Throttle unresponsive to high revving

[bigtexan99]

I really appreciate everyone's help here in getting my boat up and running!

Here's where I'm at today, and I'm not sure which direction to go.

We were able to adjust the idle out of water to 3k by adjusting the throttle cable. The 2 carbs were slightly out of sync, we adjusted them at the carb linkage bar. Then for the first time...I put the boat in the water!

1st we floated for a while and made sure it was watertight. After a few minutes, I started it up, and just like the shop manual said it would it idled in the water at about 1200. All sounded good, so I put it in F and we started moving forward. Boat was moving along pretty good at idle, so I gave it some throttle and it jumped forward and rpms went up to about 6 or 7 but we could not get on plane. Thinking there was weeds, we idled down, put it in reverse and then tried again. Once again we could not get on plane, and the throttle seemed un-responsive. I would move the lever forward, but the engine would not respond and rev up, it just stayed at about 2 or 3 rpms.

We floated for a minute then tried again. Started the boat and idled okay, maybe a little low. As soon as I gave it the slightest throttle, the engine would just die as if the stop button had been pushed. This happened 4 or 5 times. Finally, it would only idle for a few seconds then die. We manually adjusted the throttle cable up to a faster idle and let it take us back to shore.

The throttle seems to have a mind of its own....it jumps up to high revs by itself for a second or two, then drops back down.

We are stumped. Any ideas as to what might be the issue(s)?

 

[LouDoo]

I would take a close look at the throttle cable, might be stretched, check at the carbs, make sure the set screws for the cable are tight. This could be a carb. problem, or could be low compression but I would check the obvious first. You might want to check your compression on a 951 should be around 130psi.

BTW, putting your boat in reverse only lowers the reverse gate, your impeller still spins the same direction, so if you have obstructions, ie weeds it will do nothing to clear them.

Lou

 

[bigtexan99]

Okay, will do. I noticed in the manual there is a throttle sensor inline on the throttle cable. Do those ever go bad and if so, what are the symptoms?

 

[LouDoo]

I don't know, are you sure that's for a carb. model boat? Sounds like it would be for a fuel injected or direct injected motor.

Lou

 

[howie099]

Usually the jumping in RPM's is obstructions in the jet pump or you are could be sucking in air from the carbone seal or boot if you are trying to move. If you are just idling not moving the throtle and the engine is reving up you more than likely have an air leak. It does sound fuel related if you move the throtle cable and the engine dies. If you have dual carbs and they are synced you might have to open up the carbs and clean them up. Do you know if they rebuilt the carbs after the engine was swapped? That is the perfect time to do it since the carbs could of caused the engine to fail.

If you don't have proof of what they did just rebuild the carb/carbs and put in new base gaskets and check for air leaks. Could have a week fuel pump in you have the internal carb ones or fuel filter could be clogged. Check all settings you can in the fuel system first without having to take anything apart.

Cleaning the carbs is not a hard job to do it is harder to sync th carbs than rebuild them. Remeber to get new needles and seats since they don't come in the kits.

 

[bigtexan99]

Took my boat to a mechanic this morning. Bad news. The Mag Cyclinder has low compression at 90. We added some lucas oil to the cyclinder and that got it up to 120. Then with engine running, he diagnosed the clank noise as a bent crankshaft.

Options are to order a SBT engine and he will install for $1700 total.

Or, he will rebuild engine by getting a custom crankshaft and have it welded so it does not fail again. Top end rebuild with new pistons, sleeves, rings, and drill them. Machine work will e done by a specialty short block shop in Tempe, AZ. While engine apart, he will synch the carbs, remove the oil pump and block the engine to go pre-mix. $1950 out the door.

What do you guys think?

 

[LouDoo]

Nothing against your mechanic but the first thing I would do is get another opinion. In my opinion if he were a good mechanic he wouldn't advise getting rid of the oil pump and I'm not so sure about the custom crank. I personally wouldn't do anything to enhance performance of a 951, it's got all it needs from the factory. In fact early 951's were prone to failure, that's why they lowered the compression.

So with the two options you have I would got with the SBT engine and KEEP the oil pump.

Lou

 

[bigtexan99]

Whats wrong with a custom crank?

 

[LouDoo]

Honestly I don't know, but if it increases the compression I wouldn't do it. It may be just me but I believe in leaving engines stock, I'll take reliability over speed any day. That's just my opinion.

Lou

 

[howie099]

There are other buliders that will build better engines and will uphold there warranty better than SBT. I would keep it stock also less to mess with. Well I pulled oil pumps of other brands of PWC with no issues. You will use more oil and more plugs I never seen a engine run with a bent crank either that will usually eat other things up. If oil wasn't the cause of the failure i would keep it. I would look at fullbore.com, I have used him for 3 engines and have had good results. Since it's the off season you will get a better price. Most places don't like to mess with oil injection since its more work to do.


I would have the carbs rebuilt also since the thing is all a part now.

 

[bigtexan99]

Thanks... looks like I will be dropping the boat off tomorrow for a complete rebuild. New crankshaft, and top end.

I'll let you know how it turns out.

 

[bigtexan99]

Pulled the engine. Take a look at my pistons. I'm hoping a little rubbing compound will make them good as new.:lol:

 

[Seadoobuddy]

I think it will take than just rubbing compound...
They look scored. Maybe from lack of oil.
Time for complete rebuild......top to bottom. Keep the oil pump but check to be sure it works.
Carbs too (again)?

 

[bigtexan99]

LOL...

Rebuild underway. Hot Rod Crankshaft, pistons, sleeves, rings, bearings, etc. I just rebuilt the carbs last month, they should be good. We are going to kill the oil pump and go premix. I don't care if it smokes a little at idle. Not going to do this job again because a plastic oil pump breaks.