Johnson GT 300 - problem going full rpm

[TopEnd]

Hi, is currently testing my "new" Evinrude/ Johnson GT 300 out on th water, the engine starts very easy, 110psi on all cyl. with the intake closed forgot full throttle when testing psi.

Okay....from low rpm and up to 4000rpm it is running very smooth when on plane, but when giving full throttle it hits 5000rpm as max rpm and then quickly loosing rpm down to 4300rpm and struggling not like direct fuel cut outs, just jumping a little in rpm down to app 4300-4400rpm, then I am letting of the gas down to 4000 and I can hit 5000 again and the same happens, but it should also max out at min. 5700 rpm.

What can cause the problem, it's like it is not getting enough gas . is it sensitive to wrong sparks and can sparks cause a problem like this.

Also the VRO pumps is making a knocking sound, the one more than the other, should I premix instead, it does get oil but maybe the vro pumps is getting worn out.

[58Evinrude]

If the VRO pumps are noisy, I'd be taking them off, and using an electric pump. Premix the gas/oil at 50:1, and check the gas lines from the tank to the engine.. should be at least 5/16" inside diameter. The 300's need a lot of gas!!

[PanRonnie]

for a V8 you always need an electric fuel pump in addition to your VRO pumps


but like 58 evinrude said if you don,t trust them bypass them with a high volume pump and pressure regulator
i would prefer a regulator with a return hose
if you set that up right you can direct air bubbels in the return hose you get from that almost empty tank and rough water
if you keep the pressure low your less likely to jam your carburetor floats stuck

[TopEnd]

If the VRO pumps are noisy, I'd be taking them off, and using an electric pump. Premix the gas/oil at 50:1, and check the gas lines from the tank to the engine.. should be at least 5/16" inside diameter. The 300's need a lot of gas!!

58Evinrude thanks for helping me out..

Ok, I will remove the VRO's close the holes for the rods etc., It has the orignal fuel pump and it is big and sounds ok, but maybe no longer strong enough in the high rpm, I will also check all fuel lines etc.

When it comes to spark plugs, NGK BP6H S with standard play 0.7mm/ .028" - but I recall a thread about V8 that the gap need to be 0.4mm/ .016" is that correct.?....wrong plug, to hot to cold....just pulled a few today and they are very clean.

[TopEnd]

for a V8 you always need an electric fuel pump in addition to your VRO pumps


but like 58 evinrude said if you don,t trust them bypass them with a high volume pump and pressure regulator
i would prefer a regulator with a return hose
if you set that up right you can direct air bubbels in the return hose you get from that almost empty tank and rough water
if you keep the pressure low your less likely to jam your carburetor floats stuck

Hi PanRonnie

That is the fuel pump I have, but maybe I should find a new one + a regulator and return hose, the one I have could be worn out.

[58Evinrude]

Isn't that pump just to get fuel to the VRO pumps for priming? I'd take that one off, the VRO pumps, and install a regulated pump with a return hose. That's what I'll be doing with my 300.

For spark plugs I'd go with the original Champion QL77JC4 with 0.8 mm gap.

[TopEnd]

Today I removed the VRO pumps and took it out for a spin, 5200rpm and 50mph, It will be the last run with that boat, just to heavy in the front, impossible to lift the bow unless I hit a wave

Yes you are right it's a pump for the vro's only, so I will have to find another one and with a regulator & return hose.

I will also change the sparks to the original champion QL77JC4.

Its a safe feeling with the premix, one less thing to worry about.

[58Evinrude]

Glad to know that was the trouble. I'm surprised that you can't get the bow of that boat up with the 300 on the back? Maybe it's full of waterlogged foam like my Stratos was. There must have been at least 500 pounds taken out it so far, and it's not all out. Either that or you don't have the right prop on it.

[TopEnd]

The boat does have the foam, but I really didn't think it could weight so much more if waterlogged, but the boat does handle very poor and I can't lift the bow, steering also very hard, but all I was thinking was okay the engine will also be pulled to another light weight hull, but I was also thinking okay they really made this boat solid back then...lol...

Another thing that was playing me today was the rpm..again, I took a run and hit about 5000rpm and it started to struggle again when trying to push it above 5000rpm, I pulled the cowl and the lowest cylinder was smoking a little out of the carburator and gas dripping a little mayby some water, but I also noticed a little water in the bay, so maybe some of that water was sucked in by the lowest cylinder, I tryed to rev the engine, I could in first place rev it op to 5100rpm then suddenly the engine rev very clean up to 6000rpm and sounded like a Lambo ...the lowest cylinder stopped smoking out of the carb and no dripping, so either it sucked som water or a blown headgasket, I testet PSI on all and it was all between 110-112psi..the lowest cyl. was 112....

But then again I was out for a spin with 5100rpm and it didn't struggle that was the day when removing VRO, so hitting the 6000rpm must the heavy bow and prop that hinders me to reach for higher.

[58Evinrude]

It sounds like you are on the right track. It doesn't take a lot of water to add a lot of weight to a boat.. if I'm doing the conversions right, water is one Kilogram/Litre.. and the foam holds a LOT of water.