3.5 SU Lumpy Hot Idle
Posted: Sun Feb 12, 2023 11:54 am
Hi,
I have a 3.5 from a Rover P6 running SU HIF6 carbs in a MK2 Hilux. The block is marked 10.5:1 and the engine has been rebuilt at some stage recently with new pistons, camshaft, rockers etc. I can't say for sure whether or not it has high compression pistons but I am going to assume so given that the engine had been in a trialler before it was removed.
Timing is currently set at 3 degrees BTDC (runs best here), fuel mixture set blue via colourtune on both banks and the carbs are balanced. Idle speed is set at around 700-750 rpm with a laser gauge on the crank pulley.
The engine starts on the button, idles beautifully cold on choke but idles uneven and lumpy when hot.
This issue seems to be worse some days, better others. Essentially the idle will begin to drop and bog down and the engine can be felt shaking on its mounts - it is not smooth. Sometimes the idle will be smoother but still not as smooth as I'd like.
Blipping the throttle might help for a few seconds and then it will start to Peter out again.
I have checked that I'm getting a spark at all 8 plugs, all seems fine. Lifting the pins on each carb will have the engine idle rpm drop and then return to "normal". Facet fuel pump running a pressure of 3.5-4 psi and I do not have a fuel return to the tank. I have also done some quick vacuum leak checks and there doesn't seem to be any change in idle speed / sound spraying brake cleaner around the engine.
I'm running standard unleaded (E10) with an additive.
Any ideas or pointers?
Many thanks,
-Tom
I have a 3.5 from a Rover P6 running SU HIF6 carbs in a MK2 Hilux. The block is marked 10.5:1 and the engine has been rebuilt at some stage recently with new pistons, camshaft, rockers etc. I can't say for sure whether or not it has high compression pistons but I am going to assume so given that the engine had been in a trialler before it was removed.
Timing is currently set at 3 degrees BTDC (runs best here), fuel mixture set blue via colourtune on both banks and the carbs are balanced. Idle speed is set at around 700-750 rpm with a laser gauge on the crank pulley.
The engine starts on the button, idles beautifully cold on choke but idles uneven and lumpy when hot.
This issue seems to be worse some days, better others. Essentially the idle will begin to drop and bog down and the engine can be felt shaking on its mounts - it is not smooth. Sometimes the idle will be smoother but still not as smooth as I'd like.
Blipping the throttle might help for a few seconds and then it will start to Peter out again.
I have checked that I'm getting a spark at all 8 plugs, all seems fine. Lifting the pins on each carb will have the engine idle rpm drop and then return to "normal". Facet fuel pump running a pressure of 3.5-4 psi and I do not have a fuel return to the tank. I have also done some quick vacuum leak checks and there doesn't seem to be any change in idle speed / sound spraying brake cleaner around the engine.
I'm running standard unleaded (E10) with an additive.
Any ideas or pointers?
Many thanks,
-Tom