Well, it looks like I'm the Guinea Pig on this one. Since I have a wideband O2 setup and vacuum gage mounted in the car, I figure my mild 306 would be a good test platform for the “new” Summit MO8600VS 600 cfm carburetor. The package was originally set to be sent on 1/30/2009 but arrived at my door on 1/15/2009. Before the purchase, I made a couple of calls to satisfy my curiosity about this carb which, as many have pointed out, looks strangely like a Holley 4010. As we all know, the 4010 was born from the venerable and streetable Autolite 4100.
According to the Summit rep I spoke to, Holley destroyed the original molds for the old 4010 carburetor. The “guy” who designed the 4010 also helped design the Summit version… making improvements on the shortcomings of the 4010, and Holley supposedly makes these carbs for Summit. The box says “Assembled in USA but I still can’t get a straight answer on how (or if) China is involved. Anyway, I took apart the carb, checked everything out and took some pics. Enjoy.
Out of the Box
The carb was packaged as nicely as any Holley, but Summit has their own box...
And the parts that come with it...
Look inside the included fuel line…that’ll need to be filed off.
Here’s the fuel inlet on the air horn/top plate. The brass is standard inverted flare plumbing with screen filter.
That’s more like it. The aluminum portion is threaded 9/16-24 like the Holley single-feeds and Demons. This allows you to get more creative with your fuel feed options. More on that later.
The red arrow shows the air cleaner stud mount. It has to come off before the top comes off, or else the next thing you’ll hear is a crack when you start prying. The yellow arrow just shows some casting imperfections in the booster (more on that later).
I circled the choke rod. This is the only part that has to be disconnected to pull off the air horn/top. The arrows show that you can’t remove this with the choke in place…not enough clearance. The rod has a little clevis pin to hold it on the choke coil, and the choke has to be loosened (not removed) to get the rod free.
Air horn removed. The red arrow shows the now-accessible 4th screw of the secondary diaphragm. Kinda cool since you don’t really need the “quick-change” kit. Blue arrow shows where I loosened the choke screws to remove the choke rod. The yellow arrows point to all the standard Holley parts: Jets, float valves, center-hung floats, power valve, and accelerator pump “umbrella” check.
Here’s the secondary bowl. There was a little debris in there. Yellow arrow is where a power valve goes, possibly on another version. It is obviously not machined through on my 600 CFM model, nor is the secondary accelerator pump. The secondary bowl will certainly never leak.
The removable booster assembly. Primary and secondary have the same part number, but the secondaries on the 600 have no nozzles or passages. 8 discharge holes per booster. The yellow arrows show rough machining/burrs of these holes. Red arrow shows removable idle air bleeds. Summit has a kit with different sizes, sold separately.
The accelerator pump parts are standard Holley. The pump diaphragm is the turquoise Holley one, and (despite what they say) has just as much potential to leak as any other Holley accelerator pump. The pump cams are standard Holley, and the one I found installed is the black one. The pump nozzles are inside the booster “cluster” and are also removable (red arrow). Like the idle bleeds, you have to buy the kit from Summit to get different sizes, but once you have this you should be able to fully tune the pump shot.
Here’s the underside of the secondary bowl. For some reason that I’m not going to complain about, the 600 CFM version comes with a cover and gasket for the non-existent power valve. I’ll just have a spare now. You can also see where the secondary pump would be on the 750 DP. Not sure whose fingerprints those are. Also, if you look in the top right corner, you can make out the (standard Holley) secondary idle stop screw.
The yellow arrow shows a nice smooth venturi bowl. The red arrow shows some lousy casting flaws. My engineer Father-in-Law says this is typical of the casting process, but I have never seen this on a carburetor before. If this carb runs and tunes well, I’ll keep it and smooth this nonsense out. I hope mine is just a fluke, or Summit may be getting a lot of these back.
I found the power valve chamber interesting. Notice the 2 channel restrictors seem to be pressed into place there. I set one of the idle feed restrictors on top there just to show that it looks uncannily the same size as the PVCRs. Although Summit doesn’t advertise this part of the carburetor to be tunable (as I feel it should be), they seem to have left some potential to set it up that way. A fairly experienced carb guy could most likely tap these channels and set them up to accept the idle feed plugs. Then it’s just a matter of drilling the various plugs as needed to accommodate desired orifice size. At this point you’re WAY into voiding he warranty. To the left is the standard Holley accelerator pump.
