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Recurve a Distributor - Who Does it?


Ron1200

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I'm going to run a Weber 45 DCOE 13 on an L20 with a matchbox dizzy. The info I've been able to glean from the forums suggests a vacuum advance is not needed. Just run the static timing and the mechanical advance.

 

This matches the documentation that came with the Weber. The documentation states: " Weber carburetors are not compatible with vacuum advance distributors, therefore, distributor should be recurved for all centrifugal advance."

 

So where do I go to have a distributor recurved? I'm in Olympia WA.

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Truth is, part throttle requires more advance. Unless you drive WOT all the time, you will lose fuel economy. Just because a carburetor does not have a ported vacuum fitting does not mean you cannot use manifold vacuum for the distributor.

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I'm kinda dedicated to this option. I already have a cannon 'single' manifold and the carb. So I'm gonna give it a go.

 

"Truth is, part throttle requires more advance. Unless you drive WOT all the time, you will lose fuel economy. Just because a carburetor does not have a ported vacuum fitting does not mean you cannot use manifold vacuum for the distributor."

 

Daniel: So how does this work?

 

 

I dont think you want to do this. I seen to many problems with this hooking to the manifold.

 

 

Hainz: What kinds of problems do I need to be aware of?

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Hainz: What kinds of problems do I need to be aware of?

 

If you hook it up to manifold vacuum ,,it will basically work the opposite way in which it was intended

It will have alot of advance at idle but none when you give it the gas,,like going up hills and such,,

 

 

Ron1200,,, PMed you a number

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Hainz: What kinds of problems do I need to be aware of?

 

lots of pinging.Youl advance to much plus it will be pulsating as your going to run it off one intake runner right or both? either way it will give a pulsating response.

 

Fqar as I know webers were never made for this option and was considered a race carb and best get the dist ributor curved. Or just run in as I would and put the dizzy a 2-3 deg more advanve and see what happens.

YOu already got a centrufical advanve also and hooking up to the Manifold would just pull it even harder.

 

 

I hooked up one once when I dad alot of blowby on a old motor with my Mikunis and I had a port on one runner. All I heard of the clicking of the PCV valve going backand forth. I removed it. Maybe it was for a power brake port.

I had a Mikuni Toyota set up and all the ports went to a hose to PVC valve. I guess that would be the correct way. But who cares.

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From what I've been able to understand, ported vacuum from the carb is - nothing at idle and air moves past the throttle plate this is where the vacuum comes from. But when hooking up to the manifold you will have full vacuum at idle and nothing at WOT.

so at idle - full vacuum advance and no mechanical advance

at WOT - no vacuum advance and full mechanical

just flips your vacuum advance curve backwards

 

Have a single DCOE on Dallas' 521, messed with the matchbox for a while and just decided to go MegaJolt JR. Bit of a jump but it is working out well so far.

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Ported vacuum from a carburetor usually has vacuum at the port almost immediately off idle. Yes, manifold vacuum to the distributor will make the vacuum advance go "all in" at an idle, and that in turn will cause the idle to go faster, but you turn the idle back down with the throttle stop (idle speed) screw. Manifold vacuum used to be used for distributor advance, until emission requirements caused a change to ported vacuum.

 

Many factors affect how much ignition advance an engine needs at varying speeds and loads. As the engine speeds up, the spark has to occur sooner (more advance) so peak cylinder pressure occurs at the proper time after top dead center. But you may have notices centrifugal advance is "all in" by about 1/3 to 1/2 of maximum engine RPM. This is because the turbulence in the combustion chamber causes the burn rate in the cylinder to go faster at higher RPM, and I am sure there are other factors.

Like wise, the more air and fuel allowed into the cylinder, the faster the burn rate. This is why cars have vacuum advance high vacuum, slow burn rate. You will also notice on many boosted engines, (turbo or supercharged), the timing is set up to actually retard as the manifold pressure goes from below atmospheric pressure to above it.

 

Ignition advance has a huge affect on how an engine runs. Our simple old Datsun engines have basically only two tools to control it. centrifugal and vacuum. By eliminating vacuum advance, you are eliminating one of those tools. And if you drive the car on the street, you are probably needing vacuum advance as much as the centrifugal.

 

At WOT, you do not need vacuum advance, and it is not available at either the carb, if the carb is equipped with a vacuum port, or at the manifold. This is fine for a race only application, but not as good on the street.

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Wow, lot of misinformation here. Keep it simple. No engine needs vacuum advance -- all it does is give you more power at slight throttle openings.

 

If you really want to make it work with the DCOE, you can use manifold vacuum (DCOE does not have a ported vacuum tap). To use manifold vacuum, you should recurve the distributor. I wouldn't bother.

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Ported vacuum from a carburetor usually has vacuum at the port almost immediately off idle. Yes, manifold vacuum to the distributor will make the vacuum advance go "all in" at an idle, and that in turn will cause the idle to go faster, but you turn the idle back down with the throttle stop (idle speed) screw. Manifold vacuum used to be used for distributor advance, until emission requirements caused a change to ported vacuum.

 

Many factors affect how much ignition advance an engine needs at varying speeds and loads. As the engine speeds up, the spark has to occur sooner (more advance) so peak cylinder pressure occurs at the proper time after top dead center. But you may have notices centrifugal advance is "all in" by about 1/3 to 1/2 of maximum engine RPM. This is because the turbulence in the combustion chamber causes the burn rate in the cylinder to go faster at higher RPM, and I am sure there are other factors.

