Carburetor hesitation

[marks73]

I aways set it to 0 clearance.

Gotta leave a little!

 

[72HCODE]

Actually they tell you zero clearance at idle, the shot has to come on just when the throttle moves, the clearance they speak about is to make sure at wot you do not crush the diaphram.

Most new carbs have a stopper at wot so you can't really crush the accelerator pump unless the throttle linkage is bent somehow, this might be possible on some cars that use a lever and cable with a seperate return spring. Our cars have the return spring on the cable and you are limited by the cables throw with the stock linkage.

Basically you set the linkage to kiss the accelerator lever but not to put any force on it when idleing, otherwise the pump will inject fuel at idle. Then you throw the throttle linkage to wot and watch how much the accelerator lever rams into the diaphram. Usually I push on the lever at wot and the lever gives a little more just to double check there is play In the linkage at wot.

The think to remember is any change to the idle Rpms then means you have to recheck the accelerator pump lever to throttle plate to make sure they kiss, and you have to readjust the idle air bleed screws, which effect the idle Rpms again.

So the trick is using the vacuum gauge you set the idle speed, then idle air bleeds until turning the bleeds maintanes the vacuum level at the idle Rpms you want, then you set the accelerator pump linkage. Things turn into more hell when you have 4 corner air bleed screws.

Then usually after you set everything by the book you go for a drive and find out the setting doesn't work well and you end up usually on the rich side, and have to setup the accelerator pump shot again.

The joy of carbs...

 

[JimNiki]

carbs and women are 2 things we will never fully understand! Hehe.

 

[marks73]

carbs and women are 2 things we will never fully understand! Hehe.

Gotta have a little!

 

[droptop73]

The frankencarb is way more adjustable today! Now I can really mess it up. I went to a friend's shop today and we (he) added adjustable air bleeds in the primary and secondary side. We added adjustable idle fuel jets and power valve channel restrictors. The idle air bleeds were restricted by .005" and the idle fuel jet was opened up by .003". These minor changes made the transition much more sensitive and the car much more drivable at low throttle.

 

[Wolverine]

Regarding idle, I think it is important to note than when you "hype up" your car a little, the vacuum idle tends to decrease, from a STOCK 20 inches HG(typical) to 17 if you are mild or medium hyped, and maybe down to 12 if you are really hyped.

With less vacuum at idle, there is less pull for fuel. As a result, the carb needs to be adjusted for the lower idle vacuum...unless you like to shake like a fat chick on a Harley at red lights.

In any event, glad to see there are more solutions for the venerable 4300D...especially at idle.

 

[sgtjd]

Just got back from vacation and these are great posts. I notice that my linkage that lays on top of the cam was bent slightly. So I bent the linkage back on top of the cam this fixed my accelerator pump which now activates sooner like it should. I had a big gap from the linkage moving until the accelerator pump started to acitivate. This also fixed my hesitation issue somewhat.

Next problem: I still have a misfire through the secondary’s on the Holley 460 carburetor at low acceleration and when the engine has only been running about 2 min. The primary jets are sized at 31. It seems the hotter the engine gets the better it runs. The question: why is there a misfire through the secondary’s and how do you prevent it? By increasing the primary jet size to 33 would this help? Do the primary jets effect the secondary's. Also my timing is set a 8 degrees BTC. Should I increase this to 10 degrees?

 

[droptop73]

If this is indeed a 460 cfm 4 barrel... it is way too small to perform well in a 351c. A 31 jet in a holley is real small. I'm running a small jet in mine at a 65 jet. Can you provide a photo and a list number for that carb?

 

[Wolverine]

I suspect he meant a 4160 Holley...but we will see...

 

[jeffdeq]

I replaced the carb on my Q-code 351C with this Holley Carb back in December, 2007. The carb has performed flawlessly all these years. The electric choke works perfectly. Also, it sits up for months at a time between cranks and has never failed to start and run correctly after it picks up fuel. The carb was $258.91 with free shipping at that time.

Jeff

 

[travisgm]

This question might be way out in left field and I'm just going by what I was told by the owner.

The motor in question is a 73, 351 2V that over the last year has went from stock 2 barrel carb and intake to a Edelbrock Performer 4V intake with 600 carb to now the proper Edelbrock 2V intake.

I never drove the car when it had the 4V intake on it, but was told it worked fine, no stumble or hessitation.

Since putting the 2V intake on there is a very noticable stumble when hard accellerating from a stop or slow speed. When going 30mph and hitting the throttle hard when it's warmed up, it seems to work fine.

Would this stumble at start have been on the 4V intake too and maybe he just never noticed or would having the right intake on there have caused it?

