Xflow balancing reading

[Olgawld]

Hi. My xflow Supersprint feels a bit bogged down and lumpy when opening the throttle. Ive used a synchrometer to check the balance first of all and had reading of just under 3 on first and just 4 on the other. I've balanced them both to around 3.5, tickover over around 7000rpm. Question is does that sound ok? I've seen and heard other people when balancing 40 Weber's getting readings of 6 and 7. Mine sounds quite low. Car has been sat around for quite alot recently so next I'll be checking the Jets to see if they're a bit gummed up. Car runs a megajolt which I'm new to so wonder if that affects it too.

[auyt]

3.5 is good from my experience. I would’nt  suspect the jet are gummed up unless you have been running ethanol fuel. It dries and leaves a gold waxy surface.. worst bit it’s corrosive. 

[Olgawld]

Thanks for your reply Auty. I've taking the top of the carbs off to check jets etc. The floats when held vertically, Gap is 16mm, closing to 8mm, when holding it horizontally it droops down to 40mm. Does that sound right? In a book I have it states when held vertically it should be Gap of 12mm and then when held horizontally the full droop should be 26mm. Should I change it, is that what could be making it run rough? Thanks

[Olgawld]

I have the black spansil type floats.

[Roger King]

7000rpm is a little fast for idle! I'd try and drop it by an order of magnitude or thereabouts.

There is no right answer for tickover speed because it can depend so much on the engine spec, particularly the cam profile (which also has a significant effect on airflow at idle). I'd say that typically you'd see between 700rpm and 1100rpm (the latter for full race spec). There is no virtue in trying to go as low as possible; apart from possibly affecting low speed throttle response, a very low idle speed is not good for camshaft wear.

Assuming you are jetted correctly for your engine spec, the two main factors affecting idle smoothness are carburettor balance and idle screw setting.

Get the balance as near as you can and then wind all four idle mixture screws in until they seat - HUGE WARNING HERE, THERE IS ONLY A VERY SMALL INCREASE IN RESISTANCE WHEN THE IDLE SCREWS MAKE CONTACT WITH THE SEATS. IF YOU KEEP ON TURNING YOU WILL DAMAGE THE IDLE SCREW SEATS BEYOND REPAIR. Once this is done, open all of the idle screws 2.5 turns and then see how the engine runs. If it is poor unwind all screws another .25 of a turn (or you may need to wind them back in again .25 of a turn instead). Keep doing this until the engine runs as well as possible. 

Now check the balance again and adjust if necessary. Once this is done you need to adjust each idle mixture screw individually until you get the best idle. You many find that some need winding in a little and some need winding out. There are no hard and fast rules about how far out the idle mixture screws should be; it varies from engine to engine and from carburettor to carburettor - some engines might need them out by roughly .5 of turn and others might need roughly 3.5 turns out.

I should add that if you have the standard 45F9 idle jets, there is usually a huge improvement in slow speed running if you change to 50F9 instead. You will need to adjust the idle mixture screws to suit the change. 

[Olgawld]

Thanks for all that info Roger. I did find the tickover of 7000 a bit much you're right...so I lowered it to around 700 -800 instead! It does tickover very nice. It has got the 50f9 too. It just feels a bit sluggish when putting your foot down around 3000 revs onwards, hSnt got that pull I'd like. It's the standard 1700 Supersprint xflow but with megajolt. I just noticed that my float levels seemed way off and wondered if that would be contributing to the sluggish feel.

[Olgawld]

Just seen a YouTube clip by a Weber chap in Australia. He says measuring the distance for the later black barrels from the middle but my speed series Weber book says measure from the bottom. This makes a big difference when fully dropped holding horizontally.

[Roger King]

Certainly, having the correct float levels is important, although a mil. out here and there will not be the end of the world.

Have you checked the ignition timing? Ideally, you need around 12 degrees at idle and around 34 degrees at full advance, assuming you are running on 95ron fuel and have the standard Supersprint compression ratio. The above figures are only relevant if you are running 2D mapping. If you have 3D mapping, light throttle advance figures should be much greater.

[Olgawld]

Thanks for all the replies. I changed the float levels, I checked the timing, balanced the carbs, even put in some fresh petrol and now seems quite a bit better, feels more torquey so happy with that. Always something though. I've now noticed a scraping noise from somewhere. Doesn't do it on idle but when under load you can hear it especially when you lift your foot off again, quite a loud scraping sound. Hard to pinpoint exactly where it's coming from. I'll check pulleys first, maybe alternator? There's always something on these old engines, part of the fun!

[Jonathan Kay]

Hard to pinpoint exactly where it's coming from.

I recommend a stethoscope.

Jonathan