951 idles but bogs and dies if throttle advanced

[bigtexan99]

I was so sure the boat would be running today! But it did not. Well, it ran a little bit.

Yesterday we rebuilt the carbs using Mikuni OEM parts and new 2.0 Needle and Seat. Everything went okay. We started the engine on the trailer and ran it for a couple of minutes. It was hard starting, we finally put some starting fluid in the carbs to get it to run, might have been just no fuel as the carb was completely disassembled and cleaned.

Got the boat to the ramp and it started right up. Idled for about 2 minutes as we climbed aboard and got ready. Then, gave it some throttle and it just bogs and dies. It's like the engine is flooding immediately.

We set the idle (by twisting the butterfly nun on the top of the carbs) to about 2800 in the water and it putt putted around for a bit, but any attempt to accelerate and it bogs and immediately dies.

I'm not sure what to check and appreciate any advice!

 

[Dr Honda]

Did you check the pop-off pressure? Sounds to me like either the bypass ports are plugged, or the pop-off is too high.

Are you running the factory airbox?

 

[Dr Honda]

One more thing... it is very possible that you have cracked reeds.

 

[bigtexan99]

We did not do a popoff test. It got a new spring on that lever that sits over the needle. The guy who I had do the rebuild said "he had never heard of a popoff test and had never done on on any carb rebuild before".

We have left the airbox and those 2 end pieces of the airbox off/uninstalled during this testing so as to have easier access to the throttle cable and visual inspection of the carbs.

Based on this diagram... http://partsfinder.onlinemicrofiche.com/seadooforums/Seadoo_oem/Seadoo_PWC.asp?Type=13&A=76&B=16 Where is the 'bypass port'?

I will search for info on cracked reeds. What are reeds? LOL.

 

[seadoosnipe]

Tony, the carb set up on this model Sportster is using the accelerator pump.

BigTexan, check out part #63. The purpose of this pump is to squirt a small amount of fuel into your carb throat for initial take off. Once the motor starts to build vacuum from that squirt of fuel, the vacuum created is enough to start drawing fuel from your low speed jets. I don't know why they used this set up on some boats and not others. My Challenger has it and it's a PITA. I haven't done it yet, but one of my 787's doesn't have the accelerator pump, so I'm thinking of trying them on the boat. The only difference between the carbs with them and without them is the jetting.

To check your accelerator pump, you can take off your airbox cover and flame arrestor. Look down inside the throat of your carbs (use a mirror if needed). Just inside the carbs throat, you should see a small brass nipple sticking up. This is your accelerator jet. The pump is only on the PTO carb but "T's" into both carbs. You can either reach under the PTO carb or have someone help. In one quick movement, move the cam to wide open throttle. The first 1/8 of turn on the cam, you should see a squirt of gas jetted into the throat toward your throttle plate. If you don't, then this is why your motor is responding to throttle up like it is. The majority of the time, the check valve ball inside the pump is sticking. The easiest rememdy for this is to buy a small inline check valve. You can find it at a local lawnmower repair shop.

In the meantime. If you want to make sure this is your problem, when you have the boat out, start to throttle up by feathering your throttle. Give it a bit of fuel by kinda pumping the throttle. You'll be wanting to do this without stalling. Once you pass a few hundred RPM, the motor should spin up on its own, no longer needing your accelerator pump.

 

[bigtexan99]

Okay, I will check out that part 63 and I will visually check for the fuel squirt when opening the throttle. I did try several times to 'feather up' the throttle yesterday but it would not respond.

Does your comment to Tony mean I don't have the 'bypass ports' or some other issue?

Thanks!

 

[bigtexan99]

Update, I discovered today that the PTO carb is not igniting. It has good compression, its getting spark. I swapped plugs with the Mag cyclinder and the mag cylinder fired fine with the pto spark plug, the PTO cyclinder again did not ignite. It is getting fuel, plug was wet.

 

[seadoosnipe]

When you changed out the needle valve and seats, did you change the setting on your low speed jet? If your plug is saturated with a significant amount of fuel, you may be open to much on the low speed jet, causing it to flood out, which means no ignition. You also said your mag cylinder was running fine. So, make sure both the low speed needles are adjusted out the same, and that your carbs are sync'd according to the procedure in the shop manual. Being out of sync may mean your pto is opening before the mag, causing the flooding to drown out that cylinder.

Then we should go back to the pop off test. If your plug is really saturated, then you could be getting to much fuel due to the pop off being too low. If you flood the cylinder with fuel, it will not start. When you had the carb open changing the needle valve and seats, did you change out your spring? If so, and the above adjustments don't do any good, then revert back to the old spring. If you put a pop off gage together, you can just check it verses putting the older spring back in.

Tony? No, it didn't. Tony is a well versed mechanic. I believe he was referring to the small internal porting to the by-pass of the low speed jet. There are 3 holes attributed to the low speed jet. Two before the throttle plate and one behind it. This is what I believe Tony is referring to.

 

[Dr Honda]

Yes... the bypass ports are the small openings at the bottom of the carb, near the butterfly. When they get plugged, you will have a hard time transitioning past idle.

BUT... since we now know you have a wet plug, spark, and good compression... there is a good chance you have broken reads, if you were having running issues BEFORE the carb rebuild. If the problems happened after the rebuild... keep reading.

If the guys doing the rebuild said "He never has done a pop-off test"... he's got his head lodged WAY up his lower side. It is a critical test on a pressure carb. ESPECIALLY if you put in new springs and needles. When the rebuild was done... did the guy use OEM Mikuni parts, or aftermarket? Aftermarket needles, and diaphragms are known to cause flooding issues.

I'm not trying to be harsh, but the guy who did the work is obviously not a PWC mechanic.

 

[bigtexan99]

Okay, guys thanks. won't be able to work on boat for a while so I will post back when I get time to do some more troubleshooting based on this advice.

Happy New Year!