2000 GS 1/4 throttle hesitation/bog

[Mekanix]

Hey guys, First snag of the season for me.

2000 GS, first run of the season. It was well preserved last year and worked good this time out.

Here's the problem:

I've been playing with the idea of eliminating the accel pump and up jetting but thought I would try setting the pump to stock to see if it could work properly.

During that I noticed that if I held 1/4 throttle (3000 rpm) for more than 10 seconds, it will bog out and die.
I can pull the choke half on and it will recover and even the throttle response is better with half choke up until I reach 1/2 throttle. That's where the choke will get in the way and I have to push it open for it to rev.

I can just rev it past this range but its when cruising that I notice this problem.

What are your thoughts ?

Haven't troubleshot it at all yet. Just know that the choke helps allot.

When it boggs down It will not recover unless I release the throttle completely for a second or two. As in I can hold it and it will fade and stall out or I can give it more and it will still stall the same way....

 

[Dr Honda]

Your carbs need rebuilt !! STOP RUNNING THE SKI !!!!!!


With a 2-stroke... the fuel cools the pistons, and transports the oil. SO, a lean run can KILL a 2-stroke.


There's no reason to remove the accel pumps... assuming they aren't totally rotten. At my shop, I've removed a few because the parts were more than the owner's wanted to deal with... but it depends what you have. If you have the big single... then it needs to be there. If you have dual carbs... they can be removed. (don't have enough coffee in my system to remember what yours had off the top of my head)


Oh... one more thing... if you have gray tempo hoses... it's time to replace them.

 

[Mekanix]

Hey Tony.
Yes its a big single.
I only ran it long enought to discover this odd snag and get some usefull information.

(Background)
Lines changed.
Engine and carbs rebuilt.
New fuel valve.
All stock.
All done last august by me.

Ill keep the pump then, there is nothing wrong with it I just noticed that the way it was set would feel like it floodd the engine out every time it sprayed.

How do you like to set this carb for low speed, pop, and accell pump ?

 

[Mekanix]

Disassembled the carb and found it was prerty clean. Noticed that the fuel pump check valves were not holding pressure. Replaced those and all gaske. Pop off was 36 psI.
Hoping that was all thar was needed

 

[Dr Honda]

I'll have to gig a little... but on some of the carbs... there's a restrictor in the small hoses between the carb, and the accel pump. Did you replace those lines too?? If you did... you threw away a needed part.

 

[Mekanix]

Yeah i know about the restrictor in the supply line for the accell pump.
Funny thing is that last year when I rebuilt them I didn't know about that and I did throw the lines away but then I did managed to find them and reinstall them.

 

[Mekanix]

Just took it out and its the same. I have pop off set at 36. Ls at 1 turn out. Hs closed.
Feels like pop off is too high. Only reason i think that is because it will bogg out now and die unless i have the ls up to 2 turns out.

 

[jimmaki]

Just took it out and its the same. I have pop off set at 36. Ls at 1 turn out. Hs closed.
Feels like pop off is too high. Only reason i think that is because it will bogg out now and die unless i have the ls up to 2 turns out.

If too high a popoff is the problem you'd probably also have idling issues. Do you? When you rebuilt the carbs did you clean the bypass ports out and verify they were completely open and make sure the check valves sealed?

 

[Mekanix]

If too high a popoff is the problem you'd probably also have idling issues. Do you? When you rebuilt the carbs did you clean the bypass ports out and verify they were completely open and make sure the check valves sealed?

No idling issues at all. But no throttle response either even with the shot the acell pump gives..
Bypass ports are so clean I can see light through them.
Replaced the anti blow back valve since the one that was there was very flimsy and almost held open all the time.
Check valves ? You mean in the pump ? Yeah I replaced them because they were leaking.

 

[Mekanix]

News:

I've confirmed it is a pilot/pop off issue. Or at least playing with that made a huge difference.

Tried a lighter spring which set it from (36) to 26 psi. Doesn't bog out, but doesn't accelerate without ls screw at 2 turns out.

Tried the lightest spring I have which resulted in 12-14 psi pop. That made a huge difference.

I could set the LS at 1+- 1/4 and it worked properly!
No bog, smooth accel. But not as aggressive in response as when the main jet starts to contribute. At that point the throttle is as quick as a light switch



So what's wrong ?

It takes a very low pop off of 14 for it to work good when the manual says to have 36-40.

Should I leave it or up the pop to 36 again and try upping pilot jets until its right ?

