1987 Mercruser w/Alpha One Gen 1 260 - high idle - 2500 RPM

flogorfish · Sep 13, 2012

Okay, I've been searching forums and the web for a few days now trying to find help for my Mercruiser. It was running fine, hard to start sometimes, but ran good after. I was cleaning it up, getting ready to sell it (but the wife don't want me too), and started it up. It started quickly and ran up close to 3000 RPM. At this point, I had done two things, replaced the exhaust hose to the y-pipe. Earlier I'd had an impeller blow up and they got hot. I also remove the fuel/water separator (just dumped it, didn't look for water) and it looked clean. I did not fill it with fuel before re-installating it. After doing some research and thinking that there might be some object in the carb so I did a basic rebuild on it. It is a Rochester Qject. It wasn't very dirty. After installing it, I had the same exact problem. Starts quick and starts to race. I have it on the trailer and can't leave it running very long as its starting to heat up. I've disconnected the throttle cable, backed the idle screw all the way out (well - not touching the throttle stop). Adjusting the fuel mix screws doesn't seem to do anything either, but I haven't backed them out very far. I started at about 3 turns, ran them almost all the way in without much affect. I'm at a loss. The behavior is the same before after the rebuild, so I'm thinking that it is not the carb. What's puzzling is the sudden change and the only thing I touched in the fuel system before the issue was remove, dump, re-install the fuel/water sep/filter. Help!

series60 · Sep 13, 2012

Re: 1987 Mercruser w/Alpha One Gen 1 260 - high idle - 2500 RPM

flogorfish said:
Okay, I've been searching forums and the web for a few days now trying to find help for my Mercruiser. It was running fine, hard to start sometimes, but ran good after. I was cleaning it up, getting ready to sell it (but the wife don't want me too), and started it up. It started quickly and ran up close to 3000 RPM. At this point, I had done two things, replaced the exhaust hose to the y-pipe. Earlier I'd had an impeller blow up and they got hot. I also remove the fuel/water separator (just dumped it, didn't look for water) and it looked clean. I did not fill it with fuel before re-installating it. After doing some research and thinking that there might be some object in the carb so I did a basic rebuild on it. It is a Rochester Qject. It wasn't very dirty. After installing it, I had the same exact problem. Starts quick and starts to race. I have it on the trailer and can't leave it running very long as its starting to heat up. I've disconnected the throttle cable, backed the idle screw all the way out (well - not touching the throttle stop). Adjusting the fuel mix screws doesn't seem to do anything either, but I haven't backed them out very far. I started at about 3 turns, ran them almost all the way in without much affect. I'm at a loss. The behavior is the same before after the rebuild, so I'm thinking that it is not the carb. What's puzzling is the sudden change and the only thing I touched in the fuel system before the issue was remove, dump, re-install the fuel/water sep/filter. Help!

Old q-jets are notorious for leaking welch plugs which leak fuel into the intake. "basic rebuild on it" doesn't tell me much. Was it soaked in good carb dip overnight to remove the varnish from the tiny orifices? After you disconnected the linkage did it idle down properly? 3000RPMs will toast the impeller and possibly other parts.

Don S · Sep 13, 2012

Re: 1987 Mercruser w/Alpha One Gen 1 260 - high idle - 2500 RPM

Disconnect the throttle cable and see if it will idle down by holding the linkage of the carb against the idle speed adjustment screw.

flogorfish · Sep 13, 2012

Re: 1987 Mercruser w/Alpha One Gen 1 260 - high idle - 2500 RPM

Thanks guys. I hope I haven't toasted the impeller yet. I haven't let it run very long, about a minute or so max. I have the throttle linkage disconnected and no change. The flaps move easily and did not appear to have any issues when I had it apart. I put epoxy over the welch plugs before I put it back together and the carb wasn't very dirty. I soaked the carb in carb dip for a couple of hours then used berrryman spray to blow it out.

Even with the throttle linkage disconnected, it's running away. I can slow it down by closing the choke plate. If it's open, as if the engine is warm, its running high.

