Idle Issue - Carb or Fuel Problem??

[BamaBowrider 208]

Although I am sure this topic has been discussed before, I have searched the forum and still have questions. My problem is an erratic and unstable idle with this 5.7 2bbl motor. I purchased this boat in December at which time the tank was half full of unknown fuel. I added enough marine Stabil to treat it at that time. I have replaced the fuel/water separator and performed a tune up with new cap, rotor, plugs properly gapped and timing set. On the muffs, in the driveway the motor idle is inconsistent and drops down to 500 and eventually dies. I can fire it right back up and it will run for a while and then stumbles and dies.

Yesterday I topped off the tank with fresh marine non-ethanol fuel and took it to the boat launch where I could hopefully run some fresher fuel through the carb at increased rpms. It idled a little better. Not every time, but more than once when I put it in gear and loaded the motor it would die, but fire right back up. So for about an hour I ran the motor idling in and out of gear and increasing the rpms when the engine would stumble and try to die.

This morning I wanted to run it again on the muffs and record a timeline as follows;

1 throttle prime and engine fired almost instantly

Idle constantly fluctuates from 600-750 RPMs

6 min = idle increases to 850

14 min = idle starts to fluctuate from 825-850

16 min = enrichment valve starts closing, idle decreases to 750

18 min = idle decreased to 625

20 min = enrichment valve fully closed, idle drops and fluctuates from 500-600

25 min = adjust idle screw, idle fluctuates from 650-700

26 min = idle increases to 1000, readjust idle screw to bring idle back down to 700

32 min = idle fluctuates from 700-750 with occasional increase to 850 for a few seconds

35 min = idle drops and fluctuates 600-650

37 min = shut down

NOTE: the motor stumbled at the 20 minute mark, but never died today.

This is a 2007 model Larson 208 LXI, original 5.0 with a 2012 replacement 5.7 longblock. One thing I have noticed is the amount of air that the enrichment valve allows or pulls through the carb passage is very noticeable, almost loud until it starts to close. Not sure if that is normal. Anyway, I need to know where to go from here. Siphon out all the fuel and start fresh, pull the carb apart, clean out and rebuild.....maybe both??? Need some advice.

 

[GA_Boater]

Adding new gas to old, bad gas makes a bunch more bad gas.

Since you are trying to determine if you have a bad gas or carb problem, eliminate one. Use a small tank like for an outboard with fresh gas and see how it runs. After that, make your next decision.

 

[alldodge]

As GA said, run it off a gas can.
I'm thinking the carb needs a rebuild and would also check fuel pressure. Pressure should be 3-7 psi

 

[BamaBowrider 208]

Adding new gas to old, bad gas makes a bunch more bad gas.

Since you are trying to determine if you have a bad gas or carb problem, eliminate one. Use a small tank like for an outboard with fresh gas and see how it runs. After that, make your next decision.

Yep, now I have a huge mess on my hands as I filled the tank....stupid move if I have to syphon it all out. I ran it straight off of a gas can today with fresh 89 octane, when it fired up it went straight to 1200 rpms and then slowly decreased as the engine warmed up. After 16 minutes the enrichment valve closed and it idled fairly steady at 625. I adjusted the idle screw to raise it to 650........it ran there for a while, revved it up to 1200 rpms a few times, but it still wants to wander from 675 to 750 rpms when I bring it back down to idle.

 

[alldodge]

Is the throttle cable adjusted so it applies a slight bit of pressure holding carb closed at idle?

When its running, do you see any drips down the throat of the carb?

 

[BamaBowrider 208]

As GA said, run it off a gas can.
I'm thinking the carb needs a rebuild and would also check fuel pressure. Pressure should be 3-7 psi

There is no place to tap into the fuel line from the pump to the carb to check the pressure. I am going to have to get the #91-18078 pressure test fitting. I'll have to order one, doubt I can find it locally. Is it possible to measure the vacuum from one of the inlet ports to the fuel filter/water separator housing to determine if the pump is working properly?

