Engine Surge at WOT

backfoot100 wrote: levinmark wrote: Just a quick update, pulled cap off and there is no movement of the distributor shaft. This is definitely a problem. If you have no movement, you're not getting any ignition advance. That has to be freed up before you do anything else.

He might have just been checking for wobble, rather than twist.

Well I didnt get achance to get out this weekend but I did touch up a few things. i cleaned all my connections and grounds, cleaned air filter which was gummed up with alot of crud, stRted boat and watched choke plate fully open.I took the volt meter and checked at choke, coil, alt. etc. and everything was reading 12 volts. I have never messed with the carb setting screws, can you guys give me some insight if I should and how, also was going to check timing . Thanks
Mark

gun-driver wrote: GottaSki wrote: I believe you still have a fuel flow issue, not electrical. Recommend diassembling the tank's dip tube and fittings. I agree with above WOT stumble with no lower RPM problems is more than likely not enough fuel flow.

I second that for sure the only thing electrically that would cause that is if the spark is hopping around in the cap but you should see that issue at all ranges of throttle probably. does it sound like little gunshots in the engine area while running wot for a bit? if so then it sounds like detonation sometimes caused by a lean condition

I think Eddie mentioned above that maybe your advance isn't working right. Not sure what type of timing light you borrowed, but would be nice to be able to check total timing as well to make sure you are advancing to where you want to be.

Old school timing light is fine - just measure the arc length from 0-10 degrees & project it out to 30 degrees. You should get close to 30 deg. by 4000 RPM. If that is happening, then ignition advance isn't causing the miss.

Read this old post.

I also like this article which discusses the basics (non-marine specific).

David - I may be incorrect on the 30 degrees for Levin's '96 (not sure what ignition system he has). My comment was base on my old OEM prestolite which had only 18 deg of advance & it took 4000 RPM to get there.

Levin - What they suggested you do is to verify that the timing changes (advances) with increase in RPM. Not sure what your specs are but any system would have at least 18-20 degrees of advance (beyond initial setting) by 4000 RPM.

David is right about modern systems having up to 24 degrees coming in by 3000 RPM (in fact my current dizzy does this).

If you measure a curved line (arc length) from zero to the 10 deg mark (let's pretend it is 1 inch) then you can measure 1 inch from 10 degree mark & that would be 20 degrees. One more inch & you have 30.    You could use chord length (straight line) instead (this would work well with calipers), with same result.    A dot of white paint will make the new 30 degree mark easy to see with the strobe light. If you are getting advance, pointer should be near the new mark at high RPM. Have someone run the throttle up by 500 RPM increments, then you can even write down the results & draw a curve.

This isn't very precise, but you are just looking to see that it is in the ballpark.

I hope this helps.

Prestolite distributor with electronic points, right?

Back to the timing light. You removed about 6 degrees of initial timing advance, yes? That should have an impact on your total advance and any high end miss. But as mentioned above, you should check you distributor for mechanical advance correct operation in case it is stuck and the previous solution was to advance the initial timing to compensate for the problem.
Pull the cap from the distributor, see if you can look down the dist. behind the armature plate (the plate that holds your electronic points). You are looking for the limit plate and/or the springs. If you can't see it, pull the electronic points and the armature plate out, then take a screwdriver and exercise the limit plate, turning the plate and letting the springs snap shut. There are weights attached to the springs that act on the limit plate, you should be able to move them against the spring force and check for lack of a better word, linear movement. If you can't move the plate with your screwdriver, or there is some dragging and scratching going on, then your timing advance is stuck and needs to be serviced. That would cause a no timing advance condition.

Reassemble the dist., start the engine and recheck the initial timing at idle. Adjust again if it moved. Rev the motor and you should see the timing mark start to move away from the pointer. It should move about an inch or more away from the pointer. That tells you that the mechanical advance (the springs and weights inside the dist. ) is working. I believe that the total advance should be all in by 2500 rpm, you could also check and record the max advance since you are working on a high rpm problem. If your engine bogs at high load at 3000 rpm, then the total advance could be too high, and should have been addressed yesterday when you retarded the timing from 16 to 10 degrees.

Did you mention what the RPM was when the fat man pulled the rope? If the RPM was over 2500, then the mechanical advance was all in by then, and then the problem would point to the carb.

If the RPM was below 2500, then the distributor still might have a problem. This is kind of an aside, but I once replace the breaker points on a 57 MGA with the electronic points in an effort to repair a intermittent loss of spark problem. I was hoping that the laws of physics would be suspended in my case because I couldn't figure out why the engine would run fine for an hour, then suddenly quit. Turns out the distributor shaft was broken in such a way that the thing would spin fine for a hour or two, then suddenly skip and stop running. My MG mechanic found it, not I. The point being that the I tried to repair the thing using the electronic points, when the breaker points were just fine.

Speaking of that, it might do you good to pull your distributor out of the engine and inspect all the little bits. If that is all good, then I'd focus my attention on the carb.

Speaking of carbs, I've got the same engine with the red-hot marine DUI ignition in place of the factory electronic ignition, properly timed, the spark is hotter than hades, and I still have an off idle bog when hammering the throttle. My carb still needs a lot of tuning, and it's new.

Ok guys, I was able to get a chance to get to the boat this morning. I took the cap, rotor, and plate off and took screw driver and was able to move the advance weights without any limitations. Here is a pic of the balancer with the timing marks.

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You can see that it is marked over to 30 degrees as indicated with the marker. I ran it for awhile to let warm up and rechecked initial timing to 10 degrees and proceeded to rev engine taking timing readings

Here are my findings: 10 degrees @ 600 rpm idle
                      17 degrees @ 2000 rpm
                      19 degrees @ 3000 rpm
                      22 degrees @ 4000 rpm

Are these normal readings that I am looking at and are there any other suggestions. Probly wont be able to get it on the water soon, but am anxious to see how she goes.

Thanks again for the help

Just additional information from today. I warmed up engine and ran it up to 4000 rpm a couple times and did a compression test and here are my results along with pictures of the plugs.

1-150
2-145
3-145
4-152
5-150
6-148
7-148
8-159


This is plug #5, not as sooty/black as the others
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This what all the others looked like.

Any thoughts on compression readings and the plugs?
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I concur with Chris,
Something is amiss with the advance.

With initial at 10-12,
the final should be about 36 degrees and all in before 3500.

I have found that Valve Guides can be a common issue with the Chevy Engines. I have a 1987 305 that has had leaky valve guides for years. At each start-up it puffs some smoke and then it goes away. This particular car gets very little use each year.

Have you been burning any oil?

i think at the end of the day you will after all the timing,distributer fixes you are going to find a carb or fuel lean condition

I wouldn't think valve seals would cause a surge though, even if you did need them and it sounds like you don't.