Like wise, the more air and fuel allowed into the cylinder, the faster the burn rate. This is why cars have vacuum advance high vacuum, slow burn rate. You will also notice on many boosted engines, (turbo or supercharged), the timing is set up to actually retard as the manifold pressure goes from below atmospheric pressure to above it.

 

Ignition advance has a huge affect on how an engine runs. Our simple old Datsun engines have basically only two tools to control it. centrifugal and vacuum. By eliminating vacuum advance, you are eliminating one of those tools. And if you drive the car on the street, you are probably needing vacuum advance as much as the centrifugal.

 

At WOT, you do not need vacuum advance, and it is not available at either the carb, if the carb is equipped with a vacuum port, or at the manifold. This is fine for a race only application, but not as good on the street.

 

Excellent.

 

 

Wow, lot of misinformation here. Keep it simple. No engine needs vacuum advance -- all it does is give you more power at slight throttle openings.

 

 

I would change this to "No engine at full throttle needs vacuum advance" There that's better. 99.99999% of the time we run at less than full throttle so vacuum advance is a great way to increase mileage and part throttle performance. A weak cylinder filling takes longer to burn and needs more advance so that it's at maximum pressure at about 17 degrees after TDC. This is the 'sweet spot' where it most efficiently pushes down on the piston as it accelerates down the bore. Using only static and centrifugal advance which is later (but perfect for full throttle/full cylinder) the hot gasses miss the mark and simply expand into the cylinder chasing a rapidly descending piston. Much of the heat energy ends up being absorbed into the water jacket around the exhaust ports and the motor runs hot.

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No engine at part throttle requires vacuum advance. At full throttle, it's not even possible to use vacuum advance.

 

I challenge you to remove the hose and plug it, then drive your datsun. Tell us if it is then undriveable, or if it works close to the same.

 

That being said, it will give more POWER, more FUEL ECONOMY, and is a GOOD thing in general. I run vacuum advance on all my engines except my lawn mower. I have also run my Datsuns without it, tested it both ways..

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Hmmmm, well I have run my truck's L-18 and my Goon's L-16 on full manifold vac and can tell ya they both respond better in city driving (at light throttle where you would be driving in traffic and doing erands, the cylinders are NOT being filled and the extra advance just off of idle and such is a good thing for that responsive feeling and power & economy). Getting deeper into the throttle drops the vacuum (relaxes the vac adv diaphram ) back to near static so the engine doesn't ping.

Car's ran full manifold vac adv for years before emissions controls made them change it.

 

Nothing wrong with running the engines either way, ported or full vac, try it both ways (keep your static timing same, just might have to lower idle speed with full vac though) and see what your engine likes best :D

 

--Ray

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My F250 has a 460 with the vacuam can plumbed to the manifold from the factory. But it never see's more than 2800rpm. Apples and oranges my friends.

 

You cant get a good vacuam signal from one side of a dual throat carb (or 2). If you plumb them together, there wont be enough air moving to make anything happen.

Isolated runner manifold vs plenum.

 

 

Lose the can, get a good timing light with an advance/retard feature, Set the timing at 36 to 38 degrees at 3k rpm, lock it down and let it idle where it does. If you are putting a 45 dcoe on a motor, I'm guessing you are doing it for performance. So 3k and up is all that matters.

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Ray,

YOu run them on full manifold vacuum? I have fixed more cars by telling people to get RID of them.

 

GGZilla is right ,I have run them wit a Vacuum adv with down drafts and pull the hose and they seem to run the same.

 

Side drafts Dont really need to run them But my dual 40mm Mikuni set on one car it has a port. But thinking what is the porpose of one unless it hardly works as it would give a pulsating pull on the timming if at the the runner of the manifold. Meranign at idl the thing would be pulsating 1 out of 4 times or 2.(what Eric was talking about)

 

As Datzenmike sauys if you got a down draft and have it use it. and hope your dizzy is half way good.

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I have fixed more cars by telling people to get RID of them.
So have I, but that's because they are broken, like most of the matchboxes. If the distributor is in good shape, getting rid of the vacuum advance will slightly hurt. But still run OK.

 

You can get a good signal from ITB such as twin-DCOE manifold, Take a signal from each runner, run the into a vacuum reservoir, which has to be big enough to deliver a valid vacuum signal.

 

Only if the camshaft is long duration is vacuum advance prohibited.

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Eric

well only had only 1 port and assume the carbs were a matched set.Other carb in back had noting to hook up, So I thought it was for the vac adv

 

I know the toyota 18RG had 40mil Mikunis so I fured it was more a street set up and a port for the dizzy

 

I was just guessing really. I have plumbed isolated runner intakes for power brakes, but not for advance cans. Being a Weber guy, I never messed with vac cans on multiple carb setups.

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Ray,

YOu run them on full manifold vacuum? I have fixed more cars by telling people to get RID of them.

 

GGZilla is right ,I have run them wit a Vacuum adv with down drafts and pull the hose and they seem to run the same.

 

Side drafts Dont really need to run them But my dual 40mm Mikuni set on one car it has a port. But thinking what is the porpose of one unless it hardly works as it would give a pulsating pull on the timming if at the the runner of the manifold. Meranign at idl the thing would be pulsating 1 out of 4 times or 2.(what Eric was talking about)

 

As Datzenmike sauys if you got a down draft and have it use it. and hope your dizzy is half way good.

 

Yes.....like I said it's not that different from ported vac, the distributor just has extra lead during those low speed times when the engine could use it. Mileage goes up a bit AND throttle response is notably better....full throttle is not effected since the vac signal is lowered but frwy cruising is about the same as ported (of course).

Emissions did away with manifold vac not practicality :)

--Ray

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