sptcoupe
True Classic
I've been struggling with a problem on my 1986 X/19.
It's a great 1.5L motor built by the guys at Midwest. But we have had two issues that I can't seem to solve. The idle is rough and moves from 400 to 1500 rpms at will, it comes off idle with stumbling, and it backfires like crazy if you let off the throttle. The backfiring only occurs as the rpms drift down to around the 4000 rpm mark (the idle jet circuit). At WOT and once on the main/air corrector circuit, the motor is spectacular past 8000 rpms. So the issues are all in the idle jet circuit range.
I know what the problem is - after the engine reaches operating temp, the carbs begin to leak fuel through the aux venturis. You can see it pouring in at idle. And this will, of course, cause the issues and make it impossible to get it set up right. But I can't find a fix. When the engine is cold, it really is sweet all through the rpm range, and very strong (when I have the normal jetting set ups in the carbs).
This is what I've done over the past year:
- Have had 5 sets of new DCNFs on the car. I even had Weber in Spain involved.
- Have tried all manner of jetting set ups on both F24s and F36s, from way lean to way rich, swapped to brass floats and set them high and low, run 150 -200 needles.
- Have tried numerous fuel regulators and ensured I have 3psi
- Have tried 34 and 36mm chokes to try to reduce the vacuum signal.
- Have checked valve timing, and tried a wide range of initial timing settings, from very retarded to very advanced. Total advance is consistent.
- Have scoured the car for any kind of vacuum leaks and replaced the under carb gaskets and intake manifold gaskets numerous times- none I can find.
- And some other stuff as well.
The point is, every carb, no matter how it is set up, has the same leak. So the carbs are getting some sort of vacuum signal from somewhere, and no matter what I've done, they all leak, and the rough idle and backfiring haven't changed at all. So nothing I've done has touched these two issues - obviously because I haven't addressed the real issue - the fuel leaking into the bores through the aux venturi.
I think the issue is perhaps the cold start circuit leaking air, but I've had ten carbs on her, all brand new, and all with the same issue. I doubt that all ten of those carbs have the same issue with the cold start circuit (which isn't hooked up).
The engine is a mild street build. Mild cam, 10:1 pistons, header, valves - the usual hotrod stuff, but nothing radical. And, it really runs like a screaming banshee in the main/ac circuit, whether at WOT or cruising at steady throttle.
I'm no stranger to FIATS and dual weber set ups - been at it for over 50 years. But am always open to any advice or tips you guys can give me.
It's a great 1.5L motor built by the guys at Midwest. But we have had two issues that I can't seem to solve. The idle is rough and moves from 400 to 1500 rpms at will, it comes off idle with stumbling, and it backfires like crazy if you let off the throttle. The backfiring only occurs as the rpms drift down to around the 4000 rpm mark (the idle jet circuit). At WOT and once on the main/air corrector circuit, the motor is spectacular past 8000 rpms. So the issues are all in the idle jet circuit range.
I know what the problem is - after the engine reaches operating temp, the carbs begin to leak fuel through the aux venturis. You can see it pouring in at idle. And this will, of course, cause the issues and make it impossible to get it set up right. But I can't find a fix. When the engine is cold, it really is sweet all through the rpm range, and very strong (when I have the normal jetting set ups in the carbs).
This is what I've done over the past year:
- Have had 5 sets of new DCNFs on the car. I even had Weber in Spain involved.
- Have tried all manner of jetting set ups on both F24s and F36s, from way lean to way rich, swapped to brass floats and set them high and low, run 150 -200 needles.
- Have tried numerous fuel regulators and ensured I have 3psi
- Have tried 34 and 36mm chokes to try to reduce the vacuum signal.
- Have checked valve timing, and tried a wide range of initial timing settings, from very retarded to very advanced. Total advance is consistent.
- Have scoured the car for any kind of vacuum leaks and replaced the under carb gaskets and intake manifold gaskets numerous times- none I can find.
- And some other stuff as well.
The point is, every carb, no matter how it is set up, has the same leak. So the carbs are getting some sort of vacuum signal from somewhere, and no matter what I've done, they all leak, and the rough idle and backfiring haven't changed at all. So nothing I've done has touched these two issues - obviously because I haven't addressed the real issue - the fuel leaking into the bores through the aux venturi.
I think the issue is perhaps the cold start circuit leaking air, but I've had ten carbs on her, all brand new, and all with the same issue. I doubt that all ten of those carbs have the same issue with the cold start circuit (which isn't hooked up).
The engine is a mild street build. Mild cam, 10:1 pistons, header, valves - the usual hotrod stuff, but nothing radical. And, it really runs like a screaming banshee in the main/ac circuit, whether at WOT or cruising at steady throttle.
I'm no stranger to FIATS and dual weber set ups - been at it for over 50 years. But am always open to any advice or tips you guys can give me.