Hesitating/surging when accelerating

[Hunterb]

Hi all,
I am in the process of rebuilding a 1997 787 and have received a bunch of helpful advice from this site already. I now have it all back together and have taken it to the lake twice to test fly it. Here's what's happening:

It starts and idles perfectly. I have the Idle set to 1400 in the water.

Acceleration is ok Immediately off idle but as rpm's increase it starts to surge and stumble. It's like it's getting fuel and then not, and then it's back and then gone again. I have not tried to go full throttle because I'm trying to follow the proper break in procedure. It will hold steady at 4k rpm but if I try to accelerate from there it will do the surge/hesitate thing. When it gets fuel it grabs hard and accelerates well but it only lasts a few seconds and then drops off.

I have cleaned the carbs but have not put new kits on them. Pop off pressure is on the low side, about 21psi, according to my gauge but I don't trust it that much as it's a cheapie. I have the LS screws at 1 1/2 and the mag HS screw at 0 and the PTO HS at 1/2.

I'm very hesitant to keep running it in case it's lean, which I suspect it could be.

Is there a way I can determine if the surging/hesitation is a lean or rich condition or could it be something else altogether?

One thing I did check was the rectifier and it seems fine.

Any suggestions will be appreciated.

 

[Hunterb]

Hi all,
I am in the process of rebuilding a 1997 787 and have received a bunch of helpful advice from this site already. I now have it all back together and have taken it to the lake twice to test fly it. Here's what's happening:

It starts and idles perfectly. I have the Idle set to 1400 in the water.

Acceleration is ok Immediately off idle but as rpm's increase it starts to surge and stumble. It's like it's getting fuel and then not, and then it's back and then gone again. I have not tried to go full throttle because I'm trying to follow the proper break in procedure. It will hold steady at 4k rpm but if I try to accelerate from there it will do the surge/hesitate thing. When it gets fuel it grabs hard and accelerates well but it only lasts a few seconds and then drops off.

I have cleaned the carbs but have not put new kits on them. Pop off pressure is on the low side, about 21psi, according to my gauge but I don't trust it that much as it's a cheapie. I have the LS screws at 1 1/2 and the mag HS screw at 0 and the PTO HS at 1/2.

I'm very hesitant to keep running it in case it's lean, which I suspect it could be.

Is there a way I can determine if the surging/hesitation is a lean or rich condition or could it be something else altogether?

One thing I did check was the rectifier and it seems fine.

Any suggestions will be appreciated.

More info that may be helpful.
Engine was bored out 1mm over and Pro-x pistons installed.
Rave valves were shortened to proper clearance. Red caps are set flush.
Plugs are new.
I'm running with the air box off but flame arrestor on.
Idle is smooth and steady.

 

[mikidymac]

I am going to yell at you here for a minute.
REBUILD YOUR CARBS! RULE NUMBER 1 on a new engine!

You are risking a brand new engine on dirty carbs here.

New kits with Genuine Mikuni parts and needle and seats, new fuel selector and strainer.

You are going to kill theat new top end.

 

[Hunterb]

I am going to yell at you here for a minute.
REBUILD YOUR CARBS! RULE NUMBER 1 on a new engine!

You are risking a brand new engine on dirty carbs here.

New kits with Genuine Mikuni parts and needle and seats, new fuel selector and strainer.

You are going to kill theat new top end.

OK, I think that is good advice. According to the PO the carbs were recently rebuilt but a lot of the stuff he told me may or may not have been true. I'll order the kits today. Have already replaced the fuel selector and filter and all the grey lines were changed before I got it.
Off to Amazon or Ebay I go. Only genuine parts for sure.
Thanks for the reply, even if you yelled at me. Ha Ha.

 

[mikidymac]

I like to use the OSD "Back to Stock" kits since they include the correct springs and base gaskets that the actual Mikuni kits do not.

I am working on two skis that had the carbs rebuild by a professional mechanic last summer and never ran correctly.

I found that they did use genuine mikuni parts but they had the wrong springs, too many gaskets in the fuel pump block and the screws way out of adjustment. So basically don't trust anyone that has been in there before.

 

[Hunterb]

I like to use the OSD "Back to Stock" kits since they include the correct springs and base gaskets that the actual Mikuni kits do not.

I am working on two skis that had the carbs rebuild by a professional mechanic last summer and never ran correctly.

I found that they did use genuine mikuni parts but they had the wrong springs, too many gaskets in the fuel pump block and the screws way out of adjustment. So basically don't trust anyone that has been in there before.

