Carburetor 4MV quadrajet on Mercruiser 228

When I adjust either of the tw

When I adjust either of the two idle adjusting needles in or out it has no effect on the engine revs. I'm told that there is probably a blockage. I've removed both needles and tried to clear the passages with piece of thin wire and WD40 but to no avail. Any advice on what to try next? Will I need to remove the carb. from the manifold? Apart from this problem engine runs fine but does tend to cut out if tickover under 900 revs. Thanks in advance for any info.

It seems the carb's idle c

It seems the carb's idle circuits are plugged w/fuel varnish from old fuel. I am afraid that you have no choice but to rebuild/replace the carb. It builds up over time from unstablized fuel.

Fuel begins to deteriorate w/in 60 days of manufacture. After that the fuel begins to turn into varnish and a non-flammable liquid. You can smell the strong varnish odor. If you don't buy fuel from a high volume sales vendor and the vendor did not use a stabilizer then you are buying old/bad fuel.

I always add marine fuel stabilizer to the fuel that I purchase which will keep it fresh for one year. Double dosing the fuel w/stabilizer can increase the stability of the fuel for up to two years.

If you can check for a vacuum

If you can check for a vacuum leak I would do that first. If not then maybe a carb rebuild is in order. If carb is goos and no vacuum leaks you should be able to idle down to 400 rpms easily.

"Guy and Kghost, Thank you bot

"Guy and Kghost, Thank you both for your help."

frank norris
At what RPM are

frank norris
At what RPM are you trying to adjust the idle circuit needles?
Bert

Curious to hear that answer.

Curious to hear that answer.

900+ RPMs on a small block has gotta be well into the transfer slots.

"Bert, At 900 rpm. I can get

"Bert, At 900 rpm. I can get the tickover down to about 700 rpm but the engine is struggling to keep turning over at anything less than 900 revs. Makomark, Could you explain your comment in more detail. Thank you both for your help."

"As you turn up the idle screw

"As you turn up the idle screw to increase rpms, the butterfly plate inside the throat of the carb is opend and exposes a slot in the wall of the throat. as that slot gets exposes it generates a vacuum signal and will allow fuel to be drawn into the carb and it will over ride the ability to adjust the air/fuel mixture screws.

This is the normal sequence of events when accelerating. Idle, Off idle (part throttle) mid and wide open."

Guy has it right. Your idle ci

Guy has it right. Your idle circuts are plugged and your trying to idle off of the transfer circut. Rebuild/Replace the carb.
G

"I'd check the main and id

"I'd check the main and idle air bleeds first - make sure they are open and not gummed up.

As noted above, when idling "fast", the carb isn't providing fuel as designed. Another item that is usually "fixed" with increased idle speed is retarded timing - may want to check it as well as the gap if the conversion to electronic has not been done."

"Thank you everybody for the a

"Thank you everybody for the advice, Will let you know in due time how I get on."