90 HP Johnson 2 stroke surging at high power settings

[flash drive]

Hi All,
I have read many post related to this problem. My issue is not at WOT. My previous issue was cavitation/ventilation. I pulled the boat and lowered the motor as per some other threads on here. Test ride went well. The cavitation is no longer an issue. I lost about 400rpm by lowering the motor which is not a big deal since it creates more drag Im guessing. So at WOT I generate about 4700 RPM. It runs steady there but when I back off to around 4300 the motor lags and recovers over and over. If I back off a little further it runs smooth. If I squeeze the bulb while it is lagging it runs smooth and actually increases a few hundred RPM's. I replaced the fuel pump at the start of the season. I read that if the bulb is squeezed and problem goes away its fuel pump related. Could it be anything else? I know new parts can be defective but as we know its a pricey item to swap out just to troubleshoot.Any other suggestions?? By the way its on a 22ft pontoon boat....Thanks

 

[bouttime007]

Not just fuel pump related, but could be fuel related in general.

Check all your fittings on gas and vacuum lines for loose fit.

 

[emdsapmgr]

Bout is correct. The correct problem statement should be that you are likely experiencing a "fuel delivery" problem. That can include a weak fuel pump, but can also include: incorrect new fuel pump, bad fuel hose bulb (due to bad/sticking ball valves), fuel hose restriction, stuck/restricted anti-siphon valve at the tank, loose fuel pickup under the fuel tank head, fuel pickup tube that has holes in it, loose fuel hose connections on the engine, stuck floats in the carbs, etc. You did a good test, by constantly priming the fuel hose bulb. Since the engine ran better, you've confirmed it's running out of fuel by itself. You will get that up/down rpm change, as the engine consumes more fuel than the fuel system can supply. Once the carb bowls run out of fuel, the engine will slow, till the fuel system can supply more fuel, then it may take off for a short spurt. A continuing cycle.