1986 Johnson GT 200 Compression Specs

[Mhoward]

Hey folks,
I was wondering if anyone had an idea of the compression specs for 1986 GT200. I have searched the web and found nothing but conflicting information. I have seen at the top end stating that 120 psi per hole is spec, I have also seen as low as 80 psi for these as being common. I know that as long as all holes are equal, then there is not to much of an issue, but I would like a base number to go off of. I know that this engine is a 2.7L Bridgeport looper engine, with the high compression heads, but everything that I can find gives relatively low compression values for this engine. I always went with the rule of thumb of 110 - 120 on an average engine and 130 - 140 for high compression engines. Can some one please help? In addition there is mixed reviews on compression testing and whether to perform them at operating temp or at ambient temp. I see the reasoning behind doing them at both temperatures, but what is the most beneficial in your opinions?

Thanks in advance.

[perfmarine1]

110-120 would be normal GT, 130-150 would be high compression and anything over that would need race fuel in most cases. I do all mine at ambient temp, that is my base line.

[flabum1017]

110-120 would be normal GT, 130-150 would be high compression and anything over that would need race fuel in most cases. I do all mine at ambient temp, that is my base line.

The stock non-GT loopers that era were 90 - 100

[roadkill636]

115 psi on the 86-87 bridgeport GT and XP

[Mhoward]

So here is the story, I have no real engine complaints other then as of late it has been harder to hit max rpm in the boat when loaded with a few bodies. It makes no difference whether I run the speed wheel (30 pitch chopper) or the cruising prop (4 blade 23 pitch). So I was thinking that maybe I had a compression issue, even though it starts fine. So I did a compression check on the engine at ambient temp and it cleared 88 to 90 on all cylinders. Sounded low to me, but they are all the same. I checked the old interweb and found a forum saying that this was normal for the v6 loopers. Sounded weird, but I went with it. The motor still runs great, but it now feels a little sluggish out of the hole. It might be all in my head after seeing what I think are low numbers but now I am starting to question if I should be throwing some rings at it or even a full on rebuild. Any thoughts?

Thanks

[frank jackson]

Mat the hull you have is sensitive to added weight . If that boat is the boat I think it is The motor is fresh . Get the motor up to temp and butterflys open to do your comp. test .....

[davemvegas]

have someone do a leakdown on it. if leakdown is good pull heads a check the thickness of the head gaskets. I think gaskets come in .020 and .035 thickness. if you have the thicker ones you can raise compression with the thin gaskets. I have not messed with a gt motor. on a 95 looper that gaskets make about 15 psi difference. if you already have the thin gasket you may want to cut the heads a little. like I said I have not put thin gaskets on a gt. be sure you check for piston to head clearance.

[Mhoward]

So I pulled the plugs today and stuck a bore scope in to see if anything looked abnormal and everything looks fine. I performed a leak down test and there was no leakage. Frank you were right, this thing is still tight. I decided to pull off the air box and have a look and I noticed a couple of things. The first was that it had no guts to the air box (which I am told was the case for the 225hp). The second was that there was a lot fuel/oil pooled in the bottom of the air box as well all the lines and carbs were covered. I looked into the carbs and noticed that one of them had blown/pinched mounting seal sticking out. I have ordered a new seal and plan to perform a de-carb on it this week. Could this seal be the source of the fuel in the air box? Or could the jets be sized incorrectly? I would suspect that it could certainly be the source of my performance issue.

Frank hopefully some day I will be able to get that monster 3.3 built as we had discussed earlier in the year. There needs to be a couple more big johnrudes on Pigeon and Sturgeon.

Thanks

[frank jackson]

The only motors that had the gutted air box from the factory was the 200 xp/gt's . The loopers always spit back thru the carbs and the air box always has some gas/oil mix in the box . The seal will add to the gas/oil in the box and probably a lean cylinder to ...... Was that the boat and motor that was for sale on 35hyw. south of Lindsay ? If it is then I believe it was just rebuilt ...

[roadkill636]

Stock jet size is 74D

[perfmarine1]

If compression is that low I would think someone scarfed the heads and replaced them with non GT-XP heads. Check the numbers.

[Mhoward]

Frank this unit was the one a couple of years ago that was in Peterborough. The engine was rebuild about 25 hours prior to me buying it and was build by some ex-racer in the Norwood/Havelock area (so I am told). The guy I bought it from was going through a messy custody case and had owned from new. He did not want to sell it, but had to pay lawyer fees. He was really legit and straight up.


I checked the head numbers and they are the high compression heads from the GT. They have not been swapped out. The compression seems low, but the leak down was tight. Since all the numbers were even I am starting to suspect my compression tester.

[perfmarine1]

Hook the gauge up to a compressor with a gauge and see if # match.

[Captain75]

have someone do a leakdown on it. if leakdown is good pull heads a check the thickness of the head gaskets. I think gaskets come in .020 and .035 thickness. if you have the thicker ones you can raise compression with the thin gaskets. I have not messed with a gt motor. on a 95 looper that gaskets make about 15 psi difference. if you already have the thin gasket you may want to cut the heads a little. like I said I have not put thin gaskets on a gt. be sure you check for piston to head clearance.

Where can I find thinner head gaskets for the 2.7 looper?
//Anders