Yamaha 150 low compression...

[tahoetech]

:confused:I just rebuilt a 92 prov for a customer. We bored all six .030, new pistons/rings, reeds- everything. All tolerances are within spec. It runs, but not good... Its only got about 90lbs on all six.. I am stumped. Im totally up for any suggestions. Thanks guys! (Does wiesco make different compression height pistons? Maybe I got the wrong ones??):cheers:

[Ray Neudecker]

What clearance did you use?

[mcyama]

perform leak down test. thanks Martin

[tahoetech]

Thanks for the replies, guys... I was wrong on the pistons (its been about a year since I started on the project)
-Pistons, Pro Marine 1620p/1620s- (small pin 60*v) finished bore size: 3.564" .020. Ive used this brand of pistons in multiple engine builds with good sucess. Bore job measures good.
-I already took the heads off and I didn't do a leak down yet.
-The bores look great, everything looks like it should. What gets me is that all 6 are down-evenly, not just one or two.
- Ive also got a h20 problem- might have to do with excessive back pressure?? It wont pump water if I rev past 2000rpm. New imp,housing etc. It will pump up to the relief housing if I take the cover off, but not past the heads. Im thinking about everything from cracked sleeves to a bad exhaust cover. Im hoping someone else will think of a problem I havent thought of.

[skydog]

Man buddy i know my Friends Yamaha and my Bros old OMC only had about 90PSI or so.... Yes do a leak down, but it might be kind of high with the motor having no run time... I might be wrong but i dont think that is your problem.
Skydog

[skydog]

I bet your problem is with your H2o problem!

[Ray Neudecker]

I would suspect your water problem is directly related to the performance problem. Obvious place to start would be the poppit valve and next would be the exhaust divider plate and gaskets.

[tahoetech]

The popit valve, spring, grommet and gaskets are all new. I could have something in wrong (Ill double check). But what about the compression? Ive been wrenching for about 14 yrs and Ive never had this problem. What ever it is, Ill let you guys know... Keep the suggestions coming!

[mcyama]

why did you have to bore the unit?

[tahoetech]

The boat came to me smoking like crazy, overheating and fouling sparkers. I did a compression test and looked with the bore scope. After taking off the heads I realized it needed some machine work. The customer wanted some bandaids just to get the rest of the season done. Trying to save his season- I installed a new impeller, oil pump and head gaskets; it ran for about 10 more hours until fall. It has been a nightmare ever since. If I could have done it over I would have sent him down the road or repowered. The cooling system was all corroded, the emulsion tubes were eaten off inside the carbs, and the main carb's shaft was siezed after sitting a month. This boat has never seen salt- it has spent all its life in small puddles in the mid west... WTF? Anyways, here I am with a new motor that wont run and a customer getting antsy. We sold him a new boat at cost and I told him we would sell the old one on consignment when we got it running for him.. (to re-coop some of our cash) Basically his down payment for the rebuild paid for the parts, and Im eating all the labor. I just want this thing out of my hair, but I need him happy.

[Ray Neudecker]

Sounds like you are fighting the original problem. You may have to pull the powerhead to find the problem. There is a small hole with a rubber o ring on the bottom of the block that serves as a drain for the exhaust cavity. I have seen corrosion rot them out and cause the loss of water pressure.
Also check to be sure the powerhead gasket does seat everywhere. No corroded out places in either the powerhead or adapter plate.

[tahoetech]

Yeah, the cooling problem was there from day 1. I inspected everything very good before I reassembled though. The motor is getting tore down again to look for problems. What gets me though is that there was more compression with the old pistons/bores. Thanks again guys!:cheers:

[skydog]

Yea that compression will come up some after the rings seat. How much time is on the new rings?