problem at wot

[mellowjs95]

i have a question about my 1985 mercruiser 3.0 alpha 1 outdrive. a few weeks back had a problem with dead spot at half throttle and bogging and backfiring at wot. i got all got things to do to fix it wound up to be a carb rebuild. engine idles very smooth and runs great all the way thru throttle up no dead spot until i get to wot and motor bogs down really bad when i start to ease back on throttle its fine. points are new and set at 22 plugs at 35 i changed both filters one in carb and one on fuel pump.



thanks for the help.

 

[TowRoper]

Re: problem at wot

how do your plugs look? any sign of lean running conditions? fuel flow and float level can do this too.

 

[mellowjs95]

Re: problem at wot

i pulled a plug out and it had fine carbon build up on it.

 

[TowRoper]

Re: problem at wot

only other thing i can think of is maybe a defective advance in your distributer? maybe don or bondo will have something for ya.

 

[proxyx]

Re: problem at wot

backfiring probably due to retarded ignition (ar high rpm), timing advance not working, hook up a test light (1st cyl. wire) and check your base timing as well as the balancer mark advancing with rpm. Higher the rpm farther away it travels from idle mark. You may have a bad module connection or bad module itself.

 

[bruceb58]

Re: problem at wot

I disagree that it is an ignition problem. This a caused by a lean condition when the throttle is fully open.

Who rebuilt your carb?

 

[proxyx]

Re: problem at wot

lean would cause carb backfiring/popping... possible as well. Rich would backfire through exhaust bogging the engine down, we don't know which one occurs. Since carb work was done lately you may be correct pointing at carb adjustment. Simple way: put the idle screw at 1.5 - 1.75 out and test it.
Anyway it doesn't look serious.

 

[bruceb58]

Re: problem at wot

Idle screw settings will have a negligible affect at WOT.

 

[proxyx]

Re: problem at wot

I disagree, happened to me. And my carb rebuilder would disagree too. I explained in other thread. Back off your idle screw and you'll never see WOT.
We both may be wrong though, especially ordering pizza toppings, haha.

 

[jtybt]

Re: problem at wot

There are 2 or 3 little holes in the carb throat that line up with a closed throttle plate. The air rushing by the throttle plate creates a powerful vacuum that pulls emulsified fuel(air/gas mixture) from the float bowl and vent tube. As soon as the throttle plate opens far enuff that it no longer creates the venturi effect, there is no longer fuel pulled thru those holes.


And the only way to take an accurate plug reading, especially for a throttle setting that's creating a problem is to run it at that throttle position and immediately stop and pull a plug or two from each bank.

 

[Bondo]

Re: problem at wot

I disagree that it is an ignition problem. This a caused by a lean condition when the throttle is fully open.

Who rebuilt your carb?

Ayuh,... I Agree, it sounds like it's going Lean at wot....
And,...
The Idle mixture screws have Nothing to do with WOT....

 

[fat fanny]

Re: problem at wot

I would have to agree with the float setting bring incorrect and off a little from it's proper setting as little as a 1/64" could make this difference especially if your seing carbon build up on your plugs. Or one other thing could be it picked something up(debree) in one of the ports in the carb causing a restriction. Good luck

 

[proxyx]

Re: problem at wot

carbon buildup (black?) would suggest running rich not lean. Good combustion would show as brownish... I'd check your idle screw adjustment and eventually float level. Not without checking engine timing first.

 

[bruceb58]

Re: problem at wot

carbon buildup (black?) would suggest running rich not lean. Good combustion would show as brownish... I'd check your idle screw adjustment and eventually float level. Not without checking engine timing first.

Again...idle screw mixture has nothing to do with WOT. You can keep on saying it over and over but it just doesn't!

As far as the carbon buildup on the plugs, we have no idea when the plugs were pulled. Was it after idling at all? It needs to be run at WOT, engine immeditaley turned off...then read the plugs.

 

[rad1026]

Re: problem at wot

at idle fuel comes from Idle circuit, just above idle initiates the transfer tubes, cruising or mid throttle initiates the main jets, Powering brings in the power valve and hard acceleration kicks in the accelerator pump. Every step brings in more fuel. At WOT the needle valves, or idle circuit have nothing to do with fuel delivery. Definitely sounds like the carb is going lean at WOT which means there is not enough fuel being delivered. Start with the float measurement then maybe fuel pump not keeping up.