Minor hesitation

[JoDo]

First, hello to all. After lurking here for the last month, this is my first post. I will start this by pasting a post I put on iboats about 4 weeks ago. I have since worked through most of this and am on the water however I will explain the problem I need help with at the end. So here's what was going on around July 4:

1989 Maxum Cuddy 5.0 carbed mercruiser. Four barrel. I tested it all week, ran it over and over after sitting all winter with an almost empty tank. Everything ran fine. Had a stall issue when shifting to reverse so I cleaned and lubed the interrupt switch and fixed my problem there. Left for lake and put 20gal of fresh gas in. When at the lake, I backed off the dock just fine. When I went to accelerate forward it died. Then it would not idle either. I could rev the engine but the rpm's fluctuated. Whenever I tried to put in fwd it died. I was just about to flag someone for a tow when I tried "feathering" the throttle. I could creep it to the dock that way. I thought it was the fuel filter so I ran to the marina and bought one. This didn't solve it. Ended up loading it up and coming home. I wonder if the new fuel picked up some crap and sent it to the carb. I picked this up from a friend two years ago after it had been sitting unused in his yard for a couple years, I have never changed plugs, cap or rotor and don't know when he ever did. Thank you.

Then: Did the tune up: Cap, rotor, plugs and wires. Started, ran great in my driveway with the "earmuffs" on. Took her to the lake after work and launched. Ran fine, backed out, started full forward for about 30sec and she bogged right down. This tells me it's happening when the engine gets warm. I learned if I manipulate the choke plate by hand I could keep it running. So after talking to a mechanic buddy while tied to the dock, he determined it's a fuel metering issue which probably means a carb rebuild or at minimum a good cleaning. So I called another buddy to ask who could do this and turn it around by our camping trip this weekend. After giving me a name he told me to try this first: One can sea foam in the tank. Douse the carb really well with spray cleaner and let sit, start and blow the gunk out. Then while running pour one more can sea foam down the carb to see if I can knock what ever it is loose. I did this and she ran great, again, in my driveway. I let run for at least 20min and no problems. I guess we'll see tomorrow.

So I've since had 3 or 4 glorious days on Lake Pend Oreille. I pretty much cruise at around 3000-3200 rpm. The speedo tab on the boat is broke but this seems to put me at around 30-35mph on decent water. There has been a couple times now when giving more throttle the the engine "bogs" or "hesitates" at around 36-3800 rpm. Just for a moment. A couple times I stopped the boat and restarted and she cruised again at 3K, Saturday I just brought the throttle back down to 3K and she ran great. Any thoughts here? I plan to rebuild the Quadrajet when the season is over as it has had several periods of sitting unused.

[mcoffey]

Yep, sounds like fuel problems to me. Pouring the seafoam down the throat does nothing for the carb itself, just cleans the intake passage and carbon buildup on valves and cylindes. You said you used 1 can in the tank, how much fuel was in it when you did that? I have always used 1oz of seafoam per gallon of gas just to maintain and 2oz per gallon for a good cleaning.
I think they are 16oz cans so you may need a few here

Now they do call those Rochester's Quadra Bogs for a reason too
They are not half bad though so maybe just a quick rebuild is all she needs.

If it were me I would try the double dose of seafoam or maybe some good BG cleaner first. If that fails then I would go for the rebuild of the card. Make sure they soak it overnight in the carb cleaner though if you go that route.

[ImRich]

Sounds like a carb rebuild will help. Maybe the accelleration pump needs help.

Get the proper rebuild kit, go through everything when you have it apart, cleaning all passenges with carb cleaner. Blow it out, and re-assemble according to the rebuild kit.

Most important thing, when you are done, be sure to come back here and let us know what fixes the problem.

[JoDo]

Thanks everyone, since it's getting by at the moment and our boating season up here is relatively short I will wait until fall and rebuild the carb. I think I can find somebody with a sonic cleaner to really clean the body in. I don't want to mess with it now and possibly cause a problem that would make me miss precious weekends at the lake.

[rens914]

have you checked the petrol. try to suck out whatever is at the bottom of the tank.
water (condensation) is heavier than petrol and will form a bubble. (also in youre carb)
suck it out in a clear jerrycan and look if you can spot water.
it's worth the try
had this last year and this year again
symptoms sound similar
rens

[cunuck]

change the anti siphon valve cost $12.00 I have seen 2 go this year quick change.

[ss3964spd]

I have had my hands in a lot of QuadraJets and they are an outstanding carb. Fiddling with them is a pain but once they are set up correctly one rarely needs to mess with it again. It also helps to understand how they actually work. If you intend to rebuild one pick up a book - there are 2 that are really good and worth the investment.

Like MC said Seafoam down the carb - or anything else that you can squirt into the carb, won't do anything. It's possible that some internal passage is clogged but it is unlikely - QJ's aren't nearly as finicky in that regard as Holleys are. The most likely problem is as rich said - the accel pump. Another potential problem is a vacume leak - which is supported by what you mentioned about playing with the choke. If the accel pump is iffy, and or there is a vacume leak, both conditions will cause the engine to go lean as soon as you open the throttle and result in the bog you're experiencing.

Here's a zippy test: with the engine at idle and operating temp - the choke completely open, slowly, very slowly, start restricting the air flow into primary air horn (where the choke blade is) with either your hand or the choke blade and see if the engine RPM goes up a little. If they do - vacume leak. If they don't - well, no vacume leak and you can chase something else.

Lastly, ignition timing has a profound affect on how the engine runs and, if it's off, can easily result in the bog you are experiencing. In fact, I would be checking/verifying the timing WAY before I would take the carb apart. Indeed, I would check for a vacume leak and then check the timing before doing anything else.

Dan

[JoDo]

Thank you all for some great information. I will check all those things. I realize that Sea Foam procedure sounds flawed, I wondered the same thing regarding the small passages. However, whatever was giving me the issue that rendered the boat unusable cleared after doing it. Can anyone explain to me why the issue seemingly now only occurs at a certain RPM?

[ss3964spd]

Based on your description of the initial problem - bogging as soon as you opened the throttle, I would have guessed the accel pump. It's very possible that the chemicals in the SF caused the rubber cup on the accel pump to swell and therefore seal better in its bore. It could also be that the primary throttle metering piston was stuck in its bore from a build up of varnish after all the gas evaproated from sitting un used for so long and the SF desolved it.

As to what is causing the bog at only certain RPM's I'd have to know more detail of the symptoms. From your brief description above - slight bog at 3600 ish RPM, it could still be the accel pump, a timing problem - perhaps the mechanical advance weights under the distributor rotor are sticking), or maybe the secondary air valve (the enormous blades behind the choke horn on the QJ) are opening too quickly.

Dan