engine rpms drop 200 and regain 200 whlie cruising

[crazy charlie]

1988 twin mercruiser 5.0 xl with alphas.stbd engine dropped 2-300 rpms while I was cruising and came back up the same 2-300 rpms.did this a few times during a 15 minute cruise at 3300 rpms for approx 10-15 seconds each time .In attempt to remedy I ran the fuel on that engine down to almost empty and refilled with fresh fuel with startron additive.Changed the fuel/water sep cannister.Same thing next cruise.will change the fuel filter in the carb next.Any suggestions?? Charlie

 

[Don S]

Re: engine rpms drop 200 and regain 200 whlie cruising

When you get a new water seperating fuel filter, empty the old one in a clear container and look for water in it. If there was enough and there is water in the carb, that would explain it.

 

[crazy charlie]

Re: engine rpms drop 200 and regain 200 whlie cruising

That was my first thought Don.Unfortunately that was not the cure.I will need to look further.Charlie

 

[Pete104]

Re: engine rpms drop 200 and regain 200 whlie cruising

Coming & going like that sounds like plug wires. Maybe a carbon track under the cap?

 

[crazy charlie]

Re: engine rpms drop 200 and regain 200 whlie cruising

guess I can check that.

 

[crazy charlie]

Re: engine rpms drop 200 and regain 200 whlie cruising

Checked wires and cap and all looks fine...

 

[Pete104]

Re: engine rpms drop 200 and regain 200 whlie cruising

Could have a bad plug. After that, sensor under the cap is getting ready to give up.

 

[crazy charlie]

Re: engine rpms drop 200 and regain 200 whlie cruising

My thought is that a bad plug would create a miss in the engine.There is no mis.Charlie

 

[stackz]

Re: engine rpms drop 200 and regain 200 whlie cruising

did you pull the plugs and have a look at them for color/wetness/etc. plug readings can tell you all sorts of things. though, when was the last time you pulled the carb apart for cleaning?

could have a passage partially blocked, needle could be sticking, etc.

 

[crazy charlie]

Re: engine rpms drop 200 and regain 200 whlie cruising

Carb/filter is what I was thinking,but I am usually wrong.Charlie

 

[LilRedNeckGirl]

Re: engine rpms drop 200 and regain 200 whlie cruising

check the condenser inside.next to the dist cap. its amazing what a bad one will do and your symptoms could be the effect.

 

[Don S]

Re: engine rpms drop 200 and regain 200 whlie cruising

check the condenser inside.next to the dist cap. its amazing what a bad one will do and your symptoms could be the effect.

He doesn't have a points ignition system, he has the Thunderbolt IV HEI ignition.

 

[Pete104]

Re: engine rpms drop 200 and regain 200 whlie cruising

Swap out a sensor with a buddy.

 

[crazy charlie]

Re: engine rpms drop 200 and regain 200 whlie cruising

Thanks for the suggestions guys.So far no progress.Hauling boat next week so it may have to wait until spring.What are the symptioms of a carb that need rebuilding????

 

[Pete104]

Re: engine rpms drop 200 and regain 200 whlie cruising

My experience is the engine won't idle. You have to nurse it to keep it running. Run fine above 1000-1200 rpm but falls off quickly at idle.
There could be something floating around the carb. Only one way to find out.
It's great to find the smoking gun but sometimes just not possible! Good luck.

 

[crazy charlie]

Re: engine rpms drop 200 and regain 200 whlie cruising

engine idles fine after it is warmed up.Charlie