My initial impressions are actually fairly optimistic. I love the carb’s simple design and ability to use standard Holley parts on most of it. That said, I found it necessary to do some prep work before I let it near my motor. I was hoping to “Bolt On and Go” but there’s no way I would do that with this carburetor. First, there was the debris in the bowls. Plus, I would definitely want to clean up some of the machining burrs in the boosters. Second, I recorded all of the baseline settings. The driver’s side idle mixture screw was 1/2 turn out, the passenger side was 3 turns out. No way that’s ready to go out of the box. The curb idle was just barely engaged (¼ turn). The secondary idle stop was even less. I doubt it would even idle like this. I also measured the float levels as they were set and noted the squirter sizes and secondary spring type as well as stock jetting (67 pri, 73 sec).
The plan now is to clean out the passages/bowls really good. I’m going to set the idle mix screws 1.5 turns out and tune from there. I’ll probably set the curb idle so .015-.020 transfer slot is showing and tune from there. Plus I’ll set the secondary idle stop so that it actually is a stop, and throttle plates are not resting (sticking) in the bores. At this point, I will make Part 2 fairly brief, just basic hookup, plumbing and fitment. Then I plan on part 3 being actual tuning and driving with the wideband.
Part 2: Fitment and Plumbing
Now I’m going to focus on some plumbing considerations/options and how it fits on the small-block ford. I also compare some specifics between the Summit 600VS and Holley 4150 designs.
As promised, I did a good cleanup of the carb, blowing out all the passages and cleaning the debris in the bowls. I just used carb spray cleaner and my air compressor. Not shown, but I also took a pick and a fine abrasive pad and de-burred the booster discharge holes and smoothed out the boosters. I then gave them the same cleaning treatment to clear possible debris.
Here’s the test mule. It’s a 306 with the stock bottom end and pistons that yield approximately 9:1 compression with the Edelbrock Performer heads. I use the beloved Stealth intake along with a mild custom cam that focuses on the 1500-5500 RPM powerband. This is all mounted in a 1985 Mustang GT with a true-dual 2.5” exhaust system and 1 5/8” shorty headers. Let’s hope it likes this carb.
Side-by-side comparison of two carburetors. Look closer. The carb on the right is the stock Holley Motorcraft 4180. I modded the carb with a secondary metering block and center-hung 4150 style bowls. The Summit carb measures 3 1/4" from the base to the air cleaner flange. This is the same height as a 4160 or Edelbrock carb. As you can see, the 4180 is actually ¼” taller. This is due to the fact that the throttle plate is thicker on the 4180 than it is on a 4150/4160… most likely because of the different idle circuitry.
Here’s the included fuel inlet mounted to the carb. If you’re just running rubber hose to the 3/8” barb then this is all you need. Mounted to the engine, you can see where the barb comes out. There are several ways to route fuel lines, I know.
This photo shows another fuel inlet option. This is a Russel line that I got for the dual-feed setup on the 4180. The AN fittings on the 4180 are for the Holley bowl 7/8-20” threads. The Summit carb uses 9/16-24” threads, so all that is needed is the AN fitting on the rear, and the 9/16-24 to -8 AN for the front (not shown). This will allow interchange. The only potential issue with this is the narrower distance between the inlets on the Summit 600vs (8.3”) compared to the standard 4150 inlets (9 1/4"). On a similar note, Summit sells braided/AN inlet lines with the 8.3” separation that fit their carb if you want to run good plumbing. I had to use a shorter piece of braided line to make the below setup work with the narrower distance.
Here’s the carb mounted up. The included throttle lever nut snaps onto the stock throttle cable fine (red arrow). This view also shows how all the accelerator pump hardware is set up on the Summit carburetor. It is virtually identical to the Holleys, from adjusting the plunger spring to changing out the pump cams.
I finally got one of these pics to come out nice and clear. This shows one of the design changes made from the Holley 4010. The fuel inlet is angled down on the version, instead of straight out on the old 4010. As shown in the pic, this greatly aids in drop-base air cleaner clearance. The air cleaner shown is the giant dual-snorkel “460” style that came stock on the 83-85 5.0L.
