1985 Supra sunsport 350w not reaching full RPMs or top speed. Power drops off

[Nickgb55]

First off forgive me if I mess up any forum etiquette as this is my first post.

I have a 1985 Sunsport I just bought off the original owner.

It starts up and idles great and has decent acceleration. But just when you get to about 22 mph the power drops off, engine stutters/misfires, RPMs plateau and it can really only cruise at that 22 mph breaking point. If I bury the throttle nothing changes, it just remains at that performance plateau. It almost feels like the throttle cable is not fully opening the throttle.

Boat has:
New plugs/wires (plugs are not gapped just out of box and installed)
- previous plugs where badly carbon fouled all around.
New starter
New ignition coil
New solenoid
New Holley carb

I checked the timing and distributor, it still has the old point distributor but it still functions as it should and is set at 8 degrees at idle.

I think the issue may be in the carb tuning since it was just installed but never tuned. Doing a water test, the secondaries are not opening. I know it is getting enough fuel. We dusted the carb with starter fluid and manually opened the secondaries on the carb but it did not change anything performance wise.

Does this sound like a carb issue to y'all? I have been scouring the forums looking for an answer and I thought for sure it was the distributor but I just checked it with a timing light and it was all good.

[michael hunter]

If you are running points start there, As a rule they should be replaced every year or so. At minimum reset the point gap .The point gap for a Prestolite distributor is .018'' dwell is 29 to 33 degrees . Spark plug gap is .035'' After the points are set reset the timing.

[secondsupra2004]

First off forgive me if I mess up any forum etiquette as this is my first post.

I have a 1985 Sunsport I just bought off the original owner.

It starts up and idles great and has decent acceleration. But just when you get to about 22 mph the power drops off, engine stutters/misfires, RPMs plateau and it can really only cruise at that 22 mph breaking point. If I bury the throttle nothing changes, it just remains at that performance plateau. It almost feels like the throttle cable is not fully opening the throttle.

Boat has:
New plugs/wires (plugs are not gapped just out of box and installed)
- previous plugs where badly carbon fouled all around.
New starter
New ignition coil
New solenoid
New Holley carb

I checked the timing and distributor, it still has the old point distributor but it still functions as it should and is set at 8 degrees at idle.

I think the issue may be in the carb tuning since it was just installed but never tuned. Doing a water test, the secondaries are not opening. I know it is getting enough fuel. We dusted the carb with starter fluid and manually opened the secondaries on the carb but it did not change anything performance wise.

Does this sound like a carb issue to y'all? I have been scouring the forums looking for an answer and I thought for sure it was the distributor but I just checked it with a timing light and it was all good.

If the secondarys are not opening up as the throttle is increased, that is a problem. I believe that carb has vacuum secondaries, correct? If it does, make sure that the vacuum lines are connected properly and are not clogged. If you can rent,buy or borrow a handheld vacuum pump, you can attached to the secondary diaphram and check for proper operation. If the carb has mechanical secondaries, then you have a cable/linkage problem.

[Nickgb55]

Both good suggestions to look into.

I replaced the condenser today...did not seem to do the trick. I'll get new points and dial those in and make sure there are no vacuum issues. The carb does have vaccum secondaries so I'll look into eliminating that as the culprit as well.

We did manually open the secondaries and feed it fuel while on the water..the engine should have been screaming at that point but there was no change. Any thoughts on that.

What it feels like is happening you get the throttle about halfway and things are going okay but after that as you press the throttle nothing changes. No increase in power or change in RPMs as if the throttle cable is not engaging with the throttle on the motor/carb assembly.

Thanks again for the suggestions.

[secondsupra2004]

[QUOTE=Nickgb55;136425]Both good suggestions to look into.

I replaced the condenser today...did not seem to do the trick. I'll get new points and dial those in and make sure there are no vacuum issues. The carb does have vaccum secondaries so I'll look into eliminating that as the culprit as well.

We did manually open the secondaries and feed it fuel while on the water..the engine should have been screaming at that point but there was no change. Any thoughts on that.

What it feels like is happening you get the throttle about halfway and things are going okay but after that as you press the throttle nothing changes. No increase in power or change in RPMs as if the throttle cable is not engaging with the throttle on the motor/carb assembly.

Thanks again for the suggestions.[/QUOTE
So have you looked at the carb/linkage while someone is advancing the throttle? Does it travel the full amount in respect to the throttle handle movement? If so, then either the secondaries are not opeing under full throttle or there is something wrong inside the carb not allowing the fuel to flow.

[Nickgb55]

Yeah we have observed the linkage in respect to the throttle and it fully engages. Even after manually opening the secondaries at full throttle and spraying a bit of fuel in it has no effect but we can see that fuel is entering the bowl.

I have limited carb knowledge so forgive any redundancies or if I out my foot in my mouth. But based on this discussion and my observations it seems like there is an issue with the secondaries interaction inside the carb not allowing the fuel to be converted into power.

I'll do some research and get familiar with the carb and see what comes of it.

Appreciate the feedback

[Nickgb55]

Update...

Adjusted the point gap and reset timing to 8 degrees. The ignition coil and condenser where also replaced. Not sure if I mentioned that previously.

Got the boat out and still the same thing. Decent acceleration...then at about 2500 rpm engine backfires a bit and you can here it start to miss and motor/throttle go unresponsive after that point.

Also when I changed the spark plugs they where very badly carbon fouled in all 8 cylinders indicating an overly rich mixture.

I have went through every suggestion I can find on the forums.

Going to try and mess with the carb but we have opened everything up in that manually ender throttle with no effect either.

[michael hunter]

Sounds like you may be in need of a carb overhaul. Did you replace the points ? Open the gap manually and look if they are burned replace them.

[Evan46]

Maybe the secondary float is stuck. I had the same boat and it happened a lot. You did say it was a new carb though. How many RPM can you get just by reving?


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[Nickgb55]

Points look good...carb is brand new installed by previous owner who thought the lack of power and throttle was a carb problem but it ended up no fixing it.

I still think it is some issue with the timing and distributor even after I checked that it was all good. May upgrade to an electronic distributor system and see if that helps. That is money well spent regardless.

[Nickgb55]

Just trying it in neutral the ROMs are higher but it will still backfire and miss at a point.

My tac seems to be a bit finicky as well so I'm not sure I'm getting a good read. It will be bouncing at 6k but listening to the motor that is obviously not accurate.

Not sure if the tac is bad or it is a wiring issue or something else I'm not aware of with the tac read.

[Nickgb55]

Got an update that may help some folks with this issue. put a new electronic distributor in it and it fixed the problem.

It was not a fuel issue or anything with the carb. The total advanced timing curve controlled by the distributor was not fully advancing so the engine was not capable of reaching max power.

[Sixtydriver]

Try a high temp range spark plug...mine were incorrect when I bought it and had fouling...switched to highest heat range plugs and this corrected the problem...I also found the carburetor had loose bolts holding the two chambers together...has run great ever since.