Engine won't idle

[chriswilliams]

I have re-built a 1500 TI engine with no history. It has:

Hydraulic tappets.
Webber carbs.

It will not idle. The idle screws have no effect.
It will run on the mains. I took it for a drive and under power > 2000 rpm it seems to run OK. As soon as the throttles close its miss firing etc. Testing the compression gives around 125 PSI on all the cylinders the same as I get on my working SUD.

Trying to set the carbs, I get very little manifold vacuum (I have 4 vacuum gages). Lack of vacuum would seem to be why the idle jets don't work and why it won't idle!

The question is why? How is it I get so little vacuum? Lack in one cylinder could be a leak, but it seems unlikely that they all leak the same?

Any ideas are much appreciated.

Chris.

[LooLooSud33Spider]

Hi Chris. Your Compresion figures sound fine so The first thing to do is to check for major manifold leaks. That means the gaskets under carbs aswell as the manifold to head gaskets. Check this by squirting some carb cleaner in the area whilst holding the engine revs to avoid stalling. If you get no change when you spray then that would suggest gasket side of things is ok. Also Pay special attention to the servo pipe it’s not unknown for them to perish and give problem.
After that you’re getting into basic carb settings. One last thing is just check your idle jets are nice and clear they block very easily. Now The trick as I’m sure your aware is get the thing to idle long enough to actually set it up . I usually start by disconnecting the linkage from one carb . This allows you to tweek each carb without affecting the other Firstly before you start the engine unscrew each idle stop screw until it no longer contacts the throttle arm. Then slowly screw them in until it just touches the arm. Then turn 1 full turn in this should set a basic throttle opening. after that you can reconnect the linkage and slowly open the throttle whilst eyeballing the arm of each carb to see if they appear to be opening simultaneously.
Start the engine but be ready with a screwdriver in case you need to immediately unscrew the idle stop screws or open them some more. Assuming you’ve now got an idling engine regardless of the Idle speed ! If it’s too high just drop it down a bit but undo both idle stop screws the same amount this should keep your basic synch. I’d aim for an idle speed of 700-850 then with a short dumpy screwy wind each mixture screw in until the engine just starts to “go off” or sounds like it’s dropped one. Start to turn the mixture screw back out till the engine just smooths out again . Do this one at a time And recheck and adjust your idle speed and synch afterwards
It can take a bit of time and patience to get it right.
Hope this makes sense and helps.
Lou

[alfadave]

Very good tips Lou.
Just a little other thought is to check the 4 pipes from the carbs to the oil filler tube. Any splits can pull air.

[chriswilliams]

So, to report back:
I now have a nicely running engine! But to get there was a saga as follows:

I found first that cylinder 2 was not idling at all. Lots of investigation eventually found that the channel below the idle jet was completely blocked and needed a drill to clear. After that I got a badly idling engine, but it was running too fast...

I eventually realised that the NS carb was not closing correctly. I took it off and found what was preventing it closing, but I also found that the butterflies were lose. I tried tightening the screws but that made no difference. I eventually realise that the shaft was 180 degrees out and the threaded part of the butterfly fixing holes was on the wrong side, so no amount of tightening would hold the butterflies. Unfortunately I stripped one of the threads, so in the end It was easier to use a spare carb.

Now I have an idle that is rough, but I can control the RPM. I then did something I should have early on and used the "pull off each plug lead in turn" test to show that I had 3 cylinders idling, but number 3 was not. I checked the idle circuit and it was all clear but it just would not idle. By chance I screwed IN the idle jet, and as I did so the RPM increased and when it was fully IN the engine was idling MUCH better!

This suggested to much idle petrol. I took out the 2 idle jets on that carb. They both said size 52. They looked the same but I looked through my microscope and realised that the tiny fuel hole in the ends was not the same. the 'bad' jet had a bigger hole. I had a spare jet which I fitted and now:

The engine idles cleanly, and the idle jet adjustment screws work on all cylinders. I also have the vacuum reading back to normal.

The moral of the story: Don't trust a carb unless you know the person who serviced it last. I had one with the shaft in wrong and the other with an idle jet that seems to have been drilled. I also spent too long worrying about the vacuum. Once the carbs were correct the vacuum came back all on its own. Also, making sure the hole that feeds the idle jet is clear was something I missed early on.

I hope this story might help others in the future.

Chris.