need help with (fuel?) problem

[PaulK]

I did all the usual starting mods to Predator, except upgrading to billet rod, and have it running - sort of.

Changed intake, exhaust, bored Ru*ing carb, flywheel, Back Momba Jr cam, 22 lb springs, fuel pump and governor removal.

However, when I start it the engine sputters along ok but not enthusiastically.
When I goose the throttle the revs increase somewhat but minimally.
When I apply the choke the revs spike dramatically even with no throttle.
I tried turning the adjustment screw that’s on the side of the carburetor but it seems to have no impact (what is this screw, by the way? Idle or mixture?).

Any idea what’s going on or how I can adjust it to run manageably?

Thanks,
-Paul

 

[mckutzy]

Changing your spring with out changing the rod, is typical grounds for rapid rod dissection.
The springs float allowing the revs to be controlled below the threshold of the con rod letting go... A kinda gov... I'd change back to stock....

 

[bob58o]

Engine runs better with choke on. - Easy! 1 of 3 things.
Carb Jets too small.
Dirty Carburetor.
Intake Leak.

That means the engine runs better when you richen up the air/fuel mix. That means you are lean. If you haven't swapped the carb jets, or drilled them bigger - do that first. Required with Performance Air Filter and Exhaust. And while your in there, clean the carb. Poke some small wire through all the tiny holes. Spray carb cleaner after. If still having issues, check for a leak around the intake. Check the gasket and the plastic insulator. You can spray carb cleaner around the intake and listen to RPM. Don't spray into the carb or around the filter. Spray near the gasket at the head. If you have a leak, the carb cleaner may speed up the RPMs - or slow it down if you already tried to adjust the jetting to compensate for a leak.

So on second thought, check for a leak first. Then proceed to the jets if there is not a leak.


---------- Post added at 05:06 PM ---------- Previous post was at 04:59 PM ----------

Show us a picture of the carb. Guessing there are two screws. One adjusts the idle speed -how many RPMs you idle at. One adjusts the idle air/fuel mixture. It should be a "Fuel Screw" so clockwise to cut off fuel - make it leaner. Counter clockwise to allow more fuel - make it richer. This only affects the idle circuit and right off idle. Should not effect 1/4 - WOT very much.

If there is only one screw, then it adjusts the idle RPM only.

You can show us a picture of your spark plug if you want. It might show signs of being lean.

---------- Post added at 05:25 PM ---------- Previous post was at 05:06 PM ----------

Oh, I missed the fuel pump part. Some people have issues with the pressure from the fuel pump and the float needle and seat, so they add a "T" for a return line to the tank. Most carbs are set up for gravity feed which is significantly less psi than a pump. But that doesn't really sound like the problem. Maybe a picture of the spark plug tip can help us more. Google spark plug readings if you don't know what to look for, then you will know what to show us in the picture.

 

[PaulK]

update:

I got the carb from NR Racing so I asked them what jets are used in it (the bored .650 gas Ru*ing). They responded with .039".
From my limited reading, it sounds like that's probably not undersized enough for my application to be causing the problem but I'm certainly open to input on that.
It's new so I assume not dirty.

With those two assumptions, it sounds like a leak may be the most likely culprit. I'll try to check this weekend.

I also found a video from ARC Racing that clarified the purpose of the screw. I didn't even mention the black plastic one because it looked pretty clearly related to the throttle. Turns out that's the idle adjustment, which I should have guessed. The one I mentioned on the side is mixture. From what you tell me I see that if turning it were to help at all, it would be counter-clockwise. That's helpful to know. Thanks.

Didn't quite understand the fuel pump / float needle and seat thing above. If you get the chance to expand on that it would be great but if unlikely source of problem don't worry about it too much.

I still wonder why the choke would cause the revs to spike so high but I'll try to address the leak (assuming that's the problem) and maybe the rest will resolve itself. I don't have a tach hooked up but revs increase more than a little.

I'll try to post a pic or two.

I am trying to find a friend that will help me put in a billet connecting rod. I have a couple leads but it'll probably take a while to coordinate.

Thanks much for your help.

 

[Kentucky Boy]

Hitting the choke dumps more fuel into the engine which makes it rev up. It's normal for it to do that. I wouldn't waste money on all the mods and not change to a billet rod. It's only a matter of time before you break the conn rod.

 

[bob58o]

I still wonder why the choke would cause the revs to spike so high but I'll try to address the leak (assuming that's the problem) and maybe the rest will resolve itself. I don't have a tach hooked up but revs increase more than a little.

The way I view it, which might be wrong....
The mixture is lean when the choke is not on (possibly getting too much air from a leak), so it is not making as much power as it could be. Pulling the choke makes the air fuel ratio closer to where it should be. The engine makes more power with the better air fuel ratio. The RPMs increase.