I hope these pics give a good idea of how one would go about plumbing this carb, There are obviously several options, ranging from just running 3/8” rubber line with the included hardware, all the way to running a dual-feed setup with the Holley-style regulator and the fancy mount for it. Speaking of regulators, the Summit manual that comes with this carburetor recommends that fuel pressure be 5 psi, but no more than 7 psi. Above that they recommend a regulator. This seems fairly consistent with the Holley float valves. My mechanical pump operates at just about 7 psi, so I’m going to probably try it without a regulator at first. Stay tuned for another update.
Summit Carb Part 3: Driving Test
First off, my apologies for how long it’s taken to finish this review. A little back story: The original Summit
carburetor (from the first 2 Parts of the review) had to be sent back. The rear bowl was flooding and could not be adjusted down. I cleaned the float valves, and even replaced them with some known good ones from my personal bin. No luck. I tried clearing the passages again, and still no luck. After a discussion with a Tech at Summit
, we concluded it would be best to just send it back and get a replacement. I’m not sure, but I suspect a culprit was clogging something, or allowing fuel to seep past the float valve.
The new carburetor arrived March 20th, and I gave it the same inspection I did the last one. This time, I noticed the machining/casting seemed to be much better. There was no trash in the bowls, and the baseline mixture/idle settings were more realistic…1 ¼ turns out per side, with a “square” transfer slot showing under the throttles.
I mounted the carb as shown, routing the fuel line behind the carb and around to the pump. On initial startup, fuel was spewing from the primary flared fitting, and from the flare-to-3/8 nipple at the back (arrows).
Post-mortem on the Summit
fuel line showed no obvious defects where the flare sealed, but who knows. I may have gotten the 1 in 100, or 1 in 1,000 bad ones that didn’t get flared properly.
Seen below is my solution. Note the rubber line going around the carb. This is only temporary, as I plan to run AN fittings and braided line in its place, should all go well.
I used an Innovate LM-1 wideband A/F reader to record idle, cruise, and wide-open throttle conditions. I also have their RPM converter, but unfortunately I had trouble getting it to work. I tried several solutions using their forums to troubleshoot, but no luck. When you’re talking about potentiometers and calibration with computer audio output (and it still doesn’t work), then you need to take a step back and re-examine your product…rant over.
Startup and idle--
Notwithstanding the first attempt, the Russell line performed flawlessly upon the next startup. Fast idle went to 2,000 RPM, and with a tap of the throttle immediately went to 1,500 RPM…perfect. The float levels (easy to see in the big sight glasses) were a tad high and only required about a half-turn to correct. The idle mixtures were a little on the lean side and required about 1/8 turn each to get better manners. All told, the wideband was showing 13.5-14:1 and the engine idled with 17” vacuum.
Road test #1: Box stock settings—
With the idle squared away, it was time to hit the road. It was a sunny 59º day and Coastal North Carolina is pretty much sea level. Throttle response was very crisp…no doubt thanks to the annular boosters. It really reminded me of the way a well-tuned 4180 can run. On light throttle shifts, there was a very slight rich bog. I could see the wideband dip down into rich territory when I shifted, but under slightly more aggressive throttle, the bog was non-existent. The stock accelerator pump nozzles were #35 on mine. I could try a smaller shot. Otherwise, little if any tuning will be required here.
Low-speed cruise was done at 40 MPH and 2,000 RPM. A/F ratio was just under 13:1 at this speed in 4th
gear. Next I tested highway cruise, running at 65 MPH and just over 2,000 rpm in overdrive. On a level stretch of road, the wideband was showing between 13.5 and 14:1. Good…maybe a tad rich. Some may say spot on for a carb. Vacuum showed an average 15” in highway cruise.
Here’s a pic of the Innovate data logged on the first WOT run. I let off at 5k as I didn’t want to hit any kind of real speed in 3rd
gear. The red graphics are added by me as notes. You can see for yourself where the secondaries come on as it gets rich again, then starts to lean out at the top. I’ve seen better and worse curves, but all-in-all not too bad out of the box. The stock jetting is 67 in front, 73 in back.
