marine quadrajet idles not smooth

[beaug]

i have an stock 305 mercruiser i/o. timing 8 btdc fuel pressure at 5.5psi. carb# 17080565. I used cliff rebuilt kit. my idles tubes are .035".  I used recipe in his book for stock engine idle down channel .046", upper idle air bleeds .050" in air horn, lower air bleeds .070", idle bypass none, mixture screw holes .090. i have some change in my mixture screw have them at 3 turns. the problem i'm having is that then engine is not idles smooth but if i pull the hose out the secondary vacuum break creating a vacuum leak the engine smooths out any help would be appreciated thanks

[Cliff Ruggles]

Sounds too rich at idle speed.  Can you lean it out with the mixture screws?

[beaug]

I tried screwing in the mixture screw it wont lean out

[Cliff Ruggles]

I'm curious why you would have changed anything in the idle system for a stock 305 Marine engine with the stock carb on it?

Marine carburetors are not emission calibrated, and very generous for idle fuel delivery with the stock settings.

They are special units, not like their automotive counterparts, and delivery a lot of fuel everyplace in stock form.

First thing I would do would be to make sure the idle mixture screws are actually seating and not coil binding on the springs instead.  Pretty common to find this happening with the later metric pitch fine threads especially if you open up the holes under the mixture screws.

We've also ran into a number of baseplates where the metric threads weren't deep enough to allow the mixture screws to seat, or the threads galled from corrosion not allowing the screws to reach the bottom of the threads......Cliff

[beaug]

the reason I changed the idle system because I couldn't get to idle with out the throttle blade being opened too much and little change with idle mixture screws. I spoke with you on the phone about a week ago and told me to open up the upper air bleed to .050"

[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

[beaug]

It runs great except at idle is the problem

[beaug]

I removed the timing chain cover and it was off two teeth. My question is can fix my carburetor or do I have to find another one thanks

[Frank400]

What do you mean   Don't you think that by fixing the timing (two teeth off) your carb "problems" will be gone ?

[Cliff Ruggles]

Correct the timing chain issue first, then re-test the carb and make changes if/as needed.

Nothing is not reversible when modifying one for bleed hole sizes, idle tubes, DCR's etc........Cliff