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Discussion Starter #1
Would appreciate help with carb set up recently acquired 74 GTV. I’m told an older performance rebuild. 110k on odo. Compression 190 to 205. Has 306 cams which search indicates is an Alquati rally cam w/ 11mm lift.
40 DCOE 72/73 Webers I think are for a stock engine as it has normal jets, main 135, idle 55F17 and air correction 210. Think from a kit; euro manifold, rubber bushings, brace, etc. Fuel pressure is 2 ½ idling, 3 engine off.
Stock Marelli dist had original Spica advance setting. I moved it up a bit to max of 35 BTDC and idle is now about TDC. This seemed to help partial throttle power. WOT power above 3k is good. Fuel pressure is 2 ½ idling, 3 engine off.
To me running rich; new plugs black, exhaust black and fuel consumption. Also trying to improve throttle response. So went thru weber tuning per published procedure w/ throttle link disconnected. All air bypass jets were open but for now I closed those. Synced body to body. Ended up with idle mix screws out 5 ½ turns, were 6 ½. Also idle adjustment does this hysteresis thing. Turn and turn, nothing then suddenly takes off to 1800 rpm. Back out and back out, nothing then suddenly 600 rpm. Here the plates are visible in the first progression hole of the 4 total. Rev from here and it will hang up at 1800 for a bit and then drop. So can’t really follow procedure to find happy spot between lean and rich and get an 8-900 idle. Is it me or the carbs?
Right now would like to address the carb set up and later distributor, ignition, maybe cams, intake, etc. as I think I have a strong motor but the “extras” need attention. Thanks for your advice, Ken.
 

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5 1/2 turns is a long way out. It would normally indicate that the idle jets are too lean. 55F17 are on the rich side of normal but I have found them to be about right in the colder seasons in SC. I used to switch to leaner 55F21's in the warmer months.
I suspect that your distributor is part of the problem. If you have insufficient advance at idle then you have to screw in the idle speed screw to compensate and then it is hard to set up the Webers properly. I have found that an idle advance of 10 to 12 degrees works well but your distributor curve may be putting it in the wrong place when the full advance is set properly.

You are welcome to call me at 803 553 4482 if you want to talk about it. I currently have similar Webers on my Spider and I have owned a couple of GTV's with Webers.
 

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Richard Jemison
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Idle issue

I agree with Ed. Your distributor will not return to idle at the RPMs toy get with idle air screws in a reasi\onable position.
Perhaps you should look at the timing with a test light then screw in the idel screws and see where idle advanced has moved to.
Then close the throttles with the idle control screw.
Rj
 

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Discussion Starter #4
Ok, then on to the distributor. I'll advance it to ten* at idle and go through the carb adjustments. Ed I may indeed give you a call after this next step. Thanks, Ken
 

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Keep the air bypass screws closed. Set the idle speed screw about 1 turn in from where it first starts to open the butterflies. Start the motor, warm it up to full temperature then back out the idle screw to get about 800 rpm, then adjust the idle mixture screws 1/2 turn at a time to get the highest idle speed. Then hopefully you will have to back out the idle speed screw some more, then adjust the idle mixture screws 1/4 turn at a time to get the highest idle. It is usually better to have them a little too out than in.
 

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Discussion Starter #6
I got to spend some time on this today but can't say I found the cure. That distributor is touchy. I set it a 7* at an idle of 700 and was able to get minimum delay in throttle return and no spontaneous surging. IOW drivable. Idle mix were out to 6 turns. If I tried to get a 900 idle it would pop up to 1500+ rpm. I did think the distributor advance was very quick; 700 - 7*, 1200 - 20*, 2k - 30*, 3k - 34*. Ed, I read an old post when you were setting up your 123 and a quick curve seemed to give you similar issues. Does this sound right for this old Marelli or should I look at it closer? I also think I'll check for any vacuum leaks just in case. Thanks, Ken
 

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You can have a stable idle with a lot of advance at idle speed or a little advance at idle speed, but for setting Webers it will be better with more advance. For stable idle it is important that the advance is not increasing significantly at idle speed. In other words, the advance curve should be flat or have a gentle slope from about 500 to 1200 rpm. The advance at 2000 rpm will certainly influence the "liveliness" of your motor but it will not affect the idle.
What is the model number of the distributor? It is stamped on the housing.
 

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Discussion Starter #8
Unless I goofed on the data gathering my advance is pretty wild. I assume its the original Marelli but I'll go in from underneath and find a number. Two more points....
With this advance setting I think the power is down above 3K if that makes any sense.
Also, I literally drenched the manifold interfaces; carb to rubber gasket to manifold and could get maybe a 50 rpm drop. But I do mean drenched. Blocked off the intakes so nothing got in that way. Couldn't narrow to any one intake. Snugged the nuts I could get to. Also sprayed other vacuum hoses and connections with no change. I conclude there is no "problem" vac leak.
Will return w/ dist info! Ken
 

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Discussion Starter #9
Yes the distributor is the S103 BA. Noticeable movement by hand before the springs come into play.
One more data point... I opened the carbs to see if anything obvious was amiss. I did check if the brass float open/close points were correct and they were all a couple of mm greater than spec. Don't know what this could mean and didn't attempt to adjust. Did not see any crud or stuck needles, etc. with just a looksey.

