Summit 600 CFM Carb

[ArdWrknTrk]

Curious if anybody with one of the Summit (or Autolite, or Holley 4010) carbs ever had any issues with the throttle plates hanging up preventing the idle from decreasing.

My engine is running GREAT! It starts 100% perfectly, as good as any EFI. The carb seems to be very responsive, and so far the engine is looking like it is going to be good on fuel also.

I do have one nagging issue though: Idle. I can't adjust the idle below about 800-850 RPM once it is warmed up. The curb idle screw simply lifts off the throttle. I have the electric choke completely disabled...the fast idle screw is also removed, so it's not hanging up on the choke. I've done prelim check for vacuum leaks and didn't find anything. I added the thicker (0.062") carb gaskets, and have checked and torqued the carb base bolts several times. The transition slots look like they're bigger than square, but I can't adjust it any further.

With the engine cold, it idles nicely. It's when it warms up that the idle seems to creep up a bit. Once in a while, I've reached under the hood and returned the throttle by hand and the idle decreases instantly. After this I installed a return spring...not a great one, just one I had laying around. I have a proper throttle return spring/bracket kit on order arriving tomorrow.

Anyway, just wondering if anybody has any thoughts? I haven't touched the idle air mixture screws yet. It's almost perfect, but I'd love to get this idle dialed in a little better. It seems like the throttle plates are hanging up somehow, but I can't see where or how.

PS: I did not check the secondary plates...I didn't think to check them before re-installing the carb last time.

Any ideas? I was reading that a possible solution is to loosen the throttle plates and let them re-center, but I'd like to get more input on that, or other ideas first.

Edit: I also have a Holley TPS installed on this carb, and I originally thought it was an issue, but I've investigated it thoroughly and it doesn't appear to be holding anything up. When I was playing with the carb on the workbench, I had the TPS removed anyway.

I would definitely check the secondaries Cory.

The truck should die with the curb idle screw backed off the lever.

So you must be getting air from somewhere, and I know you're talented and particular enough to ferret out any vacuum leaks.

I can't say I have experience with the 4100/4010 or their clones.

Glad to hear that the old Merc is running so well!

[85lebaront2]

Curious if anybody with one of the Summit (or Autolite, or Holley 4010) carbs ever had any issues with the throttle plates hanging up preventing the idle from decreasing.

My engine is running GREAT! It starts 100% perfectly, as good as any EFI. The carb seems to be very responsive, and so far the engine is looking like it is going to be good on fuel also.

I do have one nagging issue though: Idle. I can't adjust the idle below about 800-850 RPM once it is warmed up. The curb idle screw simply lifts off the throttle. I have the electric choke completely disabled...the fast idle screw is also removed, so it's not hanging up on the choke. I've done prelim check for vacuum leaks and didn't find anything. I added the thicker (0.062") carb gaskets, and have checked and torqued the carb base bolts several times. The transition slots look like they're bigger than square, but I can't adjust it any further.

With the engine cold, it idles nicely. It's when it warms up that the idle seems to creep up a bit. Once in a while, I've reached under the hood and returned the throttle by hand and the idle decreases instantly. After this I installed a return spring...not a great one, just one I had laying around. I have a proper throttle return spring/bracket kit on order arriving tomorrow.

Anyway, just wondering if anybody has any thoughts? I haven't touched the idle air mixture screws yet. It's almost perfect, but I'd love to get this idle dialed in a little better. It seems like the throttle plates are hanging up somehow, but I can't see where or how.

PS: I did not check the secondary plates...I didn't think to check them before re-installing the carb last time.

Any ideas? I was reading that a possible solution is to loosen the throttle plates and let them re-center, but I'd like to get more input on that, or other ideas first.

Edit: I also have a Holley TPS installed on this carb, and I originally thought it was an issue, but I've investigated it thoroughly and it doesn't appear to be holding anything up. When I was playing with the carb on the workbench, I had the TPS removed anyway.

If the secondary throttle shaft has a stop screw like the Autolite 4100s did, it may need adjusting. Quick check, take a piece of vacuum hose and hold it near one ear, take the other end and insert it in the primary and secondary venturiis and listen to the hiss of air. If you have a loud hiss on the secondaries, then they definitely need adjusting.