 

[alldodge]

With a carb you can pick up a Tee fitting and carb rubber hose fitting at the local auto parts store. Get some fuel line and clamps and make up your own.

Use a vacuum/fuel pump tester which can be rented from the auto store. Connect the steel line to the Tee with short piece of fuel line.

https://www.harborfreight.com/fuel-pump-and-vacuum-tester-62637.html

 

[BamaBowrider 208]

Is the throttle cable adjusted so it applies a slight bit of pressure holding carb closed at idle?

When its running, do you see any drips down the throat of the carb?

Yes to the throttle cable

I am not sure, I will have to fire it back up and check today.

 

[BamaBowrider 208]

Ran the motor again off the bottle today. Had my assistant prime the throttle while I looked down inside the carb......a steady stream out of both squirters. Hit the key, fired right up just like the last few days......except this time the motor goes directly to 1400rpms. Warms up in 10 minutes, enrichment valve closes, still at 1000 rpms so I back the idle screw off to 750. (I had not touched the idle screw since adjusting yesterday when it was running in the 600-700rpm range) It runs there really steady for the next 10 minutes or so, only fluctuating 25 rpms either way at times. Kept an eye down inside the carb and no fuel dripping.

So there must be some debris or varnish build-up inside the carb passageways that might be getting flushed out or dislodged for it to be acting this way??? It ran much more consistent today. My rebuild kit was delivered today. I still plan to measure the fuel pressure before I disassemble anything.

 

[alldodge]

Here's to hoping the rebuild does the trick

 

[BamaBowrider 208]

With a carb you can pick up a Tee fitting and carb rubber hose fitting at the local auto parts store. Get some fuel line and clamps and make up your own.

Use a vacuum/fuel pump tester which can be rented from the auto store. Connect the steel line to the Tee with short piece of fuel line.

https://www.harborfreight.com/fuel-pump-and-vacuum-tester-62637.html

Scrounged through my old carb parts and found enough fittings to hook up my fuel pressure gauge. When the engine fired, went to 1200 and stayed there for several minutes then decreased to 1000 rpms.....during this time the gauge read a steady 6.5 psi. After it warmed up, the valve closed and the rpms dropped to 750 the fuel pressure also dropped to 5psi. Is that normal??? Let the engine run for another ten minutes or so and watched the fuel pressure needle dance around between 4.75 and 5.50psi with no noticeable affect on engine rpm or how it ran.

Then without warning, after 20 to 25 minutes of idling, the engine rpms dropped to just under 600 with no drop in fuel pressure. I haven't touched the idle screw since earlier today, but had to turn it a half turn to bring the idle back up to 750rpms.

So what do you think??? Is the fuel pump pressure steady enough?? Time to pull the carb apart and clean it out??? Also, when hooking up the pressure testing rig to the carb inlet, I pulled the filter screen out and it looked brand new, completely clean, not a spec of dirt or any discoloration.

 

[alldodge]

Fuel pressure is great
Do think the issue is your carb
Have not said it out loud, but the enrichment valve tells me its a TKS carb

 

[BamaBowrider 208]

Fuel pressure is great
Do think the issue is your carb
Have not said it out loud, but the enrichment valve tells me its a TKS carb

Yes....TKS carb.
Not familiar with them, when I first started the engine I thought there was a bad vacuum leak or some other issue until I realized where the air sucking sound was coming from, it's pretty loud when the engine is cold.

 

[BamaBowrider 208]

I wanted to follow up on this in case anyone else has this issue. It's been several months now, but my work schedule has been crazy...not leaving me enough spare time.

Pulled the carb apart for the rebuild, first thing to note was an excessive amount of debris in the fuel bowl, it was a mixture of powder and large enough granular material to get stuck in the jets. I could actually scoop up a little on the end of a small screwdriver. Both the float level and drop adjustments were way off. Cleaned it up, blew all the passageways out and installed the new parts.

I have had her out three times now for several hours of run time. Fires right up, idles steady, accelerates without hesitation......success!!!