'86 70hp rough at low speed??

[wbpope]

I finally got the water pump in and functioning properly..cool as a cucumber but, sans thermostat (for now, anyway)..at 1/2 to full throttle this baby just SCREAMS and smooth a silk but, at 2-5 knts she's might rough and "surges" a bit, intermittently but, generally just not as smooth as she should be..ideas?

 

[petryshyn]

Re: '86 70hp rough at low speed??

Lots of things can hinder an engine at low speed:

-compression
-clogged carbs
-weak ignition
-vacuum leak

Just to name a few....

What have you checked?

 

[wbpope]

Re: '86 70hp rough at low speed??

All of the above have been checked to some degree. Compression is outstanding and even. Carbs recently rebuilt. Engine fire off easily and idles fairly well.
I'm thinking timing or a vacuum leak somewhere. My first thought was fuel pump but, I'm disinclined to think that as the performance at high speed is outstanding.
Possibly even a carb adjustment issue?..'86 70 hp VRO (3 carb)..

 

[countvlad]

Re: '86 70hp rough at low speed??

All of the above have been checked to some degree. Compression is outstanding and even. Carbs recently rebuilt. Engine fire off easily and idles fairly well.
I'm thinking timing or a vacuum leak somewhere. My first thought was fuel pump but, I'm disinclined to think that as the performance at high speed is outstanding.
Possibly even a carb adjustment issue?..'86 70 hp VRO (3 carb)..

try
linc and sinc
right spark plugs?
WOT timing correct?
how recent the carbs rebuild? there could be some fuel pump issues.... have you tried to pump the primer bulb while runing?

 

[wbpope]

Re: '86 70hp rough at low speed??

I've not checked to see that the plugs are correct. Only their condition..will do.
I've been screwing with the linkage quite a bit as the person who rebuilt the carbs left them untuned..have solved MANY problems there already but, that's not to say there aren't more.
WOT timing I don't know yet...will surely check. We just put her over for sea trials this weekend so, we are discovering as we go.
Fuel pump was one suspicion I had. This engine was "converted" from VRO to pre-mix without changing pumps. There is a possibility that the old oil line is allowing air.I'll plug it and find out.

 

[petryshyn]

Re: '86 70hp rough at low speed??

Did you put new carb kit when cleaning the carbs?

Also, pull the pulse line off of the fuel pump and check hose closely for signs of fuel internally. Your pump diaphram could leak and cause rich mixture often undetected at high speed but hinders idle...

 

[wbpope]

Re: '86 70hp rough at low speed??

The carbs were completely rebuilt before I got the engine..no use at all.

The pulse line/ fuel pump is a good call....I'll check that.

The power distribution is so good I am loath to even gamble of changing the WOT timing but, there is always that....

I found that there are two different sets of plugs and two separate gaps for each recommended depending of the "sustained use" (high speed or low). Seems I have the "high speed" variety. I regapped them and that helped a bit. I'll try a set of the "low speed" ones for a comparison..also, I found that there is no actual fuel filter, just a plastic chamber with no guts. It NEVER completely fills so, that is also a possible issue.