More Carb Tuning Questions

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mopowers

mopowers

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I acquired a slightly used 650 proform mechanical secondary carb (67212) for my 340. I took it apart just to check it out and it looked great. When I put it on to fire it up on the 340, I can't get the idle lower than 1000. The primary and secondary curb idle speed adjusters are all of the way out and nothing is preventing either linkage from fully seating. The fast idle screw on the electric choke is all the way out as well.

67212.jpg


I sprayed carb cleaner around the base and can't seem to find a vacuum leak. The vacuum advance is disconnected and plugged as well and I've got a PCV hooked up. The 4-corner idle mixture screws are all about 1 turn out, which gives me 12.5 on my AFR gauge. Before I tried this carb, I had a 670 street avanger that ran fine. The only other change I made with the carb change was I installed a Carter mechanical fuel pump with pressure regulator set on 6 lbs.

What am I missing here? Any ideas why I can't get this darn carb to idle down?
 
Have you looked to ensure that the throttle blades are fully closed against the bores? If they are, I wonder if there was a casting flaw in the base plate where the idle speed screw is.
12.5 AFR at idle? That is too rich but I'm sure you know that. I recall the Holley instructions stating the "baseline" setting for the idle mixture screws is 1.5 turns out from their seats. Turning the screws in will lean it out, too much will cause a lean stumble when you tip in the throttle.
 
Kern Dog said:
Have you looked to ensure that the throttle blades are fully closed against the bores? If they are, I wonder if there was a casting flaw in the base plate where the idle speed screw is.
12.5 AFR at idle? That is too rich but I'm sure you know that. I recall the Holley instructions stating the "baseline" setting for the idle mixture screws is 1.5 turns out from their seats. Turning the screws in will lean it out, too much will cause a lean stumble when you tip in the throttle.
Click to expand...

From what I've seen from looking down the top of the carb, the blades look closed. I'm gonna pop it off tonight to verify. This whole thing has me scratching my head. I doubt it's a blown power valve because I can get the AFR way lean with just a tiny turn of each idle mixture screws. I would think a blown power valve would make it way rich. If I can't get the idle below 1000, that means air has to be going form somewhere, right? I did put on a phenolic wood spacer too. I'm wondering if it might be warped...
 
Disconnect the PCV and see if that helps. You might be pulling to much air thru at idle. When the carb is off set the transition to .020" If you are running a mild cam say 230* or less set the secondary blades to where they are just closed and not binding. If you close the secondary blades favor the primary screws when adjusting idle mixture. Example 1 turn Primary, 3/4 turn Secondary. Once throttle blades are set you can also use initial timing to dial in idle speed.
 
mopowers said:
I acquired a slightly used 650 proform mechanical secondary carb (67212) for my 340. I took it apart just to check it out and it looked great. When I put it on to fire it up on the 340, I can't get the idle lower than 1000. The primary and secondary curb idle speed adjusters are all of the way out and nothing is preventing either linkage from fully seating. The fast idle screw on the electric choke is all the way out as well.

View attachment 1715016048

I sprayed carb cleaner around the base and can't seem to find a vacuum leak. The vacuum advance is disconnected and plugged as well and I've got a PCV hooked up. The 4-corner idle mixture screws are all about 1 turn out, which gives me 12.5 on my AFR gauge. Before I tried this carb, I had a 670 street avanger that ran fine. The only other change I made with the carb change was I installed a Carter mechanical fuel pump with pressure regulator set on 6 lbs.

What am I missing here? Any ideas why I can't get this darn carb to idle down?
Click to expand...
Perhaps your throttle cable is incorrectly adjusted and just a tad short.
 
dustrr said:
Perhaps your throttle cable is incorrectly adjusted and just a tad short.
Click to expand...

That thought came to mind this morning before I rolled out of bed. I guess I just assumed the Proform would have the same linkage geometry as the Holley I replaced it with. Boy am I going to feel stupid if that's the problem!

mderoy340 said:
Disconnect the PCV and see if that helps. You might be pulling to much air thru at idle. When the carb is off set the transition to .020" If you are running a mild cam say 230* or less set the secondary blades to where they are just closed and not binding. If you close the secondary blades favor the primary screws when adjusting idle mixture. Example 1 turn Primary, 3/4 turn Secondary. Once throttle blades are set you can also use initial timing to dial in idle speed.
Click to expand...

