Fuel flow problem need HELP!!!

[Cougar_15]

I'm at the lake for a week with my boat and I'm out of ideas as to what else to try to get my motor running right, I need some help...

The motor is an 1977 70hp Evinrude. It starts great, idles great, runs great at slow speed but bogs down on wide open throttle. With the fam in the boat it only gets about 3000 rpm, with just me it hits around 4000 rpm at about 3/4 throttle. Once the thottle is advanced past 3/4 it bogs down then surges. If I repeatitly bump the choke at wot it picks up rpms and stops surging. With just me I was able to get it to 5000 rpm.

I've tried everything I can find on the fourm to fix fuel flow issues and I'm out of ideas. Here what been done so far:

• Carb rebuilt using OEM kits (completely dismantled, jets removed, soaked, new seals and floats)
• Carb have been removed and gone through an additional 2 times to look for blockages and adjust the float levels.
• Compression checked: 120,115,120
• Spark checked: will jump 1/2 gap on all three
• Linkage adjusted per the OEM manual.
• Inductive timing light show consistent pattern on all three cylinders
• New spark plugs
• New fuel lines
• New gas

The water pump and remote control were also replaced if this has any bearing on the problem...

 

[jtexas]

Re: Fuel flow problem need HELP!!!

Obviously choking should shut it off; if it doesn't, it means gas isn't getting from carb bowl into the motor. Unless maybe there's air leaking in somewhere, carb mounting gasket or intake.

 

[tx1961whaler]

Re: Fuel flow problem need HELP!!!

When it's running, with the air silencer off and someone else driving, cover each carb throat with your hand and see if the motor picks up when a particular carb is covered.
Be careful, the flywheel will do some damage if you stick yer hand in it....
If that test shows a bum carb, idle her back to the dock, 'cause that cylinder is lean and will eventually let loose on you.

 

[Cougar_15]

Re: Fuel flow problem need HELP!!!

OK, fixed one problem and created a new one.

It turns out that the high speed problem was caused by a total rookie mistake, when I reassembled the carbs I missed an small seal inside the float bowl. Now that I have that seal in place I can get plenty of high end rpm's but I lost the middle.

Now what happens is; it starts great and idles fine but when I advance the throttle it stall out. If I bump the choke before it stalls it will keep running and accelerate to wot. The other wierd this when I advance the throttle as far as it will go I get about 4,500 rpm but if I pull it back slightly I get about 5,000 rpm.

Could this mean the timing is off? Why would putting the seals in the carbs cause the low speed range to have what appears to be a fuel flow issue when that was working great before?

 

[jtexas]

Re: Fuel flow problem need HELP!!!

Re-check your link & sync, especially the throttle cam/roller timing.

Are the throttle valves going beyond horizontal when you get to WOT?

 

[Cougar_15]

Re: Fuel flow problem need HELP!!!

I rechecked the link & sync and as far as I can tell it is as it should be.

The throttle valves are right at horizontal not beyond.

I also checked the idle timing and as far as I can tell that is correct as well.

I'm completely stumped....

 

[jtexas]

Re: Fuel flow problem need HELP!!!

Ok I can't explain your symptoms (especially the part about getting 500 extra RPM by backing off the throttle), so here's some "grasping at straws" ideas that have paid off for me in the past (in no particular order):

Observe the timer base as the throttle is advanced...is it moving smoothly without sticking?

You know those ball & socket joints holding the spark advance link rod in place? See if one of those is broke.

Take it out with the airbox cover off, observe the throttle valves while under way.

Remove the intermediate jet screw plugs, and spray a good shot of carb cleaner. Stick that straw up in there & really let 'em have it. (In case a dog hair or something drifted in there while you were working on 'em).

If it's a 2-jet carb instead of 3-jets, do the idle jets instead.