Road Test #2: Slight tuning
Based on the earlier test run, I decided to go leaner in primary mains and delay the secondary opening a little more. I went with 65 jets and a brown spring from my tuning kit. The jet change/spring swap happens simultaneously, since with the air horn off, all 4 screws on the secondary spring housing are accessible. Loosening the choke to get the rod free is a pain, but after a few times, you submit and get used to it. At the top of the pic you can see where the air horn just flops out of the way (with flexible fuel line). The red arrow shows the only fuel I spilled changing the jets and spring.
The road driving showed a leaner mixture this time as expected. Conditions were 72º and sunny. 4th
gear cruise (2,000 rpm) went between 13.2 and 13.5. Highway cruise in overdrive bounced between 14.2 and 15.1, with uphill climbs creeping up to 16:1. If the car was to be kept with this jetting, a 9.5 (stock is 6.5) power valve would definitely be in order to keep away from that lean condition.
Here is the second WOT run, again up to 5,000 RPM for safety’s sake. Notice the black line. On this run I had the RPM converter connected, and even though it read double my actual RPM, I was able to use the data (notice my “corrections”). I was using the record feature and had no idea the RPM data was logging. You can see this is a leaner run. The a/f ratio went from 12.7 to 13.2 at the top with just 2 jet sizes on the primary alone. This is a bit lean if you try to imagine this curve going to 6,000 RPM. The stock jetting might not be that far off after all. With the wideband data logging capability, and the fickle yet usable RPM data, I’m going to have a lot of fun playing with this carb.
Daily Driver notes—
After the initial driving and tuning, the car sat for a week and a half. I went out one evening to fool with the RPM converter some more. Conditions were 61F with high moisture from on-and-off rain. I pressed the gas once to close the choke and turned the key. The engine fired up instantly, right to 2K, and with another tap down to 1500 RPM...just beautiful. I let the engine get up to operating temp (185º) while I was attempting to tune the RPM channel. The idle A/F showed 12.5:1, even when warmed up. I shut it off and let it sit for about 10 minutes to let it heat soak. With a turn of the key (didn’t touch the gas) it fired right back up. This time A/F was closer to the 14:1 I set, with no adverse idle issues from the heat-soak. All I used was the provided insulator gasket…no heat shield. It displayed very nice manners. I also drove the car on my 10 mile commute to the airport. This is mostly highway speeds, using the jetting from run #2, and with no ill effects.
The Good, the Bad, the Ugly: A Summary
First and foremost, this is a very nice driving carburetor. I have a well-tuned, fairly mild (280-300 fwhp) 5-liter, and this carburetor does it justice. I never sensed that the carb was holding back the engine…it just kept pulling. Plus I think that the jetting is pretty appropriate right out of the box, at least for my combo. For me, Summit
has always had a nice no-hassle customer support system. Returning the first carb was a no-questions-asked deal. I even got an email from their tech support checking to see if the replacement they sent was cutting it. Another obvious advantage is the carb’s simple design. With the exception of swapping power valves, tuning is a spill-free affair. As already mentioned most parts are Holley interchange, with the model-specific parts like gaskets, air bleeds, and fuel lines sold by Summit
None of these are really “bad” as it were, but they are things I would like to have seen. First, it would be nice to have screw-in bleeds in the power valve channel to allow this circuit to be tunable. Maybe there will be a version 2.0. In addition, I would like to see them take a page from the Edelbrock manual and make it a little easier to pull the air horn off. With the Holley design, however, I’m not sure how it could be done without redesigning the choke rod linkage altogether. Also, the included fuel feed line seems to be a hit-n-miss affair. The first one I had didn’t leak, the second one did.
As indicated in earlier parts, some casting imperfections can be found. I find, especially with aluminum parts, that this stuff can be “buffed out” so to speak. I had to remind myself that it’s not a $400 carb, so don’t expect a $400 finish. In the spirit of my venerable 5.0 GT, it is a “bang for the buck” product in which I found a nice amount of bang.
That’s it folks. I hope that this little review helped you if you’re trying to decide on a carburetor. Maybe you were just curious, and you learned a thing or two about the inner workings of this product. Please keep in mind that I had a hit-n-miss experience that required an exchange. You might also, but you might not. I can only hope mine was more of an exception than the rule, and that the majority of them are like the one I have now. In case you are wondering, I have decided to leave this carb on my car. No good review is complete without a more long-term test…right? Besides, I love the way it drives. I personally hope this line of carburetors enjoys much success, and that future parts and improvements come down the road someday. I have a few ideas myself.