mercruiser 170 bogs down > 3700 rpm

[mm82831]

I recently completely rebuilt my engine. The block is from an '85 170. The head and carb are from an my old '81 485 (both have also been rebuilt). The engine starts good, idles good, and accelerates good. It will run good < 3400 rpms. When I try to go to full throttle, the engine first starts accelerating, and then suddenly seems to act like it is starving for gas. It recovers and then does the same thing. Looking for ideas to check out. I suspect the 485 carb - it has 2 jets - maybe i need to re-jet for the 170 vs the 485 engine? I have checked the weights in the distributor and they are ok. thanks.

 

[Bondo]

Re: mercruiser 170 bogs down > 3700 rpm

It Sure Sounds like a Fuel Issue............<br /><br />Do you have a Steady, Solid 5psi of Fuel Pressure at the Carb. Inlet ?????<br /><br />I Don't Think you'll need rejetting.... Both of those motors are essentially the Same.....

 

[mm82831]

Re: mercruiser 170 bogs down > 3700 rpm

Thanks for the reply. How can I easily measure the fuel pressure - the line from the pump to the carb is metal?

 

[Bondo]

Re: mercruiser 170 bogs down > 3700 rpm

How can I easily measure the fuel pressure

Well,........ that's the Bite.... Falls into the Specialized Tools catagory......<br />You'll need to "T" the line,+ tap in a gauge......Lot's of different ways to get it done.......<br /><br />Before I get all Long Winded,.... Do You turn Your Own Wrenches ??????, <br />Or, Did you Have the motor rebuilt ????<br /><br />Either Way, Start with the Basics....<br />Is the Fuel Filter Clean ??<br />Does the Fuelpump, pump fuel ???<br />Does the Fuel flow from the Tank Alright ???<br />Is the Fuel Contaminated ???<br /><br />I Agree with you,..... It's a Fuel Starvation Issue........<br />Some of the other Guys will be along with More Ideas on Where to Look.................<br /><br />BTW,<br />Welcome to the forums here at iboats.......

 

[Richard Petersen]

Re: mercruiser 170 bogs down > 3700 rpm

Can you easily go past 3400rpm in neutral a couple of times ? If yes, It could be a bad float condition. Float should be checked with the correct gauge for engine and carb. combination. It should be closing the needle too soon. Or old fuel or dirt has started to blockoff the main fuel ckts. When you say starving, no back fireing mixed in right? Just a slower moan ?

 

[pine island fred]

Re: mercruiser 170 bogs down > 3700 rpm

Also check the tank vent line is clear, here in fl. bugs can plug up a pitot tube overnight. If you can get to the fuel pick up tube, remove it and check the finger screen in the tube. regards fred

 

[Don S]

Re: mercruiser 170 bogs down > 3700 rpm

Don't go cutting into the steel fuel line to make a Tee for the pressure tester, get a couple of fittings, one for the pump and one for the carb and adapt them to some hose from the pump to the carb. Then in a convienient place add a Tee for the gauge. Make this all out of clear hose so you can see bubbles if there are any. Any air leaks on the suction side of the fuel pump will cause lots of bubbles and your low rpm problem.

 

[rodbolt]

Re: mercruiser 170 bogs down > 3700 rpm

DO NOT rev that motor to 3700 in nutral. if all you desire is RPM remove the prop and toss said prop in the creek. all you will prove by revining in nutral is that it can do funny things.<br /> with no load it will rev to 3700 on 2 cylinders. so any data from a 3700 no load rev is meaningless. do as suggested with a fuel pressure/vacum gauge and some fittings. on the suction side of the pump you should see less than 3"Hg vacum with no air bubbles. on the pressure side you should see 4.5 to about 7 psi with no big swings. a vacum gauge is about 20 dollars or less. if vacum is high look fora restricted/crimped line, a faulty antisiphon valve, a clogged draw tube screen or a bad vent. a quick way to isolate the hull system is by simply running a buddies 6 gallon outboard tank hooked up at the suction side of the fuel pump. man you guys make trouble shooting WAAAAY to difficult. its all subsystems. test and eliminate each subsystem and the motor works. it aint voodoo.<br /> DonS I take it you see cut up and spliced fuel lines on your side too .

 

[Don S]

Re: mercruiser 170 bogs down > 3700 rpm

DonS I take it you see cut up and spliced fuel lines on your side too

Yep...."Butchers" is the name we give em.

