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question about vacuum
#1
Hey guys, I was just wondering about normal vacuum readings at idle. Im having a hard time tuning my carb. I have a 351 cleveland with a comp cams 280H magnum. My vacuum at idle is around 15-16 hg. Also I forgot to mention i have 2v heads. When I shift to drive i have only about 5hg. Is this normal?
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#2
Not too surprising with that cam profile. The big drop off is a bit weird. You might check for vacuum leaks.

'Mike'
73 Convertible - 351C/4V CC heads/4bolt/forged flat tops/comp 270/rhodes/mallory unilite/tri-power/hookers/glasspacks/c6/3.50 limited slip/Gear Vendors/Global West sub frames, strut rods and shelby style traction bars/ Rear sway bar/tilt steering (not original)

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#3
What is the RPM when you are trying to tune your carburetor? If it's too high it will make tuning difficult. As will e mentioned vacuum leaks will also make it impossible to tune.
What RPM drop are you getting when you shift into drive?



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#4
Are you running a stock torque converter with that cam?

Live, Laugh, love
ECTA, Ohio mile Landspeed record holder
C/GALT-136.36, AA/FALT 159.38 mph, New B/GALT Record 180.577mph- speed is the need !
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#5
My RPM at idle in park is about 900 and when put in gear it drops to about 650. My tourqe converter is a P330 10" diameter not sure about stall. C6 and 3.89 gears. The carb im trying now is an edelbrock 1406. It sounds great at idle in park but in gear it seems to be a bit rough. Cruising feels like its either surging or misfiring. What confuses me is that in gear with no throttle i get 5hg, then rev it up slowly and my vacuum gauge goes up slowly. Shouldn't it be the other way around. Snap the throttle in park at idle and the guage reads as it should. Could it be the step up springs need changing? My timing is at 18 initial to somewhere above 30 total, duraspark vacuum advance hooked to ported vacuum at carb.
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#6
Assuming the float level is correct, it sounds like the idle/light load/cruise circuit is lean. Are the idle mixture (really volume of mixture) about 1.5 turns out from seated? Does the vacuum gauge become erratic when placed in gear?

Chuck
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#7
I would move the distributor vacuum to manifold.

'Mike'
73 Convertible - 351C/4V CC heads/4bolt/forged flat tops/comp 270/rhodes/mallory unilite/tri-power/hookers/glasspacks/c6/3.50 limited slip/Gear Vendors/Global West sub frames, strut rods and shelby style traction bars/ Rear sway bar/tilt steering (not original)

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#8
first make sure all tuning and measurement is done with the engine warmed up, a cold verse hot engine is very different and you want to make sure all tuning is down when the car is at normal operating temperature.

next you want to make sure all tuning is down under normal load. in this case foot on brake or parking brake engaged and and for an automatic with the car in gear and headlights turned on.

turning on the headlights loads the electrical system and can affect idle.

so make sure when you tune Idle rpms with an automatic you have the car in gear and hold the brake.

what you will find is you might have a very high park RPMS but you need to compensate for the load down the transmission and accessories can cause with an aftermarket cam.

you are running a huge initial timing. depending on what you are trying to accomplish this could be the problem. Did somebody attempt to vaccum tune the motor?
are you running fixed mechanical for a turbo or super charger?

what CFM is the carb?
what spark plugs are you running?
what are the plugs gapped at?
do you have oem or aftermarket ignition?
what ignition wires are to you running?
do you have vacuum advance?

there are many different ways to approach this but there may not be a solution depending on configuration.

idle bleeds could be out of wack, butterflies could be open too far on secondaries.

on average timing is between 6 and 12 degrees more timing usually means more power but you do throw off the Idle as you increase timing.

now a vacuum leak is possible. you could have a blown diaphragm or a popped off hose or a seal leaking.
with the engine running you could try cupping your hands over the carb intake and choke off the air the engine should stall out, if it keeps running then air is getting sucked in from elsewhere and you usually can hear the vacuum leak.

when testing for a leak, you want to disconnect all vacuum accessories and cap all external vacuum tree feeds to weed them out as possible leak sources.
if you have a hand vacuum pump you can then test accessories one at a time and see what is leaking.

take a vaccum reading with the engine warmed up in gear and get a base line, cap all vaccum lines and retest and see if there was a change.

you can try to choke the carb and stall the engine to check for manifold leaks, if you suspect a leak you can spray around the base with carb cleaner and see if you have a change in idle rpms that would point to a leak.

a PCV that is clogged or incorrect can cause problems, during testing the brake booster, distributor vaccum advance and pcv should be capped off to root them out as a leak source.

if no change then you can move towards tuning changes and away from a leak.

on the tuning end i would back off initial timing check the ignition system for problems and inspect the mechanical advance to make sure it is operating correctly.

then all tuning would be under load, lights on car in gear.

when i first started to tune my car i was getting 13" in park and 7-9" in gear.
i had to change plugs, coil, ignition wires, i changed to points during testing.
i also had failures in parts i thought were good and turns out not to be.
i had to replace my brake booster, and A/C vacuum canister as they caused problems.
I found i had ignition problems even with new parts.
then it came down to setting rpms under load. on my car i have to run over 1000 rpms in park because it drops heavy under load down to 700-800rpms.

when you rebuild engines and go with aftermarket cams you can take the ford oem specs on how the engine should be setup and throw it in the trash can. you have to go by what your engine is telling you.

in your case there are still many questions, and then there is the direction you want to go with the car, tuning for traffic and normal street driving is different from performance and there is trade offs.
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#9
c9zx;198853 Wrote:Assuming the float level is correct, it sounds like the idle/light load/cruise circuit is lean. Are the idle mixture (really volume of mixture) about 1.5 turns out from seated? Does the vacuum gauge become erratic when placed in gear?

Chuck

I checked the floats and they were correct. I set the idle screws to 1 1/2 turns out from seated when i first cranked it and then i adjusted them for highest vacuum without RPM Increasing. Which resulted in me turning them out mabye another full turn. When I put it in gear the needle on the vacuum gauge does shake. In park it does not. I should also add that when I use manifold vacuum on the distributor, the idle in park goes through the roof. I've tried adjusting it but every allen wrench i stick in there doesn't seem to connect with any adjustment screw. The distributor is a remanufactured motorcraft electronic, I believe for a 1974 460. The distributor has the duraspark rotor, adapter and cap. It does drive better with manifold vacuum but when you put it in park the idle sounds rediculous. The car also tries to pull you through the stop lights.
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#10
Cleveland's do like a lot of initial timing. I found mine worked better using manifold vacuum since it added timing at idle.

'Mike'
73 Convertible - 351C/4V CC heads/4bolt/forged flat tops/comp 270/rhodes/mallory unilite/tri-power/hookers/glasspacks/c6/3.50 limited slip/Gear Vendors/Global West sub frames, strut rods and shelby style traction bars/ Rear sway bar/tilt steering (not original)

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