Powertune having no affect on rpm when sprayed into lower carb. Is this normal?

[exlration]

New to the forum and hoping someone might be able to shed some light on my experience with this motor. I have a 1996 force 120 ELPT mounted on a 96 Astro S-18FS. (about 1400lbs) I've had it on the lake twice and I'm getting 39mph at full throttle (trimmed) using an aluminum 13x21p prop with rpm right around 4900. 9max rpm is 5250) I'm not sure if that's normal or not. I've read others running upwards of 50mph with similar setups. Anyway, the other day I decided to run a can of powertune through the carbs and as I sprayed into the lower carb it had no affect at all on rpm. I could have sprayed that stuff in there all day long without stopping and it wouldn't have lowered the rpm whatsoever. Spraying the upper carb acted normally by lowering the rpm and eventually killing the motor. I wouldn't think this is normal but I have very little experience with 2 stroke engines. Also while using the fast idle lever and running the rpm around 2000 after several seconds the rpm will shoot up to 4000 and then a few seconds later go even higher to 5000 where I have to lower the lever as to not over rev the motor. I'm not sure if these two things are related but my instinct tells me something is not right. I'm hoping someone can shed some light on this and point me in the right direction. Thanks!

 

[SkiDad]

i would think you should be hitting higher than 39mph with a 21 pitch prop - i can do that with a 17 pitch and our boats weigh similar.

i do think something is going on with bottom carb - probably fuel mixture is off and you are doing OK when the larger jets open up when running wide open. I think the initial setting is 1 turn out from lightly seated - if that doesn't help you might need to clean and rebuild the carbs.

 

[gm280]

:welcome: extration to iboats. Great to have you aboard with us...

Reading your comment it would seem that your lower carb is really not the problem (at least initially). It could be after you get the lower cylinders firing, but since spraying the cleaner in and no difference in the engine's RPMs or performance I suggest you check the spark on all the cylinders being feed by that carb first. When the engine is running, remove the spark plug wires one at a time and replace the spark plug wire back again and see which ones change the engine speed. The ones that do are firing,The ones that don't are not firing. I think you will fine one or more not firing. Then swap the spark plug coil from the none firing cylinder to the position of one you know is firing and see if you still have spark. If so, then that spark plug coil is good, if not, then you just found one bad coil. Repeat that process until you verified all the good coils and all the bad coils. If all the coils prove good, you have to back up to see what is missing to the coils that are not working... It is a process of verification and illumination... Post back with your results for us to help you further if you need it... :thumb:

 

[Jiggz]

In addition to what was already mentioned, this is my diagnosis or maybe just my 2?. Since you are spraying the PT when the engine is only idling and it doesn't affect the lower carb or two lower cylinders (fed by the lower carb) but the motor is capable of reaching WOT rpm of 4900ish with 39 MPH, could be an indication of a faulty low speed jet on the carb. However, the best way to actually validate this is to spray fuel oil mix instead of PT. And if the rpm jumps with fuel oil mix then you know for sure there is a problem with the carb's low speed jet.

Or in a very unusual way could be that the lower cylinder plugs are not firing during idle rpm because the trigger wires are shorting to ground when the tower shaft and trigger advance plate are in the idle position. So eliminate this out of the equation and do what GM280 recommended.

 

[exlration]

First let me thank everyone who respond! I appreciate the warm welcome to this forum
I was spraying the pt in accordance with the instructions so the rpm were around 1600. I'm not sure if the idle jet would still be in affect at that rpm or not.
I'll check the coils this weekend and post the results.
Thanks again everyone!

 

[Jiggz]

The idle jet is always working but not the high speed jet which usually doesn't kick in until above 2000 RPM. Even at WOT the idle jet is still at work although most the of the fuel is provided by the main jets.

 

[jerryjerry05]

Do a compression and spark test.

 

[exlration]

Compressions are 130 132 140 144.
I'll be checking for spark later today and post
Results.

 

[exlration]

Ok I checked for spark in all cylinders. Pulled the wires as it was running and the second one from bottom had no affect on rpm. However the coil is getting spark to the plug because I was able to see it jump from the wire end to the plug when in close proximity. I pulled the plug and it still looks new with no carbon buildup. ( plugs were replaced right after the powertune session) Compression in that cylinder is 140. I also noticed at high idle (around 200rpm) it definitely has a random miss going on. Suddenly loses about 100 rpm then runs smooth again. It happens every two or three seconds. Seems to run smooth when revved up though. Very strange. Wish I could post a video.

 

[Jiggz]

You saw an arc over from the plug boot to the plug tip, did you try switching plugs with other cylinders just to see it is not the plug? Is there any indication of fuel spitting out of the bottom carb while in idle?

 

[exlration]

I put a screwdriver into the boot and watched it arc. I didn't think I would need to swap plugs because I had the same problem with previous plugs. There's no gas spitting out of either carbs. Now that I'm watching and listening to the video I took of it it sounds more like its momentarily dying then catches and starts running again.

 

[exlration]

I know its tough trying to diagnose a problem through a forum and I want to thank everyone for your suggestions. I'm posting a link to the video I took today so you can hear what it sounds like instead of me trying to explain it.

 

[SkiDad]

idle doesn't sound right to me unless its just the recording - i think you should be spraying that cleaner deeper into the carb and more of it.

 

[Jiggz]

There is an intermittent miss. And it is one of the hardest problem to diagnose. And the most effective diagnosis is process of elimination. Obviously you want to start with the easiest one and that is electrical. Using a timing light, place it on #3 plug wire and using a dark colored paper, bounce off the timing light strobe into it and watch for consistency. Do the same with the #4 cylinder. If there is inconsistencies, then the root of the misfiring is electrical.

If strobe light is consistent, then the next diagnosis is fuel. Compression is good. Fast idle the motor to about 2000 rpm, using a spray bottle spray directly into the bottom carbs almost continuously. If there is no change in RPM, the next move is to remove the bottom carb and inspect both reed blocks on #3 and #4.

Post your findings.

 

[exlration]

Thanks! I'll post back next week.

 

[exlration]

Ok I'm getting consistent spark to all the plugs at various rpm. I did notice a small amount of fuel accumulating in the little shroud on the bottom carburetor so tomorrow I'm going to remove it.

 

[Jiggz]

Remove both air covers on the carbs and then run it again. This time watch the carbs closely to see if they are spitting fuel. If they are then the reeds are broken.

 

[exlration]

Just out of curiosity will I notice any gains in power if it is indeed broken reeds?

 

[Jiggz]

Depending on the extent of damage on the reeds. If the engine is running fine at WOT but only idles rough, maybe not a whole lot. But with broken reeds you will eventually break some more and eventually the engine will not even start or idle anymore.

 

[exlration]

I removed the lower reed blocks today. They are intact with no chips or cracks. However the relaxed opening on most of the pedals is .015 to .017 and a couple are sealed against the block and seem to be kinda sticky when I open them up. (not real sticky but it does take a little pressure to pop it open) I also disassembled the carburetors, cleaned with carb cleaner and compressed air, and reassembled with new float pins and gaskets.
Any suggestions on the reed blocks? I know max allowable opening while relaxed is .010. Could this be the problem? Also, is it normal for the pedals to stick like some do? Thanks again!