Q Jet Question

[krisnowicki]

I am out in the water on my with my 85 305 in the 21 ft four winns and when loaded I can't get the rpms I need. IT bogs down to about 3500 but I noticed that at wot only the bottom secondaries open (the big ones) the top does not open and is this lack of air causing the low rpms under big load? the choke is opening as it should but just not the bigger ones

 

[krisnowicki]

Re: Q Jet Question

also the timing is set at 8 degrees and when not under load i get up to WOT (4800) no problem

 

[captmello]

Re: Q Jet Question

this lack of air causing the low rpms under big load? the choke is opening as it should but just not the bigger ones

Could be, I believe the butterflies are vacuum controled so it would have to be stuck closed, damaged or perhaps a vacuum leak at the manifold. Do the secondaries move easily by hand? Did you have the carb off recently.

 

[krisnowicki]

Re: Q Jet Question

yes the open and close freely with the engine off. I thought it may have been a vaccum problem. I will clean every thing up thanks guys.

 

[Maclin]

Re: Q Jet Question

The airvalve secondary plate is spring loaded and adjustable. The bottom plates open progressively from the throttle linkage. As the engine demands more air the pull on the top plates overcomes the spring tension and the secondary barrels come into play. You should be able to get them to open, but make small adjustments as you could introduce an even worse bog.

 

[Maclin]

Re: Q Jet Question

..... the lack of vacuum as the Q-Jet uses vacuum to keep them closed until it drops low enough for the spring tension to open them.

That describes how the power enrichment circuit works as it comes into play under a load but is not accurate when applied to how the secondaries are controlled on Quadrajet.

 

[MikDee]

Re: Q Jet Question

If I remember correctly, there is a small vacuum diaphram located at the back left top of the carb, that controls when the secondary air butterflies open. When engine vacuum is strong, it will keep the secondaries closed, when you suddenly open the throttle, and vacuum becomes weak, the diaphram loses vacuum, and the secondary butterflies open (lifting the metering rods in the middle, out of their respective jets) to promote fuel flow. This works in conjunction with the spring loaded secondary butterflies.

If your boat was overloaded, maybe there was too much vacuum there to release the vacuum on the diaphram, hence not letting them open, because as you say, it works fine under a light load.

Note: This may, or may not apply to your application, some versions of this carb, may not have that small vacuum diaphram, but the principles of the air butterflies not opening may still apply if you were running at high vacuum under load.

 

[Maclin]

Re: Q Jet Question

I may not understand the diaphram's entire purpose, I always thought it smoothed the operation so they would not just dump open, and that the engine's draw finished opening the airvalves once they overcame the spring tension, and since the vacuum was low once the throttle was opened that the diaphragm basically stood down so to speak. It may be more a part of the opening force than I understood, thanks for the insight.

 

[KJSmitty]

Re: Q Jet Question

^^^Yes,
The diaphram (the vacuum brake as they call it) keeps the upper secondary (air valve)plates closed - then as vacuum decreases, this vacuum brake releases allowing the plates to open thus lifting the secondary metering rods.

The diaphram/brake doesn't just pop open, thus it can provide a gradual opening of the secondary metering rods for a smoother fuel application etc.. As far as this diaphram/brake, obviously it has to be functioning and the rod leading up to the secondary plates needs to be adjusted properly. There must be a 5/64 gap at the top actuator where the vacuum brake rod connects, when fully closed etc. (hard to describe without a picture). Basically with the vacuum brake fully closed, the upper secondary plates should be able to open slightly. This allows the plates to start moving when you go to full power prior to the initial vacuum brake release.

As Bubba1235/Maclin stated, the other adjustment is the "Air valve spring windup".
All of the Q-J's that I have worked had one as well. Should be right there at the upper plate actuator where the vacuum brake rod sits etc.. This one is a little more precise and most don't have a "finger held gram scale", thus is many times adjusted by trial and error. Basically, while putting pressure on the aft section of the upper (air valve)plates they should begin to open under 70-90 "grams" of pressure.

Most likely if not operating at all you have a bad vacuum brake, cracked/leaking vacuum hose to the brake, clogged vacuum port, or something mechanically sticking etc..

- The second you start the engine you should see the operation of the vacuum brake / movement of the rod connected to the upper (air valve)/secondary plates. If this rod doesn't move you know you have issues there. If it does pull into position at idle then either the rod is out of adjustment or the spring tension needs adjustment (or something is just sticking etc.).

- With the engine/key off you can always close the vacuum brake gently with your fingers. Then manipulate the upper plates to check for ease of opening etc..

- With engine/key off you can remove the small vacuum hose from the diaphram/brake, close the brake with your fingers then place a finger on the vacuum port on the brake (where the hose connects to it etc.). Now release the brake and the diaphram should not move. Basically you are checking to ensure it holds vacuum. If it holds - its fine, if not it's bad.

