Carburetor hesitation

[72HCODE]

I've been on Cleveland net as well. The problem is when you have a combination of parts that falls outside the norm, then you stump them.

The biggest issue with Cleveland's is open chamber heads verse closed chamber heads.

The norm is running a Cleveland with closed chamber heads higher compression, the second you go open chamber on a Cleveland with modifications over stock you get into major problems. The compression is lower and adding over bore makes the issue worse. It seems this combination causes a major lean condition from 650rpms to 1500 Rpms. The vacuum is lower by 5hg as well. It seems at high Rpms this combination has some advantage but at low Rpms and street driving it's a problem. The heat range appears to be lower as well.

Open chamber lower compression is the way to go with a duel turbo or supercharger setup, and that would compensate for the low end lean issue.

The real way to figure out timing is to examine the spark plugs. Every motor is totally different. If I ran 18 degrees on my motor I would blow out my starter and have run on diesel after shutdown.

If you have a normal setup like 351-2v open chamber mild cam then your fine the minute you go v4 the larger intake volume causes all kinds of problems , then you couple that with larger exhuast and you get this weird combo of no back pressure and low vacuum and the engine is lean all the time at low Rpms.

If you go 351-4v with closed chamber heads again you are good, go open chamber and change the intake to edelbrock and you have Hesitation problems, I think the edelbrock might flow too well for street use, the original intake actually has smaller tunnel ports then the head ports this causes a disturbance of air and I think increases back pressure through the intake that would boost the fuel mixture going down into the pistons.

Cleveland net is great for telling you what parts to get and what combinations work. But when you come from left feild with problems they have no answers.

There are so many different exhaust manifolds with the same mold and cast numbers and one works and one may be slightly different and you end up in hell.

There are manifolds that appear to come from a 351 boss motor but it turns out they used them on bronco trucks the only way to tell is you have to ignore the cast numbers and measure the port size and length of the manifold. Get the wrong one and the motor won't run right.

It's such a fine balance. The best book I read said, ignore what works for other people go by what your engine is telling you. Once it's running well you start to look at the plugs to fine tune.

 

[mustbrad]

very well said and I will take this to heart thanks

 

[droptop73]

Well said. I have confirmed the lean transition condition on the 2v OC heads referred to above. To correct this condition I have decreased the size of the idle air bleed by .005" to make the transition slot more sensitive. It is much better in hot weather but it needs less air (more fuel) in transition when it's cool so restricting it another .002-.003" is on the to do list.

The 2v Edelbrock intake is considerably smaller than the head port size as well. My understanding is this has been done to increase flow velocity. When you make the runners smaller, the theory goes, you increase the speed of the "air column" which provides better signal at the carburetor and the mass of the "air column" at a higher speed would tend to fill the cylinder better (more air/fuel volume into the cylinder).

I am completely convinced setting the timing with a vacuum gauge is the way to go. This doesn't tie you into numbers for timing. If the engine "rattles" back the timing off a little. If you want to confirm your setting with a timing light so you know where it is that's good (I usually don't).

 

[72HCODE]

Vacuum tuning is the way to go, I have noticed a straight vacuum tune puts you on the lean side of the fuel air mixture.

I tend to back off the max vac about 2 Hgs to fatten the fuel a little. It seems to make the car more streetable.

 

[travisgm]

Just from reading about vaccum timing on various sites.

Would I be correct if I were to connect the vac gauge to a manifold port, advance until there is no more vaccum gain, then back off one inch hg?

This motor isn't going to be headed to any drag strips, so a "close to ideal" should be fine. Just like to be able to step on the gas and not have the stumble, while keeping the motor in a happy not too rich, not too lean condition.

Should I leave the vaccum advance connected or plugged for this procedure?

I have a timing light, but from what I have read, those "perfect numbers" aren't always perfect for every engine. Just a good idea to note them down when finished should a change or check be needed for the future.

 

[72HCODE]

for a vacuum tune you disconnect the advance, you want the idle air bleeds reset to 1.5 turns as well.

then set the idle rpms to something just under normal idle, like 600 if 650 rpms is normal to make sure the mechanical advance does not turn on.

some guys turn the engine rpms all the way down to 500 just before the engine stalls out and then start a vacuum tune keeping the rpms at 500 for each advance to the timing.

you treat it like setting the initial timing.

here is the kicker. as you advance timing the idle rpms will increase and vacuum will go up.

you have to reset the idle Rpms for each move of timing advance then you look for max vacuum reading and back off 1 or 2 degrees. to prevent too much timing when the motor is hot. you want to stay within reason as well, if you are at 22 degrees initial timing then something went wrong.

this is why many people just set initial timing at good enough and continue with a tune based that initial setting.

with a vacuum tune the key is keeping the idle rpms constant as you play around with timing and keep checking the vacuum gauge reading.

once your happy with max vac minus 1-2 HG to compensate for a hot start then you reset correct idle rpms, and now you have to continue to tune the carburetor idle air bleed screws which you reset to 1.5 turns. again you have to start the same procedure adjust idle rpms keep it steedy as you move the idle air bleed screw and tune for max vacuum... 2 hours later you may have found 1-2 HG of extra vacuum over stock.

about a week later after driving it in traffic you start to fiddle with the idle speed and air bleeds because of how the car drives low speed or sitting in traffic and that lumpy idle is driving you nuts. before you know it you sort of end up back at the stock ford settings for your car, a little on the Fat side with a lower Vacuum reading.

the only TRUE way of really perfectly tuning the motor is you need a Air/Fuel Oxygen sensor in the exhaust and a readout computer.

i've done a vacuum tune in the past for normal street driving its a waste of time better to do a standard tune up set everything to the manual maybe come up 2-6 degrees on initial timing get the idle air bleeds in the ball park and just drive it. and maybe adjust the vacuum advance as needed.

i wouldn't kill yourself trying to vacuum tune chances are that miss will still be there.

basically if slightly more advance didn't help already then either you need to get more fuel down the intake or start looking at ignition problems.

if you are using MSD 9-10mm ignition wires take them off and throw them in the trash and get normal ford oem wires, and reset timing.