Air leak, bad carb, or something else?

[bustrucket]

Hi all

Getting some strange stuff happening with my rebuilt engine, can anyone offer some advice on what to try next here? I haven't been able to give it a test drive yet, so all of this is just with the engine in the van sitting still (no load). Carb was stripped and rebuilt before getting put on the engine, all jets blown through etc.

Engine starts immediately with the choke on, runs fine and revs drop as the choke comes off but I haven't been able to get "hot" idle down below about 1000rpm without it stalling. Idle is a bit unsteady as well. Seems to respond to throttle fine, no pops or backfires. However, after about 20 minutes of idling, the temp is almost at the top of the gauge and the red light starts flashing. Fan kicks in but temp stays high.

I've changed the thermostat with a brand new 87C one (tested it in hot water beforehand), but no change. Bled the system a couple of times, got a fair bit of air out the first time, almost none the next time.

Timing is almost bang on 5 degrees BTDC at idle. Vacuum advance on dizzy works.

Vacuum gauge was showing only about 14-15 in/Hg at idle so thought it was an air leak - tested vacuum units on carb & distributor, all good. Took carb off manifold, dirko on the flanges, put some on the manifold gaskets as well, still no difference. Tried spraying water around the flanges as well but couldn't get at the carb flange very well.

I've pretty much run out of things to try short of taking the carb & manifold back off and cleaning resealing everything again - anyone got any ideas before I go down that route?

Thanks

David

[937carrera]

Let's ignore the idle problem for the moment and focus on the cooling problem.

Are you happy that the system has bled OK as it sounds like either an airlock or the water pump is not circulating coolant. Are the hoses getting warm at the front and on the return pipe to the thermostat ?

[bustrucket]

Not 100% confident! I never checked the hoses at the radiator end, just the radiator itself, which got quite hot after about 10 minutes or so. Bleed screw on radiator was letting (hot) coolant out without any bubbles, and the hoses in the engine bay were both hot as well.

What should I be doing (if anything) with the bleed screw in the engine bay? This is an early cooling system with the screw on the hose junction, I tried opening it up but it didn't seem to have much effect...

Thanks

David

[937carrera]

http://wiki.club8090.co.uk/index.php/Pe ... em_diagram

There's a diagram of the system in the wiki.

From what you have said the coolant does seem to be circulating, and the thermostat is open.

I'm not that familiar with the water cooling system on T25's so will allow others to give you more specific advice, however Simon Baxter has posted a really good how to here http://www.brick-yard.co.uk/forum/bleed ... 29710.html

I wouldn't worry about the idle yet. An engine won't normally overheat on idle even with timing miles out and weak, which is why I suggested looking at the cooling system.

How much of a rebuild has the engine had ?

[bustrucket]

Thanks for that, although the diagram applies to the later system so there are a few differences to my setup. It's probably worth running through the bleeding procedure.

Engine was more or less a total strip and rebuild, it's had new bearings, camshaft, conrods, piston rings, valves, and oil pump. Was a little stiff to turn over (by hand) after first assembly but I think this was just because of the new rings? Seems to turn over very easily on the starter anyway...

[937carrera]

Good to know the extent of the rebuild, we now know the engine has had the cases separated. Did you do it yourself, or have an engine builder ?

I can think of ways you could have these symptoms with a properly functioning cooling system, but don't want to go there for you yet. Stick to the easy / basic stuff first before more complex concerns.

[bustrucket]

Rebuilt it myself

[937carrera]

That's good news, it's all your own work, you know exactly how it was put together, you can get stuck in to fix it

[bustrucket]

Haha I'm not entirely sure about that!

Did a bit more research and read the guide on bleeding the system, I'm thinking maybe I wasn't thorough enough with getting all the air out of the system, so I suppose I'll try that first off and see how it goes.

[sarran1955]

Hello,

Hi all

Timing is almost bang on 5 degrees BTDC at idle. Vacuum advance on dizzy works.



David

Do you mean ATDC 5°...

But that should not cause overheating at idle, even on a Wetbox.

