Newly built 1608 and considering carb options

[JohnD]

Hi

I have a freshly built 1608 (big valves and 9.8 comp pistons). I have a 34dmsa that seems to be dumping too much fuel and doesn’t hold an idle. Drives great though. I am wondering what carbs you have using standard linkage for a 1608? I do have a 36 DCNF on a single plain manifold but stumped on the linkage. Pics of what you have mounted would be appreciated. Thoughts and pics on mounting the 36 would also be much appreciated!

Thanks!!

John

[Nut124]

Any carb in a performance build will likely require some jetting changes. Did you try to adjust the dmsa?

The DCNFs are designed for 1 bbl per cylinder, not a plenum manifold. Trying to jet one for a plenum could be tricky. Which way does the DCNF sit in your manifold? The DCNF is designed to be installed barrels side by side not front to back. If installed front to back it will lean out turning one way and get extra rich the other way. Annoying.

[18Fiatsandcounting]

I'm kinda thinking along the same lines as Nut124 on this one. The DMSA isn't a bad carb for the spiders, and in fact, it was a very common carb for 70s sports cars and so it's pretty easy to get parts for.

A couple questions:

- When you say it's dumping fuel, do you mean it smells like gas, seems to be running rich, poor gas mileage, etc? This could be from an incorrect float adjustment, wrong main jets, bad float needle valve or too high a fuel pressure, etc.

- When you say it won't hold an idle, do you mean the idle is "lumpy", or it stalls out when idling, or it won't idle below 1000 rpm? The first place to start here is the size of the idle jet. I can't remember whether the DMSA has an idle jet for just the primary, or both secondary and primary. It could simply be that the jet is too large or too small.

-Bryan

[JohnD]

Hello All,

Thanks for the responses! First, I would like to say I love the 34DMSA carb. The motor and the carb were rebuilt by my friend who owns a very reputable shop. I believe he said the even with the air mixture screen totally turned in the car would run but perhaps I have misunderstood.

For reference, I installed this carburetor (with single plain manifold) myself about 7 years ago on the car so I have a history with it, including resetting it and performing idle adjustments. The car ran great but I started to have issues with the car running which ultimately was related to faulty wiring to the ignition switch. Upon further investigation valve guides were worn and it was time for a slight performance rebuild. After the rebuild, the drives great but doesn't hold an idle.

The carb does seem to be running rich. I do smell gas and I need to adjust the idle near 1500 RPM so it won't stall at a stop light for example, unfortunately with an idle that high the car diesels when shutting it off. As for the idle jets, there are 2 of them. I believe the stock size is 35 (per internet) but I will keep researching. I assume I can purchase larger sizes from Redline for example. I would prefer to stay with the carb. That that be the next course action?

As for the the 36 DNCF with a single plain manifold, it does sit on the manifold in the same manner as the DMSA. I do have an manifold that has been rotated to mount the DCNF sideways but at this point I am not hell bent on upgrading to the DCNF. If I can save the DMSA I would prefer to proceed with that course of action.

[JohnD]

Quick update

Following redline reset and adjustments instructions after resetting the carburetor I had to back out the idle mixture screw almost 3.5 turns. I also had to really turn up the idle for it to run. So as a result I am thinking I need larger idle jets. I am hoping this is the result as I would like to keep the DMSA. Any comments would be appreciated.

[18Fiatsandcounting]

It sounds to me like the idle circuit in your DMSA is plugged. Symptoms of this are a idle mixture screw that has almost no effect, and a lack of ability to idle below 1000, both of which would seem to be the case. The car will "idle" at 1500, but at that point you're running on the main jets, and while it will "idle" under those conditions, the idle will be erratic and the idle mixture (and idle speed) screws won't have much effect since that circuit isn't working.

Solution: Take off the carb and thoroughly clean the idle passageways with carb cleaner or compressed air (or both). Make sure you also clean the idle jet, as that opening is pretty small and it doesn't take much to plug it up. I had that problem once, where the erratic idle drove me crazy until I found a very small speck of something lodged in the end of the idle jet. Cleaning that out solved the problem.

-Bryan

[JohnD]

Thanks Bryan! I will get on it this week and I will update the post with the results!

