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Jets and Carbs

5K views 23 replies 10 participants last post by  26634 
#1 ·
Hi all,
Let's talk jetting and carbs for bit. Have been reading til I'm cross eyed about this.
My 406, with specs as in signature has what I would call a stumble when I am idling along at slow speed in lower gears, like you would be down a local street. It occurs when you attempt to ease down on the throttle and speed up just a tad. The motor seems like it doesn't want to accelerate at all until you give it even more gas, then it jerks ahead.

Jets are as I bought the Quick Fuel Drag 750 carb, 74s in front, 76s in back, 3.5 power valves front and back, idle vacuum-7.

Question is, is this stumble caused by not enough jetting in the primaries or what. Car is pretty streetable over all, just have this issue low down. With the Quik Fuel, I can adjust idle speed on both primaries and secondaries, but not sure how to proceed. Timing is set at 20 degrees initial and the rest all in by 3000, as I recall.

Opinions and suggestions appreciated.

RJ
 
#2 ·
Another question. This carb, Quick Fuel Q Series 750, came with #74 primary jets, #84 jets with extensions, 4.5 primary power valve, secondary valve plugged. It is a race carb, has no choke, has mechanical secondaries. Question is, how does the factory determine these things. I assume it has to do with air and fuel flow testing. I read alot of guy's posts that say they used the carb right out of the box with no changes. How often is this applicable, given all the changes guys make to their engines.
Thanks
More to fololw haha
 
#5 ·
Virtually no carb will operate optimally 'out of the box'. They may work well, but the user is likely leaving a bit on the table. This is where some fine tuning will make a difference-sometimes very slight other times in a more dramatic fashion.
 
#3 ·
If its the jets I would think they just need to be cleaned and nothing more. Assuming here the car was running fine before. I wold also look/test the fuel pump (mechanical or electric) for operation. Also check the lines for any sort of blockage. Other than that it could be the idle circuit or the accelerator pump. I just don't think it has anything to do with the jets. When was the last time you had the carb rebuilt to include soaking and the idle circuits blown out with compressed air? Quickfuel carbs are good carbs to say the least. Your prob is just a matter of eliminating circuits and finding the bad one. My bet is on the idle or accelerator pump.
Hope this helps.
 
#4 ·
My diagnosis would be that you either need to install a larger accellerator pump cam or install bigger squirters, or even both. It sounds like to me you are having a temporary overly lean condition until the proper amount of fuel comes on from the jets.

If you try to hammer it instead of rolling into the throttle does it fall flat on its face or even pop through the carb?

Hi all,
Let's talk jetting and carbs for bit. Have been reading til I'm cross eyed about this.
My 406, with specs as in signature has what I would call a stumble when I am idling along at slow speed in lower gears, like you would be down a local street. It occurs when you attempt to ease down on the throttle and speed up just a tad. The motor seems like it doesn't want to accelerate at all until you give it even more gas, then it jerks ahead.

Jets are as I bought the Quick Fuel Drag 750 carb, 74s in front, 76s in back, 3.5 power valves front and back, idle vacuum-7.

Question is, is this stumble caused by not enough jetting in the primaries or what. Car is pretty streetable over all, just have this issue low down. With the Quik Fuel, I can adjust idle speed on both primaries and secondaries, but not sure how to proceed. Timing is set at 20 degrees initial and the rest all in by 3000, as I recall.

Opinions and suggestions appreciated.

RJ
 
#6 ·
carb doesnt have more than 500 miles on it. fuel pump new, lines new, everything spotless, stumble is when you just ease into the throttle, not when you nail it. When you nail it car goes sideways in 1st three gears. Just annoying with the stumble in city driving.
 
#8 ·
Okay, have you checked for any vacuum leaks? Spray some carb cleaner around the base of the carb and the throttle shafts-when you do this listen to see if the rpm changes while it is idling, and keep a medium sized towel that is doused with water handy. Spray along any vacuum hoses too.

Are you using a vacuum advance on the distributor, and if you do are you using manifold vacuum to operate it?
 
#9 ·
RJ - Carburetors are comprised of circuits or systems. Starting with the fuel inlet system, the idle system, transition, power etc... Each circuit has a function and you trouble shoot the part of the carb that pertains to the problem you are having.

Think about a car as a whole, it has a drive train, a suspension, brakes etc. You wouldn't trouble shoot the engine if you heard clunking coming from the front fender when you put on the brakes... Learn to recognize what part of the carb the symptom points to and you will become a much better tuner!