Holley 4150/4160 vacuum secondaries and the Autolite 4100s use a stop screw so the throttle plates don't stick closed. The old "Teapot" Holleys had a special procedure for installing the throttle plates in the secondaries. They were slightly asymmetrical, with the long side being the one that lifted up when opening. Procedure was to take two strips of paper and put them in the back side of the secondary throttle bores and while holding the shaft closed, tighten the screws. This meant the the secondaries seated against the front of the bores so they back side wouldn't dig into the aluminum and cause them to stick.

[Rembrant]

If the secondary throttle shaft has a stop screw like the Autolite 4100s did, it may need adjusting. Quick check, take a piece of vacuum hose and hold it near one ear, take the other end and insert it in the primary and secondary venturiis and listen to the hiss of air. If you have a loud hiss on the secondaries, then they definitely need adjusting.

Holley 4150/4160 vacuum secondaries and the Autolite 4100s use a stop screw so the throttle plates don't stick closed. The old "Teapot" Holleys had a special procedure for installing the throttle plates in the secondaries. They were slightly asymmetrical, with the long side being the one that lifted up when opening. Procedure was to take two strips of paper and put them in the back side of the secondary throttle bores and while holding the shaft closed, tighten the screws. This meant the the secondaries seated against the front of the bores so they back side wouldn't dig into the aluminum and cause them to stick.

Thanks for the tips guys. I'll check the secondaries and see what else I can find.

One thing I forgot to mention, and maybe it's nothing, but the carb seems to leak badly around the left end of the throttle plate shaft if I operate the throttle with the engine off. It did this when installed on the engine, but also on my work bench also. Not sure if that means anything or not...maybe they all do that? I don't remember my previous Holley 4160 leaking like this.

When I say that I did prelim vacuum checks, I just mean that I removed and plugged the brake booster, and I removed the vacuum tube for the wipers (truck still has original vacuum wipers...they're on the list to be electrified lol, but very low priority at the moment). I also replaced the carb base gasket (and 1" spacer gasket under that). I haven't gone around the intake and carb base yet for a proper vacuum leak test with carb cleaner or propane, etc.

[85lebaront2]

Thanks for the tips guys. I'll check the secondaries and see what else I can find.

One thing I forgot to mention, and maybe it's nothing, but the carb seems to leak badly around the left end of the throttle plate shaft if I operate the throttle with the engine off. It did this when installed on the engine, but also on my work bench also. Not sure if that means anything or not...maybe they all do that? I don't remember my previous Holley 4160 leaking like this.

When I say that I did prelim vacuum checks, I just mean that I removed and plugged the brake booster, and I removed the vacuum tube for the wipers (truck still has original vacuum wipers...they're on the list to be electrified lol, but very low priority at the moment). I also replaced the carb base gasket (and 1" spacer gasket under that). I haven't gone around the intake and carb base yet for a proper vacuum leak test with carb cleaner or propane, etc.

If you have that big a leak around a throttle shaft, I wonder if a bushing was left out? Holley used bushings on high wear areas.

[Rembrant]

That carb is jetted as: 65 primary jet, secondary side did not need any tuning, 68 secondary jet, stock secondary diaphragm spring. .035" discharge nozzles and been experimenting with acc pump cams but it seems the pink cam works best for that application. Enlarged IFR jets from .046" to .048" to get rid of off idle lean transition slot stumble. Also opened up the secondary IFR jets .002" over stock size to smooth out shaking at idle without affecting low speed AFR driving on the transfer slot.

Hey FordFETruck,

I have been doing some reading, and this appears to be a somewhat common issue with the Summit carbs. Mine is working well most of the time. It starts as good as any EFI engine, and runs down the road just fine. However, if it sits off for for a little bit, or if I get stuck in the heat in stop and go traffic, it will start to idle rough. I don't know what the problem is, and I don't have an AFR gauge, but it seems to be related to the carb getting heat soaked. I watched one video where the guy was having the exact same issue, and the cure was increasing the size of the IFR jets, in his case he went from 0.046" to 0.053", and that fixed it. He says that the Summit carbs spend A LOT more time in the transition slot than other carbs, so having the IFR jets correct is critical.

So I came back to read this thread again, and sure enough you also had to increase your IFR jet size, although not as much.

I guess I should probably install an AFR gauge to confirm that the heat soak theory/engine running rough is actually due to a lean condition...but I could also try some jetting changes first.

What jets do I order? I did see a comment online that I can use Holley Emulsion Jets. If that is all that I need to do, then I'll get a few sizes ordered up to try.