Thanks,

John Nicosia

[JohnD]

Hello All,

I have a quick update on my DMSA problem. Well the carb is off waiting for a rebuild kit and in the process of being pulled apart and cleaned. I do have the original DHSA and hate to have the car down for an extended period of time so I rebuilt the DHSA (blasted it with compressed air and carb cleaner). So bolting on the DSHA absolutely solved the idle issue. Will idle around 900rpm and drive nicely but when I shut it off it spits through the carb and diesels. The motor was rebuilt with a big valve head and hi-compression pistons. I understand that this spitting through indicates a lean condition. Overall the car drive well but shutting it off it still diesels. I confirmed the timing is set to around 10 degrees advance using my timing light. Any input would be appreciated.

[18Fiatsandcounting]

...I understand that this spitting through indicates a lean condition.

This can be a cause, but an indirect one. Dieseling occurs when the fuel autoignites, that is, the compression is sufficient to cause the fuel to reach its autoignition temperature, and so the engine continues to run (sort of) even without a spark. Causes are a high compression ratio, cheap gas, a lean condition (causes the combustion chamber parts to get really hot and act as a source of ignition), overheated engine, over-advanced ignition timing (causes heat build up), etc.

My first suggestion would be to retard the ignition slightly, perhaps to 5 degrees BTDC at idle, and run premium gas if you aren't already. If that doesn't help, you may need even higher octane gas or a gas additive. And if that doesn't help, you may need to play with your carb jets to run slightly richer, along with richening the idle a bit with the idle mixture screw. Turning down the idle speed a tad (to 800 rpm or so) might help.

What compression ratio are you running?

-Bryan

[JohnD]

Hi Bryan

I believe the pistons are 9.8 to 1 but I will confirm. I spent a good bit of time with the car today. I reset the carb again and after driving a bit the idle does degrade. I am wondering if I have vacuum leaks so tomorrow I will use carb cleaner and see if if the idle changes when sprayed with carb cleaner. Also I always run 93 octane and usually buy name brand fuel. I will try an additive tomorrow. I have been playing with the idle jets. Stick idle jets are 70 for both the DHSA and DMSA. Idles better with the 75s. Another perhaps sill observation but when I close the hood the idle seems to drop. I don’t recall that issue many years ago when I converted the car to the DHSA. Also, I started to check compression so number 1 was around 130psi. Any thoughts or pointers would be great. I have owned this car since 1989 and I can’t give up on it

Thanks

John Nicosia

[18Fiatsandcounting]

Hi John, sounds like you're doing exactly what I would do so I can't offer many suggestions, but here are a few thoughts.

1) Unless the hood is hitting the accelerator linkage somehow and changing the idle, my guess is that is just sounds like it's idling slower because it's quieter when you close the hood.

2) A 9.8 compression ratio isn't that extreme, so 93 octane *should* be fine.

3) A 130 on the compression check seems low. My tired old '71 engine with an 8.5 CR generates about 125+ psi, so yours sounds low. Here's the way to check: Remove all the spark plugs. Insert the compression gauge into each spark plug hole in turn, hold the throttle wide open (gas pedal to the floor) and crank until the reading stabilizes. For a 9.8 compression ratio, I would expect numbers well over 150 psi, maybe as high as 180.

Which makes me wonder. Given the dieseling and (perhaps) a low compression, could your intake valves be leaking?

-Bryan

[JohnD]

Thanks so much Bryan. I will confirm the piston compression ratio on Monday. I will do a full compression test tomorrow and I will check for vacuum leaks too and post back to the thread.

Thanks,

John

[18Fiatsandcounting]

I will do a full compression test tomorrow and I will check for vacuum leaks too and post back to the thread.

Awesome, John, thanks, and my (sneaking) suspicion is that we'll learn something from this.

-Bryan

[Nut124]

For how long does the engine run after you try to turn it off? 2 seconds? 5, 10 seconds? Longer?

Next time, rather than using the ign switch to stop the engine, pull the coil wire off the coil to stop the engine. If it runs any after that removal of spark, then you got major problems and need to watch for engine destruction due to detonation, l I think. If it stops immediately, then something is wrong with the ignition wiring rather.

Static CR: What block and what head? 2L head has bigger chamber. 1800 head has smaller. There are flat top pistons, 4mm dome and 8mm dome pistons. Has the block or the head been milled? How much? Also, what cams? Mild (factory) cams increase effective compression at idle vs hot cams.

I have about 10.5 SCR in mine and run good so far, knock on wood.

[JohnD]

Guys

Sorry for the delayed repose. Still need to rerun compression test. I will update the post as soon as I have the details.

Thanks so much!

John