Primer bulb when extended idling

[bware]

95 Merc 150 XRi - If I am at extended idle, like going through a long no-wake zone the engine slows/dies and I have to squeeze primer bulb to get going again. Seems worse on warmer days. I’ve replaced the line and bulb but doesn’t seem to make a difference. Does the position of the bulb matter? (is tilted downward) Higher RPM operation has no problems. SN 0G276525

[phyrefighterjay]

I had a similar thing with my opti 150. It was after i rebuilt my diaphram fuel pump. The rebuild kit was sierra, it had extra parts in it so i stupidly used them. Well, come to find out the OptiMaxx didn’t need the little rubber disc’s to go into that pump. Just the clear ones..(the merc kit dosent even include the rubber ones). The engine would work well at higher rpm’s… But would stall out and need to be re-pumped at idle to 1500 rpm’s. So I would start looking at your lift pump. Don’t get too bogged down with the bulb… Just make sure the arrows pointing in the right direction

[lpugh]

You should also check for a strong pulse from the crankcase hose that, makes the pulse pump function. Low compression or bad reeds can lower the strength of the pulse causing the problem you describe. Also check the lines carefully from the pulse pump all the way back to the tank. Pressure or vacuum test would be best. Any air being drawn in will cause this type of problem, this is critical.
When you pump up the bulb it is best to point the arrow up as the will purge the air bubble out, if not pointed up a vapor bubble can stay there. The pulse pump works by pulling vacuum on the line going to the tank and vapor bubble can expand under vacuum causing the pulse from the pump to be a little lower. Thus it will not pump as much volume. I have mine mounted so that it is always pointed up for this reason. It may work laying flat or pointed down, but it will flow more fuel volume pointed up

I am sure you have heard the term vapor lock and you have mentioned on warm days it is worse. When vacuum is applied to gas it turns to a vapor much faster and increased temperature makes it even worse, which can actually cause a bubble to form in the bulb...…...IE: Vapor Lock. The larger the vapor bubble the harder it becomes for the pulse pump to draw the fuel in, kinda a catch 22
This is why auto manufacture went to electric in tank pumps years ago. So we could push the fuel under pressure instead of pulling it. We never see vapor lock issues on those systems as a result. This can not be safely done in marine applications, pressurized fuel in the bilge area near the batteries and pumps could be hazardous

[JOE54]

and don't forget winter blend fuels much more prone to vap loc