The V8 Owners Forum -

jefferybond wrote:

Darkspeed wrote:Are the float heights and fuel pressure correct?

What advance setting are you currently using?


Andrew

See above!

Jeff

Half asleep again...

That all sounds fine - Mine is mech but runs no vac I am not after cruising economy or after any emissions reduction so starts at 16 degrees and runs to a fixed 36.

Have you put a strobe on to see what is actually happening or just assuming that the map is correct. Do you have a Throttle position pot? that maps to the Vacuum? or just the Map sensor



Andrew

The dodgy tickover after started up is exactly the same as the two RV8 lumps that had with this carb. Maybe one minute after running it goes rough and will die. The exact cause is due to carb icing and it will happen more easily when the air is full or water vapour and very cold. If you have a carb spacer that also makes it worse. I'll guarantee that when the weather gets better the problem will disappear.

As to the idle AFR you will not get a carb'd RV8 to even run at all with a ratio of 16:1. I've worked on 4 or 5 lumps with my LC1 unit, they were all happiest in the 12.2-13.5:1 range. My stage III 4.6 runs at 12.2:1 and won't run well at idle when the mixture goes to 12.5:1. (I have a 285 cam). My mates standard 3.5 lump but with an Eddy 500 will run much leaner, it will run 13.5:1 at idle.

As to the timing if you are running a standard Lucas dizzy then I'm sure that the ported vac system on the carb supplies too much vacuum when it opens, this forces you to run a less than ideal static advance which then cocks up the total mechanical advance. That is why I ditched the vac system!

If you run around 12-13 degrees at tickover and 36 all in at 2500-3000 RPM then you will get good results. (without the vac system)

Pete

sidecar wrote: As to the timing if you are running a standard Lucas dizzy then I'm sure that the ported vac system on the carb supplies too much vacuum when it opens, this forces you to run a less than ideal static advance which then cocks up the total mechanical advance. That is why I ditched the vac system!

If you run around 12-13 degrees at tickover and 36 all in at 2500-3000 RPM then you will get good results. (without the vac system)

Pete

As stated before, I'm running megasquirt to run the sparks, triggered off a crank trigger wheel and VR sensor. The dissy is still there, but only used to distribute the sparks! At idle RPM, i've set the timing to be independent of manifold vacuum, so I get a constant 12 deg timing advance at idle to help stabilise things. Vac advance comes in after 1000rpm.

I do use vac advance though, since it greatly improves light throttle driveability and economy for road use. Not so important for racing though I guess!

I need to go around the carb with some carb cleaner/WD40 to see if I can find an air leak, which I reckon is the real cause. What stops air being drawn down the threads of the idle mixture screws?

Jeff

jefferybond wrote:

sidecar wrote: As to the timing if you are running a standard Lucas dizzy then I'm sure that the ported vac system on the carb supplies too much vacuum when it opens, this forces you to run a less than ideal static advance which then cocks up the total mechanical advance. That is why I ditched the vac system!

If you run around 12-13 degrees at tickover and 36 all in at 2500-3000 RPM then you will get good results. (without the vac system)

Pete

As stated before, I'm running megasquirt to run the sparks, triggered off a crank trigger wheel and VR sensor. The dissy is still there, but only used to distribute the sparks! At idle RPM, i've set the timing to be independent of manifold vacuum, so I get a constant 12 deg timing advance at idle to help stabilise things. Vac advance comes in after 1000rpm.

I do use vac advance though, since it greatly improves light throttle driveability and economy for road use. Not so important for racing though I guess!

I need to go around the carb with some carb cleaner/WD40 to see if I can find an air leak, which I reckon is the real cause. What stops air being drawn down the threads of the idle mixture screws?

Jeff

Your problem is exactly the same as mine and my engine does not have an air leak, however it's your car if you want to blast it with WD40 that's cool.

I apologise for not noticing that you are running a megasquirt ignition system.

All that is happening is that the very small crescent shaped orifices that the air is flowing through on idle is getting blocked by a build up of ice.