Backing up a bit, I am correct is assuming I should get a good idle before the progression holes come into play? Thanks again.

Ken
 

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Backing up a bit, I am correct is assuming I should get a good idle before the progression holes come into play?
Definitely. The progression holes should not be uncovered at idle.

The lighter spring should not be loose when the distributor is stationary. The advance should only start when the engine rpm is about 900. If you set the max advance at 36 BTDC you should have zero up to about 900rpm and about 17 degrees at 2000 rpm then increasing by about 6 degrees per 1000 rpm up to 5000rpm.
 

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Discussion Starter #11
Ok, interrupted by a holiday cold. I measured the advance again and I still think it’s coming on too early. See my crude chart. On closer inspection of the distributor(I pulled it) the primary spring is not loose when static but it looks newer(shiny) than secondary one so I’m suspect it has been changed. I really don’t see anything else amiss with it other than one of the little bump stops. When on the car, advance and dwell held steady. I played with it and an angle finder and noted the secondary spring kicks in at about 25* and the total is about 32*.
So for now, I’m thinking I’d like to find an original or suitable substitute primary spring to see if that will solve the idle hysteresis. Granted there is probably a new distributor in my future but this is kinda fun and I have this originality thing. Unless someone on here has a spare it looks like it will be trial and error as I see various kits on the web and I did find a promising one at the hdwe store. Other ideas?
Ed, I’d like to give you a call Monday or Tuesday if there is a time good for you. Thanks, Ken
 

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But Mad North-Northwest
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That does seem like you're on the steep part of the advance curve too early. The curve I'm using on the 1750 is the one recommended by Tom Sahines and is:

500 rpm 6 deg
1000 rpm 6 deg
2000 rpm 20 deg
4500&up 38 deg

It's tough to go wrong with the 123Ignition programmable distributor. The thing is a solid piece of kit. If you've still got points on your current distributor it's double the bang for the buck to upgrade.
 

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Ed, I’d like to give you a call Monday or Tuesday if there is a time good for you. Thanks, Ken
Name your time between 8:00 am and 11:00 pm. I have a couple of errands to run but I am free pretty much all day.
 

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Discussion Starter #16
OK, to follow up I reworked a hardware store spring to mimic the Marelli one I borrowed from Ed, e.g. less rapid advance. I’m now able to set a 900 rpm idle which I wasn’t able to do before but I still see the hysteresis effect between 1200 and 1500 with both springs. (it’s moved or shrunk) So I think I’m in the ballpark as far as the Marelli distributor goes. I cleaned and lubricated it and it displays no erratic behavior.

As far as the rich running, I still need the idle mix screws out to 5 or 6 turns. 55F17 jet. I guess I could try a larger jet and maybe the idle could be more properly set but I’d still be moving about the same amount of gas through the carbs, correct? Car runs great with I believe better spin up from idle but still thinking it’s running rich due to low mpg, plugs.

I identified the cams above and wonder about them, specifically the valve clearances. Jim K in an old post gave a clearance of .40/.45 mm which I don’t know is a range or in/out. In any case my clearances are average in: .18mm, out: .23 mm. So what effects would this have is my next big question. Any detrimental?

The remaining unknown I have is the compression ratio but I may have to wait till an engine out to figure out that one.
Thanks, this BB and you guys have been a great help. Ken
 

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I can lend you a set of 60F17's if you want to try them.
 

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Regarding valve clearances, what you have seems tight at 0.007" intake and 0.009" exhaust. Suspect the 0.40mm/0.45mm reference is intake/exhaust not a range. But you don't need guesses. Maybe one of the engine experts will jump in and confirm.
 

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Richard Jemison
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Cam issues

Your lash settings are a bit tight. Open to .010 and .012.

These old Alquati cams had way too much overlap, and tighter lash only makes them worse.

The engine will run far better (and idle in particular) if you place the LCs at 106 int., and 106 exhaust.
 

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Discussion Starter #20
Richard, thanks. It won't be immediate but I will make your lash changes. I haven't checked lobe centers yet but will. I know nothing of the rebuild that was done and this cam stuff is out of my league.

Ed, I'll give you a call about the jets and some related questions. I'm getting like 15mpg. If I'm turning all that fuel into power, fine but not sure of that yet.

I did visit Rich last Saturday and learned a bunch while checking out his sprint and spiders. Ken
 
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