DMSA jetting for 1608

[mosu]

Hi all,

I am about to replace my tired DHSA with a nice, freshly cleaned DMSA carb on a 1608 motor. Some people suggested that the default jetting of the DMSA may be too rich for the smaller motor and I was wondering if this is the case.

Primary air correction jet - 180
Secondary air correction jet - 160
Primary main jet - 115
Secondary main jet - 135
Primary idle jet - 45
Secondary idle jet - 50

The DHSA has this jetting which I can borrow from:

Primary air correction jet - 195
Secondary air correction jet - 150
Primary main jet - 125
Secondary main jet - 155
Primary idle jet - 50
Secondary idle jet - 70

I don't know what these numbers are referring to, but it seems like the DMSA has all of them smaller (leaner jets?). What do you guys think? Should I keep the default setup, or should I take some jets from the old carb and swap them into the new one? Any help would be greatly appreciated as I'm dying to mount it on and take her for a spin.

[mdrburchette]

I would try the DMSA as it came to start with, then you can test different jets once you see how the car runs. I have two 1608s with dual 40 IDFs. The rebuilt, higher performance engine is running leaner jets than the original 1608. The best thing you could do is replace the original intake manifold with a single plane manifold from an early 1800 engine.

[mosu]

So you're saying that the jetting as-is is ok to start? My problem is that I'm relatively new at working with carbs and I'm still learning. I know that if the idle mixture screw is more than two turns out or less than a turn in, the main idle jet's too small or too big, but how to do determine if steps have to be taken for the rest? I'm sure the car will feel nice and strong regardless, but I'd like it to be as close to perfect as I can get it

The DHSA was actually mounted to an 1800 single-plane manifold by the previous owner so I scored that for free! Sweet!

[TJC]

Check out this thread on mira on this subject from not too long ago:

http://forum.mirafiori.com/mirafiorum/m ... ll&frames=

[mdrburchette]

That's a good write up and hits the nail on the head, doesn't it? I had a 34 DMSA on my 1608 and thought it was the DHSA 28/36. I gave it to a guy in AZ before I realized what I had.

[mosu]

Thanks guys...I guess I have to drop it in and see where the screw adjustment is before I know if I have to change the main idle jet.

How would I know that my fuel or air correction jets are good enough? Flat spot in the powerband when accelerating?

[TJC]

I think the concensus is to start with the stock jetting for the DMSA as per the 1800 (I have it listed on the third thread of the link above) and from there, only adjust as necessary.

For example, if the proper adjustment of idle mixture doesn't happen in the normal 1.5 - 2 turn range, change idle jet. If you have a flat spot in throttle progression from primary to secondary, start fussing with the secondary progression ("secondary idle") jet......etc. But most likely you wont have many (if any) changes to make as the 34DMSA is pretty well suited for the 1608.

However, as to your example, if you're having a problem with acceleration at WOT (and you know it's a carb problem), that usually indicates a main jet (primary and secondary) issue. Most likely, the main fuel jet.

As Brad stated, messing with the emulsion tubes is a very black box affair due to the non-standard nomenclature, so it may be better to leave those alone.

[mosu]

Thanks TJC! Carb's going onto the car tomorrow and I can't wait! The DHSA is such a turd, no power almost all the way through to redline, but at least it's smooth.

I'll see how it behaves once it's bolted on and if I have any questions I'll post them back here. Thanks again for the input, guys.

[TJC]

You're welcome, but before you outright blame the carb, make sure everything else is fine and dandy (spark, compression, timing - both ignition and valve, etc.). You wouldn't want to ruin this good mood by installing an excellent carb and still coming up with crappy results.

[msova]

Sorry for the slight hijack but my ignorance of this issue is really not worthy of a new thread....

Wanting to "drill" main jet from a spare 34DMSA carb to quell bucking in low gear as read about in a mira thread. Was hoping to just drill and swap with the dmsa on the car now. I don't even know where the main jet(s) is/are located? Exploded diagrams (Brads' excellant publications) don't really help much. I can see two jets on either side of the carb (idle jets?...which is secondary, etc?) but where is the main? Does the carb need to be taken apart to do this?

Again...sorry for the hijack and the ignorant bliss!

[mdrburchette]

Marc, you're right. The exploded view doesn't help at all. It looks like there are two main jets and they would be located on the float bowl side of the carb. You may want to email Jim Scurria for help. He's still running a 34 DMSA on his 74 beater.

[msova]

Brad pretty well helped me out. Probably not gonna mess with the mains and only up the idle jets as per his advice. Yeah...I'll have to ask Jim if he gets and bucking in low gear...pretty sure his 74 is bone stock so I don't think he's rejetted or converted to cable.

[mdrburchette]

I drove Jim's a few weeks ago and I did notice the low speed bucking. Have you replaced your motor mounts lately? My 72 has the original rod linkage and acc pedal and I have no bucking whatsoever. Ron made a solid motor mount for the left side of the engine.

[mosu]

Well I just installed it and it runs like a dream with the jetting above.

The only somewhat flat spot that I detect is in first gear at low rpms just after the clutch engages. Past that it's smooth and with no flat spots as the revs progress to WOT.

Maybe that primary idle jet needs slight adjustment as backing out the idle screw more than about half a turn from the 2 turns-out default position doesn't seem to affect the idle speed/smoothness much. Screwing it in under 1.5 turns does make it significantly worse. The throttle stop screw is barely touching the plate stopper, under a turn since contact. So I think that given the current jetting the carb is as adjusted as it can be.

Does the fast idle electrovalve affect its operation at idle? There's vacuum on its hose at idle, but I thought that should only come into play with gears 3-4-5 engaged or the test button pushed down?

PS: Mark, don't worry about hijacking the thread, it helps keep related info together for others to see.

[TJC]

Does the fast idle electrovalve affect its operation at idle? There's vacuum on its hose at idle, but I thought that should only come into play with gears 3-4-5 engaged or the test button pushed down?

The fast idle electrovalve is engaged only when the clutch is out and your in third or fourth (not fifth). It primarily is there to prevent an overly lean condition during high speed decels where a lot of NOx is produced. Why they didn't incorporate it in overdrive 5th, I don't know.