25 yamaha 4 stroke starts easy, idles, but dies upon increase idle

[TooLongACubFan27]

Problem started as loose fuel hose on backside where gas enters motor before fuel filter. . .bought new hose. . .noticed a kink in fuel line just in front of engine and replaced it and fuel bulb. . .after plugging fuel bulb into hose leading back to tank, engine would start great like b4 but stall after accelerating for a minute or so. . .looked into existing fuel line where I connected bulb and to my surprise, it had a plastic inner lining that had collapsed--so I changed out the remaining piece of hose connecting the tank. . .still started easy but now would die after a while.
Had mechanic look at it and he suggested and I bought a new fuel pump. . .next he suggested the Carb be rebuilt so "I" ordered a carb rebuilding Kit and decided to do it myself without a manual. Bought "new" gas and have used "Stabilizer" for 7 years--know I shouldn't have, but tanker was pumping gas into their tank when I arrived at the service station and bought the gas anyway. . .couldn't even get it started now. . .another mechanic is bought in and after relating the whole saga, of course he looks into tank and gas can and saw lots of junk turned the bottom of both from red to yellow. . .so cleaned carb, changed fuel filter and plugs AND bought NEW gas (no Ethanol)

Now it starts easy, idles good and either dies after a while or dies when I try to increase idle via remote control Idle shifter arm. . .rebuilt carb--cleaned, changing setting on "Prime Starter Assembly" from 3 turns to 6 turns getting various degrees of rough starting, rough idling but smooths out on high idle to starts easy, idles ok to good, but then dies upon increasing idle. . .

[boscoe99]

Where in the Yamaha manual does it address making an adjustment to the Prime Start device? I have not seen this.

[99yam40]

changing setting on "Prime Starter Assembly" from 3 turns to 6 turns getting various degrees of rough starting,.

I did not know there was an adjustment on the prime start, are you sure about this?

Of course I do not know much about the 4 stroke 25

dang you beat me to it

[TownsendsFJR1300]

I'd be checking the filters and make sure their spotless.

Then, pull the carb off and get to the main and idle jets. Pull them both out and make sure you can see thru them. If you have an air compressor some air thru the openngs wouldn't hurt. Brake cleaner will work in a pinch. Being you don't have a shop manual, if you go to the "Purchase Yamaha Parts" above, punch in your engines info, it'll bring up a parts fisch and show where all the jets are, etc. Some jets (don't know if it appies to your motor) have holes SIDEWAYS in the jet and any clogging is NOT visable unless you pull it out of the body of the carb and visually inspect it. The smaller the engine, the smaller the jets, the easier to clog up...

Sounds to me like some crap is in the carb is clogging up the jets shutting down the fuel flow... Please drain the carb into a very clean small container (I use a tuna fish can) and check for any debris. A squeeze of the primer bulb with the drain plug out will help flush any crap out as well.

Of course, look for any water in that fuel that comes out..

Good luck.

[TooLongACubFan27]

For boscoe99 and 99yam40

On the parts list I do have the Prime Start Assembly and a few other parts shown but the major spring, brass activator and many other parts are not shown. . .however in the first break-down of the carb, I wasn't "present" and the other party did not "note" the number of turns it was screwed into its plastic base--thus one of my dilemmas. . .but appreciate any input. . .thank you both.

[TooLongACubFan27]

For TownsendsFJR1300

I understand and appreciate your input and have changed fuel filters 3 times, removed jets, etc. and soaked and flushed in carbuerator cleaner, then used air compressor to further clean/remove debris. . .did note one such "sideways" jet only held in place by a very tiny "o" ring and cover rubber gasket and matching plate held by three screws. . .I am 95% sure the jets and very tiny air holes are clear of all debris as I have taken this carb apart no fewer than 10 times, however, up to the last two cleanings I did see and remove many blackish "specks" which were more liquid than solid--but I'm pretty baffled as is a couple of others who believe they are decent marine outboard mechanics. But I do thank you for your input and will AGAIN remove the carbuerator tomorrow to "clean" and stare at it. . . .

[TownsendsFJR1300]

Sounds like you got into as deep as you should have. Just draining the carb will show if that black stuff is still an issue.

The only other thing I have experianced, as you have was the fuel line inside literally breaking down (and it was marine grade). Check the rest of the lines, the primer bulb, the line/filter in the tank itself. Perhaps loosen the cap on the tank should the vent be clogged..(just a shot in the dark)

One other thing, depending on the style of the Yamaha fuel filter, my "cup" part of the filter expanded from ethonol and would NOT SEAT fully on the base, causing an air leak. A new cup, (under warranty), and it slippind in the housing easily (whereas, the enlarged one would not)

I'm not terribly familiar with the Prime start, but would think once the engines warmed up, its not used anymore and the adjustment likely wouldn't matter.