Normally I would think slightly lean would make make the RPMs the Highest, but guessing a very lean mix would slow down RPMs. Not 100% here, a pic of the spark plug MIGHT help us help you.

If you show us a pic of the plug and it looks very dark, then I'll talk about the fuel pump. If the plug looks white and clean, then check for a leak and clean the carb. Even for a newer carb. The holes are small. Not hard for a tiny piece of anything to clog up a hole in a carb.

A 0.039" main jet should be plenty big enough. That doesn't seem like the issue

 

[PaulK]

Didn't get a chance to look at the gokart this weekend but have one question about the connecting rod upgrade.
What's the bigger risk:
Not upgrading and leaving the connecting rod stock
or
upgrading and getting the lubrication gap measurement wrong. By that I mean the distance between the crankshaft and the connecting rod bearings.
I've never been good at measuring and my eye sight hasn't gotten any better with age. not only that I'd have to use plastiguage (sp?) because I don't have the expensive gauges.

 

[mckutzy]

It's seems if you can't use the springs to the full potential right now, just downgrade to the stock, and re-lash...use them later when you have a proper rod...

 

[PaulK]

That sounds like a pretty good option.
The heavier springs really just enable the higher RPMs?
Would I still feel any of the benefit from the Black Mamba Jr Cam?
If not, not big loss.
I blew up the old one and had to get a replacement of some kind.

 

[Kentucky Boy]

You don't need an expensive tool to measure the rod bearing oil clearance. The plastigauge you have should work just fine. The biggest risk of not upgrading the rod is conn rod failure and complete destruction of the entire motor. I've never measured rod bearing oil clearance that wasn't in specs when doing the rod upgrade.

 

[PaulK]

Well, I have a couple spark plug pics in case you can tell me anything about them.
But I attempted to swap the connecting rod and I'll post a related question separately.

 

[bob58o]

Looks oil fouled.
I'd get a new plug and look into why there might be too much oil in the combustion chamber.???
5 + 6 Oil-fouled
Insulator nose electrodes & spark-plug shell covered with shiny soot or carbon residue.
Cause : Too much oil in combustion chamber. Oil level too high, badly worn piston rings, cylinders & valve guides. In two stroke gasoline engines, too much oil in mixture.
Effect : Misfiring, poor starting performance.
Remedy : Overhaul engine, correct fuel-oil mixture, new spark plugs.

 

[mckutzy]

Was the motor inverted with oil in the crank case??

 

[PaulK]

Nothing strange that I'm aware.
I don't often look at spark plugs but even I was surprised when I saw it.

I'm changing the piston and rings anyway so I guess whatever was causing it should probably get addressed.
Based on the way it had been running I expected to see some evidence that it was running lean, though I didn't know what that would look like. Covered in oil didn't seem likely.

 

[bob58o]

I was expecting to see signs of running lean as well, but the plug is your window to see inside your engine while its running. Put a new plug in and see how it runs. If it runs ok for a bit, take new plug out and check it again. How old is the engine? I may have been mistaken, but I assumed this was a new engine build.

---------- Post added at 04:29 PM ---------- Previous post was at 04:27 PM ----------

Maybe the fuel pump is overpowering the float needle/seat and you are very rich because of it?????

Does the carb leak fuel?

 

[PaulK]

Ok. Changed the connecting rod and piston.
Got a new spark plug and reassembled.
Now it idles ok but if I add throttle or choke it dies.
Any ideas?

 

[mckutzy]

Check the plastic insulator for cracks aswell as all the gaskets... I reckon there is an air leak....

Did you re jet when the carb filter/exhaust was changed?

 

[PaulK]

I checked, but the seals seem ok.
I would think applying choke would help if it were an air leak, wouldn't it?

Could lashing the valves poorly cause that? I'm not sure I measured at the right top. I mean I'm not sure it was the right part of the cycle. Both valves were closed and the piston was at the top.

 

[PaulK]

Ok, I installed the billet connecting rod and piston and put it all back together.

It runs at idle, but if I give it any throttle or choke it dies.
Someone suggested I might have the metal gasket that goes between the carburetor and the air filter adapter in wrong such that it closes that fourth hole in the upper right.
I went to check it just now and see that the metal gasket is in there correctly.

However, there's no corresponding hole in the air filter adaptor.
That means no air can flow through that hole; right?
Do I have the wrong air filter adapter?

I uploaded pics of the carb and adaptor. The pic of the adapter is a little dark but hopefully you can see. There's no opening that would allow air through that fourth carburetor hole.

 

[bob58o]

What about here where there is no thick rubber?