Agreed. When I first took the carbs off the HS screws were turned out about 2 1/2 turns so who knows what's been done or why. The motor had injested something that jammed the rotary valve and stripped the gear. There was also significant damage in the top end, wrist pin clips missing, rings cooked into the pistons, scored cylinder walls etc. It was a mess.
Thanks again for the help. Now I need to find a way to get parts quickly to western Canada.
Bruce

 

[Hunterb]

Agreed. When I first took the carbs off the HS screws were turned out about 2 1/2 turns so who knows what's been done or why. The motor had injested something that jammed the rotary valve and stripped the gear. There was also significant damage in the top end, wrist pin clips missing, rings cooked into the pistons, scored cylinder walls etc. It was a mess.
Thanks again for the help. Now I need to find a way to get parts quickly to western Canada.
Bruce

Hi again. So I sorted out the carb issue and I have the seadoo up and running well. My question now is about oil. I have read the million or so posts about the correct oil to use so I know I need a full synthetic, low ash, API TC rated oil. What is in the tank now is xps 2T full synthetic. My problem is that I cannot purchase that where I am and the nearest Seadoo dealer is 1 1/2 hours away. So I found another oil made by Shell: full synthetic, low ash, API TC rated as well as JASO FD and a bunch of others. So I'm pretty sure it is suitable oil. I'm not trying to save money as I think this oil is about the same price as the XPS stuff ($62.00 for 5 liters). My question is, finally, is it advisable to mix the two oils? I am premixing as well as injecting as I break in the motor so I could just use this new oil as premix if it's really dangerous to mix oils. It seems to me that the average user probably does that all the time buying whatever is local and cheapest.
Anyway, let me know what you think. I didn't want to start another oil debate.
Thanks
Bruce

 

[mikidymac]

No, never mix two different oils in your oil tank as they can gel and plug up the oil injection system.

You only need to premix for the first full tank on a new engine.

If it was mine and I had no other options I would use the existing XPS full synthetic int he tank and premix with the Shell oil.

 

[andy.262]

is it advisable to mix the two oils?

I asked the same question not too long ago, and was also told that the answer is no. So I siphoned all the old oil out and put the new oil in, thinking I was done. Wrong. Even what little old oil was left in the tank reacted with the new, and it created this thick sludge at the bottom of the tank. Thankfully I caught it before it did any harm.

Based on my experience, I would take the oil tank out and rinse it out with gasoline to make absolutely sure the 2 oils don't mix. It's not terribly difficult to remove the tank.

That being said... I'm in a similar situation to you where the closest place to get XPS full synth is about an hour away. I buy it from Amazon, if you have prime you can even get 2-day shipping. Whenever I finish a bottle I order a new one, and I haven't been stuck without oil yet. My 2 cents on the situation.

 

[Hunterb]

No, never mix two different oils in your oil tank as they can gel and plug up the oil injection system.

You only need to premix for the first full tank on a new engine.

If it was mine and I had no other options I would use the existing XPS full synthetic int he tank and premix with the Shell oil.

OK. Thanks for the quick reply! I'll do that. It's strange that 2 full synthetics can't be mixed. I can see not mixing Dino and synthetic. I guess it's about the additives being different.
Wow, does that ski ever go now that I sorted the carbs out. Still a bit strange if you hold steady in mid throttle; kind of a brap, brap, brap thing, but add a little throttle and it jumps like crazy.
I'm assuming I need to complete the break-in process before doing any plug chops to see how it's burning?
Thanks again for all your help on this forum. I read a ton of your posts.
Bruce

 

[Hunterb]

I asked the same question not too long ago, and was also told that the answer is no. So I siphoned all the old oil out and put the new oil in, thinking I was done. Wrong. Even what little old oil was left in the tank reacted with the new, and it created this thick sludge at the bottom of the tank. Thankfully I caught it before it did any harm.

Based on my experience, I would take the oil tank out and rinse it out with gasoline to make absolutely sure the 2 oils don't mix. It's not terribly difficult to remove the tank.

That being said... I'm in a similar situation to you where the closest place to get XPS full synth is about an hour away. I buy it from Amazon, if you have prime you can even get 2-day shipping. Whenever I finish a bottle I order a new one, and I haven't been stuck without oil yet. My 2 cents on the situation.

Thanks for the reply. Unfortunately I am in the great white north and I cannot find the XPS oil on Amazon. I'll continue searching. This Shell oil certainly looks like quality stuff but if mixing is a bad idea I'll either have to do as you suggest or take it back and make the journey to get the XPS stuff.