thoughts on low vacuum?

[luna negra]

Just swapped in a new to me 350 in my K5. Never heard it run before today but was told it had a bigger cam in it from seller who I trust. He had no idea of the cam specs.

Getting around 12 Hg at idle but the motor sounds happy with the timing set at 12 initial and 36 total. Vacuum gauge is hooked up to manifold port for the tranny modulator. Fires up at first click of the switch. Here's a video and pic of setup. Thoughts if vacuum is too low or would a bigger cam be a cause of no concern?

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[PrairieDrifter]

Big cam, and no pcv system would be some factors. Have you tried lower initial timing? Valve adjustment would be another thing.

Also please change out that fuel filter!!! Get a wix, plastic or metal! You're gonna burn your truck down. It's gonna smash on that valve cover perfectly.

 

[dvdswan]

Bigger cams do have less vacuum. Closer intake/exhaust valves opening/closing. I've heard roller cams can help with this but I have no experience with it.

 

[RustyPile]

Longer duration, additional valve overlap -- all lead to low manifold vacuum.. Cams of this design tend to make their best power and torque in the upper RPM range..

 

[luna negra]

Initially set base timing at 10 but it sounded happier at 12. Also, 36 degrees total timing was at 3800 RPM. I only checked the vacuum one time at 12/36.

10-4 on fuel filter - hasn't run in awhile and wanted to see if I was getting crap from the tank. Will change before I'm roading it.

 

[75gmck25]

Most big cams need more timing to operate effectively. I would advance base timing to at least 12 at 650 rpm, but not more than 16. Then check all-in timing (base plus mechanical) at about 3500 rpm to verify it does not go higher than 34-36. Also check the vacuum advance to see what it adds.

 

[Ricko1966]

Cams with lots of overlap cause low vacuum. If you can't find any vacuum leaks,and are happy with how it runs,all is good. If you want higher vacuum readings,you will have to change out the cam. Run it drive it see how you like it,if the low end,mpg and power at the RPM you most often drive at aren't good enough for you,change the cam.

 

[Snoots]

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[Ricko1966]

K. I'm going to explain why valve overlap,lowers your vacuum reading,which makes a normal vacuum chart reading deceiving. If you stick a straw in a drink and suck on it you get a mouthful of whatever your drinking,put a hole in that straw above the liquid line and draw,depending on the size of the hole you get less drink or no drink. Your vacuum gauge reading works by the piston is traveling down intake valve open,exhaust valve closed,creating suction. The throttle plates are closed so it draws on the gauge and you get a reading. Open the throttle plates a little now the suction can go somewhere else,so your vacuum gauge reads less. Create a vacuum leak well some of the suction can draw there,so you get a lower vacuum reading. Put in a cam that hangs intake and exhaust valves open ( valve over lap) and during that period of overlap the exhaust valve becomes a vacuum leak,the piston down stroke can draw through both valves during that period,so the gauge on the intake reads lower. Its normal and the only way to change it would be change the camshaft. So don't jump to any conclusion based on a vacuum reading on a modified engine. If you are running the distributor to ported vacuum,try switching it to manifold vacuum. That should bring up idle speed. Now readjust mixture and lower the idle speed with the throttle stop screw. Closing the throttle plates further will bring up your vacuum reading.

 

[luna negra]

Thanks for the responses. Drove it around the pasture a little today and it hesitates when getting into throttle under load. With RPMs up, it runs great. Need to fiddle with the carb to get that dialed in but still running only headers with a 12" collector pipe. Need to get some exhaust on before I start a fire out in the fields.

 

[Ricko1966]

Thanks for the responses. Drove it around the pasture a little today and it hesitates when getting into throttle under load. With RPMs up, it runs great. Need to fiddle with the carb to get that dialed in but still running only headers with a 12" collector pipe. Need to get some exhaust on before I start a fire out in the fields.

I wouldn't immediately assume it's the carb,there's a good chance you need lighter springs in the distributor.

 

[luna negra]

I bought a cheap chineseium distributor just to get it running that didn't come with extra springs. What would be the best way to determine a spring vs carb issue?

 

[Ricko1966]

I bought a cheap chineseium distributor just to get it running that didn't come with extra springs. What would be the best way to determine a spring vs carb issue?

If your cheap Chinese distributor takes GM HEI parts buy a recurve kit,I'm going to look up the part number for you and post it so check this post in a bit. The reason I'm suggesting this particular kit is it gives you the ability to change how fast the centrifugal comes in,what vacuum the vacuum comes in and a separate stop to limit the amount of vacuum advance. Instead of a compromise on the vacuum advance,trying to use just the can for the rate and amount. And it comes with good instructions. Anyway performance tuning,it's ignition first,then carbureator. Biab Crane 996001

 

[luna negra]

Ok, I dbl-ck timing this weekend then buy the kit. Thanks for the part #.

 

[luna negra]

Crane 996001

Ricko, not seeing this kit available anywhere - it's sold out or not available. Any other suggestions or just get a lighter spring kit?

Went back and adjusted total timing to 36 degrees at 3000 rpm instead of 3800 and it's more responsive but still feels sluggish to about 1800-2000 RPM, then it kicks in.