Great info. Thanks. I definitely plan on trying it with the pcv disconnected.
 
A)You said the primary blades '"look closed", and you said the mixture screws are very sensitive. Well then I agree, the blades are near to closed, on the front at least.
B)You said all the speed screws are backed out, and nothing is preventing the blades from closing,I'm assuming that includes the secondaries. Because of A, I agree the secondaries are probably closed
C) you said you could find no air leak and it ran fine with the previous carb. Because of both A and B, I agree, there are no air leaks, including the PCV.
D) you said you can get the AFR into line with a simple adjustment of the mixture screws.Because of all of the above, I would be scratching my head too.
Q1) how much Idle-timing are you running
Q2) What size is your cam?
>I don't know anything about that brand of carb. So I would be yanking it and checking the T-port sync (as already mentioned). If the Transfers are not at least .020 or a little more, and the idle timing is in the range of 14 to 18, and the PV diaphragm is known to be good, then I would be searching for an internal air bypass; like holes drilled through the blades.
>But if the idle-timing is more than 18* I would crank it back first, cuz with no other options this carb doesn't like excessive timing.
>If the unit has an electric choke with an internal choke pull-off like some old Holleys have, they have a small air passage to manifold vacuum that on a Holley is sealed behind the choke housing with a small cork gasket. The hole is very small, and I doubt that if you drilled an equivalent hole in the T-blade it would make much difference, so I don't know why I even mentioned it,lol.
>I have seen on some carbs that the air-cleaner hold-down bolt,if removed,opens a passage all the way down to the intake.


FWI,
I recently ran across an old Holly 2bbl that had an engineered idle-air bypass circuit that started just above the parked throttle blades, traveled forward, then down, then back rearwards to exit underneath the blades. It was non-adjustable. It looked just like any other often-seen, horizontal,rectangular, port.Yeah, same deal, engine idled too fast, and could not be slowed down, right after it went on, as compared to the one that just came off. Customer came to me with $1000, and said the unit had been to two other well-known shops, with no results. Fix it he said.
 
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AJ/FormS said:
A)You said the primary blades '"look closed", and you said the mixture screws are very sensitive. Well then I agree, the blades are near to closed, on the front at least.
B)You said all the speed screws are backed out, and nothing is preventing the blades from closing,I'm assuming that includes the secondaries. Because of A, I agree the secondaries are probably closed
C) you said you could find no air leak and it ran fine with the previous carb. Because of both A and B, I agree, there are no air leaks, including the PCV.
D) you said you can get the AFR into line with a simple adjustment of the mixture screws.Because of all of the above, I would be scratching my head too.
Click to expand...

A) A 1/4 turn on all four mixture screws will send the AFR from 12.5 to 17-18 range.
B) Yes, that includes the secondaries. The linkage arm on the secondary shaft isn't even touching the small adjustment screw at this point. (still have to check that the throttle cable isn't the culprit holding up the linkage)
C) I couldn't find a leak when spraying starter fluid around the base. Although, I did change the 1" spacer to a phenolic wood type, so, even though it's new, I guess it may be the culprit as well. I'll take it off to eliminate that possibility.
D) Yes, the mixture screws are extremely responsive. Damn thing just doesn't want to idle below 1000-1100

AJ/FormS said:
Q1) how much Idle-timing are you running
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24 degrees initial 35 total. I haven't put a light on it since swapping carbs, but I'm sure since the idle is now through the roof at 1000, it's already into the mechanical advance. I'll be checking this as well.

AJ/FormS said:
Q2) What size is your cam?
Click to expand...

Mild single pattern HFT 231* @.050, .561" 108LSA in at 104ICL
 
In my 367,I run a similar cam; 230/237/110/in at 106& 549/571 lift.
I run 14 initial. This puts the T-port sync spot on at 750 rpm, with just a tiny bit of bypass air..No spacer, just the AG.
 
