3.5 SU Lumpy Hot Idle

[Pos]

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

[stevieturbo]

3 deg sounds really low for timing ?

And equally, 4psi sounds quite high for a lot of SU's

Whilst I did once own a colourtune, I've never actually used one. And given you would probably only check it in one cylinder.....you never really know what the rest are doing ?

For anyone trying to do any tuning at all, I'd always say buy a wideband. They are just so cheap these days, it makes no sense not to have one

Ideally one per bank, but even one you can switch between banks, or at a minimum one that monitors all 8, after any collectors etc in a single pipe.

[Pos]

I did a bit of reading on timing for 10.5:1 engines running E10 and the general consensus was anything between 0 and 4 degrees BTDC. The engine seems to pull nicely at 4 degrees (presumably down to ignition advance curve). It doesn't pink running a higher advance either. I trialled 8 degrees earlier with no noticeable difference in power. The idle is higher naturally but still a little uneven.

When I've tuned using the colourtune. I have always set the front cylinder on each bank first and then moved the colourtune back to check all 8 cylinders. The flame has been consistent across all 8 cylinders.

Double checked fuel pressure this afternoon. It's sitting at 3psi.

Do you believe it may be a mixture issue?

[ChrisJC]

Are the carbs balanced properly?

You might be idling mostly on just four cylinders.

Chris.

[Pos]

They are balanced yes. Drawing the same on each throat at idle and through the rev range.

[Pos]

Just wanted to post a follow up after a bit of exploration this evening:

Rerouted all of my HT leads as per Rovers recommendation (various diagrams on Google). 5 and 7 were running parallel / touching eachother which I've read can cause crossfiring / tracking.

The idle seemed slightly smoother but there was still a bit of bogging down / stumbling.

I took the truck for a good blast to get it nice and hot. Parked up back at home and adjusted the mixture on the left carb slightly richer. The stumbling became worse. I kept adjusting it leaner and low and behold the engine smoothed out. I repeated the same procedure on the right carb and the engine seems to be idling extremely smoothly.

My assumption is that it was running a little too rich at idle, even though the colourtune and the dashpot lift pins suggested otherwise.

I'll monitor it over the next few days and see how it goes. I suspect I may need a slightly richer needle off idle. Currently running BBW with k&n cone filters straight off the inlets.

[jp928]

My 9.25:1 CR 3.5L runs fine at 10 BTDC, on 98 Octane . Recently rebuilt HIF6 carbs - new needle & seat and jets, reset per manual. Mix fine tuned with wideband. 10.5:1 CR will be a bit more critical on timing, but really should be run on highest octane fuel you can get, IMHO.
jp 74 P6B S