From as far as I've been told, the carb hasn't been adjusted, just take out of the box and put on the car.

If the carb does need a tune, can I follow the same steps in this post to tune an edelbrock carb too?

Carbs are new to me, and I am not against taking it to a pro if it's something a beginner shouldn't tackle.

 

[c9zx]

The short answer is yes, it should behave differently with the 2V intake vs. the 4V intake. the mis-match in port sizes, with the 4V intakes at the head should cause reversion and turbulence The larger volume of the 4V runners should also lower velocity and yield a weaker "signal" at the carb.

Still check the obvious first, accelerator pump and AF mixture volume screws. If you are tuning use this, section 2 http://www.edelbrock.com/automotive_new/mc/carbs_acc/pdf/carb_owners_manual.pdf

Chuck

 

[72HCODE]

i just want to say i have absolutely grown to hate the edelbrock performer intake for the 2v and 4v. every single person i have spoken to has the stumble issue with it.

Just one thing, when you use the edelbrock intake, you must increase initial timing 2-4 degrees. I don't know where i buried my instructions for my V4 intake, but one of the last steps required is bumping up the initial timing from stock.

Since your car was stock then the intake was changed along with the carburetor if you have not changed the initial timing i would, and then go out and road test.

if you add headers you have to bump up initial timing as well.

if your at 6-8 degrees go to 12-14.

from there you may need to vacuum tune the carburator.

 

[Jeff73Mach1]

I'm at 16-18 with no ill effects, but I have a lot of cam

 

[72HCODE]

what you need to watch out for is total timing.

this is set in the distributor. There is a stopper for the mechanical advance. There is a plate with a 2 numbers and L next to it. you may have a plate that reads 16L and 12L on the other.

depending on which side the stopper is on determines how much total timing you have.

for example if your your distributor is set to the 16L side that means 16X2= 32 degrees mechanical + lets say you run stock 6 initial that gives you a total of 38 degrees total timing.

lets say you start modifying the car. you drop a new intake, headers and carb, and you increase timing from 6 to 14.

now your total timing just on mechanical is 46, this is well pasted what is considered the danger zone for a Cleveland.

most hypertonic pistons in the aftermarket warn about exceeding 34 degrees of timing total.

if you have a manual and start doing high rpm launches and high rpm gear shifts you may blow out a piston or melt the tops.

so you want to be careful in this case if you flipped the distributor 180 removed the timing plate turned it 180 then you would be using the 12L on the stopper.

that is 12X2 = 24 + 14 that puts you at 38 total timing a much safer number.

a lot of guys hunt for 10L and 8L plates for the distributor in the junk yard or they start welding up there distributor mechanical stopper plates and cutting custom slots in the metal.

now even the original plates can be horribly out of wack, for example my distributor plate was 16L but it was acting more like a 18L i used a piece of rubber tubing on the stopper to limit total mechanical movement down to around 12L

so when the engine builder gave my engine back and he claimed he dynoed it, i was getting total timing around 56 Degrees and i had to seriously start to pull the total timing back down into the 35-40 range. the sucker was pinging like CRAZY.

just another thing to worry about when you start messing around with the aftermarket.

basically you get 2 ends of the spectrum

you keep advancing timing until the car Diesels when you shut down, or it won't start hot, then you have to knock down total timing or hit 5400rpms and KAboom.

oh how i wish i could go back to a stock 351v4 configuration would make street driving so much better.

the performer intake causes other issues as well, you get fuel vapor lock from it as well since aluminum conducts heat faster then the original cast iron intake.

after reading books and books on the subject about how a dual plane is better then a single and how you get more low end punch doing ABC verse DEF,, or all the Pandora's box of problems you start to open up when you start ripping the original parts off for the sake of seeing a 5 hp increase, for the street and keeping the car 100,000 miles keep it stock if you can.

I still fart around with tuning 5 years later looking for that perfect running motor that i will never attain compared to how it was from the factory. The aftermarket is great when you need to maximize performance for the strip or high rpm constant running, these 2 situations will never occur on a street sitting in traffic averaging 30mph a year.

but anyway if you bump the initial timing just go in little steps if you don't see a change in hesitation then turn the timing back down and add more fuel.

 

[travisgm]

Thanks for the advice, The timing will be my first step when I get to the car again.

One other question for the vac advance, what port shoud I be connecting it to on the front of the carb, the passenger side ported vaccum port or the driver side full-time vaccum port?

I had read that I should be using the ported port due to the low compression of the motor, is that right?

