Rebuilt Carb now Surging

I have a 1975 4hp johnson motor, I just rebuilt the carb last week and replaced the water pump and now when I start it at idle it surges pretty crazy and will eventually die if I don't give it gas. The surging settles down as I give it a little gas. I can't really adjust the low speed needle at all either I don't think this would cause the problem though, I have set it 1 1/2 turns out from snug and I have played around with it a bit but its very hard to setup with the surging. I have tried pumping the bulb while it happens and does nothing at all, I don't think it has to do with the ignition as it smooths out very quickly as I raise the rpm a bit. I can't remember as I rebuilt it over a few days but I don't think I checked that the float was parallel with the housing when the carb is flipped upside down, I think I just left it to however the float came. Could this cause a surging problem if it isn't parallel? I will probobly take it off and double check either way but its starting to bug me as I am already into this motor much more then I want to be and it isn't running very smoothly yet.

Re: Rebuilt Carb now Surging

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Re: Rebuilt Carb now Surging

I have read through most of those, some of them many times and have some printed off as well. But it does not help me at all with my surging problem at idle. I have done lots of seaching on the problem and most of what I find comes to either the fuel pump or the ignition system, my fuel pump appears to be ok as pumping the bulb when surging does nothing, spark appears to be good with my spark tester I have brand new sparks plugs and a nice blue spark, and the surging goes away as soon as the RPM is raised a little bit beyond idle which takes away the ignition system problem. After doing all of the research I thought back to when I rebuilt my carbs and don't think I checked the float level at all so however they set it up if at all where it was made is how it is it could be fine or off alot I don't know and thought that it might cause the problem.

Re: Rebuilt Carb now Surging

Ok, time to inform a few people about boat carbs.

Unlike older cars, most boat carbs do not have internal boost pumps. When you roll the throttle you are giving the motor air not fuel. the increase in flowing air drags in more gas(siphons) from the carb.

So, if you slightly open the throttle and it smooths, you are increasing enough air flow to drag the needed fuel out of the carb. overcoming a restriction.

Recheck your work - Perfectly clean high speed orifice. Strong flow through idle passages under pressure. Use a can of CRC carb cleaner.
Make sure the air bleed opening is clear. it allows ari to purge from the fuel blow and be replaced by gas mix.

Double check float level and function.

Re: Rebuilt Carb now Surging

I am curious as to how you might describe the 'surge'.

a. an immediate bump in power (rpm) (like a spark plug kicks in/out electrically)
b. an undulating rpm ramp up then down (ever repeating and consistant) occuring over several seconds

Is there any boat shake associated with the undulation?

Re: Rebuilt Carb now Surging

You arn't trying to set the idle to low are you?

Re: Rebuilt Carb now Surging

MAW31 said:

I am curious as to how you might describe the 'surge'.

a. an immediate bump in power (rpm) (like a spark plug kicks in/out electrically)
b. an undulating rpm ramp up then down (ever repeating and consistant) occuring over several seconds

Is there any boat shake associated with the undulation?

Click to expand...

It goes up and down probobly within a second and is continious, if you say vroom vroom vroom vroom vroom vroom vroom at a normally reading speed out loud it is a similar speed to how the engine goes up and down, as soon as I increase the throttle a bit it goes away and runs fairly smoothly.

Re: Rebuilt Carb now Surging

Daviet said:

You arn't trying to set the idle to low are you?

Click to expand...

I could be Iam not sure its very hard to tell with this motor as I have no tach for it and its always in gear, but setting it to low wound't cause a surge would it, I can't really adjust the low speed needle right now either because of the surging?

Re: Rebuilt Carb now Surging

crxess said:

Ok, time to inform a few people about boat carbs.

Unlike older cars, most boat carbs do not have internal boost pumps. When you roll the throttle you are giving the motor air not fuel. the increase in flowing air drags in more gas(siphons) from the carb.

So, if you slightly open the throttle and it smooths, you are increasing enough air flow to drag the needed fuel out of the carb. overcoming a restriction.

Recheck your work - Perfectly clean high speed orifice. Strong flow through idle passages under pressure. Use a can of CRC carb cleaner.
Make sure the air bleed opening is clear. it allows ari to purge from the fuel blow and be replaced by gas mix.

Double check float level and function.

Click to expand...

I will check it all again but iam pretty sure its clean its a very simple carb, soaked it over night used airsol carb cleaner after it had soaked and an air compressor as well. I will check the float level this time as well.

Re: Rebuilt Carb now Surging

remove air box, start motor and look into mouth of carb to see what's going on there. the surging is almost certainly a fuel delivery issue, like fuel pump, primer bulb, fuel tank vent, fuel tank check valve, fuel line quick disconnect on front of motor. May be an in-leak of air into fuel line. Good Luck!

Re: Rebuilt Carb now Surging

robert graham said:

remove air box, start motor and look into mouth of carb to see what's going on there. the surging is almost certainly a fuel delivery issue, like fuel pump, primer bulb, fuel tank vent, fuel tank check valve, fuel line quick disconnect on front of motor. May be an in-leak of air into fuel line. Good Luck!

Click to expand...

Yes I checked it out today and I forgot to put the clamp back on the fuel line that goes to the carb, although the bulb would pump up nicely it would drip fuel from that connection, I hadn't noticed it the first few times I fired it up to play with it, thanks, I hope this is the cause of the surging.

Re: Rebuilt Carb now Surging

I had to turn my slow speed needle in.You got to go slow with it.It will level out ,but 1/8 turn then wait .I just got thru with a 6hp and did the same thing.

Re: Rebuilt Carb now Surging

I replaced the fuel line today and connected it properly and it runs like a champ now. Thanks for the help and hopefully this thread might help someone else in the future.