johnson 300 low end bog

[jd98x]

I bought a Johnson 4L V8 300 and put it on a Baja 220 sport with a gil braket and jackplate. I have tryed 5 very different props and the consistent problem i have is the motor wont put down low. Is this normal on one of these? Ive been as high as a 29 four blade and have no trouble pulling it on top but its a bitch to get on plane. I tryed a 23 three blade and had the same low end bog.. Lookin for ideas, been threw all the carbs and there clean, running 93 non ethanol gas to with good Merc oil..

[kimswang]

Merc oil????? Good Merc...??? LOL.... Just kidding of course.


Check compression first. Then check timing. Then check plugs to see if you're lean or rich. Post results.

[jd98x]

Compression is 125-130 rite across the board, leakdown was less than 5%, plugs are Carmel colored and timming hits the mark at 16 deg every pull. Plugs are a champion heat range 7. We also tryed a 27 with big holes in it to see if it would spool up at a lower rpm and same thing. Pulled the plug wires off the plugs while it's idleing and there all fireing. When the motor is idleing and you trim it all the way in it seems to load up a bit, do you think the float levels could be off? I'm guessing at this point.

[Action Dave]

Compression is 125-130 rite across the board, leakdown was less than 5%, plugs are Carmel colored and timming hits the mark at 16 deg every pull. Plugs are a champion heat range 7. We also tryed a 27 with big holes in it to see if it would spool up at a lower rpm and same thing. Pulled the plug wires off the plugs while it's idleing and there all fireing. When the motor is idleing and you trim it all the way in it seems to load up a bit, do you think the float levels could be off? I'm guessing at this point.


125 lbs is high compression for a V8. Stock is usally in the 100-105 range. It's possible that it was jetted super rich to compensate for the compression boost. It's very possible you're loading up at idle and it takes a bit for it too clear out when you plane off.

[Miss Isle]

I'd definately change to NGK sparkplugs, champions suck bad.

How's the link and sync? Maybe timing is advancing too early in the lower throttle postitons.

[wideopen23]

drill out the exhaust snout if you havent. I was having this same problem relieved the exhaust and holy moly she came alive

[jd98x]

did some research this weekend and this is what i found.. Motor seems to have a crazy powerband allmost like a dirt bike, not quite sure why but it certainly does.. That gave me a two part problem, 1 when you put a big enough wheel on it to be able to leg it out and not blow it up you couldnt get on plane cause the motor wouldnt pull it down low. 2 when I put a small prop on it would spool up to quickly and blow the prop out before i could get on plane. With the Gil bracket and jack plate my X dim is realy high so it dosnt get much traction tryin to get on plane when the bow starts to drop. Theres a plate mounted on the bottom of the gil bracket that im guessing is used to help this but when u angle it to much it pushes the water away from the prop.. So what i did was brought that plate allmost to the top and put an ugly whales tail on the end of the motor and a 27 tempest with huge exaust holes in it.. I hate those stupid tails but it worked very well!! 79 mph with awesome holeshots i can deal with that ugly thing hagin off the back!! Thanx for all the advice and help guys i realy do apriciate it!!

[racer]

Are you sure it is a 4.0 what you are describing sounds exactly like a 3.6 acts. It may be floating with the exhaust tuner in the water try starting with the trim level and triming in as you hit the throttle to see if that helps.

[jd98x]

That's a good question. I was told it's a 4L by the guy I bought it from but I don't know how to tell there's no tag on it anywhere so i'm not sure what it is. I was told to take numbers off a frost plug and try that, they didn't match anything we saw in any books or on the computer.

[racer]

Look in the cylinder a 4.0 will have a slight dome in the piston while the 3.6 is flat

[kimswang]

Glad to hear you found a solution. Post a picture of the top of the headgasket (small lipp sticking out on top) and we can tell you wheter or not it is a 4.0.

[racer]

Yes on the gasket assuming the right one for the block was installed, better to verify the piston top then make sure it has the right gasket.

[jd98x]

Got pics of the gasket and will upload them tonite. Pistons look flat but realy hard to tell. Gonna pull a head off so I can figure it out. Guy I bought it from had it listed as a 4L 250 with big bore carbs that make it a 300? Does this make sense?

[kimswang]

Got pics of the gasket and will upload them tonite. Pistons look flat but realy hard to tell. Gonna pull a head off so I can figure it out. Guy I bought it from had it listed as a 4L 250 with big bore carbs that make it a 300? Does this make sense?

Nope, the 300 had midbore carbs in that era. The 250 4.0L is the favorite motor to start working on though.

[jd98x]

pics of the head gaskets..

[kimswang]

4.0, assuming correct gaskets are used, like Racer mentioned.

[jd98x]

Well I'm glad to hear that it's a 4L. It's been a great engine so far and I'm very happy with it. I'm useing Quiksilver premiem oil and 93 non ethanol fuel just switched from a Champion plug to an NGK plug. Anything I should watch out for with this thing? I'm useing the vro for fuel and oil with a Carter fuel pump feeding the vro.

[jd98x]

It had Champion QL78YC and I switched to an NGK BZ7HS-10. Haven't run the NGK yet but the plug is shorter. Is this the rite plug?

[Miss Isle]

Well I'm glad to hear that it's a 4L. It's been a great engine so far and I'm very happy with it. I'm useing Quiksilver premiem oil and 93 non ethanol fuel just switched from a Champion plug to an NGK plug. Anything I should watch out for with this thing? I'm useing the vro for fuel and oil with a Carter fuel pump feeding the vro.

That makes no sense to me at all. If your still using the VRO, that is a pump all in itself. All you need to continue to use the VRO is a primer bulb. That may be your whole problem. I.E. too much fuel pressure at mid throttle and flooding condition.

I'd consider eliminating the VRO and going to pre-mix and fuel pump, as the unit is getting some age on it.

[jd98x]

I know what your sayin. It made no sence to me either but I think the pump is a transfer pump only I unhooked that pump and tryed to run it on the vro only it wouldn't run. Guy I bought it from said the vro worked fine. It's def not overfuelin I ran it at for 2 hours last weekend at 1100rpms towin somebody in and it never missed a beat.