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Old 08-24-2009, 05:30 PM
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Edelbrock to Q-jet conversion

I just converted my 72 Chevy 350 from an Edelbrock 600 cfm with EPS Performer intake to a stock GM aluminum intake with a Jet Performance Stage II Quadrajet. I tried using a carb adapter but was not getting the performance so I changed manifolds. The Q-jet does not have the throttle response that the Edelbrock did. I was using a 4 hole spacer on the Edelbrock but don't have one for the Q-jet. I plan on using a spacer of at least 1" hoping to increase torque and throttle response. Is 1" enough with the stock manifold? Is there something else I need to look at to help cure the lack of throttle response when cruising. I set up the Q-jet for maximum vacuum. I am at about 16 to 17 inches at 700rpm which is slightly better than with the Edelbrock. I have not changed the timing. I am at 14 initial with 36 with vacuum and about 51 total with mechanical.

Another odd thing happened when adjusting the carb. It was at about 1 turn out when I got it. I did have a ported vacuum source but after I opened it up to 3 turns I lost the ported vacuum. It now has full manifold vacuum at the connection that was ported.
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Old 08-24-2009, 05:48 PM
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Vac advance should be connected to a manifold source. If the ported source became a manifold that sounds like the primary butterflies are open too far at idle. Move the timing back to 12 degree`s before top dead center with the vac advance disconnected and plugged, relock the distributor and connect the vac advance to a full time manifold vacuum source. I just use the one on the rear base plate of the carb that is right beside the power brakes vacuum.
Recheck the spark timing, it should have around 20 to 24 degrees at idle, this will aide in moving the idle speed down further closing the butterflies.
Even so the GM intake is not great, it works, the aluminum Q-jet intakes are known to be warped due to high heat from the EGR so it`s possible you could have a vacuum leak at the intake gaskets. If you remove the carb and the plenums full of oil you have a leak.
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Old 08-24-2009, 05:59 PM
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Are you confusing the basic difference in apparent engine throttle increase relative to your foot input of a QJET/spreadbore design with small diameter primarys compared to a square flange carb with much larger primary throttle diameter?
The much smaller primary throttles will take much more foot input to get the same engine airflow increase. The foot input/throttle progression is a lot different. Takes a bit to get used to when you switch designs.

Edelbrock makes a Performer RPM that allows direct bolt on of a QJET carb.
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Old 08-24-2009, 08:57 PM
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The idle screw is all the way out. Idle is at 700. I will take some timing out but it was working great with the Edelbrock at 36 plus mechanical. I do have the vacuum advance connected to manifold vacuum using the upper port on the left front side.

Yes it is the foot pressure that seems weak. I have lots of pedal travel to increase speed until I get the pedal pretty far down. I am worried about a loss of torque for pulling my trailer. I hope to get a little better gas mileage for the rest of the time.

The secondaries were sticking when I got it but I got them loose. Hopefully the rods weren't bent. I got them to open partially when I rev'd it in the driveway.

I will check for leaks when I install my spacer. I used the newer metal gaskets. I was not sure how good these work but it was hell to get the old ones off and these will definitely be a bit easier should I go the Edelbrock spread bore manifold.
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Old 08-25-2009, 06:47 AM
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If your referring to the idle adjustment on the side of the carb and it`s all the way out and it only idles at 700 RPM you have a problem somewhere. With the recalibrated idle system of Jet`s Q-jets opens the idle circuit up for engines that need more fuel than the stock Q-jet system could provide. Also with the idle all the way open it`s pulling fuel out of the primaries instead of the idle circuits indicating there`s a problem elsewhere. If you were referring to the idle adjustment screws they only adjust the air/fuel mix of the idle.
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Old 08-25-2009, 09:07 AM
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I am talking about the idle adjustment at the drivers side of the carb that is used to set idle speed once the air/fuel mixture screws have been set at the base of the carb. When I first adjusted the carb I opened up the air/fuel screws from about one turn out to 3 turns out and that sped up the idle. I then backed out the curb idle screw to reduce the idle speed. Right now it is out so far that it does not touch the throttle lever and it is idling at 700 rpm. I can't close it any more with the throttle linkage connected or with it disconnected.

The funny thing is that I had ported vacuum when I first installed the carb with the timing and everything the same as it was with the Edelbrock. Opening up the air/fuel screws seems to have cause the lack of ported vacuum. I thought it might have been that I didn't have the brake vacuum hooked up on the first run but I pinched the line last night and still didn't have ported vacuum.

