Summit M08600VS Carburetor opinion

[Cantedvalve 128]

There are plenty of reviews out there already.  I am just going to give you what I have experienced with this carburetor.  I am by no means an expert in carburetors.  Hopefully someone finds it helpful.

I ordered and received the 600CFM vacuum secondary carburetor.  It came packaged well.  It included the fuel feed line and wires for the choke.  It might have had something else in the box (a funky carb base gasket), but those were the important parts.  They advertise that they wet test these carburetors... I can confirm that as mine still had some gas in the secondary bowl.  The finish on the carb isn't perfect... it is tumble polished, so around casting features and such (like the logo), you can see where it didn't polish.  I am probably going to have mine bead blasted for a satin finish, so I am not terribly concerned with it.  Since receiving it, I have replaced the vacuum diaphragm housing with one that has a quick change top on it for the springs.  That is the only modification I have made so far.

The good things:

• Its based on the Autolite 4100 design, so it kinda looks at home on a Ford engine
• There is exactly one gasket between body segments - Holleys have 5 (base plate, fuel bowls x 2, metering block x 2)
• Changing gets no longer requires you to spill gas everywhere (vs. a Holley), but changing the power valve will
• Out of the box, the car started and ran.  I did end up taking the carb off so I could do some double check of settings, and I will take it off when I switch out power valves
• Aluminum construction - Zinc has always bothered me, and it "corrodes" quickly (don't they use that as sacrificial anodes).  Aluminum I should be able to keep looking nice.
• Once I tuned idle, it runs well.  I can start and drive it without issue.
• A lot of the parts I used on my Holley (jets, power valves, float valves, accel pump diaphragms, accel pump cams, secondary diaphragms, secondary springs, small cork gaskets) all work with this carb, so no need to stock up on more pieces if you already have them.  Parts that are different are gaskets (lid gasket and venturi assembly gasket), accelerator pump jets instead of nozzle assemblies, and air bleeds.

The not-so-good things:

• The feed line that came with the carb... doesn't fit well without persuasion.  Leg to leg distance was too short, meaning when I screwed in one leg, the other wouldnt reach.  A little creative massaging and persuasion was needed to make it work (I think it is made of chromed copper), but I would just plan on getting a separate feed line.
• The carb wasn't terribly well tuned when it arrived... it ran, but I had to reset the throttle blade position so that it would idle without using up the transfer slot.
• The choke has a vacuum port on it (like Holleys do), but it didn't have a gasket between it and the throttle body, and there was a gap.  Potential for vacuum leak.
• There is a port on the back of the carburetor for brake booster or PCV valve hookup.  I couldn't get the plug out of the port.  Still haven't.
• I had an issue with the throttle sticking... not closing all the way.  I backed the curb idle screw all the way out, loosened the throttle plate screws, and let the throttle shut all the way.  I then tightened the screws, and the throttle hasn't stuck since.

This isnt a drawback on the carb as much as a learning... the Lokar throttle cable bracket won't work as-is.  The set screw doesn't have a surface to contact, so the bracket wants  to rotate.  I made a new, longer set screw that makes the whole thing work. 8-32 screw or some threaded rod will get the job done.

A little about my combination.  I have a stock 351C long block with a new cam.  I did throw some new valves and springs at stock 2V heads, but other than that, the motor is as it was from the factory.  The cam I have in there is a hydraulic roller with nearly .600" lift and a god bit of duration.  I have a 2500 stall convertor in an AOD and 3.89 gears.  The engine has welcomed the new carburetor, and so far the two are playing well together.  It is a good carburetor, cost aside.  When you toss in the price point, it is a great carburetor.

[Cantedvalve 128]

I finished my tuning carburetor (for now). I changed the power valve from the stock 6.5 to a 3.5. No more bog.

[1969_Mach1 333]

Nice review of those carbs.  I've looked at them, then move on to a Holley because I've always wondered what the quality would be.  Any reason why you went so small?  I always thought Cleveland motors ran better with larger carbs.  My 351W runs great with a 650 DP but seems to pull harder through mid range with a 700 DP carb.

I wouldn't be critical of the tweaks you had to do to get it setup.  With Holley carbs, before installing them I've learned it's best to do things like check the throttle plates for being centered, preliminary set the primary and secondary idle speed screws, etc.  Adjustments are just not preset very well from the factory.

[Cantedvalve 128]

I went with a 600 because mine is a street car that won’t see the far side of 6000 RPM very often. 600 CFM is plenty of carburetor for what I’m doing.  My engine probably would do more with a racier carb, but for my uses, this works.  Chances are, if my son ends up getting a car, that I’ll use this one there and get a 750 for me.

Im not disappointed with the tweaks I had to do, just disclosing what I found. I think it’s a great carb and would recommend it for any street engine

[Cantedvalve 128]

Final update. Put in the 3.5 power valve and it pulls hard and smooth now. No more tuning required for now.

[barnett468 418]

On 7/26/2019 at 5:16 AM, Cantedvalve said:

Final update. Put in the 3.5 power valve and it pulls hard and smooth now. No more tuning required for now.

Your main jets are likely too small. 99.9% of the engines use a 6.5 power valve.

