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Johnson 4HP 1999 J4RLEEA runs then slowly dies

R

richierni

New member
Hi, I've recently obtained this motor. It starts ok with some throttle and sometimes some choke but when I reduce the throttle it'll run for about a minute then the revs get lower, lower, lower and it dies. It'll usually start again ok straight after.

I overhauled the carb, needle valve turn and a half out, checked the compression, replaced the plugs, checked spark, cleaned out the tank, new fuel, breather open, made no difference.

I'm running off the internal fuel tank and I've cleaned out the hoses between it and the carb. Given I'm running off this tank, is the fuel pump doing anything at all? It's connected to the connector for an external tank and the carb. Could I disconnect it from the carb and block off that nipple to try and rule out that it's affecting the operation in the carb in some way? E.g. pumping extra air into the carb from a dodgy fuel connector?

Wondering what else to try next?
 
racerone said:
Little red ( ? ) bladder in good shape in carburetor ?
Click to expand...

Yeah, replaced it as part of the carb overhaul, it was included in the carb repair kit. All the original parts actually looked in good condition apart from the packing washers around the needle valve but I replaced everything that came with the kit anyway.
 
richierni said:
And, I've noticed that even when it is running the revs are inconsistent, speeds up, slows down, without touching the throttle.
Click to expand...

Thinking filter, like Racerone. .... The one in the feed from the integral tank.

Have you checked fuel flow to the carb?

Have you tried running from an external tank ?
 
Forgot to mention, I replaced the inline filter from the internal tank when I did the carb overhaul and also cleaned the fuel lines but they were already clean. I have also tried an external tank, same issue.
It's a pain to check fuel in the bowl as there's no drain but I will do that next time. It does start again immediately after though so I suspect there is fuel there.
The inconsistent RPM generally is maybe a symptom the underlying problem? I had it running, out of gear, at about 1400, then see it drop a couple hundred RPMs without touching the throttle, sit there for a while then come back up a bit, then drop again ...
I also notice if I run it multiple times I'll get different RPMs from the same throttle position (using the idle speed adjuster as a set point)
Starting to wonder if it could be something electrical. Is comparing the tach output on each HT lead a reliable way of checking each cylinder is getting similar pulses?
It did decide to idle for a few minutes today on one test run and I put it into gear and it seemed pretty happy for a while. Maybe I need to pay it more compliments.
 
I had a similar problem with a 9.8 mercury that I had for a bit. Turns out there was particulate in the fuel line upstream of the fuel filter. It would start fine, run for about 10min and then slowly lose steam. It would help somewhat to pump the primer bulb but it would fizzle out if I kept it at WOT. It didn’t take much grit for this to happen….about 1/8 of a teaspoon of fine grit dispersed on fuel filter mesh.
 
The other thing I've thought it might be is a vacuum leak. Anyone know where that would likely occur on this motor? I'm not really equiped to do a proper leak test, any tips for more basic testing methods?
 
Running lean means that it slows down until it stalls.----Sorry , You need to post the actual compression numbers.-----" good compression " means nothing to me.-----Could be an issue with crankcase compression as well.-----Reed valves checked ?
 
Been away for a week ... Compression is 80psi on each cylinder. Reed valves, good call, I guess if they're not good it's a form of vacuum leak. I'll take a look at them next.
 
Took the carb off again and looked through at the reed valves. They look in great condition (I get the impression this motor has done very few hours). They cover the holes completely and I can push them in and they spring back with a nice air sealing sounding "plop". From a bit of reading it seems that they don't often go wrong so given the above is it worth taking the intake plate off to examine them further? I don't have gaskets for the plate and am a bit worried about making things worse. What's the chances of it actually being a reed valve issue?
 
Question? When the rpm,s die down and the motor slowly comes to a stop, is the butterfly fly in the carb slowly closing along with the loss of rpm?
 
Usually it dies from idle so the butterfly is already closed. I have seen it die from higher revs though so I'll check out what's happening there. I don't think it would be closing because I'm not moving the twist grip so the throttle cam shouldn't be moving but will check, I guess it's possible something might be slipping. Might have to run it without the air box on to get a decent view. I guess that will also let me check it's not spitting fuel out of the carb due to reed valve issues. The air box has always seems very clean when I've removed it though so hopefully I won't see that.
 
I recently resolved a similar issue with a 1967 6 hp. The tension spring found in the throttle twist handle was missing that keeps tension in the linkage. Without that spring it allowed the cam follower to slowly rotate through slow idle than shut down. Maybe the spring is missing or weak? Something to check out. Part no.19
 
Some more experiments today. Redid the compression test with engine slightly warm and throttle wide open, got 100 from bottom cylinder and 95 from top.
Ran it without the air box and didn't see/feel any fuel spitting out the front.
Checked the bowl immediately after it died, it had plenty of fuel in it.
Checked if the throttle butterfly was making any uncommanded movements, it wasn't.
Used a spark gap tester (previously I just had one of the neon bulb inline things) At 10mm I was getting good strong sparks. This decreased until at 15mm I was just getting 1 single spark from the top coil per pull of the starter rope and no spark at all from the bottom. What's the opinion on that? is 10mm good enough?
Tried running it on each single cylinder. It did seem to run better on the top one but a bit inconclusive, would need to repeat that one a few times to be certain it wasn't just the inconsistency I'm seeing generally.
 
Updates - I made a DVA adapter and checked the voltage form the sensor coil and the powerpack, all good. I used some propane to do a basic check for vacuum leaks, none found. Only place i could make it affect the revs was in front of the carb as you'd expect.
So, at a loss. I'm going to take the carb off again and soak it in chem dip overnight (even though it's the cleanest carb on earth already ...)
Anyone know if it's safe to put the plastic float bowl, which includes the mechanism for switching the fuel on/off from the internal tank, into the chem dip?
 
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