compression on rebuild

[walleyehed]

The boys on the forums have me thinking maybe I've got compression- or the lack of, on my fresh rebuild 97 crossflow 115 Jonny. I bypassed the VRO, went to 50:1, but for break-in, 25:1.<br />After the first 6 hrs run time, 3 90's and a 95.<br />now at 20hrs and on 50:1 after 8 hours, I've got 3 95's, and 1 at a 100lbs....Comments????

 

[G DANE]

Re: compression on rebuild

I would recommend another gauge - maybe your gauge is readin low. Looks fine equal, but a bit low. Does this have lowcompression heads or gaskets.

 

[walleyehed]

Re: compression on rebuild

I bought this engine out of Florida, and had 1 piston siezed. Bore was too tight for .020 pistons, as bore only measured .018 over std.<br /> I went to .0205 as recommended by wiseco. And as far as I know, it has stock heads, and I installed a standard gasket set. I guess I will try another gauge, but it reads in the 120's on another older 115. Holy CRAP, I don't want to pull this thing down again....I'm an aircraft Mechanic that specializes in powerplants, and you would think I could get this right the first time, although this is about the 18th outboard I've done?????????

 

[Walker]

Re: compression on rebuild

How does it run? If it runs good, who cares what the compression is.

 

[walleyehed]

Re: compression on rebuild

Walker, overall, it runs real good...the boyesen reeds I put in made the idle smoother than I've ever seen in this vintage. I just don't think it's turning the right prop, the right rpm. We sold about 40 of these 115's in the late 70's and early 80's, and on 17' or 17.5' they always were fit to a 13.25X17 (5500-5600), mine is an aluminum starcraft, although 18', I was thinking because of the boat weight compared to the Glastrons we were setting up, this would swing the same prop, but I only get 5300 with a light load and trim very carefully to optimum performance. Is it just a drag infested boat because of rivot heads?????

 

[G DANE]

Re: compression on rebuild

walleyehed : I have a 91 115HP sitting i am going to put on my project boat (17' light runabout ) when finished. It sat for 4 years now, but read 125 125 125 and 135 on a really low battery. I put oil in it now ( after test ) to loosen rings up - hope to get more equal reedings. Its been owned by same guy from new, never decarbed. I only have the 90-95 manual, dont know if any changes to a 97. I expect to turn a 19 or 21 prop, boat only weighs 650 lbs + crew and gear. Be aware HPs changed in 84 to proprated, putting out around 10 HPs more. Didnt the 90 deg crossflow end in 95 ? Is yours a 60 deg ?

 

[Dhadley]

Re: compression on rebuild

What heads are on it? What is the timing set at? Standard head gaskets or thick ones? What spark plugs?<br /><br />Let us know!

 

[walleyehed]

Re: compression on rebuild

Stock heads, 29 deg timing, stock head gaskets, all stock but jet size (1 lower).

 

[walleyehed]

Re: compression on rebuild

Oh yea, QL82C's gapped at .040

 

[posuelo]

Re: compression on rebuild

is a 110 psi good compression when is even in al six cilinder?

 

[dazk14]

Re: compression on rebuild

Those Boyesens tend to run lean on the bottom and rich on the top.

If your motor is "broken in" enough - new plugs, high speed run, chop ignition - read plugs.

See if you can get a measurement on the head gasket. I've have 4 different brands of v-4 head gaskets and they varied by .012". That is roughly 10psi.

Are you checking compression with all the plugs out? Short ignition wires to ground or pull out the lanyard.