 

[Aquadzac Captain]

Re: mercruiser 170 bogs down > 3700 rpm

Just to feel you pain, last year my 350 with a new Elderbrock 600 CFM is doing the same thing. I would run great for a few hours then I would slow troll then add gas and she would like to Bogg Down. This has happend on 2 carbs, the old Rothenbreg too, I have replaced all fuel lines, filters. Last thought was to add a fuel line presssure gauge to see if the fuel tank is clogging up? Not sure but Feel you pain?

 

[mm82831]

Re: mercruiser 170 bogs down > 3700 rpm

first, thanks for all the input. i turn my own wrenches - i have owned this boat a long time and it has never seen a repair shop. my kids got older and the boat endied up 'resting' for a few years - a friend expressed interest in the boat, and he and i have been working to give it another life. the fuel tank was pulled, MTed and the fuel line replaced to the pump. i think the vent line is ok, but will check. the carb was completely cleaned and rebuilt, and i think the float is ok, but will check (will also check the jets, but i think they are ok). the engine is not backfiring at all - from stop under full acceleration, it goes good. if the carb is ok, i will remove the hard line to the carb, and make a soft line (clear tubing) i can tee into to check the fuel pressure and add a pc of clear just before the pump. didn't get to work on the boat last weekend, and probably won't this next weekend (daytona bike week you know). i will keep you posted. if you have any other thoughts, please let me know.

 

[Richard Petersen]

Re: mercruiser 170 bogs down > 3700 rpm

I go for float level or a weak fuel pump that can't keep up with higher demand. Sitting around for those years does not help pumps.

 

[Aquadzac Captain]

Re: mercruiser 170 bogs down > 3700 rpm

http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=24;t=010078 <br /><br />Working the same issue the same time you are. I plan to clear tube the fuel line to look for bubbles while double checking my pressure then check my gas tank inner screen.

 

[mm82831]

Re: mercruiser 170 bogs down > 3700 rpm

ok, finally got around to working some more on this issue. fuel pressure is 5-7 until wot. it then goes down to around 2 psi. looks to me like the fuel delivery is not keeping up and i need to replace the fuel pump. where is the best place to get one? also, would it be better to just replace it with an electric fuel pump? thanks.

 

[Bondo]

Re: mercruiser 170 bogs down > 3700 rpm

Before you replace the fuel pump,...<br />You should make sure there's Not a Restriction between the Tank,+ the Fuel Pump.....

 

[Mahoney]

Re: mercruiser 170 bogs down > 3700 rpm

Orlando mike, I had a previous post about a similar problem, and it has seemingly come down to a fuel issue. After testing the fuel pump I realized it was only a little under 4 lbs, way to low for my application. Make sure you check that there isnt a restriction first in the feed line and all the way up to the carb itself, also that there are not air leaks.<br /><br />You can get all the parts to make a t-fitting at your local hardware store for about $5 not including the pressure gauge. Just unscrew the fitting that feeds fuel to the carb fuel line from the pump and bring that with you.<br /><br />I was fitted with a male to male brass fitting that went in the pump, then a brass T fitting with all female threads that connected to the male to male fitting in the pump. On the top of the T goes a male to hose-barb brass fitting connecting to your pressure gauge and then screw your original fitting from the pump to the other end of the T, connect your fuel lines and check for leaks.<br /><br />Bending the metal fuel line might be necessary to account for the extra space generated by the fitting.<br /><br />Good luck!

 

[mm82831]

Re: mercruiser 170 bogs down > 3700 rpm

i will check for a restriction - but, i doubt it. the line from the tank to the pump is new, and the tank was emptied and cleaned. also, is there a equivalent auto fuel pump that could be used? i am getting pretty high prices for the marine one. thanks.

 

[Mahoney]

Re: mercruiser 170 bogs down > 3700 rpm

Crap I just saw your last post. I was able to find really cheap OEM fuel pumps from a place called boating associates or something, I can update with the url when they send my shipping confirmation.<br /><br />My fuel pump(most expensive) was listed at anywhere from 130-205(Napa) and I found it there at 125.00.

 

[Bondo]

Re: mercruiser 170 bogs down > 3700 rpm

Fuel Pumps MUST be Marine..... Safety Issues.....<br /><br />That,+ Even if you find a pump that'll bolt on,... the Ports won't line up......<br /><br />Go with What Belongs on it...... Wrong place to be cutting corners......

 

[mm82831]

Re: mercruiser 170 bogs down > 3700 rpm

thanks, mahoney. i have already basically done what you reccommended - i have a pressure guage now in line between the pump and carb. where did you buy your fuel pump?