1987 Cobra 4.3 won't rev

[itsathepete]

Hi all, first post here.

I have acquired a project boat. It's an '87 Seaswirl with a 4.3 Cobra. The engine was blown and I replaced it with one from a Chevy pickup. Good compression on all cylinders, ran great, was actually a rebuilt engine and in really good shape. After installing, it starts and idles fine but wont rev past 3500 rpm per the tach although it doesn't even sound that high. It revs right up under light throttle till 3000-3500 then just bogs down.

I have tested the coil and replaced the points and condenser and made sure the distributor terminals are all clean. No misfires, timed at 6 btdc. Tried different ignition timing, tried adding fuel, tried removing fuel, tried different fuel, disconnected ESA. I tried it on the water and it won't get on plane just plows with the bow up.

It seems like an electrical issue because, as I said, I have tried both adding and removing fuel and neither helped, both actually made it worse. Is there some sort of rev limiter that I'm missing, or maybe a bad ground that could be causing a problem. I even disconnected a spark plug wire and using a screwdriver in the socket and holding it close to the engine had a big fat spark at all rpm ranges so I don't know what is going on. I'm not the most experienced with points ignition systems, but I've worked with them before and never had this kind of problem. Any help is greatly appreciated. Thanks

 

[Lou C]

Could be the anti siphon valve on the gas tank. If it gets sticky or is corroded inside you can have exactly those symptoms...

 

[Lightwin 3]

How much of the engine did you switch over from "pickup" to marine? A list.

 

[Lou C]

Also these engines came with a Prestolite points distributor with mechanical advance. If the advance weights are rusted up and not moving then you can also have performance problems.

 

[Scott Danforth]

first, welcome aboard

next, its never the coil

when you change the motor over, did you strip it down to just the block and heads, then swap out the head gasket for marine and the core plugs for brass?

regarding the motor, check the fuel system, check the points. look at the stickies at the top of the forum and look for the thread on low WOT and on adjusting points

did you just set the gap on your points or did you actually properly use a dwell meter?
check your anti siphon valve
check the contents of your fuel filter
did you rebuild the carb?

 

[itsathepete]

Lou C
i ran it off a portable gas can and it did the same thing

Lightwin
block and heads swapped
intake on up timing cover balancer front accessories and oil pan

 

[itsathepete]

Lou C
The advance is not kicking in, I assume the springs are too heavy am trying to find a new set

Scott
yes to gaskets and core plugs. no to dwell meter, I dont have one. have tried different point gaps but I'm not sure it would prevent the engine from revving under no load, carb is putting fuel in I can look down the barrels of the carb and see it pouring in with no increase in rpm

 

[Lou C]

From my OMC 1988 shop manual....
4.3 V6
point gap: .019 "
dwell: 37*-41*
Base timing: 6* BTDC
Full centrifugal advance: @3200 rpm, 12* of centrifugal + 6* of base advance = 18* total. You need an advance timing light to check this.

If the springs are original I doubt they are too heavy, the advance assembly may be frozen from rust. When you take the rotor off, there is a wick inside the dist shaft, you are suppose to oil that once a year. I took the points plate off this year and took a good look at them and they are moving fine.

 

[Scott Danforth]

. no to dwell meter, I dont have one.

then go to your local FLAPS and borrow one. you cant set points without one. you can even use some multi-meters.

 

[itsathepete]

From my OMC 1988 shop manual....
4.3 V6
point gap: .019 "
dwell: 37*-41*
Base timing: 6* BTDC
Full centrifugal advance: @3200 rpm, 12* of centrifugal + 6* of base advance = 18* total. You need an advance timing light to check this.

