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Discussion Starter · #1 ·
Hello thanks for looking, I recently just built a 82 Chevy small block 350 with 98 vortec heads. The motor fires and runs but think we have a vacuum leak because it won't stay idling long! It fires runs for about 5 sec then putters out! Will continue running if you keep on Throttle. We have installed a mother thumper cam and roller rockers. It has a quadrajet carb and lots of vacuum ports on it. Could someone please send me a diagram or something to tell me what I need for this motor to run reliabley without during out? It's not a street truck its 4x4 strictly toy off road. We have the vacuum advanced hooked up to what we think is the right vacuum to the carb. And the break booster as well. Any help or hints tips would be great thanks! If you need more info I'll do my best to answer them thanks
 

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Plug all the line(temporarily)set timing for 35 total at 3k.turn idle screw up to 1400 rpm( w/e it takes to keep running. adjust carb,turn down idle . check all tune up specs and continue down until it idles at 850 ish.
test for vacuum leaks with carb cleaner or other aerosol spray.
 

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Help on 350 small block vortec head build! Please!

If you think you have a vacuum leak, here's how to check. Using a can of starting fluid spray sparingly along the intake where the gaskets are. Spray around base of the carburetor too. If you have a leak the idle will increase. What type of intake do you have? When you installed the intake did you use any sealer on threads? Here is a link to an article to read about setting your timing. http://www.crankshaftcoalition.com/wiki/Hot_rodding_the_HEI_distributor
 

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Discussion Starter · #5 ·
I don't thinks it's quiet a leak but I think more I'm either pulling to much vacuum or not enough we aren't sure what goes where off the carb. Vacuume wise. And the intake is an off brand that is made for the 350 with the newer vortec heads and yes we sealed everything with gasket sealer thanks for the thread/ web site
 

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Discussion Starter · #9 ·
What about re tuning carb to have no choke I mean we can get it to stay running to warm it up. I can hold the pedal at let's say two grand and it will run there constant! But when you let off it will idle for about 5 seconds and it tries to stay idling but eventually dies out. So still leaning towards choke?
 

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What about re tuning carb to have no choke I mean we can get it to stay running to warm it up. I can hold the pedal at let's say two grand and it will run there constant! But when you let off it will idle for about 5 seconds and it tries to stay idling but eventually dies out. So still leaning towards choke?
Its not going to be a choke issue, I rarely have a choke on any of my junk, get it started, get it to 1500ish for a minute or 2, should idle down and stay running...and that's in chilly Canada climate and with the heat risers blocked. The mother thmpr is a pain in the ***, ask me how I know....get all the vacuum ports plugged, get your base timing to 24°, adjust idle down to 800, and see what it does....but yes, this is after you have plugged everything and confirmed no vacuum leaks....if it runs decent, and stays running, then we can worry about where all your vacuum should be routed. That cam is going to want alot alot of initial...
 

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Plug all the line(temporarily)set timing for 35 total at 3k.turn idle screw up to 1400 rpm( w/e it takes to keep running. adjust carb,turn down idle . check all tune up specs and continue down until it idles at 850 ish.
test for vacuum leaks with carb cleaner or other aerosol spray.
Didn't even see this Vinnie lol....oops.....
Yep....this will work....
 

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Discussion Starter · #13 ·
When you say plug all the vacuum ports you do mean everything right? Vacuum advance and vacuum for brake booster? Everything on the carb? Just wanted to clarify thank you
 

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I forgot about your vacuum lines. If I remember correctly, the Q-jet has a threaded fitting on the backside of the carburetor. This is where the vacuum line from the brake booster goes. The brake booster should have its own vacuum port, do not tee anything else to it. There should be a 3/8 port on the front of the carburetor for the PCV valve. You should have a vacuum port fitting that screws into the manifold behind the carburetor for your vacuum to modulator valve on the transmission. There will be other vacuum ports on the front for the distributor vacuum advance. Just remember the ports that have vacuum at idle are manifold vacuum and the ports that don't are ported vacuum. I have a picture from a website that might help you. Look at post #7. Good luck. PALEEZE help me hook up my QuadraJET and vaccuum stuff. - THE H.A.M.B.
 