 

[jimmaki]

No idling issues at all. But no throttle response either even with the shot the acell pump gives..
Bypass ports are so clean I can see light through them.
Replaced the anti blow back valve since the one that was there was very flimsy and almost held open all the time.
Check valves ? You mean in the pump ? Yeah I replaced them because they were leaking.

I don't know if that's just a figure of speech you're using, but I'm not aware of any way to see light through the bypass ports. They're just upstream of the throttle butterfly when it is closed. When you open the butterfly the high draft of the manifold pressure immediately drops requiring the bypass ports to provide more fuel. If they are not completely open, you'll get a bog.

I think what might be happening in your case is the very low pop off you've created is allowing a surplus charge of fuel to collect in the crankcase during idle and the increased updraft when you open the throttle on acceleration is pulling that reserve fuel up helping feed the cylinders. Since it's a limited reserve, it won't substitute for full working bypass fuel supply. Nor will the accelerator pump.

The check valves I was referring to are not the ones in the pump but the ones on the bottom of the part covering the jets. If they leak, the low speed circuit can pull air from the upstream high speed circuit before the high speed circuit develops low pressure from air velocity through the gunsight.

 

[Mekanix]

Ok now I know what you mean. The plastic part covering the jets is called the anti-blowback valve i think i read. The original one was bad so I replaced it. But no change.

What I meant by seeing light was that by using a flash light I can actually see light coming out of the bypass ports.
I did also run half a can of carb cleaner through them both ways just to be sure.

I like your theory.

 

[Mekanix]

Well some good news and some bad.

Found the problem, leaking around the needle O-ring

Changed the needle and seat and that was it..

 

[jimmaki]

Great! Good work!

 

[Mekanix]

Thanks Going to pull the head off to check for any signs of seizure but I think I got it before anything happened.

 

[Dr Honda]

Umm....

Normally a leaking needle and seat (or it's seal) doesn't cause the issue you are talking about. But I'm glad you nailed it down. Good work on the digging !!

I'll have to keep that one in the back of my head.

 

[Mekanix]

Thanks Doc, Its been a pain and I still don't understand it yet lol. I'm going to guess the needle was jamming.

I had a look inside each cylinder with a bore-scope and there are signs of a light 4 point seizure. Still has 150 psi cold in each cylinder though.
I might be paranoid but I think I hear piston slap or just a light tick at idle and low rpm.

Going to gear up for a top end rebuild. Shame something so small can cause such a problem.

 

[Mekanix]

Well, looks like its alright, just the way I had the low speed set made the idle feel lean.

Now I'm having a slight flat spot transitioning between pilot and main jet around 1/2 throttle. Should I try cleaning the pilot circuit again or up the pilot jet or lower the pop off pressure ?

 

[krazykarpet]

I'm having the same problem on my 96 GTX. If you run around 5000 RPM it seems to fall on its face 4500 all good, 5500 all good. If you feather the throttle it works OK but if you hold it constant it seem to fall flat? Both carbs have new OEM Mukuni kits in them over the winter.

 

[Mekanix]

I'm still in the process of going out and trying some things. But for that I was thinking of lowering the pop off or upping the pilot jet. On mine, just a tiny bit of choke will make the transition happen really well so its definitely lean until the main jet kicks in fully I think.

If you open the low speed screws another turn you might notice an improvement. If so that's another indication its too lean.

What are your carb settings, jets, pop off ?
I think its the low speed circuit that is partially clogged or corroded or just settings too lean.

 

[Mekanix]

Finished cleaning the carbs and the main problem is fixed, no more bogging or stalling.

Still have a small issue transitioning between 4500-5000.
One thing I did during the rebuild is replace the Anti-blowback valve under the half moon shape block.
I noticed that the old one was quite soft compared to this new one.

Could that cause the issue ?

If I hold 1/2 throttle and rev it up to 4500, after a while it will rev in smoothly to 5200 and if I let off it will drop down to 4500.

 

[Mekanix]

UPDATE,

Since I like to add solutions in my posts here is the fix for the above post with the hesitation between 4-5000.

Check valve in the carb was tooo stiff.

I used one out of a brand new kit and I don't think it was the right one or it was a cheap aftermarket one. ( I thought they were all the same ! )
But no... I swapped that small piece of plastic out with one that was very flimsy and now its amazing how smooth the transition is between the low and main jet. I really can't feel the difference.

So in the future I'll keep this one in mind if I have a hesitation getting onto the main jet after a carb rebuild !!