Omc 150gt

[rockycarr]

I have a 1988 Johnson 150GT. It is on a 18ft bass boat. I am running a 14 1/4 x 24 raker prop. It runs 60 top end. I would like to get it close to 70. I was told that I could change the comp. on the heads. Is this correct and would it get me any more speed? The comp. now stock is 95-100psi. I have thought about going to a 26. Need to know how much to take off the heads if I were to shave them and how much am I going to gane. Just looking for ways to get more top end. It comes out of the hole great and I think that I have a little to gain there by changing the prop. The jack plate if up as fare as I can take it and it sits about 2" bellow the pad now. Thanks for any help that someone can be.

[mcyama]

Stock reeds, carbs and heads off of a 235 1978-1982 will give you more than what you ask for. Thanks Martin

[roadkill636]

Stock reeds, carbs and heads off of a 235 1978-1982


88 johnson is a looper

[rockycarr]

So if it is a looper does that mean that the heads off a 235 will not work as mcyama has suggested?

[roadkill636]

NOTHING at all is compatable between a looper and a crossflow powerhead

what exact RPMs was it turning at 60 mph?

[Riverratt]

88 johnson is a looper

A 1988 150 is a crossflow not a looper

[roadkill636]

I thought Johnsons were loopers in 86 and up, and evinrudes were crossflows for a few more years on?

[Riverratt]

I thought Johnsons were loopers in 86 and up, and evinrudes were crossflows for a few more years on?

The 150 remained a crossflow for many years

[roadkill636]

I stand corrected

[ChrisCarsonMarine]

The 150 gt is and always was a crossflow,3.5 bore.Heads from a 78 or 79 235 will fit,they are also 3.5 bore.In 1980 the 235 jumped to 3.625 bore,and stayed that way...these heads wont fit your 3.5 bore 150.Small bore small chamber heads,78-79,1 3/8 carbs from the 235,glass reeds and some working with the jets will wake it up,chris

[EMDSAPMGR]

Some of the 150GT/XP heads make decent compression. They are similar to the old 456/457 heads. Definitely the early 235 heads are much better. Close to 120 lbs compression. Also, add the rubber intake filler blocks from the early 200/235 engines. They are NLA, but will att 5-8 hp for free. You already have the large 1 5/16" carbs, which are solid. With heads, composite reeds and the intake filler blocks, you should jet the carbs up a little. You may find the big bore 1 3/8" carbs are too much for a small bore, low port crossflow like you have. If you convet to them, your holeshot may be worse. These large bore carbs are better on the big bore powerheads. You won't see 70 with these changes, but you may find additional setup changes will help-for instance you may have to go to a higher pitch prop after you make the engine changes. Also, filling some of the lower unit water pickup holes will let you run the engine higher=more speed.

[76baja18ft]

88 johnson is a looper

1988 gt150 is definately a crossflow

[rockycarr]

At this time I do not know what the RPMs are as it did not have a tach. I installed one but I have not been able to run it this year yet do to the snow and ice. Where can I find the things that are mentioned as fare as the spacers and reeds. If I wanted could I get the heads milled down to get more comp.? Thanks for the help. It looks as if I have alot of resurch to do to find a set of heads off a 78 or 79 crossflow. Thanks again.

[ChrisCarsonMarine]

we have anything you need,chris

[rockycarr]

PM me the prices shipped to 48189 and I will see what I can do. Also a phone number would be great. Thanks for all your help.