flat spot

[deejaycee_2000]

hey guys, I bought a brand new merc 55hp with tiller .... I have just done the ride in hours and ready for some serious speed, I am not really much familiar with working on merc, mostly yamaha, suzuki and tohatsu ........ but when I open the throttle slowely, I get a flat spot and the motor dies, and then I have to choke it to get it started, otherwise it starts with one pull ....... I have to open the throttle WOT right away to miss the flat spot ..... any ideas how to get rid of the flat spot? ......

 

[deejaycee_2000]

Re: flat spot

bump

 

[JUSTINTIME]

Re: flat spot

could be the carb

 

[deejaycee_2000]

Re: flat spot

carbs are spotless ...... even ran fuel system cleaner in fuel ....

 

[MikDee]

Re: flat spot

Wow, It's a brand new motor with issues? It should be warranteed, No? What does the dealer say? You didn't say if it's 2 stroke or 4 stroke? Anyway, It's a process of elimination. 1st thing I'd suspect is bad fuel,,, or if it's a 2 stroke (your fuel mix proportions?) then your fuel delivery system, filter, fuel lne, tank line, tank filter, & antisiphon valve, for debri, or a pinched line, Next, check the timing, retarded timing would do this. I'd also say maybe your thermostats not working, if the engines not getting warmed up to operating temp, but seeing where your from I'd imagine it's a temperate climate there all the time? Next I would check your flame arrestor, just to be sure it's clean. Finally, check the spark plugs, maybe too much "extra" oil during break in might have fouled them some? After this I'm plum outta idea's? knowing it's a new motor. Good Luck, Mike

 

[JustJason]

Re: flat spot

55HP rope start???? ugh...... if its brand new take it to a dealer....
however if your in cape town.. it may be black market...

i'd do a comp test and have a lookie at the plugs. then do a fuel restriction test and test the fuel pumps ability to pull at idle. if everything looks good i'd have a lookie at the fuel filters and then the INSIDE of the carbs. Fuel system cleaner doesn't do much... especially if you have a few specks of your african sand floating around in the carb bowls.

 

[JUSTINTIME]

Re: flat spot

i bet it was not set up right

take it back to the dealer like everybody said

 

[deejaycee_2000]

Re: flat spot

I can't take it back to the dealer, this is a race motor, in other word no warrantee, fuel system is clean, brand new fuel tank with lines, plugs are good, timing is perfect .... this motor have been blueprinted (tuned to spec) ...

maybe i should just give the carbs a clean again .....

 

[JustJason]

Re: flat spot

a 55hp tiller race moter??? hmmmm doesn't sound to likely.
Post your serial ##.
check the compression. post your actual numbers. check the timing, and post what it is set at at idle and at wot.

 

[Sig_Mech]

Re: flat spot

If it's a race motor then it was built and tuned to do one thing: go fast! If you are experiencing a flat spot at slow speed and you have checked your spark and ignition circuit then you need to suspect your fuel system. Were the carbs rejetted to work best with the higher RPM's of a race motor? If so you'll either have to live with it or retune the motor for better operation at lower speeds.

 

[j_martin]

Re: flat spot

If it's a racing motor, somebody set it up. That person can tell you how to fix the flat spot, or why you can't.

Normally you could replace the throttle on a racing motor with a switch. You only use the 2 end positions, ei on or off.

I know in the V6's, you get top end by putting on bigger carbs. (WH20's) You kiss low end performance goodbye to get more at the top. Usually they have to idle so high to keep running that they are rigged to start in gear to protect the LU.

BTW, what are you racing it on?

John

 

[old-gubbins]

Re: flat spot

Hi J_M

I don't think you have seen Deejays posts in the Yamaha section:

http://forums.iboats.com/showthread.php?t=215447

B*lls harder than steel (and he still floats)!

See his signature.....

O-G

 

[JUSTINTIME]

Re: flat spot

sounds like jets and timing is not right

what is the compression?
what RON r u running

will have a flat spot if not running the right RON and the low jets are not right

what is idle RPM?

 

[Laddies]

Re: flat spot

deejay, if that engine uses the same carb as the XR50 then it probably has the wrong low speed orifice in it for the XR 50 you need a .80 jet not the .60 a fishing carb comes with other wise you get a flat spot of idle (lean) you can ck the numbers on the BAM Marine site--Bob