wont run at high speed after bad gas

[Strahm]

1973 mercury 850, thunderbolt ignition

motor has ran fine, ever since i rebuilt the carbs. ran it several hours since the carb rebuild, no problems whatsoever.

a couple days ago i ran a tank of gas that had sat over the winter and until now. i did add stabil to it before storage.
as soon as the motor picked up this gas, it wouldnt run at high speed.
runs great at idle and 1/4 throttle. i can push the throttle all the way up but the motor will only run at 2500 rpms, and about 5-10 mph (normal full speed is 35)
got fresh gas in a different tank, same problem, but i did run the engine long enough ( at least 1/2 to 1 gallon of fuel) to get the old fuel out and new fuel in.

so i went home and changed the plugs. they did look awfully dirty. they are the style that is just flat on top, no gap to set (hope this makes sense)
went back out with the same new gas. motor ran great for 5 minutes, then bogged back down, doing the same exact thing.

havent checked the pluggs again yet, as it is dark.
any suggestions?

 

[Cal_MI]

Re: wont run at high speed after bad gas

Most gas nowadays has ethanol in it. It attracts moisture more than pure gas does and makes gum in older engines that do not have up-to-date hose and other rubber material that resists ethanol. Marine ethanol grade Sta-Bil ( a fairly new product) helps some, but probably will not help old rubber much. Got Gum?

 

[Outsider]

Re: wont run at high speed after bad gas

Did you run the new fuel with a different hose? If not, try it that way. I once chased a reduced RPM problem to the tune of about a thousand dollars. Turned out to be a $15 hose that was deteriorating on the inside and restricting the bulb outlet ... :facepalm:

 

[Strahm]

Re: wont run at high speed after bad gas

i wouldnt think it would be the hose, since it happened right after the bad gas, unless something in the gas clogged the hose, which is doubtful.
also it shouldnt be gum in the engine as the old gas didnt sit in the engine at all. it only sat in the gas tank, then burned in the engine for minutes, then flushed with fresh gas.

i will check out the hose though.

 

[Chris1956]

Re: wont run at high speed after bad gas

That motor will have banjo fuel filters on top of the carb covers. Maybe they got clogged from the old gas? I do not think the motor had any stock other filters. It also likely makes sense to check the fuel pumps for junk/dirt.

 

[Strahm]

Re: wont run at high speed after bad gas

well there is one fuel filter on the engine block between the fuel pump and carbs. i checked it and it is spotless as always.
checked the fuel pump and it is spotless. there is no filters on top of the carbs.
i took the spark plugs out (brand new), and the top two cylinders had grime on them bottom two still looked new.

i cleaned the plugs off, and plan on running it tomorow, and plan to put some seafoam and or stabil in the new gas.
unless someone has a better idea of what to do?

 

[Chris1956]

Re: wont run at high speed after bad gas

Stram, Normally a 1973 Merc 850 will have round cylinder-shaped covers on top of the carbs. A single bolt holds these covers over banjo filters. if you do not have these covers, the motor is older or newer, or a Frankenmerc. Newer motors have backdrag carbs with flat carb covers. These have a stainless screen under the brass fuel inlet fitting. You might check serial number and let us know what you have.

 

[Strahm]

Re: wont run at high speed after bad gas

i dont have the serial number on me right now. but i know for a fact it is titled as a 73. and i know for a fact it doesnt have the filter on top of the carb, as i just rebuilt the carbs, and they dont have the filter on top. i was wondering why they didnt as my kit came with a new filter "cartridge" and all the drawings showed them on there, mine just didnt have them.

i dont think its a frankenmerc as my family has owned it most of its life, but it wouldnt surprise me if the carbs are off of a different year.

either way i checked the fuel filter i do have and it is clean, no debris. i wouldnt think something made it past the filter and clogged the carb, but odder things have happened.

im going to go run it today after cleaning the plugs and adding fuel supplements.

 

[aussieflash]

Re: wont run at high speed after bad gas

Just to make sure,I would OHM test the Stator just to rule that out.

