AQ130d mid-range hesitation

[cconger]

Hi all, I have used this forum many times to search information on multiple projects. This is a great forum. I have searched many hours all over the internet trying to find a solution to this problem. I have Volvo Penta AQ130D motor with the two Solex 44 carburetors. The boat has been completely rebuilt including the motor. The boat originally had a AQ131 motor and was transplanted with a complete AQ130D with outdrive out of another boat due to the AQ131 throwing a rod. The boat starts great. Will idle smooth all day. Top end perfect.

The issue is I have a hesitation at 2600-2900 RPM when I accelerate. Other than this "dead spot" it runs great. As I accelerate and hit that RPM I feel loss of power and RPM's drop to about 2000 rpm. The boat wont stall. I can push through that spot to higher RPM and the boat runs fine. For the life of me I cant figure this out. Last year I checked the timing and tried slightly different adjustments any where from 8-12 degrees BTDC. The distributor is advancing properly up to 2000 RPM. Cleaned the carbs three times. Installed Pertronix Ignitor module with Flamethrower coil. Changed fuel pump. Synced and adjusted the carbs mixture screw multiple times.

This year I purchased a rebuild kit for the carbs. Even tried putting a different needle seat washer/shim to change float height. All I can do is make the hesitation slightly better or worse. I can get the engine running fantastic all the way around other than that dead spot. I even tried adjusting the accelerator pump rod nut in and out with not much change other than it causes the hesitation to start earlier or later. I am not completely sure how the accelerator pump works or when it is supposed to kick in. It almost seems like the injector runs out of steam to soon so there isn't enough gas.

I can also tell you it seems if I accelerate hard it isnt as bad of a hesitation as it is when I accelerate slowly.

Please someone help. Thanks to all in advance.

 

[PiratePast40]

Re: AQ130d mid-range hesitation

I'm not sure about the hesitation but can tell you how the accelerator pump is supposed to work. As the throttle advances, the lever is pulled inward and pushes the diaphram against the spring. That action pushes fuel up through passages to the "candy cane" you see on top of the carbs. If working correctly, you should observe is gas squirting out of the accelerator nozzle (candy cane) as you advance the throttle from idle.

Other things that you might want to check are that the linkage is operating both carbs the same amount at the same time, the main jet is clean, and the emulsifier jet (tube in the center of the carb) is also clean.

Also check for vacuum leaks at the manifold and on the carbs with carb cleaner. Might also take a close look at the top plates of both carbs and check for cracks around the screws that would also cause vacuum leaks.

I'm by no means an expert but have the same setup as you and have had my share of "re-learning experiences" :redface:.

 

[cconger]

Re: AQ130d mid-range hesitation

I appreciate the reply. Is there a certain spot (RPM) the accelerator pumps start and stop injecting the fuel into the carb throat. How does that little adjustment nut on the accelerator pump rod effect the actual function of the pump.

Is it possible that the jets in the carbs I am using are not the correct size for the bigger boat I have the engine in? It came out of a 16 foot boat. It is now in a 19 foot boat. Which jet would effect the mid-range RPM?

 

[PiratePast40]

Re: AQ130d mid-range hesitation

The accelerator pump should start as soon as the linkage at the carb starts to move. The nut on the adjustment rod will determine when the diaphram begins to push fuel up to the nozzle. There will be a finite amount of fuel available in the chamber that can be pumped up into the nozzle. The spring inside the chamber helps dampen the diaphram action and allows the spray to continue longer than a short squirt.

If you've rebuilt the carbs, try to remember how you positioned the spring in the accelerator pumps. In the instructions, there is a drawing that shows the spring on the lever side of the accelerator pump diaphram. That diagram is incorrect. There is a photo that shows the spring on the opposite side of the diaphram, i.e., inside the chamber. The photo is the correct position of the spring.

The main jet size is based on the engine, not the size of the boat. Unless you somehow got carbs from one of the newer and higher HP DOHC engines, that shouldn't be an issue. If you didn't remove the main jet holder during the rebuild, I'd really be inclined to pull it now as well as pulling the emulsifier tube from the center of the carb throat and clean the jet and passages to see if that helps.

 

[cconger]

Re: AQ130d mid-range hesitation

Thanks again for the response. I did take the carbs apart a few times and cleaned them well. The last time was this year and put a rebuild kit in them as well. I did notice in the on diagram the picture was wrong for the accelerator pump spring. I am not new to carburators at all and have rebuilt numerous ones. These are however my first Solex set. I am new to accelerator pumps however. The accelerator pumps are working they will squirt fuel when I advanced the throttle. However are they supposed to squirt fuel everytime I advance the throttle or just when I rapidly accelerate. It almost as if the engine is getting to much fuel and floods out for a second but its hard to tell. That might explain why it is much worse when I accelerate slowly. If I accelerate fast the hesistation is almost non-existant. I am not getting any popping from the carbs like when its lean.

I also had a question about the flame arrestor. I have a rectangle metal "air box" as well as round mesh screens with metal tops. Are these two types of flame arrestors or are they intended to be used together? I dont want to be over choking the engine. Might be the whole problem!!

 

[PiratePast40]

Re: AQ130d mid-range hesitation

The metal screens are the flame arrestors and should be cleaned so dirt and grit doesn't restrict flow. The metal box is called an air silencer and helps reduce air noise from the carbs.

If you look at the linkage for the accdelerator pumps, you'll see that the actuating rod only goes so far. Past a certain point, there's no more travel to actuate the diaphram. At that point, the butterfly's should be opening and engine vacuum would be sucking fuel through the main carburetor circuit. So to answer your question, no, the accelerator pump does not operate through the entire engine operating range. Just when going from idle into the power range. As far as the spring - glad to hear you caught the error in the instructions.

For adjusting the accelerator pumps - you should see approximately half the adjusting nut length of thread on the adjusting rod. That should make sense when you look at the linkage and the long adjusting nut.

If you're sure that there are no vacuum leaks, the main jet holder is snug, the passages are clean, there is plenty of clean fuel, and that there are no restrictions, then I'm afraid I've come to the limit of my knowledge and it's time for the big guns to come to the rescue.:redface:

 

[sqbtr]

Re: AQ130d mid-range hesitation

Try posititioning the dump tube so the fuel spray hits the emulsion tube holder strut, it helps atomize the fuel. +1 on syncronizing the carbs

 

[cconger]

Re: AQ130d mid-range hesitation

Ok so heres an up date. I sprayed carb cleaner on all possible vacuum leak locations, no change in RPM. So I am assuming no leaks. I did put a thinner copper washer behind the float needle as well as putting to gaskets on top of each other under where the top plate of the carb meets the base effectively increasing float height. I also richened the mixture screws a little. Doesnt idle as good due to the mixture adjustment but wouldnt you know it the hesitation is almost non-existant. Only if I accelerate very slowly can I notice any hesitation present. It makes me wonder if the fuel pump isnt keeping the fuel bowl full of fuel like it should leaning the mid-range out. I never got a new gasket kit for the fuel pump. I have two fuel pumps that I cleaned thoroughly. I changed one out for the other without a change in the problem last year so I wrote it off as not the problem. But it sure could be possible that they are both a little weak.