There is another idle screw at the base of the carb on the passenger side the opens the primary plates. I don't want to touch that one if it was set by Jet Performance unless I have to. I did buy the carb used so I am not sure what adjustments were made but this adjustment in very rarely touched by the normal guy. I had ported vacuum with this adjustment as is.
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Old 08-25-2009, 09:26 AM
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Okay Mertz,, Now that I have a bit of a better understanding on what you mean lets see what we can come up with. It`s puzzling to me a ported source turned into manifold source with the butterflies in the fully closed position, so this makes me wonder if fuel is getting in from another place. With the engine at idle, shine a light in the primary side of the carb and see if you see fuel dripping or pouring out of the primary boosters. This also can indicate the spark timing is too high causing to high of idle.
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Old 08-25-2009, 10:56 AM
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Just so you know the area that normally drips on the Q-jet have been sealed and the bowls seem to stay full since it starts right up. Maybe something is wrong at the air/fuel screws. I can't figure why they were set at 1 turn out when normally they should be at least 2 1/2. I opened them up a little and got a big improvement in idle speed and vacuum. I will check for drips this evening.
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Old 08-25-2009, 12:14 PM
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I know what area`s drip also, the main wells and i`m not referring to them. My experiences with Quadrajets over the years have gone pretty far. I learned you can`t use epoxy to seal the main wells as they`ll still leak. The 2 sean murphy built Quadrajets I have both leak out of the main wells and both were epoxied. I learned on my own to remove the epoxy and restake the main well plugs that are only little balls with a hammer and a punch. afterwards I set the carb on top of a empty bucket, fill the bowl and wait to see if it leaks so I know which area to hit again, so far everyone I`ve done has worked. The Stage 2 Jet carb you have has a reworked idle circuit, in other words the passages were made bigger to flow more fuel than a stock quadrajet could pass for larger or competition engine. Since your engine is close to stock and now I give it more thought it could be the reworked idle circuit is too big for your engine and even with the blades completely closed the larger passages are giving it too much fuel, this is why it`ll idle at 700 with the curb idle out of the picture.
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Old 08-25-2009, 01:59 PM
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I think I will try turning down the air/fuel screws again and stopping when the vacuum start to come up and holds. That said I don't have a problem having it idle at 700 and since I am on manifold vacuum for the vacuum advance anyway I don't really need ported vacuum. I just need the best performance and mileage I can get out of it.
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Old 08-25-2009, 04:42 PM
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Are one or more of the throttle blades bent? (dropped carb)
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Old 08-25-2009, 09:18 PM
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The carb could have been dropped the secondaries where sticking and I got them free. The blades don't appear to be bent. I can check it when I pull it to put the spacer in. I tried the propane trick to see if I was getting vacuum leak I got no change in rpm. I took it for a test drive this evening and it ran fine and appears to be getting better gas mileage. It could be that the carb was partially gummed up. It is running better all the time. I did not check the idle speed today but it might be running a little faster. Vacuum was at 20 today.

I read somewhere that some Q-jets don't have ported vacuum. The only reason I want it to be sure that the carb is working right. The probably need more of a load to open the secondaries. I will check with Jet to see if they have a solution.
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Old 08-26-2009, 05:57 AM
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the secondaries won't open just reving in the driveway
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Old 08-26-2009, 01:53 PM
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The carb must have been slightly gummed up when I got it because although the secondaries aren't supposed to open up in the driveway they did slightly when I had ported vacuum indicating to me that there was a restriction causing the need for extra fuel.

I found a 1" spacer I am going to install this evening and see if it has any affect. I am looking for a little more torque so I got the 4 hole unit.
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Old 08-26-2009, 08:51 PM
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I talked to Jet tech and he said that the ported vac could be on the left side at the base plate. He wasn't sure why the right side port wasn't ported. He also wondered why I was using a stage II on a basically unmodified engine. He said they normally go on something with a bigger cam but thought it should work if adjusted correctly.

I pulled the carb to put in the spacer and found something that looked like oil or grease and water on the gasket where the plugs are that normally leak. That seems very odd unless the guy I bought it from left it outside or otherwise got some water in it. That could explain why it is running better the longer I drive it.

I tried power braking the engine and there was a significant stumble on the first try. It did not do it on the other tries. It takes off fine but really lacks power until I get up to speed and then it seems to have a little more power.

The spacer didn't seem to help much. No real noticeable difference. The mileage still seems to be good. My gas gauge doesn't drop nearly as bad as when I was changing rod on the Edelbrock. I'm still not happy with the performance.

BTW I put the Edelbrock carb on my Catalina 428 and it seems to be working fine.
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