DMSA Issues

[aj81spider]

Background: 1974 with an 1800 engine with a performance head from John Logan installed by me. DMSA carburetor also rebuilt by me. Car has seemingly not run with ideal power since I finished it. However I'm comparing it to a 2liter fuel injected spider I previously owned, so it may not be a fair comparison.

The DMSA stock idle jets are 50/60 primary/secondary. A while ago I switched to 60/70 because of the lack of power and hesitation on acceleration. It seemed to run better, but also seemed to be running rich. My basis for that was popping when decelerating.

The problem: Today it was running poorly (stalled at idle unless the choke was pulled out a little bit - even after a long ride) so I decided to address the jetting. I have a wireless O2 sensor so I can see the air/fuel ratio. I started by switching to the stock idle jets. With the idle screw at 2 rotations the a/f ratio was at 17-19. I put the 60/70 back in and the a/f ratio at 2 rotations was still high. By screwing it out about 3/4 it got down to 14.7 or so.

I didn't have an 80, so I switched the jets so the primary got the 70 and the secondary got the 60. It ran at 14.7 at about 2 turns on the idle screw, but not substantially different from the 60. At one point it jumped to 18 and stayed there for a while. I revved the engine up - then it stalled when going back to idle. On restart it was back at 14.7. I took it out for a ride and it ran better than earlier in the day, but about the same as it has all summer - although it wasn't popping on deceleration today (to be fair that is intermittent).

The questions:

> Is something seriously wrong with my carburetor given that I'm at a 70 idle jet versus the 50 stock size? I have a spare DMSA, but I rebuilt that too, so any screw ups in the rebuild are likely the same. It also has stripped holes where the air filter housing studs mount, so putting it in the car will require a reasonable amount of work.

> Any advice on what I should do to tune the carburetor for better performance generally? I have the Artigue manual and have used it to set the idle screw and the throttle stop. It idles OK (with the big idle jet) but overall performance is not that great.

[Nanonevol]

Are not there other jets to consider in your tuning? Main jet, air corrector, emulsion tubes, accelerator pump? I did some trial and error with my carb (34adf) and finally hit it right. I would say don't worry about using a jet that is other than stock as long as it works with your motor.

[aj81spider]

The other jets are all the stock sizes right now. Before tinkering with them I'm trying to get the idle jets right first.

It seems wrong that the idle jet would be almost 50% bigger than stock (0.70 vs 0.50). Not knowing much I also am questioning how much the bump in size should affect the turns of the idle screw. It doesn't seem like it's affected it much. Bumping 0.20 has changed the idle screw setting by about 1 turn.