 

[72HCODE]

you want to connect the vacuum advance on the 71-73 mustangs to the ported vacuum port.

it had nothing to do with the compression of the motor it has to do with the configuration of the mechanical advance in the stock distributor and the emissions laws at the time.

at Idle rpms the carburetor is configured so that vacuum is 0"HG through the ported port. this turns off the vacuum advance at idle. this means at idle only the mechanical advance is working which at idle rpms should be adding 0 degrees of advance so the motor is only using the set initial advance which stock is between 6-8 degrees. this lower timing forced the exhaust temperature to be hotter, thus at Idle the emissions from the car was reduced due to higher temperature burn off of the car exhaust.

you can run the car off full manifold vacuum if you want want, what will happen is at idle the vacuum advance will now be turned on since the vacuum at idle can be as high as 25"HG the advance might be fully on, this will significantly raise the idle RPMs and change the tune of the car, you may have too much timing for the range of throttle 0-30mph.

so you would have to retune the car to use full manifold vacuum.

At idle your exhaust temperature will be low you will also have more timing this will make the car more punchy and have more power off the line, because of all the adding motor timing advance. you will lose some MPG and when the car runs hot it will get very bitchy and be a pain in the butt when sitting in bumper to bumper traffic.

there is another thing ported verse full manifold vacuum was used for.

If your car had air conditioning Full manifold vacuum to the vacuum advance was used as a engine over heat fail-safe.

i wrote an article about the Distributor Vacuum control valve.

http://www.7173mustangs.com/thread-under-the-hood-mysteries-distributor-vaccum-control-valve

basically this was a heat sensor switch that changed the vacuum going to the vacuum advance in the case of engine overheating at idle.

when a A/C car would overheat sitting in traffic the DVCV would turn on switching ported vacuum to full manifold vacuum at idle.

this would raise Engine RPMS and increase the cooling Fan speed of the motor, this would cool down the radiator faster thus lowering engine temperature which would cool the motor and turn off the DVCV returning the car to ported vaccum at idle. it was sort of like an emergency overheat fail safe to protect the motor.

many people ripped the DVCV off the cars thinking it was part of the evil emissions system. It sort of was but it was a good thing.

 

[Jeff73Mach1]

Well, I learned something.

I check total timing at 3000 rpm's with an adjustable timing gun. I am at 36 degrees total advance. I am running an HEI distributor from Performance Distributors in Memphis.

I did not mean to imply that anyone should just set their timing to 16 degrees initial, only that it is working for me.

Being mildly familiar with the vacuum port issue, almost any car set up for performance will be happier with full manifold vacuum to the advance canister.

 

[72HCODE]

Yup every motor is different at one point I was running 18 degrees of advance myself and well my motor began to diesel, so I backed off. What I learned from that was performance verse drivability is a trade off as well.

The deal with ported verse full manifold comes down to how you curve a distributor.

Most aftermarket performance distributors are designed for full manifold vacuum and take advantage of it. The problem is when you start throwing the rules of performance onto stock distributors which were design after 1968 to use ported vacuum to comply with government regulations.

When people say hey my car runs cooler and has more power under full manifold vacuum verse Ported this is true. But you have to understand the trade off will be at the fuel pump and at emissions testing that some states still require on cars older then 1960.

Full manifold vaccum will advance timing at idle which lowers exhaust tempatures, increases nox emissions at idle, and may lower mpg because you may need to curve more fuel to dump during acceleration to prevent pinging or detonation.

The other issue depending on configuration of the motor might be low speed drivability. It may be more susceptible to a surge when holding idle in traffic.

That's the trade off when you go down the performance route with a distributor and carburetor. When you change one area it effects something else.

I used to have performance mods to my motor pretty much everything in the book, Msd ignition, headers , hot cam, over 3000rpms it was great. Sitting in traffic or tring to daily drive was a nightmare 2500 stall converter on a c6 urg....

Every time I took something off and went back to stock I dropped performance but the car became easier to drive and less whiny.

It's a choice we all make either you start chasing 9 second 1/4 miles or try for 200,000 miles on a engine rebuild

Well, I learned something.

I check total timing at 3000 rpm's with an adjustable timing gun. I am at 36 degrees total advance. I am running an HEI distributor from Performance Distributors in Memphis.

I did not mean to imply that anyone should just set their timing to 16 degrees initial, only that it is working for me.

Being mildly familiar with the vacuum port issue, almost any car set up for performance will be happier with full manifold vacuum to the advance canister.

 

[mustbrad]

Clevelands love timing I also frequent 351Cleveland.net and a lot of the post talk is about timing these guys know there stuff about Cleveland from stock to race. I also run 16-18 degrees no issues at all except the stumble from off idle which I now believe is the Edelbrock. Just to clarify this is in no way a shot at any info on this site between this site and theirs any answer should be solved.