Carb question

ID89GT

ID89GT

10 Year Member
May 26, 2008
1,659
18
49
Spokane Valley, WA
#1
Okay so I have a carb related question. I have a Holley 600 on my car right now, its a 351w bored 30 over. The question I have is is my carb to small for my engine? I know nothing about carbs, and I know there is a lot of experienced people on here with them. I plan on getting it professionally tuned or at least have someone who knows what they are doing this Spring so I can actually drive my car this summer.. Should I just keep my carb I have or get a bigger one? There is a Holley 750 on craigslist for 65 dollars right now.. has electric choke and what looks like vacuum secondaries. It is also a lot newer model than the one I have which has a manual choke with mechanical secondaries. Ive had a few people tell me my carb is to small for my engine but I have no idea.. What would you guys suggest?

Also I plan on converting it over to EFI within the next couple years so I really don't wanna drop a ton of money on a new carb just to get rid of it a couple years later. I just want something that works for now while I collect the parts for my swap.
 

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#4
That 600 probably gives you some crisp throttle response, but if you have anything more than stock heads, good chance it's limiting on the top end. If you were to upgrade to a 750 vacuum secondary, it would open up your top end some and still maintain decent mileage as long as you kept your foot out of it. :burnout:and tuned the primary circuit well.

$65 is an awesome price...I just hope its in good shape. What all is wrong with it now? backfiring? sluggish? off idle response? falling on itself?
 
#5
Barely idles, when I get off the freeway and drive in town it wants to die when I come to a stop. Also doesn't really pull hard at all. Its a pure street car so low end is where I want my power. I can roast the tires just fine but overall drivability just sucks
 
#6
Ok-how much cam are you working with?
is it a vacuum secondary or a double pumper (model #)?
have you verified the power valve is functioning correctly?
have you adjusted the idle mixture screws?
do you have any vacuum leaks?
have you tried upping the jets or squirters?
 
#7
It has an elgin cam in it now, ill have to go to tge shop to get the exact specs for it. Mechanical secondaries right now. And ya i can turn the metering block scews in all the way and it dies like its suppose to so i know the power valve aint bad. As for jets i havent touched them since idk if im makin it better or worst. And i have no vac leaks
 
#8
Ok, if you have a 600 double pumper, and it has that many issues, you might want to find a place with a wideband so you can get your A/F dialed in...just like with FI, your fuel curve will probably have to be messed with. If memory serves, you set your idle mix screws in all the way to stop, then out 1 1/2 turns for inital adjustment, then up or down as necessary-achieve highest idle, then in very slowly until you detect slight dip in idle speed. Depending on your idle vacuum, you may need to change your power valve...I believe they're made from 2.5"-10.5" hg for enrichment circuit-this is where a vacuum gage comes in really handy. You could also try different pump cams, and squirters to see what gives you the best throttle response. The jet size will of course enrich or lean out the mixture during part throttle cruise/acceleration as well as WOT...I'd almost suggest you drop the money on a jet kit than a bigger carb...but ultimately, that's up to you-and a bigger carb would probably cure your issues, but might make other issues arise where you'd need to drop a jet size or a squirters or some other issue may exist.

How do your plugs look? Normal tan? White? Sooty? Glazed? Does it run fat at all?
 
#9
ID89GT said:
... As for jets i havent touched them since idk if im makin it better or worst...
Click to expand...

Only way to tell is to change them and drive it...of course, that's the old fashioned way, and isn't as precise as a wideband and a set of rollers. I'd almost bet you'd benefit from some bigger jets though. If the carb has never been opened since new, it's probably jetted for a smaller engine (300 cubes or so) with less demand...you're running a 351 with bigger cam, and I'm guessing free flowing exhaust-this will increase your fuel needs-especially when you mash the go pedal.
 
#10
Ya idk what size power valve it has in it right now. It barely pulls 5 inches of vacuum at idle but if i snap the throttle it shoots up to 20 on decel. Only vac line i have is to the dizzy, manual brakes so no booster, and all vac spots are capped off. Ill probably pick up one of them jet kits and just mess around with it. Gotta learn somehow, also will keep an eye out for a wideband since i wanted one anyway
 
#16
You have more problems than just a carburetor.

Idle vacuum at 5" is terrible unless you have one hell of a cam. The fact that the vacuum is so low is probably because you cant get it to run right. The carb is probably a large part of that, but you gotta work on the parts that are the problem, (the idle) not throw money at it on a used/ 75.00 C/L carb that is probably just as screwed up.

#1 On the choke horn of that carb is a "list number". A 600 DP will be 4776. The last number dictates how big the carb actually is.
4777 is 650; 4778 is 700; 4779 is 750 CFM etc., etc. Get that number, it'll tell you exactly what you have.

#2 If it is in fact a true 600 DP, that is a 350.00 carb. (brand new)

#3. It could be rebuilt, as there is every part available for that carb to make it like "brand new."

There is a formula for determining carb size vs intended usage/engine size/ and demand. Right now however, your problem isn't because the carb is too small, it's that there's something wrong w/ it.
Do not change jets now, they have nothing to do w/ the idle circuit. Do not change the power valve, it has nothing to do w/ the idle circuit.
Do you have a friend that has a known, working carb that you can swap on? if you do, bring him to your house buy him a coke, and swap his carb. (unless it's like a 1050 dominator or something)

If you dont, you gotta work w/ what you got.

Does your car start nicely? Or does it need to be cranked? Do you know what the float level is set at? Do you know how to adjust it?

Get this book if you are gonna keep it carb'd.
books?id=M3gq1v61qiYC&printsec=frontcover&img=1&zoom=1&l=220


It'll cost about 15-20 bones. You'll "know it all"after you read it.
 
#17
Ive had the carb rebuilt already. I bought one of them Holley trick kits for it and had a guy I worked with at the Toyota dealership rebuild it for me.

The car normally has to be cranked when I first start it, after it has been started once it usually starts right away after that.
 
#18
ID89GT said:
Ive had the carb rebuilt already. I bought one of them Holley trick kits for it and had a guy I worked with at the Toyota dealership rebuild it for me.

The car normally has to be cranked when I first start it, after it has been started once it usually starts right away after that.
Click to expand...

The reason I asked about how it starts is that a bad power valve will leak all of the fuel out of that bowl overnight. Do you have an electric pump? Does your carb have clear sight plugs on the float bowls? Next time, right after you shut it off, either check the float level by unscrewing the sight plug and rocking the engine (fuel should just barely slosh out) or looking at the clear plug. Then check it the next morning to be sure it's all still there. If it's gone. the PV could be leaking. A leaking PV will make the car run like a pig as it will allow too much gas in all the time.

The list number? What is it? And lastly,...(don't take this the wrong way) Does your friend know what he's doing w/ that carb?
Seriously, That book I told you to get states that there are alot of "experts" out there that do more harm than good.
Hell, I bought my first Holley DP for my 351 in 1975. I've had a dozen since then.
Even so,..I'm certainly no expert. I have that book.
 
#19
Ya I have a universal electric fuel pump on it right now. Ill get the list number off it when I get home. No the sight plugs are not see through. The guywho rebuilt it was a master tech at toyota soi would hope he knew what he was doing lol.
 
#20
Like Mike said, if the floats are low, it'll have issues...all over the rpm range. Didn't intend to cloud anything with my description of the pv...I was only asking if you'd checked it, and it has to be matched to suit your cam. Too high vacuum opening, and it'll constantly run rich due to low idle vacuum, too low opening and you'll starve it on acceleration. You'd do well to get that SA book...there's tons of info in it for troubleshooting and modification.

Did you get the cam specs yet?
 
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