GX200 go kart motor

[abtheman]

Hey guys,

I have a highly modified GX200 go kart motor. The issue I am having is I can get the motor to idle but it will cough and splatter then stall under light to medium throttle while the motor is under load. A list below shows what I have done so far to diagnose the fault, but after carrying out all the below things.....I still ha e the fault!

*Remove carb, clean and blow out all jets, and ports (no issues found)
*Change modified carb to stock carb, (same motor running issue)
*replace carb gaskets with new ones and check carb insulator with no cracks found
*replace ignition coil with a new one
*sand slight rust off flywheel magnets
*replace spark plug with new one
*check flywheel key way still in tack (no issues found)
*loosen fuel cap lid while motor running (made no difference)
*change fuel with new fuel
*change fuel lines with new ones and replace fuel filter
*reset value clearances
*remove cylinder head and check valves not burnt (all appears ok, including rocker gear) replaced head gasket
*bypassed ground connection from coil (no difference found)
*check fuel flow from tank to carb, (good flow seen)
*Tried running engine with no air filter (made no difference)

Pretty sure that covers everything I have done so far, and as I mentioned earlier the motor still runs like [censored] anywhere above idle and will soon after stall....but idles fine!

Yesterday I tested the cylinder compression and I got 55psi to begin with (best I could get) then 76psi after adding a bit of oil to the cylinder. So my question is, could the issue with cylinder compression cause my motor to run like [censored] above idle?? I really am beginning to run out of options but I know the fault lays in there somewhere. Any help would be much appreciated! Thanks

 

[EdwardC]

Don't those motors have a compression release on the cam? I think those numbers are about right with the compression release. I think you can do a compression test by spinning backwards (or something like that, google it). Sorry, can't help too much else, good luck!!

 

[old1]

what kind of modifications have been done to the engine? And did it run fine with these modifications till recently, or has it not run right since modified???

 

[abtheman]

Sorry I should have mentioned that. Yes the motor has been running mint for the last 2years while using all the mods on the motor up until the other day then all of a sudden it just started to cough and splatter then died. As mentioned prior I can get it idling but that's about it. Anywhere above idle while under load it just coughs and splatters.

Mods as follows

Shaved gx160 head 18cc
18lbs Valve springs
Chrome Molly push rods
Rockers etc on head to suit

Race flat top piston
Billet alloy connecting rod
Billet alloy flywheel
Governor removed
Bored (oversized stock carb and jets etc to suit)
Free flow exhaust header pipe)
Race cam shaft (higher lift) BSP2 from memory?

Running motor on 98 octane (10% ethanol in fuel blend)

 

[abtheman]

Cheers mate

yup they run a compression release on the cam. Will look into it

 

[old1]

Almost sounds to me like it is one of those things that you have already checked, and checked out good, but it is bad anyway. Like the engine I could not get to start, not even a pop, and it finally turned out to be the brand new spark plug. Sounds like a fuel problem (maybe) try slowly adding some fuel (from a squirt can) while accelerating. Still no help, try to set the plug gap down to maybe .015, in case of weak spark. I know it is a new coil, but what I have found is that if everything is new or tests ok, SOMETHING that checks ok ISN'T.

 

[abtheman]

Thanks for your help

yeah it's honestly so frustrating as I've spent ages going through each piece off the motor taking my time especially with the carb/carbs, I have cleaned out the modified one numourous time now just to prove it flows etc... even used carb cleaning from one end of each port to the other just to confirm fluid flows which it does with ease.

So you recon based on my symptoms it would have nothing to do with cylinder compression?

 

[old1]

With a compression release, those are normalish compression readings. Personally I never use a compression gauge. I just put a finger over the plug hole and pull the rope. If it pops my finger off, art is good enough to run. And once it is running, low compression won't make it not accelerate. Low power perhaps, or oil burning, yes.

 

[abtheman]

Cool thanks for your help. I'll have another play with the motor tomorrow and see if I can achieve something.

