1990 Johnson 70 surging at WOT

[jhotch]

Hi all my boys have gone through this 1990 Johnson 70 to put behind their pandemic project. restoring a Boston Whaler Mischief 15 we found abandoned in the woods at a home remodel project. It starts right up. Replaced spark plugs, starter, selenoid, and tge previous owner of the motor got rid of the vro with an electric pump so we put a manual fuel pump on it. First time in the water, it opened right up then after 30 seconds it began to surge. opened the cowling and the face plate of the new pump had come loose and gas was spilling out. tightened it back on but it was still surging and continued to get worse.
Got back and after research I found out that they came straight out of the pulse port with a reqular 5/16 gas fitting. Also they were running 1/4 inch line and 3/8 fuel line variously throught the fuel system. Got a pulse limiter and rebuilt the fuel harness with 5/16 to the filter and then to the pump. Then 5/16 from the pump to the first tee and 3/16 from there on to the carbs. Now it runs at wot for about 60 seconds and then starts surging again. Any ideas? new to posting on these forums, but would appreciate some help!

 

[flyingscott]

What fuel pump did you install on it? Did you clean the carbs? The pulse limiter is only required on VRO pumps. It can cause problems with the regular pump so you may want. to ditch that.

 

[jhotch]

yes cleaned and rebuilt the carbs. This is the pump. We initially did not use the pulse limiter and I was thinking that we may have damaged this pump because of it.?

 

[racerone]

The pulse limiter is a " flow fuse " to protect the VRO from pressure spikes.-----It is NOT NEEDED for the conventional pump.

 

[jhotch]

ok removed pulse limiter and put straight valve back in. Still surging. I tried both manually pumping bulb and pushing key to prime, neither solved the problem. Does this mean I need to start looking at ignition and then timing? I have followed the manual in terms of adjusting linkage and the idle runs between 700 and 750 and the the forward Idle speed stays around 600-650.

 

[Bosunsmate]

Perhaps your floats are malfunctioning and flooding her. Try priming the fuel bulb with the engine off until it goes firm and checking that the bulb holds firm for at least a minute (with the engine off)

 

[flyingscott]

Are the carb butterflies synced? All closed at an idle and open to 90 deg?

 

[jhotch]

yes they are all synced as per the manual.

 

[jhotch]

Perhaps your floats are malfunctioning and flooding her. Try priming the fuel bulb with the engine off until it goes firm and checking that the bulb holds firm for at least a minute (with the engine off)

I pumped the primer bulb and it stayed hard for a minute and then I noticed a small amount of gas at the bottom of the case. I pulled tge air cover and noticed some leaking. As I pressed the bulb more there was a small flow coming from the carbs. The engine was tilted and I was pressing pretty hard... any thoughts?

 

[Bosunsmate]

Try putting it level and trying again, If you get more flow then something is wrong. If a float/valve is worn it will cause surging.
Dont press too hard, if you are straining its more than you need too

 

[jhotch]

Try putting it level and trying again, If you get more flow then something is wrong. If a float/valve is worn it will cause surging.
Dont press too hard, if you are straining its more than you need too

We rebuilt the carbs last month, it was our first carb rebuild, but ww carefully followed the instructions. Soaked them, used a drill bit and compressed air to blow them out etc... but the seat threads were a little snug and the needle valves as well. I recently went back and looked at the reviews of the kit and they were not great. May need to rebuild again. Thank you for the advice. I’ll post an update after I level and retry.

 

[jhotch]

Ok. compression was good and equal, spark was good on all cylinders. Switched the coils betwwe the 2nd and third cylinders and the second cylinder still caused no change when I pulled the plug at idle. Squirted fuel in the second cylindee and the idle immediately smoothed out. Pulled the second carb, cleaned it, checked the float setting. put it all back together and voila. idled great on muffs in the driveway. I took it to the lake, started and opened it up, ran great for about 2 minutes and then started surging again at about 3500 rpms. I think I’ll pill the othwr 2 carbs and give them a once over and then redo the linkage. Thoughts?
oh also Timing at idle on muffs in the driveway was 4 btdc

 

[Bosunsmate]

Id just look at the second carb again, messing with the others is likely to just add another problem as they seem to be doing just fine. It might be a sticky valve

 

[Bosunsmate]

Whats the timing spec for that engine? Its seems quite advanced for idle but its being a while since ive checked mine

 

[jhotch]

I think the spec for the engine at idle was 4 ATDC. I’ll check. I was wondering if that could cause just one cylinder to becweak? Another thing to add is that when I adjusted the linkage by the manual I ended up with the Idle screw almost all the way in (about an inch farther in then where it was when I bought it.
Also by a sticky valve do you mean the reeds? I visually inspected them and pressed on them when the carb was off, they looked clean and seemed to all move ok, but I was not sure how to diagnose them beyond that.

 

[Bosunsmate]

Normally if you need spark that advanced at idle theres something else wrong. Especially likely with that idle screw so far in. You could try a drop test on each cylinder and see if you can track down a weak one.
I meant a sticky inlet fuel valve. Bad reeds will usually cause backfiring and a finicky so are best left alone
Also look for differences on your spark plug tips