4.3L 4BBL Secondaries

[turbinesurgeon]

I have a 92 4.3L with the Rochester 4BBL. The engine runs smooth all the way up until around 4600 RPM and after that is starts to cough and sputter. I believe this is when the secondaries are opening. The secondaries are mechanically linked but the air valve is vacuum and I think my problem is somehow related the this airvalve. Any ideas? I am not sure if I am in for a rebuild or if there is something common with these carbs thats an easy fix?

Thank you

 

[Limited-Time]

Re: 4.3L 4BBL Secondaries

Sounds like a fuel delivery issue. However I doubt the secondary's are just coming in at 4600 RPM. I'd be looking for a bad fuel filter, or other restriction.

 

[180shabah]

Re: 4.3L 4BBL Secondaries

Secondaries opened long before 4600. Look at fuel condition and fuel delivery.

 

[Bondo]

Re: 4.3L 4BBL Secondaries

Ayuh,...

Change your Fuel Filters,+ Try it....

 

[turbinesurgeon]

Re: 4.3L 4BBL Secondaries

I just purchased this boat a few months ago and I discovered that the fuel pump was bad on it, (leaking through the discharge line). I installed a new fuel pump. I also intalled new fuel filters prior to our first trip out to the lake. So it shouldn't be a delivery problem. Looking at the carb when I move the throttle to about 80% travel the secondaries start to open so I was guessing that the RPM was around that range. Maybe the problem is when the secondaries are almost all the way open. But when I go past 80% throttle to full she will start to cough. Maybe I should run it without the air cleaner and see if that air valve is opening.

 

[Bondo]

Re: 4.3L 4BBL Secondaries

I also intalled new fuel filters prior to our first trip out to the lake. So it shouldn't be a delivery problem.

Ayuh,...

The Secondaries are Wide Open,...
Looonnnng before the motor gets to 4600rpms.....

'ell,.... That's danm near Redline.....

You can get enough Water into a fuel filter in 1 Day to cause this Issue,....

But,.. Hey,...
If you think it's the Carb,... Get a Kit,+ Rebuild it.......

 

[MikDee]

Re: 4.3L 4BBL Secondaries

The air valve on the Qjet secondaries has the rear main jets metering rods suspended off it, when it opens the rods lift, and open the jets letting the gas flow from the float bowl out the venturi tubes (the brass looking tubes), into the rear barrels, It's That Simple! When you manually lift the rear air valve you can see these rods towards the middle, hanging (suspended) off it.

So I doubt there is a problem there, unless the main jets in the float bowl are clogged.

 

[turbinesurgeon]

Re: 4.3L 4BBL Secondaries

The air valve on the Qjet secondaries has the rear main jets metering rods suspended off it, when it opens the rods lift, and open the jets letting the gas flow from the float bowl out the venturi tubes (the brass looking tubes), into the rear barrels, It's That Simple! When you manually lift the rear air valve you can see these rods towards the middle, hanging (suspended) off it.

So I doubt there is a problem there, unless the main jets in the float bowl are clogged.

Does vacuum open the valve? Its spring loaded shut. I see the choke vacuum pot will stop the valve from opening during high vacuum and releases the linkage at some point during low vacuum to allow the secondaries to meter the fuel.

I have little experience with Rochesters, the air valve thing is a little wacky and is probably why I started there. But I need to understand it a little better to accuratley troubleshoot the problem.

If it is a fuel delivery problem and the pump and everything is ok, would a low float level cause this?

Thanks

 

[jerryjerry05]

Re: 4.3L 4BBL Secondaries

The secondaries are mechanical,no vacume unless someone has added a car carb and put in a vacum line.They open somewhere after 3400rpm. Have you done a compression/leak down test?
If your running at 4600rpm you are past what is suggested.4400 should be about max you should operate.If you can go past 4600 then you need to re-prop and slow your motor down.Jerry

 

[turbinesurgeon]

Re: 4.3L 4BBL Secondaries

The secondaries are mechanical,no vacume unless someone has added a car carb and put in a vacum line.They open somewhere after 3400rpm. Have you done a compression/leak down test?
If your running at 4600rpm you are past what is suggested.4400 should be about max you should operate.If you can go past 4600 then you need to re-prop and slow your motor down.Jerry

Yes the secondaries are mechanical but I was wondering about the air valve above them, I think that must open on vacuum. This boat seems to top its RPM at about 5,000, it is 19 foot Chaparral with the Volvo Penta Duoprop, with original props. I thought that these lower units ran higher RPMs? Anybody know the RPM range for the duoprop?

 

[MikDee]

Re: 4.3L 4BBL Secondaries

Yes, the air valve on top is vacuum operated, and spring rebound return, if the carb is matched to the engine, & boat, and the valve is not binding, the spring tension should be just right, don't mess with it.

As all the knowlegable previous posters mentioned, and I agree, it sounds like a fuel delivery issue. There are a few items to check before you get to the carb, such as, the fuel tank strainer, and antisiphon valve there, the fuel hose itself, the filter(s), especially the last one under the Big nut on the Qjet Carb.

 

[turbinesurgeon]

Re: 4.3L 4BBL Secondaries

Yes, the air valve on top is vacuum operated, and spring rebound return, if the carb is matched to the engine, & boat, and the valve is not binding, the spring tension should be just right, don't mess with it.

As all the knowlegable previous posters mentioned, and I agree, it sounds like a fuel delivery issue. There are a few items to check before you get to the carb, such as, the fuel tank strainer, and antisiphon valve there, the fuel hose itself, the filter(s), especially the last one under the Big nut on the Qjet Carb.

You gave me a good starting point. I am going to leave the carb alone for now. This boat has a little less than 300 hrs on it and the prior owner didn't use it the last two seasons. I went through everything mechanicaly and discovered the fuel pump diaphram was rotted so I cleaned everything out north of the pump, new filters and everything but I didn't do anything with the tank. I did notice that when filling it up with fuel, when fuel gets to the top it splashes out the air vent. I will investigate the tank first.

Thanks

 

[turbinesurgeon]

Re: 4.3L 4BBL Secondaries

I took the standpipe off of the tank and cleaned it out, made sure the check valve work and cleaned out the line from the tank to the filter and found nothing. back to the drawing board.

 

[turbinesurgeon]

Re: 4.3L 4BBL Secondaries

I took the secondary metering rods off and found one of the tips bent and scarred. I straightened it out and cleaned them up with scotchbrite. I took the boat out on the lake yesterday. It ran much better, it looks like at WOT it runs exactly 4800 RPMs.