96 GTX Runs bad at about 1/3 to 1/2 throttle

[jjsinaz]

I posted something about this in another thread, but the subject goes not address this problem specifically.

I just installed a new engine and am in the process of breaking it in. I'm having an issue between 1/3 to 1/2 throttle. It almost seems like it's dropping a cylinder in that range. Runs good from idle to 1/4 throttle starts missing intermittently above that, runs like crap from 1/3 to 1/2 throttle and runs great beyond that. Coming back the other way, it starts acting up at about 1/2 throttle down to about 1/4 and smooths out again. Since I am still breaking it in I really don't know how it runs at WOT but I think it will run fine there.

Carb settings: 1.5 N/S, 142.5 main jet, 70 pilot jet, pop off 38 psi on each carb, HS screws 0, LS screws 1 turn out, idle in water 1500 RPM. Took the carbs back off and thoroughly cleaned the low speed circuit paying particular attention to the transfer ports, as it was suggested this could be causing this problem (thanks mikiD).

Put in new spark plugs with new engine install.

This problem was not occurring before the previous engine blew (lost a rod on the Mag cyl due to rust in the crank)

What am I missing?

 

[mikidymac]

I would rebuild the carbs and fuel system with the correct parts an any new engine.

 

[GGuillot]

When you went back through it looking at the low speed circuit (3 +1 small holes in barrel), how'd the jets under the metering block look? I'm referring to the 2 small brass jets screwed in the carb body. I think people refer to them as pilot jets. I'm in the process on mine (97' GTX) finishing up the total RESTO and rebuilding carbs (using new genuine kits) last week the little bity one on the PTO carb was completely blocked. Also, that mylar film check valve under the block could be stuck, the one underneath with the very small screw holding it. I found it on one carb glued from dried fuel.

 

[jjsinaz]

I agree that the Carbs and fuel system should be rebuilt for any new engine but that was all done just last year with all Mikuni parts, so I think that should be okay. N/S were replaced then too and aren't leaking.

The little bitty jet is the pilot (low speed) and the larger one is the high speed jet. I did have them both out and made sure they are not blocked. Also checked the mylar film to be sure it wasn't stuck down.

Thanks for your responses. Even though I have already checked the stuff you both mentioned, I still appreciate your feedback and suggestions. If anything else comes to mind, please share

 

[jjsinaz]

Okay, so I got the cavitation issue solved by replacing the carbon seal, but I'm still having this carb issue.

I took the carbs back off, completely disassembled them, cleaned them out again with carb cleaner, made sure the transfer ports were clear, rechecked pop off and still at 38 PSI

Had it out this evening and it idles good runs good on the high speed end but I'm still having this issue. It seem like it is occurring at the transition from low speed to high speed circuit (around 4K RPM

Seems like the low speed circuit is rich, as I had to close the LS mixture screws about 1/2 turn to smooth out the idle and when it is in the range where it runs poorly, if you pull out the choke slightly, it gets worse. Everything is stock setup so I am not sure what the problem could be. The only thing different now from last year is the air box. I went back to the stock air box setup.

I'm thinking about maybe going one size smaller on the low speed jet.

Any ideas are appreciated

 

[kmarts]

...

I went back to the stock air box setup.

I'm thinking about maybe going one size smaller on the low speed jet.

so you we running aftermarket flame arestors before?

if so did you up the jetting to compensate? if that's the case then yes you're going to have to run stock jetting.

 

[jjsinaz]

I ran for a season with the aftermarket FA on it. (they were on it when I bought the ski)
It has the stock jets in it 70 LS/142.5 HS Funny thing is it ran good with the aftermarket FA on it with the stock jetting.

 

[jjsinaz]

After re-reading the carb adjustments sticky, it appears the symptoms point more toward the HS jet, due to the throttle opening where the symptoms occur. Wondering if the PO could have drilled the jets to compensate for the aftermarket FA. The HS jet definitely has 142.5 stamped on it.

What I am going to try next is removing the flame arrestor screen temporarily to see if the problem goes away

 

[esahlin]

Copy from a different thread I responded to.

I had something similar on my 96 787 GTX that anything at light part throttle seemed excessively rich or lean. Everything at 3/4 to full throttle was fine. I went higher and also lower on the jets in the pilot circuit with little change. Turned out it was the exhaust water control valve. The retaining clip had rusted out and the diaphragm had a hole in it. Fixed this and my part throttle improved dramatically.