Any Carb Gurus out there want to comment?

[Pman1961]

Motor 445 sbf, 611hp@5900 with 750SS Quick Fuel DP mechanical secondaries carb, Brodix 210KC heads, victor jr. style intake, 10.4.1 comp, hydraulic roller .609 lift intake, exhaust, 254 intake 262 exhaust @ .050, 104 intake centerline, 108 lobe seperation, 9 inches of vacuum at 1,000rpm idle, msd ignition, TKO 600, 3:46 gears, Superformance Cobra. Okay currently it has a Super Street Quick Fuel 750 dp mechanical secondaries which was used on a 408. Motor likes and needs a lot of initial timing. With my primaries set showing about .020 of transfer lot at the bottom of the primary throttle plate and not adjusting the secondaries it idles decent at 26 degrees initial advance. Car cranks up fine at 34 degrees initial and seems to run decent at idle as well. Go any lower in timing I have to adjust the secondaries to get it to idle. Also, when I run a lower initial timing even at 20 degrees I get a little stumble off idle. I can take care of the advance limiting situation but I do have an issue with the air/fuel idle mixture screws. The screws are currently 1 full turn out. Turn about a 1/4 in the motor will lean and begin to drop idle. However, when you turn the mixture screws out it seems to have no effect. I'm sure it is an air bleed size issue but I have a question. With the motor description I have given would it not be better to use a race style carb such as the Quick Fuel 'Q' or Race 'Q' and increase cfm to 850? I've been fighting the cam with the timing and carb. Getting close but I want to see if anyone out there is running a race style carb and working well. I don't know if is worth retuning my carb if I would have been better off with a race style carb in the first place. Thoughts???

[elmariachi]

I am no guru. But the golden rule of tuning is you always set your timing first and you never tune the carb and change the timing at the same time. You need to know what the total timing is for the engine, and if you have the base timing that's good too. You ignore the carb quirks as much as you can and get your initial and total timing set first and lock it down. Then you can start adjusting the carb and if that means you have to adjust the idle screw beyond .020 then do it. If that adjustment then induces a stumble, then fix it in the carb, not by adjusting the timing. If the car won't idle below 26 degrees, fix it in the carb, not by going back and advancing the timing. That engine should idle well below 26 initial.

I suggest you spend $10 and buy Don Gould's "Tuning to Win" eBook. No affiliation, other than its the best $10 I ever spent and taught me how to tune carbs and diagnose problems.

EDIT>>One other item, if this is a double pumper with mechanical secondaries, you should approach the carburetor as "square" and make equal adjustments to front and rear at the same time. Primary and secondary jetting should be square, both pump lever arms adjusted equally and both transition slots set identically.

[Dana E.]

Perry,

Adjust both the primary and secondary throttle plates so they are open the same amount. Adjust all four of the idle mixture screws-start with one turn out and go from there. How does it run if you set the idle speed to 1100-1200 rpm? If it stumbles/bucks off idle driving at light throttle, that sounds like it's too lean.

Give quick fuel a call and see what they have to say about the fuel curve of the race carb. No doubt that race carb is plenty rich. A bigger carb will no doubt make more power up top, but it might be a bit "soggy" down low.

I'm curious-what's the cranking compession of your motor?

[Gaz64]

Your SS carb is too lean, you need at least a Q series:

Q-Series Carburetor 750CFM CT

What is your timing curve? Or is it locked out.

I'd think something around 20 at idle.

[blykins]

Perry,

If I were doing that engine, I would do a Q-850. I use Q-series carbs on almost every engine I do.

With that being said, I have no clue why your engine needs that much initial timing. Was this engine completely assembled when you got it? Maybe the balancer marks are off?

I guess I'm also not understanding why you need to richen up the idle mixture. You can do it, but it will require different idle air bleeds and different idle feed restrictors. We change those right off the bat in our Quick Fuel carbs.

Sounds like you have several issues going on. Bottom line is that your SS-750 will do a fine job, but you're probably leaving a bit of power on the table. I don't use the HR or SS series carbs unless somebody HAS to have a choke....otherwise, they get a Q series.

[vector1]

thatsa a big meatball!!

i would be looking to idle in the 1200 rpm at a minimum, with the 108lsa and duration you are going to get a lot of reversion in the intake at lower rpm's, probably why you need a lot of advance, with the higher rpm you might not need as much advance initially. i'll bet your cruising is going to require a 2500rpm range to be happy or more than the 408 will or than you are used to.

i think the street carbs are usually set up to be used on higher vacuum engines, probably using dual plane intakes, yours is not pulling much vacuum which the race carbs are usually tuned for.

you have a lot of information, i would sell the present carb, call aed and give them the info and have them make a carb, pay the money, be happy, and enjoy the driving season. i'll bet you end up with a bigger carb also.

[Pman1961]

I have been discussing my timing and carb situation with a good adviser currently but he was unavailable yesterday evening and just wanted Club Cobras opinions. What Vector1 has commented is what I've been told that the street style carb is designed for higher vacuum. I previously had been idling around 1100rpm. When I put more timing in the motor it seems to have better idle quality and therefore I tried the idle at 1000 to 1050 with the fans running. The motor was assembled at KCR and used a new SS750 Quick Fuel that I had bought for a 408w. When the initial advance was 20 degrees or lower having a wider timing curve it required opening the primarys up a whole bunch just to idle and the idle would get really lazy quick below 1100 rpm especially when the fans kicked on because it was idling with initial timing. Last night was my test to see how much initial advance I could take before any starter drag and I was able to go to a full 34 degrees without any issues. Only issue is the idle mixture screws were not sensitive turning them out so I'm thinking the setup within the carb is not compatible with my motor with low vacuum. I feel as others the Q850 would be the correct carb for my motor. Just was curious how these perform on the street if they are more of a race style carb. By the way my timing curve currently is 20-34. I can modify for 26-34 which seems to be okay or lock it out. Opinions on locking out are welcomed also.