As a side note, if there is ANY varnish or such, Chevron Techtron (Walmart and most automotive stores carry it-black plastic botle) makes an excellent fuel system cleaner, cheap enough, won't hurt anything and can only help.

Other than that, I'm stumped.

@Rodbolt should chime in have some accurate ideas on what to do next.


Good luck..

[TooLongACubFan27]

The black stuff is/has been a continuing question mark. . .as I have changed every fuel line with marine grade and changed out the primer bulb to no avail--the problem persists. . .my last mechanic went so far as to completely loosen the gas cap (which I thought strange at the time), but that did not affect the performance. . .he also felt the Prime start was a non-factor once the engine warmed up. . .seems like the both of you were on the same page (so that is comforting to me), but he left my home scratching and shaking his head as well!!! Today is another day--and thanks again for your incite, much appreciated.

[99yam40]

On the parts list I do have the Prime Start Assembly and a few other parts shown but the major spring, brass activator and many other parts are not shown. . .however in the first break-down of the carb, I wasn't "present" and the other party did not "note" the number of turns it was screwed into its plastic base--thus one of my dilemmas. . .but appreciate any input. . .thank you both.

I have never heard of someone taking apart the prime start module.
All that I have read in manuals is to measure the distance it is extended , then hook up 12 volts to it for a period of time and the measure to make sure it has extended a certain amount. Have you followed procedures in service manual for anything dealing with the carbs?

[TownsendsFJR1300]

I'm out of idea's.

PM Rodbolt if he doesn't find this and ask for assistance. He doesn't generally answer PM's but will address the problem and likely have an answer..

[TooLongACubFan27]

To boscoe99; 99yam40; TownsendsFJR1300. . .many thanks to you all--took 12 tries, but upon taking carb completely down--I discovered the primary idle jet (the one with the rubber stopper covering it) was partially blocked with the "bad gas" yellow gook and the Plunger and spring were stuck (not free floating). Upon resembling carb and installing--motor fired right up and ran like a kitten. . .as far as the "prime start monule" is concerned, I tried to recall spacing between base and "L" shape part--as well as the color difference on exposed threads and settled on that for the "test". . .ran very well, but intend to review this "setting" at a later point in time. Anyway, thanks again for your time and consideration. Respectfully, Gerald D. Freeman

[TownsendsFJR1300]

To boscoe99; 99yam40; TownsendsFJR1300. . .many thanks to you all--took 12 tries, but upon taking carb completely down--I discovered the primary idle jet (the one with the rubber stopper covering it) was partially blocked with the "bad gas" yellow gook and the Plunger and spring were stuck (not free floating). Upon resembling carb and installing--motor fired right up and ran like a kitten. . .as far as the "prime start monule" is concerned, I tried to recall spacing between base and "L" shape part--as well as the color difference on exposed threads and settled on that for the "test". . .ran very well, but intend to review this "setting" at a later point in time. Anyway, thanks again for your time and consideration. Respectfully, Gerald D. Freeman

Glad to hear it got straightened out Gerald. Sounded like the carb..

Besides finding and repairing the actual hard mechanical issues, I would strongly suggest using a fuel stabilizer to keep the yellow gook and varnish from accumulating, just preventive maintainance. Even if you run the engine dry, un-hook the tank, a small residue remains in the engine/lines/fuel pump, etc. IMO, pulling drain plugs on carbs is a PIA, especially ones hard to get to.

IME, using one of the below additives, will prevent the crappy fuel we have nowadays from gooing up your carb. That carb can be on a boat engine, mower, weedeater, etc..

I've had good luck with "Gas Shok", "Seafoam", and for large built in tanks with water issues, K100. Also as noted previously, Chevron Techtron works well for cleaning (not stabilizing) fuel systems.


True story re a clogged set of carbs and Seafoam.
My friend got a very low mileage 500cc Kawaski motorcycle that hadn't been run in years. The two carbs were completly clogged and wouldn't run.

I offered to pull them, clean the jets and varnish but he didn't want to do that. Instead, we drained the fuel from the float bowls, (same as an outboard carb) and feed straight Seafoam into the float bowls and let sit for several hours. We did NOT do anything else.

Eventually we drained the Seafoam, and added fresh fuel. Cranked it some, one of two cylinders starting firing, smoking like a b...ch, the the second cylinder shortly thereafter, started firing. After 10-15 minutes of smoking and some revving, it was running great, idleing, accelerating, un-believeable.

I knew it worked well but certainly didn't think this would work as the carbs were really bad (inside the fuel tank was rusted out from being parked under a tarp with a partially full tank, literally rust falling off inside).

Just something to consider..

[TooLongACubFan27]

Thank you again for ALL your input. . .the Chevron Techtron sounds good too. . .will do it!!!