Cordialement,

[kevtherev]

Early DG is set 5 deg BTDC no vac.
V notch on the crank pulley

The cooling system needs to be bled as per the Baxter method.
Lets get that under control first.

As for the idle
Choke pull down
Secondary vac throttle
Distributor
All have diaphragms check that they are not leaking.

Has the mixture been checked?

[Mocki]

its early cooling system, so you will need to raise the front despite what others may tell you, the engine coolant pipes will hide air , as will the heater .
make sure the revs are high , 2000rpm at least and when you think you have all the air out, let it cool, and do it again!!

as for the revving, are you sure the carb throttle is set correctly against the stepped cam?
also the right hand vac pipe on the back of the carb that should go to the air box is sealed, its known for raising the revs for some reason .

[bustrucket]

Apologies for the wall of text, since I can only get to the van at the weekends I thought I'd better try and include as much as I can!

OK, first time around it turned out I had set the timing with vacuum connected, so corrected that first off. Now set to 5 deg BTDC with vac disconnected and sits around 11-12 deg BTDC (vac connected) at 1000 RPM.

Raised the front of the van and tried bleeding the system again (twice), got a few tiny bubbles out but no indication of anything more serious. Pipes get hot front to back, radiator gets hot top to bottom and evenly across the way as well. Unfortunately my heater fan is bust, but got my hand behind the dash and the heater feed and return hoses got hot as well so don't think there's air in there either.

Gauge still reads hot though. However I did try jumping the fan switch to keep the radiator fan running, and the gauge stabilised at around 3/4 with no sign of going higher. Return hose got quite a bit cooler and I think the temp could have come down a bit further if I had given it enough time. Unfortunately I didn't have time to try running the fan continuously during initial warm up to see if it would stabilise at a lower temp. Possibly just a combination of old narrow radiator and poor airflow here?

Vac units - I tested the them a couple of weeks back with a pump, they all hold vacuum OK, although they do leak down very slowly. All vac hoses are good, right-hand pipe is currently used for the vac gauge. All others connected as normal. Vac gauge still bounces around between 14 - 17 in/Hg at idle.

Mixture - hard to tell, I got hold of a Gunson CO meter but the reading fluctuates a bit, near as I can tell it sits at about 1 - 2% CO at idle (1st test - 1.3/1.4%, 2nd test 2%+). This is with the CO screw about 2 turns out. Any further out than that and the idle speed starts to pick up and CO drops down to 0.5 - 0.9 or even lower. Any further in and the idle becomes very unstable.

[Mocki]

next check is sniff test for zorst gasses in the coolant then.....

i had this many years ago, turned out i had put a head on with a pin hole tween combustion and coolant

[937carrera]

next check is sniff test for zorst gasses in the coolant then.....

I was hoping we wouldn't need to frighten the OP about having to go down that route when I said I could think of ways he could have these symptoms with a properly functioning cooling system, but, as you've opened the bottle.....

Yep, if the cooling system is properly bled, then the next area to consider is combustion gases getting into the coolant. Could just be barrels not sealed correctly.. Best way to test for this would be a combustion gas tester to sniff the coolant.

However, the system may not be bled properly yet. I recently did another car with front rads / rear engine that's a nightmare to bleed. Thought I had it done, temps all OK, lots of running up to temperature before taking onto the road. The car ran for a full day with no issues, multiple heat cycles. Next morning temp gauge went up to red after 10 minutes running. It needed 2 litres of coolant, there was an airlock somewhere, which had finally burped itself clear.

I'd suggest the OP goes through the bleed procedure again, making sure that water levels in the header tanks are where they should be. Bridge the fan so it's on max and allow the van to get up to temperature. I would open and close the heater valve a few times too. Then see if the water level has dropped when the van has cooled. Time for coolant sniffer then.

Many's the time I have got unnecessarily concerned about a repair because I overcomplicated the problem, I don't do that much these days, so I must have learnt something. The answer can quite often be the simple one.

Bustrucket, you got a wall of text back , more suggestions to come before the weekend I suspect.