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I've got a set of 40 DCOM's on my Alfa/FNM 2300 in a 102 Roadster. They run great with outstanding behavior and performance in all modes except idle.
They were originally delivered with an odd idle jet that I've never heard of. Something like 69F119. Car wouldn't idle at all. I changed these to 60F9, which I happened to have laying around and the car would idle, but seemed to be too rich, taking just a nanosecond to catch as it came off of idle. This was NOT the dreaded bog-spot so often experienced when putting Webers onto the original Solex manifold for this engine. I have an original FNM manifold which is intended for Webers or Solex ADDH carbs, not the PHH primary/secondary units on the original 102.
With the 60F9's it ran great as soon as it had cleared its throat, which took less than a heartbeat. The idle seemed rich, but I could maintain a 1200 RPM idle. Anything lower and it would tend to die.
Today I dropped in some 45F9 idle jets. Absolutely cured the need for "throat clearing", with immediate pick-up from idle and smooth transitions through all revs and throttle settings. However, the idle behavior has degraded from not-so-good, to weird.
With various fiddling I can get it to idle at 1,200, but after about 15 - 25 seconds it will start to slow down and eventually die. I can prevent this by adjusting the idle stop screw up to about 1500 RPM, but this is clearly a bandaid, presumably using fuel from the first transition hole to keep it going.
Adjusting the idle mixture screws doesn't seem to make much difference, regardless of throttle stop-screw adjustment. Turning them too lean will speed up the cut-off and opening them to what is probably too rich will slow down the cut-off.
I'm going to order some 50 and 55 F 9 idle jets, but I've always been under the impression that these jets have far more to do with the transition than the actual idle. As the original idle jets wouldn't allow any idle at all, clearly they have some impact.
Note that when I went to the 45F9's, I started getting a few sharp pops from the tailpipe upon deceleration. This was not evident with the earlier 60F9s. I wondered if this was not due to a too-rich setting on the idle mixture screws (which were at about 2 1/2 turns), but efforts to lean them aggravated the slow-down and die.
Lastly...
The DCOM's have 4 brass standpipes that stick up just inward from the transition holes. I have no experience with these, but was advised that in some applications connecting them with a balance tube can help the idle. I have tried this, but it seems to make no difference. Might they be intended to have a separate single-point air-bleed screw that inserts equal air to all 4 barrels during idle, allowing the throttle plates to be set more fully closed? Perhaps this approach is what calls for the original weird idle jet?
Comments anyone?
They were originally delivered with an odd idle jet that I've never heard of. Something like 69F119. Car wouldn't idle at all. I changed these to 60F9, which I happened to have laying around and the car would idle, but seemed to be too rich, taking just a nanosecond to catch as it came off of idle. This was NOT the dreaded bog-spot so often experienced when putting Webers onto the original Solex manifold for this engine. I have an original FNM manifold which is intended for Webers or Solex ADDH carbs, not the PHH primary/secondary units on the original 102.
With the 60F9's it ran great as soon as it had cleared its throat, which took less than a heartbeat. The idle seemed rich, but I could maintain a 1200 RPM idle. Anything lower and it would tend to die.
Today I dropped in some 45F9 idle jets. Absolutely cured the need for "throat clearing", with immediate pick-up from idle and smooth transitions through all revs and throttle settings. However, the idle behavior has degraded from not-so-good, to weird.
With various fiddling I can get it to idle at 1,200, but after about 15 - 25 seconds it will start to slow down and eventually die. I can prevent this by adjusting the idle stop screw up to about 1500 RPM, but this is clearly a bandaid, presumably using fuel from the first transition hole to keep it going.
Adjusting the idle mixture screws doesn't seem to make much difference, regardless of throttle stop-screw adjustment. Turning them too lean will speed up the cut-off and opening them to what is probably too rich will slow down the cut-off.
I'm going to order some 50 and 55 F 9 idle jets, but I've always been under the impression that these jets have far more to do with the transition than the actual idle. As the original idle jets wouldn't allow any idle at all, clearly they have some impact.
Note that when I went to the 45F9's, I started getting a few sharp pops from the tailpipe upon deceleration. This was not evident with the earlier 60F9s. I wondered if this was not due to a too-rich setting on the idle mixture screws (which were at about 2 1/2 turns), but efforts to lean them aggravated the slow-down and die.
Lastly...
The DCOM's have 4 brass standpipes that stick up just inward from the transition holes. I have no experience with these, but was advised that in some applications connecting them with a balance tube can help the idle. I have tried this, but it seems to make no difference. Might they be intended to have a separate single-point air-bleed screw that inserts equal air to all 4 barrels during idle, allowing the throttle plates to be set more fully closed? Perhaps this approach is what calls for the original weird idle jet?
Comments anyone?
