radiator is new, thermostat is new and tested, water pump has been inspected (it's fine), fan belt is tensioned properly, coolant is new and clean, radiator cap is new, fan shroud has been fitted and timing is set to 12 deg btdc.

My suspicion at this point is the fuel mixture, but I don't have a lambda meter to know for sure. I have replaced the old weber with a new one (34ICH) and have jetted it to the recommendations. I have a 165 main, F6 tube, 50 idle jet, 40 back bleed valve, 55 pump jet, and 170 air corrector jet. I did have the 190 ac jet, but dropped to the 170 today to try to make the top a little richer.

I suspect the mixture because it runs fine up to 35 or 40 mph. If I hit the highway and run at 55-60 for a few minutes, the temp runs nearly into the red. After a 10 minute run at 35, the temp is normal again. Also, the footwell next to the exhause will become unbearably hot after a run on the highway.

The pump and thermostat are working correctly and this has been verified with a new radiator cap with a thermostat built in. After the run on the highway, the water temp will read between 95 and 100c.

I don't think that the coolant passages are plugged, but I'm not certain. The coolant is still clean, but the truck did sit unused for about 30 years. I have flushed the block, but I suppose it's possible I missed some crud.

I'm open to suggestions if you guys have any ideas.

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Is the radiator OK? If it's been sat for 30 years you could find the old coolant has dried out and the radiator tubes could now be blocked with concrete-hard scale and decades of bits crumbled off the insides of perished coolant-hoses..

Equally, the gills could have corroded to paper-thinness and have lost around 90% of their heat-shedding ability.

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12 degrees advance is a bit excessive.

Are you sure its been shaved to 8:1 and not more? UK spec 2's and 2a's were 7:1, later 2a's (I think) and 3's were 8:1 as standard. Recommeded ignition timing is TDC when using 90 octane fuel and 3 degrees AFTER TDC when using 85 octane for an 8:1 engine and a 7:1 engine is different. Special instructions relate to emmission controlled engines - but seem to start form about 6 degrees AFTER TDC.

Google may help you find a copy of the S3 workshop manual (covers both 7:1 and 8:1 engines and emmission controls.)

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In one of my old S2a's, i ran a 2.25p that got a head skim to get 9:1 CR. I can't remember the exact figure but it wasn't anywhere near your timing figures. I think the timing or mixture is causing your issue.

I just set the timing by ear once the motor had been rebuilt.

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  • 2 weeks later...
  • 2 months later...

Head skims and modern fuels change the flame front (passage of fuel in gasses out) on petrol motors..additives purely cushion the valves.unleaded conversion is a grey area...put jetting backnto standard specs and start your timing at 6..strobe it..suck it and see..if it was water related you soon know it would escape somewhere...or drive at 40

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