Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

marine quadrajet idles not smooth

<< < (2/3) > >>

Cliff Ruggles:
With .090" mixture screw holes you need to make sure the idle mixture screws are actually seating.  Did that carb have mixture screw holes that big before you modified it?

beaug:
The idle mixture screw was not the big before I changed it.. I checked and the idle mixture screws are seating.

Cliff Ruggles:
Working with Marine Q-jets is a little different than automotive.  They are not emission calibrated and very generous for fuel everyplace.

The tiny upper idle bleeds have the engine pulling a lot of fuel from the idle system.  It also feeds a lot of fuel to the transfer slots as well.

They lack bypass air and not uncommon to see a little nozzle drip from them.

Marine engines are also different.  They use cams with decent duration and wide LSA, so they tend to be a little "soggy" at idle speed not making as much vacuum at low rpm's as their automotive counterparts. 

Marine engines also operate at very heavy load so no vacuum advance is used and the main system has tiny airbleeds to put more fuel to the engine across the entire load/speed range.

One of the biggest problems we see here with Marine engines are folks rebuilding them with automotive camshafts and larger cams on tighter LSA's.  Makes carb tuning quite a challenge in some cases.......Cliff

beaug:
How do I find out if I have the right camshaft

Cliff Ruggles:
No way to really tell, but Marine engines at idle will be smooth, with a "deep/heavy" sound.  The power curve is very broad as well, like it should be. 

I will say this, if a Marine carburetor is completely and correctly rebuilt, it works fine on the stock Marine engine it was originally used on.

When we see problems with them it's typically when the engine under it has been modified, or it getting tired, such as low compression, stretched out timing set, etc......Cliff

Navigation

[0] Message Index

[#] Next page

[*] Previous page