Have a read of this book, it actually describes the problem in it:-

http://cgi.ebay.co.uk/CARTER-EDELBROCK- ... 5886219671

The large pressure drop at the butterfly plates is cooling the air in the same way that a fridge works or in the opposite way that a diesel engine works.

You could remove the insulator plate that the carb is sitting on as this will allow some heat to work its way up the carb or you can leave the plate in place and live with it in cold weather. As my car is a Cobra replica for sunny days I have left my insulator in place.


I don’t know what stops the air leaking past the pilot screw threads but in any event they don’t actually control the air anyway! They control the total amount of a pre-mixed air/fuel emulsion that is delivered into the main air stream. Maybe because of this the threads are not subject to a high vacuum. Or, maybe there is a small O ring in the carb where the needle part of the air screw goes into the carb. (I know I have just said that that they are not air screws and I keep calling them “air screws” but then again loads of people call vacuum cleaners “Hoovers”) At the end of the day I’m sure that the manufacturers have engineered a good carb that does not leak air where it should not leak air!

I guess it could be ice build up, especially the problems I see during the warm-up phase (when the carb is cold). It still does it when hot though, even when left idling, where I would have expected engine heat to thoroughly defrost it.

I've had carb icing on an old cavalier before (with weber DMTL carb + K&N filter). The symptoms on that were totally different - you'd be cruising on the motorway and get less and less part-throttle response, until you had either no power or full power! Most amusing to drive. Pulling over and letting the engine heat work it's magic always fixed it.

Maybe I should run the thing up to temp with the air cleaner removed, so I can peer down the venturi's and look for ice. I guess I could stand by with the hair dryer and see if that provides a temporary fix - that would be conclusive evidence I suppose.

Thanks for all your help chaps!

Jeff

jefferybond wrote:I guess it could be ice build up, especially the problems I see during the warm-up phase (when the carb is cold). It still does it when hot though, even when left idling, where I would have expected engine heat to thoroughly defrost it.

I've had carb icing on an old cavalier before (with weber DMTL carb + K&N filter). The symptoms on that were totally different - you'd be cruising on the motorway and get less and less part-throttle response, until you had either no power or full power! Most amusing to drive. Pulling over and letting the engine heat work it's magic always fixed it.

Maybe I should run the thing up to temp with the air cleaner removed, so I can peer down the venturi's and look for ice. I guess I could stand by with the hair dryer and see if that provides a temporary fix - that would be conclusive evidence I suppose.

Thanks for all your help chaps!

Jeff

Aha, I did not realise that it was also running badly when hot! There is no way that it will be ice when the engine is hot, well not if the car is just standing still ticking over so maybe you do have another issue as well!

The hairdryer thing might work just to prove that the bad running during warm up is ice. On my car if I let the engine stall when it starts to run badly then leave it for about 30 seconds then fire it up again it will tickover fine straight away.

Good luck, keep us posted!

Yeah, still does it when hot, but it's much better than when cold, and mostly tolerable.

When hot, the idle will vary slowly between 500rpm and 1000rpm (i've tried to set it to 750rpm). AFR also varies between 12:1 and 14:1, and it's a bit lumpy (it's a light car and you can feel the lumpiness jiggling the car). It responds straight away to the throttle with no hesitation etc.

It would just be nice to get a smooth idle when hot!

Jeff

jefferybond wrote:Yeah, still does it when hot, but it's much better than when cold, and mostly tolerable.

When hot, the idle will vary slowly between 500rpm and 1000rpm (i've tried to set it to 750rpm). AFR also varies between 12:1 and 14:1, and it's a bit lumpy (it's a light car and you can feel the lumpiness jiggling the car). It responds straight away to the throttle with no hesitation etc.

It would just be nice to get a smooth idle when hot!

Jeff

Hi Jeff, what manifold are you running?

sidecar wrote: Hi Jeff, what manifold are you running?

Edelbrock performer (dual plane).

Jeff

jefferybond wrote:

sidecar wrote: Hi Jeff, what manifold are you running?

Edelbrock performer (dual plane).

Jeff

Nowt wrong with that manifold so that blows another theory that I was working on!