 

[cronk]

The only things I can think of that you have not already checked are:
The fuel outlet valve, they usually have a screen on them that can clog an reduce fuel flow.
Possible sticking valve, carbon buildup around the valve stems could be causing valves to stick. Could be slightly sticking and run ok at idle but be more of an issue as rpms increase.

 

[old1]

By the way, Have you checked the oil, and is it at least reasonably clean, and not overfull or low.? Probably not a problem, but when you can't find anything else, grab at straws. As I said before when all the normal stuff has been checked check the stuff it can't possibly be.

 

[ironman_gq]

I had one doing something similar and it turned out to be a bad compression release mechanism. It wasn't disengaging at idle speed, motor would idle but would spit and pop if I gave it the slightest throttle.

 

[old1]

Originally Posted By: ironman_gq
I had one doing something similar and it turned out to be a bad compression release mechanism. It wasn't disengaging at idle speed, motor would idle but would spit and pop if I gave it the slightest throttle.


Could be that, but how do you determine that without complete dissassembly?

 

[abtheman]

Thanks mate. Yesterday I pulled the engine cases apart and everything appeared ok. The dots on the cam and crank gear were aligned and the compression release mechanism works good, (when I push on the small lever it sprung back with ease. The lobes were good too. Thought I would go ahead and pull the cases just to confirm that nothing obvious was wrong in the bottom end.


So I am guessing your compression release mechanism was not returning itself under the spring tension after you pushed on it? I'm just needing more info before I put it back together

Thanks for your help

 

[abtheman]

I ended up draining the oil yesterday and cleaned out the whole bottom end with brake cleaner after inspecting the cam (including compression release mechanism), crank etc. the oil was quite black but was definitely not low or too high as I checked it on the dip stick a few days prior. Was siting right on the full level mark on the stick. I will go ahead and reassemble it today as I found nothing wrong In that area. The next thing I will do is strip the head again but this time inspect each valve stem for carbon build up and make sure this is no issue with the valves moving as they should do.

Just for further information I have been reading online regarding these motors that there appears to be a lot of guys out there with modified motors like I have that run there valve clearances set at 0 lash, as they mention the valve clearances will adjust themselves as the motor warms up....so set them to 0 at room temp. What's people's thoughts on this?? I was setting them at .003 exhaust and .002 intake set at room temp

 

[abtheman]

Thanks mate. Yeah forgot to mention I did pull the small filter screen out of the bottom of the fuel tank which attaches to the hose and it was good. Although I did clean it anyway. I have also tried removing the fuel hose from the carb and fuel flows with ease. So no issues with fuel flow.

I will pull the head today and inspect the valve stems and give them a possible light sand. At least that's then another thing I can eliminate

 

[old1]

Originally Posted By: abtheman
I ended up draining the oil yesterday and cleaned out the whole bottom end with brake cleaner after inspecting the cam (including compression release mechanism), crank etc. the oil was quite black but was definitely not low or too high as I checked it on the dip stick a few days prior. Was siting right on the full level mark on the stick. I will go ahead and reassemble it today as I found nothing wrong In that area. The next thing I will do is strip the head again but this time inspect each valve stem for carbon build up and make sure this is no issue with the valves moving as they should do.

Just for further information I have been reading online regarding these motors that there appears to be a lot of guys out there with modified motors like I have that run there valve clearances set at 0 lash, as they mention the valve clearances will adjust themselves as the motor warms up....so set them to 0 at room temp. What's people's thoughts on this?? I was setting them at .003 exhaust and .002 intake set at room temp


0 lash? I have never heard of anything running 0 lash except with hydraulic lifters. As the engine warms up, the lash will get tighter, so I would think that the valves wouldn't even go closed. Personally, I would use factory spec on valve lash.

 

[ironman_gq]

Often the cam manufacturer will give you lash specs and I wouldn't be surprised if they're a little looser than factory.

 

[abtheman]

This is what I found online yesterday. Was an interesting read