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Discussion Starter · #17 ·
I forgot about your vacuum lines. If I remember correctly, the Q-jet has a threaded fitting on the backside of the carburetor. This is where the vacuum line from the brake booster goes. The brake booster should have its own vacuum port, do not tee anything else to it. There should be a 3/8 port on the front of the carburetor for the PCV valve. You should have a vacuum port fitting that screws into the manifold behind the carburetor for your vacuum to modulator valve on the transmission. There will be other vacuum ports on the front for the distributor vacuum advance. Just remember the ports that have vacuum at idle are manifold vacuum and the ports that don't are ported vacuum. I have a picture from a website that might help you. Look at post #7. Good luck. PALEEZE help me hook up my QuadraJET and vaccuum stuff. - THE H.A.M.B.
Ok man, got the truck running and idling ok..... Now we have another issue that's kinda stopping us. I tuned the carb I set the timming good it will run and stay running fine but now when I put it in gear it dies out.... I can keep it running in gear with the idle turnt up or foot on gas and keeping it running! As soon as I let off it kills. I have a correction it is a comp thumper cam NOT a mother thumper. We were told we shouldn't have to have a stall with this cam. Are there any tricks am I missing something? There are no vacuum lines to the trans nor cables atached to the motor/carb from the 350 turbo trans. Thanks for any input!
 

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Ok man, got the truck running and idling ok..... Now we have another issue that's kinda stopping us. I tuned the carb I set the timming good it will run and stay running fine but now when I put it in gear it dies out.... I can keep it running in gear with the idle turnt up or foot on gas and keeping it running! As soon as I let off it kills. I have a correction it is a comp thumper cam NOT a mother thumper. We were told we shouldn't have to have a stall with this cam. Are there any tricks am I missing something? There are no vacuum lines to the trans nor cables atached to the motor/carb from the 350 turbo trans. Thanks for any input!
The key to making the thumpr cam behave is more initial timing. I'd start w/about 22 degrees BTDC and work up or down as needed from there.

Vortec heads only need 32 degrees total timing, not counting vacuum advance. More on setting up the timing correctly is linked to below. Until you get the timing sorted out, you're going to have a hard time w/idle quality, nozzle drip, off idle response, etc.

Use a vacuum advance limited to 10 degrees, use ported vacuum for it.

If you find you're still having a problem getting a balance between idle quality and initial advance (advance isn't helping enough), you may need to add idle bypass air.

Not all Q-jets have a threaded rear port for a power brake booster. Use a manifold port if yours doesn't. The front large vacuum port is for the PCV valve.

Carb vacuum port ID

Advance curve

You most likely do need more stall, but sort the timing out first, then see what you need if anything.
 

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Discussion Starter · #19 ·
The key to making the thumpr cam behave is more initial timing. I'd start w/about 22 degrees BTDC and work up or down as needed from there.

Vortec heads only need 32 degrees total timing, not counting vacuum advance. More on setting up the timing correctly is linked to below. Until you get the timing sorted out, you're going to have a hard time w/idle quality, nozzle drip, off idle response, etc.

Use a vacuum advance limited to 10 degrees, use ported vacuum for it.

If you find you're still having a problem getting a balance between idle quality and initial advance (advance isn't helping enough), you may need to add idle bypass air.

Not all Q-jets have a threaded rear port for a power brake booster. Use a manifold port if yours doesn't. The front large vacuum port is for the PCV valve.

Carb vacuum port ID

Advance curve

You most likely do need more stall, but sort the timing out first, then see what you need if anything.
This truck is strictly an off road truck it will probably never come out of 1st and or 2nd gear... Fuel milagr doesn't bother me... So can I completely get rid of or plug off vacuum advance? Will this help the truck from dying going into gear? I'm sorry I'm kinda virgin to the tuning part of things. Thanks again
 

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This truck is strictly an off road truck it will probably never come out of 1st and or 2nd gear... Fuel milagr doesn't bother me... So can I completely get rid of or plug off vacuum advance? Will this help the truck from dying going into gear? I'm sorry I'm kinda virgin to the tuning part of things. Thanks again
You need more initial timing first. I edited my original post to include the vacuum advance. Having it might make the engine run a little better during the times you are under light throttle. If these times are rare, you can run w/o it.

If you are using a vacuum advance connected to manifold (full time) vacuum, this can cause the engine to stall if you don't have enough initial timing. When it goes into gear, the vacuum drops off. This can cause the vacuum advance to drop off as well, and you lose the timing it was supplying.

That's why I said to add initial timing- as much as the engine needs- then to use ported vacuum for the vacuum advance.
 
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