Choke use with cold engine

RoverP6B · Post by **RoverP6B** » Tue Mar 10, 2009 5:54 am

When my P6B was running with the original 3.5, for cold starting maximun choke was required followed by several minutes of gradually decreasing choke before the engine would run properly. If I pushed the choke in before such time, the engine would either stall if idling or run poorly if above idle.

The engine was fitted with twin SU HIF6 carburettors. I have also read that the 3.5 when fitted with a Weber will also behave in a similar fashion until sufficiently warm.

Why did/does this happen?

Now my P6B is fitted with a 4.6 running twin SU HIF6 carburettors. For cold starting I use full choke. The moment the engine starts I can push the choke fully in and within 5 seconds take my foot off the accelerator and the engine will idle smoothly at 500rpm. I can then drive without hesitation, the choke no longer required.

It behaves just like a modern fuel injected engine, but why?

The 4.6 does not run rich when idling, no sooty black exhaust or fouled plugs.

The engine was tuned on a rolling road,..the carburettor needles polished to provide essentially an air / fuel ratio of 12.5 : 1 when being accelerated from off idle to 5500 rpm. The camshaft is specified as being for a fuel injected engine.

City running 18 to 20mpg. Country running 25 to 29mpg.

Your thoughts are most appreciated.
Ron.

ChrisJC · Post by **ChrisJC** » Tue Mar 10, 2009 8:16 am

I don't know the answer, but when I robbed the 2" SU's of a 4.2litre Jag 6-cyl at the scrapyard, I noticed that there was no choke mechanism at all. There was a solenoid to lift the idle speed, but absolutely nothing to alter the mixture.

So maybe it's not uncommon?

Chris.

Quagmire · Post by **Quagmire** » Tue Mar 10, 2009 11:21 am

My 3.5 on HIF6's requires very little choke at all, usually on a frosty morning i need to pull it about halfway to get it to fire and then after trundling down my lane (30secs or so) it can come off. TBH i think its probably running a little rich anyway so that probably helps

I would probably have taken the time to get it spot on if i didnt run LPG 99% of the time.

I do find the v8 much more sensitive to choke than my old 4-pot. On that you could do pretty much what you liked. With the '8 i find that it is very picky about how much you give whether its too much choke, or not enough!

Thinner cylinder walls in the 4.6 allowing faster temp rise perhaps?

kiwicar · Post by **kiwicar** » Tue Mar 10, 2009 3:13 pm

If you are running 12.5 to 1 at idle I am not suprised you need little or no choke, at 14.7 or even 15 to1 you would need alot more choke, also with the bigger engine, with the bigger peak flow in the middle of each pulse from each pot, the mixture, though giving you an average of 12.5 to 1 probably is much richer at the bit of the flow coresponding to peak flow through the carb. Finally just properly ballancing the carbs for flow and mixture as you will have dne on the rolling road means all 8 cylinders are running more efficiently so will run better on a leaner charge han if the carbs are unballanced.
Mike

Quagmire · Post by **Quagmire** » Tue Mar 10, 2009 5:09 pm

kiwicar wrote:Finally just properly ballancing the carbs for flow and mixture as you will have dne on the rolling road means all 8 cylinders are running more efficiently so will run better on a leaner charge han if the carbs are unballanced.
Mike

This has a bigger effect than you might think! Mine needed much more choke before i balanced the carbs, anything up to 5 mins of runtime...

RoverP6B · Post by **RoverP6B** » Wed Mar 11, 2009 9:32 am

Thanks for the feedback gents!

I can also add that the carbs when on my 3.5 were also balanced, running factory specified needles, but lots of choke was always needed.

The 4.6 must be much more efficient, more power and less fuel consumption.

Ron.

ChrisJC · Post by **ChrisJC** » Wed Mar 11, 2009 10:24 am

It might also be that the intake charge is moving faster (because it's a larger capacity), so the fuel vapour has less time to condense out before being ignited....?

Chris.

CastleMGBV8 · Post by **CastleMGBV8** » Wed Mar 11, 2009 10:43 am

Ron,

Seems like youv'e got it well sorted and pretty good consumption figures for a 4.6

With my new 4.35 engine which is still on an Edelbrock 500 it's an absolute pig to get it started whereas with the original engine it would start after three or four tries even after being unrun for some time, at the moment am not sure if it will start at all but once it fires up it run a little rough at any choke setting until it warms a little. I think this is partly because of the high overlap cam and big valve heads which is reducing vacuum, also carb is set at a high idle speed as engine has only done approx 100 miles. Interestingly the car will run down to almost 30MPH in fourth gear and run along very smoothly which the old engine didn't but even a small touch of throttle will qlmost stall the engine which is probably not surprising in view of the state of tune.

I haven't re-jetted the carb yet so it's probably far too lean on the 3.5 jet setting, having just had to move the car to another garage all my stuff including spare jets is safely packed away somewhere ie cant find them, but will have another search and probably go back to the carb base setting to see if that improves things.

Kevin.