Feels flat on full throttle

[John Simister]

My 1600 HF used to have a very high idle speed, caused I think by general wear of butterflies and barrels. A pair of C42 DDHFs came up on eBay which seemed to be in better condition judging by conversations with the seller, so I bought them. They were indeed better but I took them to a carburettor specialist for a rebuild to make them as perfect as they could be - slightly worn bushes replaced, thorough chemical clean, new needle valves, gaskets, diaphragms etc etc.

I fitted them, set the balance and the idle mixtures, all seemed good and the idle speed is now fine. But at big throttle openings and high revs, where before the car would leap forward with crisp enthusiasm, now it feels flat as if misfiring. It's as if there are one or more bad spark plugs but they are new and the engine ran very well before (apart from the idle).

Float level wrong? The floats are indeed set to a lower level than on the old carbs, judging by the degree of bend in the floats' arms ( I haven't got the measuring tool). So I put the old floats in the 'new' carbs and it made no difference. Unless a sudden ignition fault has appeared coincidentally, it all points to something wrong in the carbs. Does anyone have any idea what it might be? A blockage in a main jet feed or something? My next action will be to swap all the jets from the old carbs into the 'new' ones and see if that makes a difference, but I've already blown through the main jets and the car runs fine on part throttle, when the main jets are presumably in use.

Help!

John Simister

[andrea]

are the two accelerator pumps (at the bottom of the carbs) working OK? Do they have the correct springs and membranes?

[Ed Levin]

John,

Swap all the jets and air correctors immediately. I know Webers better than Solexes, but it sounds like a classic case of running lean at high revs, which would be down to the jetting.

I would never assume that the jetting on another used carb is the same as what you were previously running, even if they came from the same type of engine, and I wouldn't trust that the jetting in the replacement carb was right for your set-up. (The jetting I inherited in the Webers on my HF was way out of factory spec and a long way from what I'm now running; different people can come to very different conclusions about what ought to work.)

From your description, I'd be surprised if changing the "new" carbs to your "old" jetting doesn't cure the problem.

[John Simister]

Thanks very much Andrea and Ed for your suggestions. I'll check the pumps and swap all the jets this coming weekend and see what happens. I noticed from a Lancia technical data sheet that the float level is 20mm, but which dimension does that relate to? The level of fuel below the top of the opened float chamber, that is the distance from fuel surface to gasket face?

Thanks, John

[Peter Cripps]

Might also be worth checking those new needle valves. If for some reason they aren't delivering adequate fuel flow as the floats drop, you'd get exactly the symptoms described.

I mention this because a year or so back Omicron had to replace defective needle valves that were included in 35PHH rebuild kits. (Not slamming Omicron, they acted promptly and responsibly to a problem that was not of their making.)

Peter

[tim]

John

I can send you details of a tool that I made from info of Tony from Oz gave me. Or you can come over to essex and we can have a fiddle. The tool helped sought out a flat spot on my HF and I "calibrated" it on some untampered carbs and was found to be spot on. I also think you may have a leak on your inlet manifold gasket (I use a copper loaded silicon gasket cement which cure a leak I had), and more recently I found the cam timing out and this was the major source of flat spot. When timed to the correct opening/closing with a DTI +-10degress can make a difference. I now run 5 extra degrees ignition and no pinking or flat spot.

Tim

[tim]

Just one thing else. Solex unlike weber or dell orto did not provide any/many alternative jets for their carbs and only way was to solder up and redrill. I doubt that has happened to your carbs (either set). Unlike weber/DO you have to get to the jets from the bottom - ie carb off. Be careful to replace the drip tray incase of any leak - no barbies under the bonnet!. I also doubt a blockage as the carbs were refurbed and thouroughly cleaned/inspected etc. Could be accelerator pump problems but this is easy to see with air box off. What colour are the plugs. My HF (on standard carbs) runs quite rich at all times. I have never had a brown electrode - always black.

Tim

[P. de R. Leclercq]

Fuel pump is a possible candidate for this one; I have just posted about this on the thread about "Fulvia won't start"

Paul

[Leonard Zapala]

I had a similar problem with my 2S Fulvia. I did most of what has already been suggested. The problem turned out to be a pin hole in one of the Solex's gaskets. The way I found out was that the hole eventually became large enough that I could hear it and if I put my finger on the spot of the noise the revs jumped up.

[Ed Levin]

John,

I honestly don't know how the tech manual measures the float level.

It’s quite possible that some of the other suggestion are correct. I think the problem is too lean a mixture at high revs, and this could be from any number of causes. I would think that the fuel pump is an unlikely culprit; Solexes don’t require a lot of pressure, and you didn’t have this problem with your “old” carbs, which would have had the same fuel flow requirement. If you changed out the floats to your original ones, then fuel supply to the jets is probably not the problem. Which comes back to those jets and air correctors.

Of course, Webers offer a lot more options for changing jetting and emulsion tubes (read: a lot more chances to get it completely wrong), but I had a similar problem, and after some trial and error, a reduction in the air corrector solved it.