[Wbulk]

Elmariachi is right, set engine timing first then work on the carb. If this is a street car I don't think you are going to be happy with this cam. With that high of duration and a narrow LSA you have a ton of overlap which drops your vacuum way down. That being said a higher compression engine needs less total timing also, but there in lies the conflict, they need higher initial timing at idle to run, but lower total timing. This engine should be able to run with the initial idle timing at just about 18 deg. As you know once you are opening the blades into the idle transfer slots you start affecting adjustability. This is one of those case where an engine my benefit from a vacuum advance dist. hooked to manifold vacuum that is adjustable. I met two guys in the last month that have pulled their very expensive high rpm race type engine for a more street friendly engine. In the long run I think you would be a lot happier by changing the cam.

[blykins]

Originally Posted by Pman1961

I have been discussing my timing and carb situation with a good adviser currently but he was unavailable yesterday evening and just wanted Club Cobras opinions. What Vector1 has commented is what I've been told that the street style carb is designed for higher vacuum. I previously had been idling around 1100rpm. When I put more timing in the motor it seems to have better idle quality and therefore I tried the idle at 1000 to 1050 with the fans running. The motor was assembled at KCR and used a new SS750 Quick Fuel that I had bought for a 408w. When the initial advance was 20 degrees or lower having a wider timing curve it required opening the primarys up a whole bunch just to idle and the idle would get really lazy quick below 1100 rpm especially when the fans kicked on because it was idling with initial timing. Last night was my test to see how much initial advance I could take before any starter drag and I was able to go to a full 34 degrees without any issues. Only issue is the idle mixture screws were not sensitive turning them out so I'm thinking the setup within the carb is not compatible with my motor with low vacuum. I feel as others the Q850 would be the correct carb for my motor. Just was curious how these perform on the street if they are more of a race style carb. By the way my timing curve currently is 20-34. I can modify for 26-34 which seems to be okay or lock it out. Opinions on locking out are welcomed also.

Q series carbs are absolutely fine for a street engine.

Also, on locking out the advance, I'd do it. With a Cobra, you usually go with a quick curve anyway, and it doesn't take much to get past initial timing once you get rolling. If your starter can handle it, I'd do it.

[scottj]

For street, with a big cam, I always ran locked-out and a custom Holley HP. Engines were 434cid and 264/272 @ .050 solid rollers. Carb size was 1.45 venturi with 1.75 throttle bores. Timing was 35-36 degrees depending on the heads and compression was 9.5 to 10.8, again depending on the heads. Throttle response and drivability rivaled my current fuel injection.

[elmariachi]

My 427 FE has a similar cam profile as your engine. I am at 10:1, I run 18 initial and 36 total and my Powermaster starter spins my engine like a running chainsaw. If it were me, I would start by setting the total time to 34 degrees all in by 3k. I would then lock out the advance and set the idle timing to 18 and bolt it down and not touch it again. (You can come back later after all the carb tuning is done and experiment with a little more total timing, provided you don't let that bump up the initial timing and alter your idle.) Then I would remove the carb and make sure (based on your 9" vacuum reading above) that you have nothing larger than 6.5 power valves installed, and confirm that both transition slots are square. Open the mixture screws 1.5 turns, put the carb back on and work to get it to idle just under 1k rpms. I would use the vacuum gauge to tune the mixture screws for max vacuum, and then open the mixture screws another 1/8-1/4 turn from max vacuum. After all that, I'd unlock the advance and work on using heavier springs first and experimenting with lighter spring combos to get to an advance curve that feels good in the seat of your pants and doesn't affect idle. If all that doesn't work, then its time to either have that carb looked at or try another carb with which to make some comparisons and decisions.

Unless they are plugged, I cannot imagine air bleeds causing any of your problems and I don't think its a cfm issue at all.



.

[Pman1961]

After a few tests it appears the motor likes total timing and I have no issues cranking the motor so I'm going to lock out the distributor and then look into a carb specifically for this motor. Question is I have removed all the advance stuff, bushing and the roll pin at the drive gear of the distributor just as the MSD tech suggests. How do you remove the gear so I can rotate the shaft 180 degrees and lock the distributor out? It seems very tight. Is there a specific tool you would recommend and does the gear only come off or part of the shaft also? Want to make sure I am securing properly prior to beating on it. Thanks

[blykins]

You shouldn't have to remove the gear. The roll pin you want to drive out is in the aluminum sleeve up above the gear. If you drive the top roll pin out, then slide that sleeve down, then you can slide the shaft up far enough to put the pin in the advance stop hole, rotate it 180°, then push the shaft back down so that the pin goes in the hole in the distributor.

[Pman1961]

Thanks Brent. MSD instructions specifically states remove drive gear. Must be generic. Anyhow, distributor is locked and the car sounds and idles so much better and cleaner. Throttle response just blipping the throttle has no stumbles compared to the occasional with an advance curve. I'll give you a call down the road about a carb. Roads are wet so no driving today