1995 XP 717 starting question - plugs a bit wet

[orbit398]

Recently purchased a nice XP with motor issues, low compression. Along with fixing the motor (honed, new piston/rings), I rebuilt the carbs with Mikuni kits following directions, removed the choke plates and installed a primer, replaced grey gas lines, filters ect. Now compression is over 135psi on each cylinder. It starts and runs great when cool/cold. But once hot, it's hard to start and I have to hold the throttle wide open for 10 to 15 seconds for it to start. Plugs look ok in color, a bit rich, but they seem kinda wet. I did find I had over pressurization in the gas tank due to the gas tank vent check valve not working, thus I thought the issue was the pressure build up in the tank for forcing gas into the carbs. I removed the check valve to see if it had an effect but it did not as it still hard to start when warm. Not sure what to check...looking for ideas. thanks

 

[mikidymac]

What is the compression when it is hot?
Typically when compression is low just installing new pistons and rings will not fix it due to the bores being worn and excessive clearance.

 

[orbit398]

Hum, have never checked a motor compression when it was hot. Cylinders didn't look that bad, thus honed and put new piston/rings. In the many past projects I have done, with cylinders looking ok, honing them allows for the new rings to seat well and being new rings, there is more material to press on the cylinder walls, which normally bumps compression way up. I believe 135+psi is pretty good for these engines.

But, as you suggest, next time I am out (probably this weekend) I will do a compression check and let you know the results. Now I am curious on this. Thanks for the suggestion. And I sure hope the compression isn't significantly less when the engine is hot.

 

[burtshaver2021]

Hum, have never checked a motor compression when it was hot. Cylinders didn't look that bad, thus honed and put new piston/rings. In the many past projects I have done, with cylinders looking ok, honing them allows for the new rings to seat well and being new rings, there is more material to press on the cylinder walls, which normally bumps compression way up. I believe 135+psi is pretty good for these engines.

But, as you suggest, next time I am out (probably this weekend) I will do a compression check and let you know the results. Now I am curious on this. Thanks for the suggestion. And I sure hope the compression isn't significantly less when the engine is hot.

Did you measure the cylinder bores? To see if they were within spec

 

[orbit398]

Did not as don't have the tool to do that, so you may be on to something. Is there a minimum psi where the engines start to have issues when starting? I was thinking is more a carburation issue of some sort.....

 

[burtshaver2021]

You may be right, it may be a carb issue. See what the compression is when hot. I didn’t look back to see what engine you had but for the 587, 717 and 787 I would say anything below 125 to 130 will start causing issues.

 

[orbit398]

Well crap, we've been on four good days at the lake since the engine redo etc. Each time, the XP is getting harder and harder to start. Once going, it runs great. So today was the worse day for starting. When home, warmed it up on trailer with the hose. Check compression and man, what a difference. One cylinder is a 120 psi, the other at 115 psi. Same gages; have two and both read the same. So, guess I found my issue. Had no idea compression would get worse with engine warm. I thought it'd be better hot but the more I think about it, when metal gets cold, it contracts. I have froze bearings before that need to get smacked into car engines at time, thus when the metal cylinder gets hot on the rotax, it expands. Sure wished I would have had the cylinder measured by someone prior to me doing all that work. Bummer....