Low Top End GT40

After putting a new prop (ACME 422) on my boat and posting results in the sport prop thread , it was confirmed what I already knew. My GT40 is missing a few ponies.

I've decided to create a new thread to document my search rather then keep posting in the prop thread.

I did a compression test before buying the boat and had a couple low cylinders. Below are the results.

1. 120
2. 100
3. 120
4. 110
5. 120
6. 110
7. 90
8. 120.

My 95 Sport with GT40 current WOT RPM is 4500 and top speed is 43.5 with the ACME 422. I also feel like my gas economy isn't great but I don't really know how to confirm this, I just seem to be using more gas then I'd expect but this may be due to all the WOT runs to see if I've 'run the rough out'. I also have a loping idle when the boat is warm which for now I assume is a separate issue to the top end.

I posted when originally looking at the boat and was told to run the boat for a while and it might correct itself. I've put about 30 hours on the boat since buying it and want to revisit the issue.

I am going to redo the compression test myself and if I still have low compression I'll do the test with a teaspoon of oil and get a leak down test done by my Marina.

I also plan on doing any recommended maintenance/tune up items that haven't been done yet.

So far since owning the boat I have done the following
- Oil and filter change
- ATF changed
- New FCC fuel filter
- Cleaned flame arrestor.
- New spark plugs.
- Can of Fuel cleaner (BG44K).

This weekend I plan to
- Change the plug wires, cap and rotor.
- If time permits I'll redo the compression test.

I am new to all of this so any guidance or suggestions are welcome.

a leak down is going to tell you a lot more than another comp test. i wouldn't even bother with that. You can watch a few youtube videos i'm sure, and it'll tell you how to do a leak down and you can probably rent the fittings you need from autozone.

Absolutely do the leakdown or another compresson test to see what you have now compared to those earlier numbers, but I'm not sure I'd get too concerned about one cylinder under 100 and overall pretty balanced numbers. I got 4,940 rpm at 45.4 mph in my '98 Sport with the 422, not sure of total hours but I'd guess in the 800-1,000 range. So you are looking for maybe 400 more rpm if the hulls are expected to perform the same, and after some really interesting threads on the Sport hulls I still don't know what to expect there.

Would be interesting to know, but not sure I'd do too much to a good running GT-40 if all I was lacking was WOT. Does it come out of the hole strong? I also have the loping idle when warm issue, only in neutral, only after idling for a while. Starting and stopping between ski runs it doesn't happen at all, and it really only is an issue after extended running when trying to dock, will surge in neutral and settle down once in gear. I've ignored it, really a non issue for me.

There is no TSC sport... They are all NWZ based.

you don't have to disconnect the battery to swap the cap and wires.

The timing light will work in the sunlight, but do double check that you can clearly see the timing marks on the crank pulley.

I replaced cap and rotor this weekend. Cap had some corrosion on the inside. I also confirmed the base timing was set to 5 degrees.

No change in performance. Boat still maxes out at 4500 rpm (43.5mph).

I'm getting a leak down test done this week. Whatever the result I don't think it's in the budget to get it fixed this year but at least I'll know what the issue is (assuming it's a compression issue).

Paul, I'm guessing you could get 5 more seasons out of it if you really wanted to! What you are describing, in my opinion, isn't something that will cause engine failure. It's some rings that aren't seating/sealing like they should, causing low compression in one cylinder. I'd expect you to be burning a bit of oil as well, but with those numbers could just be slight, and with the low amount of use boats get it could be less than a quart a season.

I also completely understand if you want to get it fixed, I'm the same way. Hopefully others with more rebuild knowledge will offer their opinion, if you are pulling the engine, dropping the pan, removing a piston, I don't know if it makes sense to just re-ring and hone that piston vs. doing a complete refresh of the engine.

What do you see for oil pressure at idle and at speed? What oil are you running?

=Blamey I also confirmed the base timing was set to 5 degrees.

Was this set @ idle ? It should be set with spout removed and 2,000 rpm
Your numbers are considerably low across the board. Just to see what mine is getting I threw a gauge on a cylinder. I do have slightly different and newer heads.

rule of thumb is that you shouldn't have more than 10% difference between cylinders.     I see your compression as an indicator of overall engine health, and that you need a set of rings and a hone or bore.    Since it runs smoothly still, you may be able to run it for a while, but I would certainly plan on an engine rebuild...   It seems to me that this is inevitable.    Rusty cylinder walls may clean up if the rust is only on the surface, but if there is any pitting on your cylinder walls, the rings will eat themselves up, and you won't ever get compression to return. Loose or bad rings can wear the walls unevenly, or scar them and could cause the next rebuild bore to be larger to get the damage to come out.     

cost is dependent on your local area rates, and the work that needs to be done.    you tackling the tear down and assembly will help your budget a good deal.    My Machine shop quoted me 1500+ parts for this work, I assume that is a bit on the steep side as they are well known around here for performance builds, and they had an 8 week backlog. (translation, they didn't need my one off 351 build to keep them busy, especially with the racing season coming to an end in the next few weeks)    
All in, I spent 2300 (including shipping and tax) on a roller long block, gt-40.   While I was interested in doing the build work myself as well, time and cost were prohibitive to me, I figured I'd spend more than 800 in parts and tools to get this thing right in my garage, and many hours that I should be doing something else, so for me, experiencing the build didn't make sense.     If it runs now, and you can get another season out of the motor, I don't see a lot of reason not too, I would venture that you'll probably purchase a long block if you're budget conscious on getting the new engine.

Engine should arrive today or tomorrow.     Since your on the right coast, you might check into Rapido Marine,   it looked like they could do a marine 351 long block for about 1500+ shipping.   Might be more with GT-40p heads, if they can do that. Michigan motors was another name that came up during my search, but I think they were more expensive.

Also, how do I go about deciding whether i need to fix what I've got (not even sure exactly what this would entail) or buy a new short block/long block and then move everything over .
Sounds like the easiest would be to buy a block and move thing over then sell my old block. Since as far as I can tell my heads seem fine, would a new short block do?

Kris,

Thanks for the help. I have a lot to learn before I make any moves but you've have really me understand some of the options, so thanks.

Let us know how it goes putting it all back together.

I'd really like to do the work myself because i want to learn how it all works.

One more question, as I have a GT40 with EFI are there any additional considerations vs Carb when choosing an engine.

So I have been trying to read up as much as possible about what my leak down results mean and I need a bit of help to determine what my next steps should be.

For those that don't want to read the whole thread I have low compression in two cylinders. Results from when I bought the boat last year are as follows:
1. 120
2. 100
3. 120
4. 110
5. 120
6. 110
7. 90
8. 120.

Low compression overall with 2 and 7 being particularly problematic. I did a leak down 2 weeks ago which confirmed the results. During the leak down I didn't hear any air escaping from the Exhaust or Throttle body. Which I believe points to either the rings or possibly the head gasket being bad.

So I am looking for advice on what to do next and if there are any low cost options as I am not quite ready to pay for a short block or a rebuild.

If there is anything I can do over the winter to the engine to either try improve performance or confirm that the issue is with the rings/block. I am new to working on engines but I am happy to try anything myself.

So far I have determined the following as Options

1. Uses some snake oil and see if it helps. I purchased some Seafoam and some Restore and plan on adding the Restore to my oil and adding the Seafoam to the Gas and the running some through the Throttle Body.

2. Perform the compression test again, this time add some oil to the cylinder to confirm that the issue is with the rings. My thinking here is that is could still be a head gasket issue and this is the only way I can confirm this.

Any other suggestions on thing for me to try other then a rebuild/re-power.