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AQ131 Stumbles above 4000 rpm

R

robj

Regular Contributor
"Hello,

This problem has b

"Hello,

This problem has been nagging me for the last 3 years. Engine runs great upto about 4000 rpm and above that is starts to bog, stumble. I have done all the obvious, carb rebuild, compression is good, new plugs, Sierra fuel pump , Pertronix unit, check dwell, check advance springs, check distributor shaft play etc. Everything checks out OK. I know this is definitely fuel related. I am thinking fuel pump. The boat did not come with an external fuel filter, I installed that. The filter creates more resistance and I am thinking that the pump cannot supply enough fuel, which is why it will only run for 5 to 10 seconds above 4000 rpm until the fuel runs out and then stumbles. Throttle down abit and it runs great.

I know I can test for this, but that would involve getting another fuel line, cutting it to accept a pressure gage, and taking it out on the water to load the engine up and check the fuel pressure. It is easier just to buy another fuel pump. Or another idea, there is a plastic spacer between the pump and engine block, are these available in different thicknesses? Reducing the thickness of the spacer would increase the stroke of the pump resulting in more fuel.

Thanx for your suggestions and have a great day

Rob."
 
"Hi Rob,

I think we did deb

"Hi Rob,

I think we did debate this thing to death last year

http://www.marineengine.com/discus/messages/12487/284903.shtml

You may want to review it to see if you overlooked to test or to do something that was already suggested.

Now regarding your post above.

The fuel pump should pump far more fuel that your engine would need at WOT. Since I hear you have installed a new one, so that is unlikely the problem. I also don't think the fuel filter is the problem.

Be careful with the spacer: too thin and you will break the pump lever!!! I don't think that's your problem, either.

Have you considered trying another carb, or even installing a Weber kit?

And have you borrowed a new or used distributor to see if it makes a difference?

Regards."
 
"Hello El-P,

You are right

"Hello El-P,

You are right we did debate it to death. The only thing left to look at is the fuel pump, since it sounds like it is not getting enough fuel above 4000 rpm. Not sure what to do because I have checked everything else.

Regarding a Weber, can't justify spending the big bucks right now, especially when I don't think it will resolve my problem. The wife would freak if she found out that I spent $1k for a new carb. Now if I found a good used one...hmm different story.

Have a great day

Rob."
 
"Hello again,

Did a bit of

"Hello again,

Did a bit of reading and even the Seloc manual suggested that based on my symptoms it is fuel starvation and most likely the fuel pump.

Will get a new one. Not sure if I should go OEM or aftermarket. Any suggestions?

Thanx and have a great day

Rob."
 
"Didn't you just change th

"Didn't you just change the pump? Remember the saying: if you keep doing the same things over and over, you will get the same results.

What about looking for an obstruction or an air leak in the suction line? That sounds much more logical to me. Or even a wrongly set carb.

BTW, nothing wrong with the Sierra pumps."
 
"re: "What about looking f

"re: "What about looking for an obstruction "
Sticky or stuck anti siphon valve?

If you can find a short length of clear fuel-rated tubing, you could install it temporarily just in front of the carb inlet fitting. Some mating brass fittings and hose clamps would complete the test link."
 
"I have gone over this thing w

"I have gone over this thing with a fine tooth comb. Thinking back to an earlier comment and when I initially installed the pump, I did forget to put in the spacer and installed it without it, only to later notice the spacer, removed the pump and put the spacer in. Maybe I damaged the pump when I did this?

Last year I drained the tank, to look for a plugged intake strainer if there is one. No blockage. No air leak in the suction lines, they are new. The only thing that I did differently was add a water seperating fuel filter, it did not have one. Not sure if the addition of a fuel filter could be causing my problem?

Not sure how could it be a wrongly set carb. There is only one setting that is the idle mixture, that is it. Can't even set the float on this carb. Anti siphon valve was replaced last year as well.

Kind of at my wits end on this one, that is why I am thinking to try a new fuel pump.

Thanx for your ideas. Have a great day

Rob."
 
"Hi Rob, Perhaps a temporary i

"Hi Rob, Perhaps a temporary installation of an electric fuel pump in line before the engine driven pump might tell you if you indeed have a fuel starvation issue."
 
"It seems the easiest diagnost

"It seems the easiest diagnostic technique would be the temporary installation of a six gallon 0utboard motor fuel tank complete with squeeze bulb primer. The tank could first be installed on the suction side of the pump to check for tank or line problems and later on the carburetor side with the fuel pump pressure line blocked to prevent leakage, during the test. I think the constant pumping of the primer bulb should keep up with fuel demands even at WOT."
 
"Hello,

Thanx for the ideas

"Hello,

Thanx for the ideas. I was thinking of the external tank and still may do that. The only problem is that inorder for me to do the test, I have to take the boat out onto the water and test there. The engine has to be loaded up for the test.

I also looked at electric fuel pumps. They aren't cheap, more than double what a mechanical one costs.

Have a great day

Rob."
 
"Rob, I just looked through my

"Rob, I just looked through my paperwork and found a receipt for an electric fuel pump, Facet I believe is the manufacturer, 43.99 at auto parts store. This is a sealed unit with a seperate ground wire for marine applications. It is installed in my boat, AQ-145A, and works great. Hope this helps,Chris"
 
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