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Discussion Starter #1
Hi everyone,

After my introduction here
https://forums.vintage-mustang.com/new-member-introductions/1164950-69-gt-convertible-says-hello-switzerland.html

I was wondering if I could get some help here :smile2:


I have a Summit 600 carb running on my 69 mustang with a stock 351w 4V and FMX.
The carb works great so far but there's one annoying bug i just can't get rid of!

It idles fine hot/cold, but when i shut it off and let it sit for 10+ minutes then restart, it starts well with ½ throttle but idles terrible with AFR 16-18:1 dead lean.
After driving it a minute or two, everything is back to normal again.
Anyone an idea??

Here before shutting it off:

And restart after 10 min:


-Engine is fresh rebuilt
-Choke was swapped to manual because of this problem. It's definitely all the way open.
- Prim. jets 66; Sec. jets 68; Idle feed restriction smallest 0.043, PV original 6.5
-idle vacuum steady at 18-19 and gaskets changed a hundred times so no vacuum leak
-idle in park 750; in gear 550. All good.
-timing 12° BTDC, ~36° total
-Transferslot is correct, secondary blades are shut
-PCV valve is new

I'm struggling with this problem for over a year now and I’m totally out of ideas...


Thanks

Oli
 

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a) Secondary idle adjustment.... loosen secondary stop screw until not touching arm then tighten until it just touches plus 1/4 turn. Yes, the secondaries have an idle adjustment.

b) Fuel percolation emptying the idle circuit when shut down hot. Explore insulating carburetor spacer and/or reducing fuel temperature (do you have all steel fuel line between pump & carb?).
 

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Discussion Starter #3
a) That’s pretty much how I adjusted it actually. Although a bit less than ¼ turn.

b) Fuel temp is definitely not the problem. I’ve replaced the aluminum 1” spacer with an plastic one and on top of that I’m using the summit insulator gasket. Steel fuel line was removed and replaced by a rubber hose routed away from the engine. When the engine is hot, I can always touch the carb. It’s warm but not hot.
Floats bowls are filled when starting hot
 

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Steel fuel line was removed and replaced by a rubber hose routed away from the engine.
^^^ Rubber hose will not let the fuel inside shed heat. It will absorb the ambient heat from the engine compartment.

From Engine Builder Magazine:

"The under hood temperature of many vehicles will reach 230°F or higher during a hot soak (after the engine is shut off) so 50% of the gasoline (the most volatile parts of the fuel) in the carburetor may boil off. This heating and subsequent boil off of fuel components can and does wreak havoc on fuel curves and ignition timing requirements of a carburetor equipped engine plus it will create vapor lock issues if the gasoline boils in fuel lines or the carburetor bowl(s). The answer, particularly in a carbureted engine, is to minimize the exposure of fuel system components to heat in every way possible."
 

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Even in warm weather, engines need some choke, in order to correct a lean situation when the engine's started. Anytime it's had a chance to cool down, this will happen. It's not enough for a lot of people to mess with, especially in places like southern California, but then again, most people didn't run A/F meters either!


Your assertment that the choke is 'wide open' with it being (let's call it a 'cool start', if not cold one) is the bit that makes me think this is the problem.
 

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Next time it’s hot and you shut it down, take the air cleaner off and see if fuel is percolating out of the carb boosters. If this were a multiple choice test in Woodchucks garage I’d choose (b).......it doesn’t have to be ultra hot for modern fuel to boil out.
 

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69GTconvertible

I've got no great suggestions here... but I do have a similar issue (just not as bad). Idles at 13.5AFR normally but after a ~20min soak the idle is at ~15.5 - 16AFR. There is gas in the bowls. Carb with phenolic spacer is warm but not so hot that you can't keep a finger on it, etc. After ~1min of driving its back to 13.5AFR. Since the engine still idles reasonably well for me with a 13.5 - 16 AFR, I just live with it (actually, I probably would not have even noticed it except that I have an AFR gauge).

Since you have a manual choke, per what Grimbrand suggested, did you try choking the carb to get the AFR down?

Also here is an interesting video on how one can handle vapor lock (which might? be somewhat associated)...

 

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Sounds like one of those non-problems from analysis paralysis.
Can you get pure gas or have you and seen any difference with different gas?
 

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Discussion Starter #10
Thank you guys for all your input!

It is definitely not fuel starvation or boiling.
The float bowls are always full and there’s no percolation or boiling. I had this problem before I installed the insulator spacer when fuel was boiling and flooded the engine.
Now it’s fine. The temp. gun never shows more than ~40° (~110F) at the carb when the engine heat soaks.