If the springs are original I doubt they are too heavy, the advance assembly may be frozen from rust. When you take the rotor off, there is a wick inside the dist shaft, you are suppose to oil that once a year. I took the points plate off this year and took a good look at them and they are moving fine.

the advance weights seem to have quite a bit of movement without even pulling on the springs. it takes a lot of torque on the shaft to pull the springs at all. I'm going to pick up a new set of springs and beg, borrow, or steal a dwell meter to try some more

 

[Scott Danforth]

most likely the springs are fine on your dizzy. plus, dont know where you would get springs for an old prestolite marine dizzy anyway.

turning the shaft isnt how you test, you use your finger to move the weights

go back to the dwell. that is set when cranking the engine with the cap off.

 

[itsathepete]

Thank you all for your suggestions and insight. She is running good now. The points and condenser I had previously put on were from an auto parts store and the contact was not an exact match but as close as I could get. The local marine service center was able to get the correct one but it took a few days. Funny the condenser from the marine store was not correct so I am using the one from the auto parts store. Also, no one has distributor springs for the old prestolite as was expected but I was able to get a set of weights and springs for a GM HEI distributor and got the springs to work, albeit some improvement can me made I'm sure. I was not able to find a dwell meter locally so that is something I still need to look into. Set the point gap at .017" and she revved right up nice and smooth in the driveway so I took her to the lake. After short runs followed by timing adjustments she was running pretty good. There is more in there I'm sure, just will take some tinkering.

Thanks again for your help

 

[Lou C]

If you have an advance timing light Id check the total timing advance at 3200 rpm. Just want to make sure you are not getting too much advance.

 

[Scott Danforth]

e. Also, no one has distributor springs for the old prestolite as was expected but I was able to get a set of weights and springs for a GM HEI distributor and got the springs to work,

you cant do that without checking the advance curve of the distributor. this curve is mapped on a sun distributor machine. the HEI dizzy uses different weights and springs because an automotive advance curve is completely different than the marine engine advance curve. now you dont know what you have, which can be bad as it can lead to improper timing which leads to detonation

also, based on your comments, you still didnt set the dwell, did you?

 

[itsathepete]

If you have an advance timing light Id check the total timing advance at 3200 rpm. Just want to make sure you are not getting too much advance.

in the driveway it went from 6 to about 8 or 10 at 3200. I advanced it a little more during my test run in the water. Yes I need to recheck it and probably change springs. thanks

 

[itsathepete]

you cant do that without checking the advance curve of the distributor. this curve is mapped on a sun distributor machine. the HEI dizzy uses different weights and springs because an automotive advance curve is completely different than the marine engine advance curve. now you dont know what you have, which can be bad as it can lead to improper timing which leads to detonation

also, based on your comments, you still didnt set the dwell, did you?

Unfortunately there is not a sun distributor machine in my area. Small town, limited resources. Yes I know the springs are not correct, but I did use the original weights. And the dwell meter is not immediately available so I'm working with what I have for now until I can get the correct tools. Thanks

 

[Lou C]

Get a simple Actron engine analyzer with Volts, ohms, RPM, dwell; and an advance timing light.

 

[Scott Danforth]

Unfortunately there is not a sun distributor machine in my area. Small town, limited resources. Yes I know the springs are not correct, but I did use the original weights. And the dwell meter is not immediately available so I'm working with what I have for now until I can get the correct tools. Thanks

if there is an advanced auto or autozone or other FLAPS, you have a dwell meter.

if you have a DMM, you can measure dwell

To use the DMM to measuer dwell, first measure percent duty cycle (%DC) and convert to dwell using this formula:

Dwell = (360 divided by # of cylinders) x (%DC divided by 100%)​

 

[itsathepete]

Get a simple Actron engine analyzer with Volts, ohms, RPM, dwell; and an advance timing light.

I didn't know they did all that and neither did the guys at Advance Auto

 

[itsathepete]

if there is an advanced auto or autozone or other FLAPS, you have a dwell meter.

if you have a DMM, you can measure dwell

To use the DMM to measuer dwell, first measure percent duty cycle (%DC) and convert to dwell using this formula:

Dwell = (360 divided by # of cylinders) x (%DC divided by 100%)​

thank for the info about using the multimeter. I'll look at mine after the holiday and see if it can do that