OMC looper top end surge

[Machinist76]

94 Venom 200 STLERK
89 Stratos 201
Beat up 24P Raker and/or 25P Shooter (mint condition) though still junk from what I read.....

Fresh rebuild, I did.

Starts instantly, holeshots dead perfect...zero complaints.idles in gear well, doesn't load up

Top speed is 45-50 mph,. Lugging...tach broke...sounds like maybe 5000 rpm to me, definitely sounds flat. Engine surges like it's lost one or two cylinders.,. intermittently....

Compression good, lynked and synced.


If I had to guess, one or both of the lower carbs are not staying open....or lacking fuel. VRO gone feed by electric pump at 4 psi

Anyone got a quick "hey, mine did that it's...." or need I check all fuel, and electrical?

Are the timing magnets under flywheel able to cause this issue? It's only running issues are at WOT.

[Machinist76]

Found culprit I believe.

Max timing was maybe 10-12*, due to tension spring I installed between trigger linkage and CDI box.
I did this because hot foot won't seem to allow both WOT and completely closing to idle...

Test run tomorrow and see results

[Machinist76]

Ran it today, no real improvement.

Guess I'll go all through the carbs, can't see it being electrical affecting only upper rpms

[baja200merk]

Confirm All pistons are upright and in their appropriate side.

If it won’t get to 18* at 5k under load Check center hub of the flywheel make sure it has not spun.

[Motv18]

Holding 5 and surge up or hold 5k and drop down? Up look to spark down look to fuel

[Machinist76]

Pistons oriented and location is correct.

It will reach 18* now.

I'm still just guessing on the rpms. MUST GET tach to work/replace.

Thanks for the insight guys, will report back as I can....

[Gorilla0178]

Had a similar prob too. WOT surge, mine was an issue with the original fuel pump. Have someone pump the primer ball when it starts surging and see if it goes away. If it does its a fuel issue, I put an electric pump on and had no more issues.

[Machinist76]

Update:

Tach fixed. She turns 4800 @ 45mph. Trimmed way out 4950 no speed gain.

Side note, I'm convinced this boat is severely waterlogged. Little bow lift, not fast enough to chine walk although it does porpoise a bit mid throttle 35-40 mph

Have full rebuild kit for carbs to install, I personally never yet had them apart...

FWIW, lots of black soot on prop... I think it's jetted fat pig rich

[warriorcookie]

What main jets are in the carbs? Is it the stock carbs or have you swapped them to something else? Make sure the mains haven't backed out too.

When you do your link and sync you should have a good idea wether or not the bottom 2 carbs are staying open or not. If your butterfly's have screws make sure they are tight.

Does your timer base rotate freely? Sometimes the stator melts and the gunk seizes the base.

How are you checking timing? You need a test prop in a tank or on the trailer at the launch with 5k+ rpm and the quick start disabled.

Use the guides at https://www.cdielectronics.com/ to test your ignition components before you start throwing parts at it. You need a multimeter and to either purchase or build a DVA adapter.

[FMP]

Read plugs

[Machinist76]

Update:

Guys I'm stumped. Test ran a brand new 21P aluminum prop, same rpms 43 or so mph.

I should have gained 900-1200 rpms from the 24 Raker.

Carbs coming off tomorrow afternoon. However, im convinced it ain't gonna matter.

If it we're lean it would have done melted piston.

To add to my above mentioned list of what it's not....

Sparkbox/ powepack
Stuck trigger
Melted stator
Trigger magnets loose/moved
Charging magnets loose/missing
Fuel pump/primer bulb...have electric @4 psi
Low compression
Pistons in wrong side or upside down
Mismatched carbs or intake/way wrong jets. I haven't got numbers in front of me
But from memory the are 2 down on idle and 4 down mids. (Remember that's richer, they meter air)

[FMP]

Because you haven't mentioned it , check replace the spark leads.

[Machinist76]

Plug wires are Prestolite 108717 7MM. Not sure if they are correct.

I have read about RFI/MFI causing the rev limiter to happen at my exact (problem) rpm, 4900.

Hopefully it's the wires are junk....

I'll give a bit more background of this boat/motor here shortly...for entertainment and educational purpose.. HAHA

[Captain75]

plug wires are prestolite 108717 7mm. Not sure if they are correct.

I have read about rfi/mfi causing the rev limiter to happen at my exact (problem) rpm, 4900.

Hopefully it's the wires are junk....


I'll give a bit more background of this boat/motor here shortly...for entertainment and educational purpose.. Haha

rfi/mfi??

[Machinist76]

Electro magnetic interference.

Noise.

Oscilloscope stuff...

Way above my electrician abilities

https://maxrules.com/fixomcoisignition.html