I have not tried different gas yet actually. We have 95 and 98 octane here in Switzerland. I always used 98 due to my compression of +10:1
You think that could have an impact?

I understand it might be normal and the engine needs some choke.
But really already 5min. after shutting it off in hot summer weather? I had definitely noticed it even without an AFR gauge since it runs so poor it almost goes out in gear

I can of course correct it with the choke, for that reason I’ve replaced it with an manual one. But to me it was always a stopgap solution as I thought there was a problem.

If it’s normal, I accept it and live with it, probably try another carb once. It’s really annoying and this stupid manual choke always get stuck in pulled position so I have to get out of the car, reach under the hood and open it by hand. :LOL:
 

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you say teh float bowls are full, but is the fuel level set properly? in hot weather it can be very fine line to toe. living in southern arizona, i have had this issue, and i can tell you that fuel boil can happen when the thermometer read above 90, depending on conditions. and you dont even have to have fuel boil in the float bowls to have a lean mixture issue. remember the fuel lines, and the fuel pump can also experience fuel boil, and that will cause a lean mixture, when coupled with a low fuel level.

and the fuel level in the bowl doesnt even have to be very low either. remember that carbs are not sealed units, they vent vapor all the time through a number of orifices, and in hot weather, the fuel vaporizes easily, and actually doesnt have to boil.

and dont think yo can just raise the fuel level slightly above where it should be and solve the problem either, as the fuel expansion will cause a dead rich mixture. damned if you do and damned if you dont.

in the end i think you have a few places causing your issue.
 

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Discussion Starter #12
Yes the floats should be right. As summit advises, fuel level is set to mid of sight glass.
I noticed though that after cooling down, the level is down at the bottom of the glass. When I give the car a shake, the fuel swashes up.

After I start the car, level is up again immediately.
 

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I don’t know much about it but, are you sure the O2 sensor is accurate when first started? Don’t they need a warmup period?
 
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Just a side note but running a primary power valve, 66-68 is too tight between the primary and secondary jets. Normally, there is a spread of around 6 numbers in size larger on the secondary jets because it doesn't have a power valve. I checked the spec's I could find for out of the box config and the Summit chart said 67 and 73 for jets. If you go up or down on the primaries you would want to keep around that same ratio difference on the secondaries. I don't think this has much to do with your leaned out a/fr though. It still sounds like hot fuel, thinned out, puts it lean until you get fresh fuel flowing into the carb. It doesn't have to be boiling to still be a bit too hot. I have this problem some with the 302 in my truck.
 

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Discussion Starter #15
@RestoMike66
They do. You can see it in the second footage when the gauge is flashing the first 20 sec. the reading starts when the light stays on.
I’m sure it’s accurate as the engine runs like garbage:LOL:

@ macstang
Stock conf. was 68/73. The problem is it ran (still is) too rich under WOT so I decreased the sec. jets. I'm still in tuning phase along with the vac. Spring for the secondaries.
I’ve heard that some guys in Germany run their summits with same jest in prim. and sec. so I wanted to try it out… Still a bit too rich with 66/68.

Fiddling with the jets hasn’t changed the problem though.

The car is now stowed away for the winter. Next year I try the lower octane gas (95) and see if there’s any difference and maybe try a different carb.

What you guys think would be the better carb for my stock 351W with FMX? Autolite 4100 or Holley 4150/4160?
Still have the stock 470cfm 4300 laying around from before the restauration but I’m not intending throwing this one on the car
 

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What you guys think would be the better carb for my stock 351W with FMX? Autolite 4100 or Holley 4150/4160?
Still have the stock 470cfm 4300 laying around from before the restauration but I’m not intending throwing this one on the car
personally i would run a 1406 edelbrock performer carb, if i were going to keep the engine stock, or step up to a 750cfm carb in the same performer line, not sure what model number it is. they run great out of the box, and i have only had to jet one down out of the 15 or so that my friend and i have used over the years. they are easy to tune, and have plenty of parts available to tune them.
 

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Street holley carbs work better it you relocate the Idle jet to the bottom. They have been putting them up on the top of the metering block for years now and if you relocate them to the bottom they are submersed in fuel..Not hard to do you just need a tap and 4 brass set screws...
 

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Discussion Starter #18
thanks again you all for your hints and opinions.

I was very sure that vapor lock isn’t the problem since there’s always fuel in the bowls and the carb isn’t really hot.
But after reading a lot about it, that must be the problem.

I have the cheapest fuel pump on it and I don’t know it’s pressure rate so I think I’m gonna get a fuel pressure gauge and check it’s pressure. Then probably buy a good quality Holley pump. Maybe that helps too.
 
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