If the Street Avenger ran fine, why did you change it?
 
Have you tried cranking the timing back?
I tell you what;
if you were to pull off the carb and flip it over, and if you saw almost no transfer slot exposed, then you would know without a doubt that she's running too much idle-timing. Reset the slot to; square to a little taller than wide, using the curb idle screw. After that, do not touch it again. Put it all back together, Start it up,warm it up. Take out idle timing to a minimum of 12*, or until the rpm comes down to 750 ish. If it still won't idle down, NOW, this is almost proof that shes pulling in air somewhere. If you now reset the mixture screws and find them quite far open, that is almost proof that the extra air has no fuel in it, so it is coming from an outside source, or from below the blades. But if you find the mixture screws nearly closed, then the extra air does have fuel in it, and that fuel almost certainly has to be coming from the bowls,lol. So the question then would be where's it coming from. Since the T-Port sync is right, we know it can't be from anywhere but the low-speed circuit.
The first place to look is the fuel level,
then the 4-corner idle system if the carb is so equipped,
then the air bleeds, and finally the PV circuit.
After this if you still have not found it, you will have to blow the thing apart and go look for an internal bypass, engineered or not.
For a stick car, I prefer the DP carb too. I'm a sucker for all barrels blazing at any ol' time!If the tires are spinning too fast,as if, I can always throttle back. I got no patience for vacuum secondaries,lol. By the time they wake up, the urge may have passed.
 
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Well ! Did you check your throttle cable yet ? These guys are wasting their advice, if that"s the problem. Rusty has a good point.
 
Well, the problem seems to be fixed. I took the carb off and replaced the cheap thin gaskets that came with the 1" phenolic spacer with a couple thicker one, put the carb back on, and it idles really well now after a couple adjustments.

I noticed something about one of the manifold vacuum ports on the base plate though. The port is not being used. When I pull the vacuum cap off while the engine is not running, a little bit of fuel comes out. Not sure why this is. Any one have any ideas??? I've circled the port I'm talking about in the picture below.

vacuum.png
 
How is the rear float set? It might be a touch high.

mopowers said:
Well, the problem seems to be fixed. I took the carb off and replaced the cheap thin gaskets that came with the 1" phenolic spacer with a couple thicker one, put the carb back on, and it idles really well now after a couple adjustments.

I noticed something about one of the manifold vacuum ports on the base plate though. The port is not being used. When I pull the vacuum cap off while the engine is not running, a little bit of fuel comes out. Not sure why this is. Any one have any ideas??? I've circled the port I'm talking about in the picture below.

View attachment 1715017369
Click to expand...
 
RustyRatRod said:
If the car is an automatic, you had the right carburetor on it.
Click to expand...

In your opinion... I've had great luck with mechanical secondaries on automatic cars. Have you ever used a Street Avenger Carb? The fuel curve on them is way too lean for any motor hotter than stock IMO.

RustyRatRod said:
How is the rear float set? It might be a touch high.
Click to expand...

The port in question is on the primary side. The primary float is probably a tad high. Just a smidge over the bottom of the sight glass. I'll be sure to adjust accordingly.
 
You can use float level to help with slightly rich or lean conditions in a carb just off idle. Higher when lean, lower when rich.

A car shouldn't care the type of carb it has on it, mech or vac, when tuned correctly. For the most part, the right foot has a bigger determining factor in all things related to fuel consumption than anything else.

IMO, vacuum carbs are for taxi cabs... :)
 
crackedback said:
You can use float level to help with slightly rich or lean conditions in a carb just off idle. Higher when lean, lower when rich.

A car shouldn't care the type of carb it has on it, mech or vac, when tuned correctly. For the most part, the right foot has a bigger determining factor in all things related to fuel consumption than anything else.

IMO, vacuum carbs are for taxi cabs... :)
Click to expand...
While I agree with everything
It's hard to find fault with a TQ, which is an air-valve secondary, which I suppose is a form of vacuum secondary.
I love TQs
 
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