Motor runs fine out of the water.

[Brandenton]

So I finally had a chance to put my boat in the water after having it for more than a month. It runs great at home, when I have muffs on it. When its in the water, it would bog down and die every now and then. I had to give it a little bit of gas to keep it idleing. It was idleing around 1500 RPM in the water and right at 3000 RPM when I have the muffs on it. I am assuming the change in RPM in and out of the water has something to do with back pressure from the exhaust being in the water. So question being, do I need to do some adjusting to my idle or could it be a bigger problem? The motor is a 1984 Chrysler 90HP.

 

[roscoe]

Re: Motor runs fine out of the water.

The first thing you need to do is get a Repair manual.

Second, do a compression test.
Then a spark test.

I suspect you are going to be doing some work here.

Idle speed, in gear in the water, should be 800-900.

 

[Brandenton]

Re: Motor runs fine out of the water.

Already done a compression and spark test. Got 117-120psi for the compression test and spark is good. Manual is on order as well, just getting impatient and ready to get this motor running good.

 

[Frank Acampora]

Re: Motor runs fine out of the water.

Is that the 4 cylinder 90 with two carbs or the three cylinder with three carbs?

In neutral, in or out of the water, the idle speed should be the same. There is an idle relief circuit to reduce backpressure while sitting low in the water. That's what the two holes in the back are. At idle in the water (and even on muffs for that matter) some exhaust and water should be venting from them.

Your idle numbers just don't jive. A 1500 RPM difference when on muffs just should not be happening. When you get the manual, synchronize the timing and carbs. then try again. check all the plugs. These engines are prone to foul them and the first indication is idle problems.

Double check the compression; 120 is a tad low, so check it after running the engine and while it is warm and has had some fresh oil run through it.

 

[john from md]

Re: Motor runs fine out of the water.

I agree with Frank, your idle rpms don't sound right. Have you done a basic carb/timing rigging check? If your adjustments are off, that will cause your kind of problems. Read the book when it comes and check the carb and mechanical timing step by step. In the end it will all come together.

John

 

[Brandenton]

Re: Motor runs fine out of the water.

It is the 4 cylinder with 2 carbs. And it does have the two holes in the back. I am going to take another look at the plugs but other than that I will just wait for than manual. I do appriciate all the input. Thanx

 

[Frank Acampora]

Re: Motor runs fine out of the water.

Prior to 1982 or so, that engine was the chrysler 105. It was rated at the powerhead. When outboards were changed to rating at the prop, Chrysler downrated the 105 to 90. Nice engine and should give you years of service once you get the bugs out.

Just a thought: That engine should have small WB carbs with 1 5/16 diameter venturis. You can gain a couple of horsepower by finding the bigger manifolds and a set of TC carbs with 1 5/8 diameter venturis. It won't be anything dramatic but you might gain about 1-3 MPH.

I know this seems contrary to my constant refrain that carb changes will not increase horsepower but this is a different case. Most people want to put 85 carbs on a 75 and gain 10 horsepower. This can not be done because of the porting of the blocks. Although the 90 (105) is ported differently than the 120-125-140-150, it is still possible to gain a few ponies because the carbs are so much larger in area.