(diaphram or diaphragm - spelling?)

Be safe!!


Good luck

 

[John_S]

Re: Q Jet Question

Whats your timing at 3500rpms? Should be near 30 BTDC. ie is your advance working correctly?

While you may have a carb problem, I doubt your problem is the air door. On a 305 the air door will not be open that much at wot. The primaries alone should take it above 3500rpm.

 

[krisnowicki]

Re: Q Jet Question

of course it bounces around but it is usually between 28-32 degrees as far as timing the ignition is all new TBIV

 

[John_S]

Re: Q Jet Question

DonS has a list posted on things to check if you can't reach wot. If you haven't already, I would click through them. One of them will be fuel supply and carb.

Given what you have said about the airdoor (it moves freely open and closes), it is unlikely to be the problem. Spring windup doen't just change unless you were already in their adjusting it. If spring breaks, door will open all the time and you would have very severe bog. If it has been doing it from new or rebuilt carb, then maybe, but all adj are suspect then.

T4 shouldn't bounce too much. At a set rpm it should be rather steady. T5 has adaptive capabilities, and will bounce around in "normal" mode. You probably have some play in distributer, but very unlikely that this would be causing such a power loss.

 

[KJSmitty]

Re: Q Jet Question

Given what you have said about the airdoor (it moves freely open and closes), it is unlikely to be the problem. Spring windup doen't just change unless you were already in their adjusting it. If spring breaks, door will open all the time and you would have very severe bog. If it has been doing it from new or rebuilt carb, then maybe, but all adj are suspect then.

True, the spring windup typically only need adjusting if someone has previously messed with it.

However, given the fact he said the Air valve doesn't open at all at WOT the vacuum brake definitely needs to be looked at. Now, if at WOT and he can push the air valve open and nothing happens - then theirs definitely something amiss either elsewhere mechanically or in fuel delivery as you mentioned..

Cheers

 

[John_S]

Re: Q Jet Question

However, given the fact he said the Air valve doesn't open at all at WOT the vacuum brake definitely needs to be looked at. Now, if at WOT and he can push the air valve open and nothing happens - then theirs definitely something amiss either elsewhere mechanically or in fuel delivery as you mentioned..
Cheers

The vacuum break is easy to check, but if he does what you say with the air valve, he will quickly drop from his already low 3500rpm. That would create a very big bog. It is easy to do, as well, but just make sure nothing is close to your forehead, like a windshield frame.

If nothing else was holding it back, he should come close to 4K on just the primaries. I can get more than 3500rpm on a larger 350, with the primaries open just to the point just before where the secondaries start to open.

 

[KJSmitty]

Re: Q Jet Question

just make sure nothing is close to your forehead, like a windshield frame.
QUOTE]


No kidding -good point

That would be an eye opener for sure if it reacted as it normally would....

 

[CharlieB]

Re: Q Jet Question

Gentlemen please,

To go back to his original postings we see that

"when not under load i get up to WOT (4800) no problem".

How LOADED is this boat?

Any waterlogged foam?

4800 UN-loaded?

Doesn't anyone begin to suspect that this craft may be just a bit OVER-propped?

It's fine to check the timing and spark plugs to ensure that the motor is in tune and capable of performing as required, but before condeming a Q-Jet look at the whole boat and I think that in this case the carb/engine is NOT the issue.

 

[MikDee]

Re: Q Jet Question

Gentlemen please,

To go back to his original postings we see that

"when not under load i get up to WOT (4800) no problem".

How LOADED is this boat?

Any waterlogged foam?

4800 UN-loaded?

Doesn't anyone begin to suspect that this craft may be just a bit OVER-propped?

It's fine to check the timing and spark plugs to ensure that the motor is in tune and capable of performing as required, but before condeming a Q-Jet look at the whole boat and I think that in this case the carb/engine is NOT the issue.

Overpropped, Yes,, but apparently not when loaded up as he stated.

 

[Maclin]

Re: Q Jet Question

I thought krisnowicki meant he sees 4800 when in neutral no load on the engine at all.....what say ye kris?

 

[krisnowicki]

Re: Q Jet Question

yes underload i meant with the drive and a couple people in thew boat and a tank of gas. I have a 17p prop and am getting 3800. When in neutrual and the OD not engaged I get about 4800 but i dont do that for the health f the engine

 

[MikDee]

Re: Q Jet Question

yes underload i meant with the drive and a couple people in thew boat and a tank of gas. I have a 17p prop and am getting 3800. When in neutrual and the OD not engaged I get about 4800 but i dont do that for the health f the engine

And I thought, as well as others here, that you were at WOT under power at 4800rpm, I guess my crystal ball is a bit rusty? You should be between 4200-4600 rpm at WOT under load. I avoid opening up the throttle in neutral at all costs. You definitely have issues, there's a lot of good info here from previous posters for you to check out.