 

[1969_Mach1 333]

The 3.5 power valve would not open as soon under acceleration.  I'm thinking that would lean the mixture, at least until it opens, then back to the same as it was before.  I agree, the main jets are probably too small.  I also wonder if the idle/transfer circuit is extremely rich which would explain why a power valve opening later made a difference.

[Cantedvalve 128]

First off, I had a big flat spot in it when I had the 6.5  Went to a 3.5 and it went away.  My engine idles in park at 9.0.  Rule of thumb is either 1/2 of in park vacuum(9.0) or 1/2 of in drive vacuum (7.0) (I have seen both).

Normal driving it does fine (it didn't before the change).  When I floor it, I have a small flat spot.  I might need slightly larger jets, or a 4.5 valve.  I have both to try.  Not a big priority at the moment as it runs well.  I just want to get some miles on it.

[barnett468 418]

1 hour ago, Cantedvalve said:

First off, I had a big flat spot in it when I had the 6.5  Went to a 3.5 and it went away.  My engine idles in park at 9.0.  Rule of thumb is either 1/2 of in park vacuum(9.0) or 1/2 of in drive vacuum (7.0) (I have seen both).

Normal driving it does fine (it didn't before the change).  When I floor it, I have a small flat spot.  I might need slightly larger jets, or a 4.5 valve.  I have both to try.  Not a big priority at the moment as it runs well.  I just want to get some miles on it.

hat is the engines static compression ratio?

What is the engine size?

What is your elevation?

I am very good at jetting carbs and have jetted well over 1000 of them in the last 45 years, and have never used a 3.5 power valve and neither has any of the other professional tuners I know. As I stated, your main jets are likely too small or you have some other problem. Also the rule of thumb you use is not correct and no professional tuner uses it, but if one does use it then they must first set the idle timing to the optimum level.

A 4.5 valve will not remove a flat spot in your case, and if anything, it will make it worse.

Running lean is a huge priority to fix if it was my car. Also setting the timing curve so it is optimal for the engine is a priority, and incorrect timing can cause flat spots when accelerating or surging at steady throttle or can cause an engine to run hotter than it otherwise would etc.

Sounds to me like you need to go to at least a 4.5 power valve and up 3 sizes on your jets as a good starting point after you set your timing curve to the optimum level.

PROPER tuning is critical for any engine if one wants to get the most performance and the most life out of it and the most mileage from it.

 

 

 

 

[Cantedvalve 128]

I’ll give that a go this weekend.

[1969_Mach1 333]

2 hours ago, Cantedvalve said:

First off, I had a big flat spot in it when I had the 6.5  Went to a 3.5 and it went away.  My engine idles in park at 9.0.  Rule of thumb is either 1/2 of in park vacuum(9.0) or 1/2 of in drive vacuum (7.0) (I have seen both).

Normal driving it does fine (it didn't before the change).  When I floor it, I have a small flat spot.  I might need slightly larger jets, or a 4.5 valve.  I have both to try.  Not a big priority at the moment as it runs well.  I just want to get some miles on it.

With that low of a vacuum signal, you might also need to start tuning the accelerator pump circuit.  Flat spots are commonly caused by a momentary mixture that is too lean.  Sometimes with aggressive cams and low vacuum signals more or longer duration accelerator accelerator pump shots are needed to compensate for the longer time it takes for the main circuit to start up.  It's a good thing you went with only a 600 CFM carb.  With a larger carb the issue would be more severe.

[mwye0627 44]

23 hours ago, barnett468 said:

Your main jets are likely too small. 99.9% of the engines use a 6.5 power valve.

 

Perhaps your statement should read: 99.9% of STOCK engines use a 6.5 power valve!!!   

He did state that his cam is nearly 0.600" lift, with "a Good Bit Of duration"...   Any camshaft, depending on Duration, that allows the Vacuum signal to fall significantly at idle or under  moderate acceleration would require a Lower Value, eg. 3.5" Hg power valve.  We have had instances with fairly mild BBC race engines that when pulled into gear with an automatic based transmissions that the vacuum would fall low enough that we would use Power Valve Block-off plugs.   Typically, we would then jet the carburetor up 4 jet sizes.   It's really much more common than most people would believe!!!

[barnett468 418]

22 hours ago, mwye0627 said:

Perhaps your statement should read: 99.9% of STOCK engines use a 6.5 power valve!!!   

He did state that his cam is nearly 0.600" lift, with "a Good Bit Of duration"... 

We have had instances with fairly mild BBC race engines that when pulled into gear with an automatic based transmissions that the vacuum would fall low enough that we would use Power Valve Block-off plugs.

No, it shouldn't because that is not what I meant .

"A good bit of duration" is a subjective statement which is not very useful because it can mean vastly different things to different people, and I prefer not to "guess" at what someone means by undefined/unspecific statements like that. He stated the cam specs on another thread so i know exactly what the duration of his cam is.

This has nothing to do with his specific issue, and if your idle drops more than around 200 rpm when you put your race engine cars in gear, then perhaps you should try a converter with a higher stall which will also keep the idle vacuum at a slightly higher level.