Jet boat bog

[Monster12valve]

Hi everyone,

I am having trouble with my 720 sportster. It starts, idles good, give it throttle and around half way it bogs bad. If i keep it throttled when it’s bogging it back fires through the carb. Here is everything I have tried....

Rebuilt carb

After reading on here about junk rebuild kits, I rebuilt it again with a “back to oem” kit, no different.
I have changed plugs
Cut wires back 1/4 inch
Unplugged red wire in electrical box
Took off front cover, all looks new, no shaft play. This motor has very low hours (been out 4 times since professional full rebuild) carb gaskets have been replaced 2 times (thought maybe pulling air) I am at a loss. Compression is 144psi in both cylinders. When it idles I don’t see or smell any exhaust leaks either. Air intake is stock, jets are all the factory numbers, lows are 1 1/3rd out, highs are closed. Any help would be greatly appreciated, I am out of ideas And frustrated. I have also swapped intake manifolds out with the old motor, to make sure it wasn’t cracked in shipping on new motor where I couldn’t see it.

 

[Monster12valve]

Some history on the boat.. I bought it with a hard starting issue (very cheap), the prev owner just had motor put in (with receipts) he used it 2 times, but hard starting both times. I got it home ran compression... one cylinder was dead, cracked piston. I called company, took it to them and they put a new top end In it, even though it was obvious the issue was a lean cylinder. I then rebuilt carbs completely, put it all together and back in. Fired right up, fast and easy... but now has a terrible bog and back fire.

 

[Monster12valve]

Rv timing has been checked also, forgot about that. The mpem and flywheel are off old motor.

 

[Dr Honda]

THe carbs still aren't right. Take them apart, and verify EVERYTHING. Make sure all the bypass ports are clear. Make sure the main feeds are clear. Verify that the pop-off is right. Make sure the high speed check valve is flat. make sure the meter block fits and seals.

When my shop was open, I rebuilt 100's of carbs remotely, and they all worked when they went back on. BUT... with that said... I did get a few in that were rotten, and beyond rebuild. (leaking shafts, eroded internals)

 

[Monster12valve]

Ok, I will take them apart again today and fully check everything again, and report back. Thank you for the response!

 

[Monster12valve]

Any other ideas? I took carbs off, blew through every jet, hole using a piece of welding wire to losen anything up, same thing. Called a local friend with same Boat I have but twin engine speedster. Swapped carbs, exact same thing. Bad bog... back fire. Idles great! One thing I did notice after rebuilding again... rear carb drips fuel from Venturi slightly, even at idle. I also pulled the head, just to make sure everything looked good, and it did.

 

[Dr Honda]

So.... with a known good set of carbs (from your buddies boat) you still get a bog?

 

[Monster12valve]

Yes it was. But I think... hopefully I got it. I bought a new box of plugs from seadoo (ngk) and changed them, even though it was running good. I did it because I was rebuilding carbs while motor was getting done... so motor came back, put new plugs in... not the old ones. So for the hell of it, I put the old plugs back in just now, and no bogging or backfiring! So to prove it, I put other (new) plugs back it... bogging. So for the heck of it... I put the other new pair in (box of 4)... back to bogging and backfiring. Old (original) plugs back in, runs perfect! The plugs are the same part number as the new box that isn’t working right. NGK br8es. I think I got a bad box of plugs!!!!!

 

[UNYboater]

NGK br8es. I think I got a bad box of plugs!!!!!

There are two types of BR8ES plugs though (different connectors maybe?). No way you ended up with the wrong type is there?

 

[mikidymac]

You want to get the NGK 3961, they are a BR8ES but with the solid top electrode so they don't rattle loose. O'reilly has them for about 2.50 each. Make sure you ask them for NGK 3961 or they will sell you the other BR8Es's.