Engine racing
 

My 122s, 1966 has developed a fault in that when the engine gets hot it is racing. Switch it off for 5 minutes and its back to normal then a few minutes later it speeds up and is racing again. This is the 1800 engine with twin SU's.
Both throttles are closed and both pistons are fully down so its not a fuel issue and must be an air leak I suspect. If I disconnect the carb to servo tube I get the same effect so I have fitted a new tube. Also when its racing, if I put my finger over the tube it makes no difference. Both carbs have been overhauled last year and new gaskets fitted as well as the manifold gasket.
So wheres the air leak coming from? As its only when hot I am beginning to suspect the inlet manifold.

Get yourself one of these :

https://www.ebay.co.uk/itm/DRAPER-HY...4/331343015448

When the engine starts racing, clamp the servo hose. If this brings the idle back to normal, there is an air leak from your brake servo.

Next, when you say racing, what rpm is it running at?

Have you checked the choke control cables? Do the choke cams on the carbs return to the fully off position when the choke is pushed fully in?

What fluid are you using in the dashpots?

Have you check-tightened the inlet manifold nuts?

What plugs are you running and at what gaps? When were the HT leads last checked? Are the HT leads fully on each plug?

Check your distributor: take the cap off and grab the rotor arm, then twist it. It shoudl have good, springy resistance and when you let go, it should spring bang with a good 'clink'. If it's floppy, the springs have had it and as soon as the engine's running, the timing will go to full advance causing the engine to race at at idle.

I'd eliminate that first before touching anything on the carbs.

This was going to be my next suggestion after elininating the brake servo and/or NRV valve in the servo hose.

With the dizzy cap off, it's also worth checking the carbon brush inside the cap and the dizzy shaft for wear on the bearings - side to side movement will show this up.

Tf;

I get the most from "...throttles are closed and both pistons are fully down..."...this suggests to me an air source other than the throttles (because that would lift the Dashpot!), so the first thing I think of is: False Air...so look for any leaks/openings into the intake downstream of the carbs...and I'm with LS...the first place I'd look/eliminate is the vacuum connection to PCV/Brake Assist unit...the fact the symptoms are temp dependent is interesting, and telling

Your statement "...so its not a fuel issue..." bothered me a bit at first, but in thinking about it some more, I guess this is your (short) way of saying false air must be coming in downstream of carbs, and I totally agree with this...!

A failure of the Cent Advance would likely not be thermal.

Ref: https://www.sw-em.com/su_carbs.htm#SU_Carbs_False_Air

Good Hunting!

As I said originally, I fitted a new hose from the carb to the servo and when the engine was racing I put my finger over the hole and it didn't reduce the revs so nothing to do with the servo. I have checked the distributor and it springs back fine but it will continue to go a bit more after it has sprung back but it does that whether hot or cold so that's not the problem. So I think it's back to an air leak somewhere between the carbs and the cylinder head. Will continue to
Look. Thanks for the replies.

Which hole? The one on the manifold or the one on the end of the brake servo hose?

If it kept running with the servo hose removed, it must be running as rich as hell - perhaps as i previously suggested, the choke is stuck on on the carbs, rather than the cable.

Carb overhaul may not have gone well and there could be leaks elsewhere. Once hot try spraying Easy Start on the spindles and other possible ingress areas. If the car revs up more you have the leak. Please be careful and be lenient with amount so as not to set yourself or car on fire. Found similar issue with a leak at my spindles.