The behavior is the same as it was before I removed and reinstalled the carb.

oldjeep · Sep 13, 2012

Re: 1987 Mercruser w/Alpha One Gen 1 260 - high idle - 2500 RPM

Dumb question - but did you look at the relief hose from the fuel pump for fuel to see if the diaphram has blown?

flogorfish · Sep 13, 2012

Re: 1987 Mercruser w/Alpha One Gen 1 260 - high idle - 2500 RPM

oldjeep..no dumb questions for me as I'm far from competent. So please, anyone, no assumptions that I have checked things. I did a fair amount of research first, but ...I'm a rookie.

So, oldjeep, no I haven't check that. Is there an easy way to do it. Is that the relief line that comes down from the carb, above the fuel inlet?

Thanks for your input.

oldjeep · Sep 13, 2012

Re: 1987 Mercruser w/Alpha One Gen 1 260 - high idle - 2500 RPM

Depends on the carb, but it is the extra line that comes from the fuel pump and dumps the fuel into the carb if the pump diaphram fails.

Don S · Sep 13, 2012

Re: 1987 Mercruser w/Alpha One Gen 1 260 - high idle - 2500 RPM

flogorfish said:
Even with the throttle linkage disconnected, it's running away. I can slow it down by closing the choke plate. If it's open, as if the engine is warm, its running high.

The behavior is the same as it was before I removed and reinstalled the carb.

Might want to look at the throttle plates and see if they are closed (Look at the secondaries also). The idle speed adjustment may be turned in too far as well.
Did you use a new gasket when installing the carb?

If the fuel pump was dumping fuel in the carb you would have a flooding situation and a lot of black smoke. You have excess air and fuel that is causing your problem.

akorcovelos · Sep 13, 2012

Re: 1987 Mercruser w/Alpha One Gen 1 260 - high idle - 2500 RPM

hard starts and high idle sounds like a timing issue to me, have you varified initial timing?

flogorfish · Sep 13, 2012

Re: 1987 Mercruser w/Alpha One Gen 1 260 - high idle - 2500 RPM

Don S, yes the kit I bought had three gaskets w/it. used them all and they seemed to be the same as the ones I took off. I have the idle screw back out and throttle cable disconnected. I'll check the plates to make sure since I put the carb back on. It's idling high, but it is smooth. before I rebuilt the carb, it would diesel a little but the last couple of times I tried it, it's running smooth and shuts down smooth. But I don't leave it running for very long. It starts, I'd guess around 1000 rpms and then speeds up. I shut it down in 30 to 45 seconds. I'm not seeing any sign of smoke, or poor performance. Just running too fast. Of course I don't have it under load either...

flogorfish · Sep 13, 2012

Re: 1987 Mercruser w/Alpha One Gen 1 260 - high idle - 2500 RPM

akorcovelos, No I don't have equipment for that. But it starts quick and easy now. Before the issue started, it seemed like fuel (lack of) was the problem. With regular use, it started easy, almost always. And ran good too. Until the all of the sudden racing at idle problem which started this piece of the journey.

flogorfish · Sep 13, 2012

Re: 1987 Mercruser w/Alpha One Gen 1 260 - high idle - 2500 RPM

oldjeep said:
Depends on the carb, but it is the extra line that comes from the fuel pump and dumps the fuel into the carb if the pump diaphram fails.

ah, so its the one that would have fuel in it if there was a problem, but dry otherwise?

oldjeep · Sep 13, 2012

Re: 1987 Mercruser w/Alpha One Gen 1 260 - high idle - 2500 RPM

flogorfish said:
ah, so its the one that would have fuel in it if there was a problem, but dry otherwise?

Yes, but Don says that would not cause your problem.

flogorfish · Sep 14, 2012

Re: 1987 Mercruser w/Alpha One Gen 1 260 - high idle - 2500 RPM

I'll check it today anyway, but when I took it loose to remove the carb, it was dry.

flogorfish · Sep 15, 2012

Re: 1987 Mercruser w/Alpha One Gen 1 260 - high idle - 2500 RPM

Today, I'm going to look for vacuum leaks, but still looking for any tips/advice/suggestions....Thanks!

flogorfish · Sep 16, 2012

Re: 1987 Mercruser w/Alpha One Gen 1 260 - high idle - 2500 RPM

No luck and I think I may have toasted my new impeller despite efforts to prevent that from happening. With ski season pretty much done for us this year, I'm winterizing and putting this one on the back burner for now.

1987 Mercruser w/Alpha One Gen 1 260 - high idle - 2500 RPM

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