Get a Holley book like Amazon.com: Holly Carburetor Handbook 4150 & 4160 Hp473 (0075478007509): Mike Urich: Books


Jets are part of the power circuit and not likely to cause a part throttle stumble. Look at the opening on the top of your carb that lets air into the mixture. When you step on the gas far more air can rush into the carb than fuel so you have an accelerator pump (2 if it's a mechanical secondary) that operate and squirt extra fuel to compensate for the overpowering amount of air. This is part of the transition circuit along with the discharge nozzles and is most likely where the trouble is...
 
#13 ·
Well that is why-or at least part of the reason why- you are having problems at part throttle or rolling into it. A distributor without a vacuum advance is designed really for either idling around the pits or WOT.

Can you find a distributor with a VA to try?
 
#17 ·
I'm with Mike on this one. A vacuum advance would help considerably. It is fairly easy to try larger squirters or different acc. cam profiles, Holley has a chart which shows pump volume based on cam plastic color. Just make sure the butterflys are closed and use a rag when swapping out acc. pump jets. It is really a pain when the screw or jet falls into the intake manifold.

alan
 
#18 ·
Carb came with a 4.5 power valve and you are running a 3.5 if i understand correctly. And when you are rolling along, low speed, high vac situation and ease into it, as vac drops below your 7", it stumbles. Sounds like it is going lean when the power valve should be coming in. I would go back to the 4.5 and try that. That pv will come in sooner.
I went thru a similar problem tuning a Holley 4360 but i had my A/F meter which was a big help. Tuning the power valve got it smoothed out.


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#19 ·
If it stumbles at slight throttle openings but is fine when the throttle is opened more it sounds to me like the throttle pump arm isn't in contact with the cam. This results in slight rotation of the cam without movement on the accelerator pump.

What cam and discharge nozzle are you using?
 
#20 ·
No slop in accelerator pump linkage.
As for the mechanical advance distributor, if I had it to do over again, I would get a dizzy with vacuum canister, but I'm probably not going to pull the MSD distributor and spring for big bucks on another one.
 
#21 ·
Just a thought with more info. How much vacuum at idle and just off idle? Two words though, Vacuum canister !
This may be applicable to your situation. Just my 2 cents. Its worth a look. This is from a magazine article. Not saying to drill your throttle blades, but you may want to look at your idle transfer slots. How much throttle opening is your carb set to get it to idle ?
Hope this helps.

Setting Idle Speed
The problem with a long-duration camshaft with lots of overlap is low manifold vacuum that requires more throttle opening to set the proper idle speed. The ideal position of the throttle blades in relation to the idle transfer slot is where the primary throttle blades just barely uncover the bottom of the transfer slot-roughly about 0.020 inch. However, long-duration camshafts often demand much more throttle opening. This uncovers too much of the idle transfer slot delivering more fuel from the idle circuit. This creates an off-idle hesitation or bog that is difficult to eliminate. If the curb idle speed position of the primary throttle blades uncover to much of the idle transfer slot, the quick fix is to drill two small holes in the primary throttle blades adjacent to the transfer slot. Start with holes of roughly 1/32 inch in diameter and then readjust the idle speed on the carb. Adjust the hole size, idle speed, and throttle-blade position until you achieve a blade that uncovers no more than .020 of the idle transfer slot.
 
#22 ·
Thanks for the good ideas and info.
I went back to 4.5 power valve in front and removed the plug in the secondaries and put in 4.5 power valve there too, and upped my primary jets to #74s, and decreased the rears from #84s to #74s.
idol vacuum is 9", and the car now eases of of idol with no hesitation at all, much better than what it was. I still have "roughness" from 1500-on up to 2500 or so, maybe a touch more, and then it runs very smooth from then on up to 6500. The roughness i speak of is similar to the rough cam idol, but I would have thought that would disappear by 1500rpm max, but maybe I'm wrong, I never had this aggressive of a camshaft before, soo wasn't sure what to expect.
Also had a fouled #4 plug. replaced it, checked the resistance on the MSD wires which was good, and then checked the MSD distributor cap and found that the terminals in the cap were quite corroded. I cleaned them with some sandpaper, pup it back together with a new spark plug, and the misfire was gone. That of course helped alot with the idol.
Not sure if the roughness up to 2500 is normal, or if there is further adjustment necessary. I dont have the extra case to spring for a vacuum distributor right now, or I would make the change.
Thanks all.
RJ
 
#23 ·
If you are gently rolling up to cruise and getting roughness below 2000 rpm you are running mostly on the t slots and idle circuits. Probably a touch lean. If you have already properly adjusted idle you would have to tweak your IFR's or IAB's to change that. Without an AFR curve I'd just be guessing, I'm not as good as the old school guys...


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#24 ·
I was thinking about going up to #76s on the primary and giving that a try. Before, I was having issues with the plugs fouling. That's when I replaced the rear plug with a 4.5 power valve and reduced the rear jet size from #84 to #74. Really cleaned up the plugs after that.
 
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