 

[fishon79]

Re: wont run at high speed after bad gas

guessing water from condensation has either damaged fuel pump system or is causing a viscocity issue by remaining in fuel line at horizontal to vertical bend not allowing enough fuel top pass I had the up stream side of primer bulb hold water and starve engine bad gas is the worse of the worse what do ya do with it If fuel pump(s) have diaphrams look there water tears em up

 

[fishon79]

Re: wont run at high speed after bad gas

one other thought if your using new tank now read info from merc on them
http://www.mercurymarine.com/parts-and-accessories/product-overview/portable-fuel-system/

 

[Chris1956]

Re: wont run at high speed after bad gas

Stram, Does that motor have backdrag carbs? If so, you might remove the brass fuel inlets and check the stainless screens under them.

 

[Strahm]

Re: wont run at high speed after bad gas

im not sure if it has back drag carbs or not, im not sure what a back drag carb is.
i googled it and it looks like i do have them. i dont remember there being a screen under any screws when i rebuilt the carbs though.

update, i ran seafoam through the engine. did it how the bottle says. put the seafoam straight into the fuel line until it killed the engine let it sit for 5 minutes, started up, lots of grey smoke burned off. did this three times, and got the boat to run 3/4 of what it used to do.

i think my next plan will be replace the fuel line, as when i was taking the ends off to do the seafoam it looks awfully worn, and blackish in the hose even though its onky a year old.

then im going to drain both tanks make sure they are clean inside, refill from a gas station i have never had problems with. and hope for the best.

and i will look into the screen in the carb, but i dont remember seeing one in there

im curious why would the stator have anything to do with this? if i remember correctly my stator isnt even hooked up as the wires were so corroded it couldnt be repaire, so i just cut them and taped them off. several people on here said it would be ok, and the boatr ran fine after doing so.
and i checked the fuel pump diaphrams, look fine.

 

[Chris1956]

Re: wont run at high speed after bad gas

Stator has no effect on Distributor Ignitions. Screens are under brass elbows and tees in carb covers. You must remove the brass fittings to see them. Do you have the brass nipples on the carb throat and carb covers?

 

[Strahm]

Re: wont run at high speed after bad gas

here is a link to what my carbs look like
http://www.google.com/imgres?q=1973...191&start=0&ndsp=19&ved=1t:429,r:14,s:0,i:104

im not real sure if i have brass nipples on the carb throat and covers. i didnt think so but i could be wrong.
when i rebuilt the carbs i took everything apart that was easily removed, and cleaned it all. then replaced the float and jet. i dont remember seeing any screens/filters.

 

[Chris1956]

Re: wont run at high speed after bad gas

You have the original carbs. If you remove the hex bolt (with a 5/16" wrench) at the top of the carb cover, the cylinder fitting is supposed to have a banjo filter inside it. The fuel lines with the crimped on copper fittings are NLA, I have heard. You may need to source some brass hose barbs to replace them, if you want new fuel lines.

Did you replace the carb floats? The originals were foam which is disolved by the alcohol in today's gasoline.

 

[Strahm]

Re: wont run at high speed after bad gas

yes i replaced the floats, though they had been replaced before as well as they were plastic and in good shape.

I dont have a cylinder on top of the carb with a banjo filter. i remember seeing pictures of them, and the kit i ordered came with the new banjo filter, but my carbs do not have that filter. if i remember right they are supposed to be right above the float "chamber" my has nothing above it.
aside from the filter missing my carbs look just like the carbs in the picture i posted a link for.

also i replaced all the fuel lines with automotive fuel hose. which includes the fuel line connecting the carbs together, and the lines coming from the fuel pump, to the filter on the engine block, to the carbs.

i think we are overthinking this at the moment. im betting, if i clean the spark plugs again (they are probably fouled again) change out the gas and check my tanks, and change out the main fuel line from the tank it should be fine.

 

[Chris1956]

Re: wont run at high speed after bad gas

Well, if you don't have the caps, you have some other kind of carb cover, or there would be no place to connect the fuel lines. A picture will help, if you need further assistance.....

 

[Strahm]

Re: wont run at high speed after bad gas

ok, thank you for all your help so far. once i get time to do some work on it and get some pictures i will put them on here.

thank you very much for all the help

 

[aussieflash]

Re: wont run at high speed after bad gas

Just to make sure,I would OHM test the Stator just to rule that out.

OOPS forgot it was that old(distributor ignition).
With some good pics of the carbs,someone will work it out.