Bog - rpm restricted after 15 minutes

[Bruce1983]

All,

I have done some searching on the site but I am limited as I am accessing the Internet via cell network. Hope you can help me out.

I have a 88 mercruiser 4.3 with mechanical fuel pump and 2 barrel carb. I can run for about 10-15 minutes cruising at 3600 rpm and then it starts to bog. If I don't drop the throttle back it has stalled, but if I do I can keep cruising around 3000 rpm for as long as I want (at least 25 minutes anyway to get back home).

If I drop to idle for a minute or two I can get back to 3600-4000 for a minute or two until it bogs again.

I replaced the pump filter and carb inlet filter, checked the line into the tank and the vent line which seem fine.

Thanks for any help.

 

[HT32BSX115]

Re: Bog - rpm restricted after 15 minutes

Howdy,


It sure sounds like a fuel delivery problem.

If the filter is good and the vent is not plugged, you may have a plugged anti-siphon valve or the fuel pick-up screen in the tank is partially blocked with debris.

 

[180shabah]

Re: Bog - rpm restricted after 15 minutes

I agree, start at the tank and work your way through...

 

[Bruce1983]

Re: Bog - rpm restricted after 15 minutes

Thanks for the feedback. I pulled the tube out of the tank and there is no screen on the bottom. It doesn't appear to be an anti siphon valve either .... Unless it is contained within the tube or the 90 fitting??

 

[jerryjerry05]

Re: Bog - rpm restricted after 15 minutes

Check the distributor advance.Points set right?
Fuel restriction?keep looking.
Check coil once you get it running,If it gets too hot to touch,it's bad.J

 

[180shabah]

Re: Bog - rpm restricted after 15 minutes

A/S valve typically is screwed INTO the 90* fitting on the tank. Externally it just looks like a FPT to barbed fitting, but on the inside there is a one way check valve.

 

[180shabah]

Re: Bog - rpm restricted after 15 minutes

'88 shouldn't have points.

 

[superbenk]

Re: Bog - rpm restricted after 15 minutes

You can get a 1/4" NPT hose fitting for a compressor hose at an auto or hardware store and temporarily replace the anti-siphon to see if it helps (fixed my same issue & saved our vacation). I'll be ordering a new anti-siphon valve when we get home. You may also be developing a vacuum in the tank.

 

[superbenk]

Re: Bog - rpm restricted after 15 minutes

You can probably test for a vacuum situation by loosening the gas cap when you are having your bogging issues. Should clear up I that's where the problem is.

 

[Alpheus]

Re: Bog - rpm restricted after 15 minutes

You may want to check your fuel pressure also...

 

[a70eliminator]

Re: Bog - rpm restricted after 15 minutes

I have heard although rare that a tightly sealed engine compartment can actually starve the engine for air, make sure there are no restrictions in the fresh air supplys, exhaust leaks make matters worse, rare but worth a mention if everything else has been ruled out.

 

[joenapier]

Re: Bog - rpm restricted after 15 minutes

Check also your fuel / water separator filter. If the paper is loaded with water, it will restrict ful flow. Change it once a year.