Thread: 94 750 bunning pistons
04-14-2010, 10:11 AM #1
- Join Date
- Apr 2010
- Southlake, Tx.
94 750 bunning pistons
Neighbor had a 750 polaris and rode with no problems for a season and then put a hole in the front (stator) piston. Being young and no money asked me to help out. Tear down was complete including spliting the case and flushing debre. Inspected lower end, it was within spec. Oil injection system was removied in favor of premix. Barrels were cut and honed to accept +.10 and unit was reassembled. He could not afford new reeds or carb rebuild so against my better judgement I put it back together, checked carb adjustment and took it to the lake for break-in.
Shortly after the ski was again in my drive with a burnt piston, same one. The owner had the chance to buy a house and needed money so after one jack to many, I bought the ski. But in a new piston, new low lever power reeds and rebuilt carbs. The front cylinder is still burning hot. Timing is good, carb pop off is good, water flow is good, it starts easy and runs smooth. Secondary fuel pump was added to front carb and both pumps appear to flow well.
Now that you have lived through my sad story can any body tell me what is going on? Any suggestion will be welcomed. My wife already suggested throwing a match on it, so that one's been used.
04-14-2010, 10:39 AM #2
- Join Date
- Jul 2007
- near Toronto, Canada
Welcome to the Hulk
The links below might be helpful
Secondary fuel pump?
Sounds like you need to install a Mikuni triple outlet fuel pump.
Also, make sure there is a fuel flow restrictor in the fuel return line from the carbs to the tank. If the fuel lines have been changed, often the small brass restrictor gets tossed out with the old lines. Then the fuel pressure is low, regardless of what fuel pump is in there.
On the early years, the restrictor is a small brass slug inside the fuel return hose. Later years the restrictor is built into the last carb's fuel exit fitting.
Polaris Fuji (blue) engine
Did you check the carb jet sizes while you had it apart?
During the carb rebuilds, did you flush out all the tiny internal passages with cleaner?
04-14-2010, 05:22 PM #3
- Join Date
- Oct 2006
- Cleveland OH
Welcome to the hulk rob.
A leak down test of the crankcases would be in order to verify no air leaks.
As mentioned above, make sure the restrictor is installed in the return fuel hose. Especially if the hoses were replaced. If they weren't, might be a good time to do it. Including the pulse hose.
Also use the triple outlet fuel pump instead of that secondary fuel pump set up.
Make sure the autocock was removed per Polaris bulletin.
What's the compression on the cyls? Too much compression can add heat and detonation.
What spark plugs are you running? NGK BPR8ES are recommended instead of the 7's the book calls for.
05-09-2010, 01:54 PM #4
Crand index checking tool
The other one ran great, never a problem
The dealer had/made this 2 -piece collar and degree wheel, that clamped over the P.T.O. end, was able to diagnose it without engine removal. It's a pretty simple set-up, you can probably fabricate one for cheap$$$$$$
Hope this helps
05-09-2010, 09:04 PM #5
Get a fuel pressure gauge on that thing and verify you're getting fuel to the carbs.
Fuel selector switch, leaking hoses, bad clamps etc. could all be fuel supply issues contributing to your problem.
I'd certainly verify the crank index as well.
05-10-2010, 12:53 PM #6
- Join Date
- Apr 2009
- Lubbock Texas
I have the same ski, bought it for my wife and it did the same thing. I added a second pump and still did not work. I ordered the Tripple outlet pump that comes on a SL780 and put it on my 750 and now it runs great.
05-17-2010, 01:44 PM #7
I've tried the second pump but didn't work as well as the tripple outlet.
the fuel system is the main cause of a lot of jet ski engine failures. theres no telling how many supply lines i've pulled off of carbs and hooked my vacuum pump up to and watched a train of air bubbles flowing at even low flow.
start at the tank and work your way to the carbs, bypass as much as you feel comfortable doing, my personal skis; i baypass everything, i run an in-tank filter and a strait line to the carb. no potential for air leaks but you don't have reserve but my filter sits on the bottom of my tank.
don't forget about the thermostat in the polaris skis either.
05-18-2010, 08:40 PM #8
While going through the fuel system in mine for the same problem on the same machine you have, I found grass in my fuel selector valve.... This was discovered when shooting WD-40 through the valve to determine it's effectiveness in blocking fuel lines that aren't in use.
Like you, I bought the ski as a fixer upper with the hole in the piston from the previous owner.
Users Browsing this Thread
There are currently 1 users browsing this thread. (0 members and 1 guests)
By gprmike in forum Yamaha PWC Performance (2-stroke)Replies: 9Last Post: 11-18-2007, 05:58 PM
By quadzilla in forum Yamaha Open DiscussionReplies: 1Last Post: 09-19-2007, 12:05 AM
By jmule98 in forum Polaris How To & FAQsReplies: 9Last Post: 08-16-2007, 10:52 PM
By vanlohd in forum Polaris Open DiscussionReplies: 10Last Post: 05-20-2007, 10:50 PM
By TXslt950 in forum Polaris Open DiscussionReplies: 30Last Post: 04-16-2007, 12:45 AM