Variable idling 1275 tuned Marina engine

[MMT1968]

Hi,

My 1275cc tuned Marina engine drives me crazy.

It has got twin HS4 carbs and an electronic distributor with vacuum advance. When started up from
cold idle is at 1100 to 1200 revs, idle adjusting screw fully turned out. As the engine warms up idle is rising steadily and after a run on the motorway when the engine is hot it‘s idling at 2000revs. There is nothing to do against it except turning the distributor left. But when it then idles at 1100 again with altered ignition timing, hot oil and coolant it doesn’t pull from stand still and has backfiring.
The carbs have been fully restored and are as new. The distributor is new as well.
Does anyone have an idea what I‘m doing wrong?

Chris

[Bill_qaz]

Have you check for air leak on inlet, manifold, carb flanges? Do you have a brake servo connection. Is the throttle cable or link holding the carb(s) off idleYou should be able to slow the idle to stall point without retarding the distributor.

[MMT1968]

Thanks Bill,
Yes throttle cable and choke cable are loose enough and I have checked for air leaks with some brake cleaner but there are none. That was the first thing I checked because of the thermal connection.
You‘re right, I should be able to slow it down to stall point but this is only possible by turning the distributor left (retarding). The idle adjusting screw is fully out. The engine doesn’t run properly with idle lower than 1100 revs.

Chris

[Bill_qaz]

How about setting the static timing so you have a reference?
Are both pistons free in your twin carbs?
What about the interconnection are both butterflies closing together and have you balanced the carbs.
When did the issue start.

[MMT1968]

Thanks for the input and questions.
The issue was from the beginning after I had rebuilt the engine from bits and pieces. All parts exept block, crank and conrods are new. So Pistons, head camshaft, valves a.s.o. are new. The carbs came out of a Healey BN100 and have been overhauled completely. Their pistons are running freely and close completely, the needles are perfectly centralized. The butterflies are closing completely and are synchronized perfectly.
Timing of the cam was made to specs on the markings.
The engine is running well and powerful and with exeption of the idling I would not have any complaints. It has some 85 to 100hp I‘d guess. I‘ll know exactly after the dyno tests to fit the correct needles to the carbs.
As far as I can see there ar not any air leaks at the carb flanges and inlet manifold or butterfly axles. That’s what I have argued first. The connection of the brake servo vacuum hose to the intake manifold has been sealed with silicone tape and there is no other hole to close in the intake manifold.
I would not have asked if I have an idea.

[Bill_qaz]

Have you tried disconnecting the servo pipe and blanking the manifold, if your servo diaphragm is leaking it could be pulling air.

[MMT1968]

Good idea! I have not tried that and will do that tomorrow. But the brakes are working perfectly also, shouldn’t they be affected also? Anyway I’ll try it.

[Bill_qaz]

Good idea! I have not tried that and will do that tomorrow. But the brakes are working perfectly also, shouldn’t they be affected also? Anyway I’ll try it.

Not necessarily you can have a vacuum leak that can effect carbs without you feeling it in the braking assistance. It only takes a small amount of air to increase idle.
You seem to have checked most other things.

[MMT1968]

I thought to have checked most other things, yes. I suspected a vacuum leak or a secondary air intake somwhere as you do also. What bothers me is the thermal correlation. If it’s a vacuum leakage that increases with temperature would mean the issue must be in a region that warms up together with the engine. So far so easy. The brake servo is mounted a bit further away but the connection of the pipe is at the manifold. I’ll check that.

Chris

[MMT1968]

I found a small hole in the top surface of the inlet manifold beneath the vacuum hose to the brake servo. That hole has been closed with a rivet and it‘s head filed off. Probably there could be a leakage so I covered it with some JB-Weld. I‘ll check when that has hardened if the problem still occurs.
The hose to the brake servo is perfectly sealed at its connections but I still have to check if the brake servo membrane is Ok. But obvious things first then I go the next step.

Chris

[Bill_qaz]

Just a thought, are you sure the butterflies are the correct way around in your rebuilt carbs?
Reference chamfered edge.

[MMT1968]

Yes, absolutely. I‘ve checked it before mounting the carbs. And they close completely, checked with a torch.

Chris

[Bill_qaz]

What breather system is fitted to your engine, is it connected to the inlet in any way?

[MMT1968]

No, it’s K&N see pic.

[oliver90owner]

I think Bill is on the right track - re air-leaks or carbs not in synch.

A few points/observations/suggestions.

I might be checking the spark plugs after reasonable periods of good, bad or ugly running. They may indicate where the problem lies.

Adjusting the timing by turning the dizzy to reduce the revs is a non-starter/crazy/useless;. One would normally be altering it for increased, not decreased, rpms.

You will never ever have a vacuum leak. Think - there is nothing in a vacuum to leak!

Fuel level in the carbs could be something to check.

[Sleeper]

" You will never ever have a vacuum leak. Think - there is nothing in a vacuum to leak! "

Air could be leaking into the vacuum....

John ;-)

[oliver90owner]

" You will never ever have a vacuum leak. Think - there is nothing in a vacuum to leak! "

Air could be leaking into the vacuum....

John ;-)

Eggzackerly. That is an air leak, not from the vacuum. It could equally be any gad, vapour or liquid. But it won’t be nothing, which is what a vacuum represents.

Much like people who think cold leaks into their homes - it doesn’t, heat energy leaks from regions of higher temperature to those of lower temperature. Cold draughts is another matter.

Thermal conductivity is exactly the same - heat travels fro the hotter to the cooler parts.

Water does not flow up-hill unless forced to do so.

All simple Physics.

[myoldjalopy]

"heat energy leaks from regions of higher temperature to those of lower temperature"
That is why you get hypothermia if you stay in cold sea temperatures for too long.

[Bill_qaz]

Thank you to the pedants for the physics lesson
Both myself and the op both know what we meant that if the vacuum pipe or its associated connections leak the AIR enters not vacuum leaking out.
Don't know how I survived 70+ years without these lessons.

[MMT1968]

Bill, you made my day