Weird hesitation issue

[FiveSeventyZee]

So I just picked up a 71 4 door 510 and it starts and gets me around every time. but it has this weird (to me atleast) problem where if I stomp on the pedal it does nothing and returns to idle....if I push the pedal down slower it pulls hard all the way to 5k rpms which is the highest I really dare rev this thing. the hesitation is sometimes short then itll kick in and sometimes it will just stay at idle until I let off the gas and press it again (slower)

 

Technical info (to the best of my knowledge)

L18 (I'm assuming stock internals/head/cam)

weber carb. not sure what type. it's a downdraft type but that's about all I know

 

no real engine mods that I know of.

[yello620]

Some people are going to tell you that this is just the typical weber bog.

 

Yet it doesnt have to be that way, when the timing and carb jetting is happy, this is not a problem.

[datzenmike]

With engine off.. look down the carb and work the throttle. You should see a strong squirt of fuel into the primaty barrel. This is the accelerator pump and helps transition from idle to part or full throttle be adding raw gar to richen the mixture. Without the accelerator pump the sudden opening of the throttle will cause a sudden vacuum drop in the intake. Because the engine is turning so slowly at idle there isn't enough air passing by the venturi to cause a vacuum to draw more fuel in. The engine will experience a sudden lean bog and stutter and often will not rev up properly. Adding throttle slowly allows the motor time o rev up and draw more air in past the venturi to et things going.

 

The fix is to replace the accelerator pump or get it working.

 

When the accelerator peddle is released, the pump piston moves up and sucks fuel in from the fuel bowl past a one way valve. The valve prevents gas from returning to the fuel bowl. When the throttle is opened the pump piston moves down and pushes the gas out a nozzle into the primary barrel past another one way valve. This valve lets fuel out but when the piston moves up to draw another shot of gas in it closes to prevent air being sucked back in.

[Laecaon]

Also pull the primary idle jet, and the fuel/air mixture screw. Spray carb cleaner into the idle jet opening until it leaks out the bottom mixture hole.

[ggzilla]

5k rpms which is the highest I really dare rev this thing

 

The L18 is an OHC engine, which will run somewhere above 7k in stock form. Do not be afraid.

 

 

When you are looking down the carb throat, slowly move the throttle. You should see a steady stream of gasoline. If instead it drips, dribbles, or starts/stops, then the acclerator pump needs replacing. It is part of the normal $20 rebuild kit.

[banzai510(hainz)]

sometimes the accel pump lever can get stuck in the IN mode.

But sound like a a Accel pump iisure to me.

 

weber DGV you never really want to just go from idle to slamming the gas as it will not create enough vaccum at low RPM. stock carbs have a vacuum 2nd to prevent this

[FiveSeventyZee]

thanks for the responses guys. I'll check out the accelerator pump when I get a chance. that sounds like the culprit.

 

as for ggzilla, my car starts to vibrate at 4....so 7 seems dangerous to me. but it's good to know my engine can handle way more abuse than I'm giving it.

[ggzilla]

It should not vibrate at 4,000 RPM. You might have a problem, something unbalanced.

 

Weber 32/36 can be slammed if tuned right -- but they often are not. The stock hitachi doesn't have vacuum secondary either, but instead has a counter-weighted air flap to do the same thing. Weber 32/36 is not as sophisticated as the hitachi.

[FiveSeventyZee]

Well, I don't know if vibration is the right word...it just gets loud, like sound vibrations....if that makes sense. it sort of reminds me of the sound my buddy's dc5 makes when it does the whole vtech thing. don't worry I'm not a Honda/vtech guy :P