>Drive it up to and into the vibration zone and immediately stick it into neutral, and let the engine idle down. If the vibration stays, it obviously is not the engine at all. But if it stopped the instant you put it into neutral, then it is in the engine for sure, or something to do with the engine.
>What is the rpm? Figure that out. Then free-rev it in your driveway to that rpm, or a little higher. If it's not there, this rules out ignition,mostly.And it rules out an engine balance problem. But if the issue remains,then;
By now, we have figured out that it is a loading problem. The vibration is related to how hard the engine is working. Or it is related to how far the throttle is opened.
Normally the engine is on the low speed circuit at 65 mph. This would be the transfers.You said below 65 it is smooth. That tells us the low speed is working just fine. And you did not say you had any problems with accelerating to 65. This tells us that the power circuit and mains are working fine. Since it didn't happen at any other speed or rpm, it kindof tells us that it probably is not rpm related, nor that the ignition cannot keep up.
Again this all points to loading.
So that means either the fuel system is crapping out, or the exhaust system is not letting the exhaust out.
Since you said it begins by surging, my first thought is a low fuel level or the engine has been leaning out all the way up to 65 and now it is just too lean to continue.
So, I suggest to go look for a fairly large vacuum leak, If you do not find it, then you will have to perform a fuelpump output test.This involves disconnecting the fuel line and redirecting the fuel from the pump into a large container, as the engine continues to idle. The test is timed and the delivery rate is calculated. And this is a really good time to inspect the contents of the container and make sure it is 100% gas, and reasonably fresh. Fresh gas is very nearly clear. as it ages it goes thru yellow to orange to red. Slightly yellow will be ok. red is not even good to start fires with, and orange MAY run in your lawnmower, but get it out of the tank.
The delivery rate should be 1quart in 1 minute or less, at 500 rpm.
If your pump can do that, then put it all back together.
Then you are gonna have to look at the exhaust. There are two ways of testing it. You can drill a small hole in the pipe anywhere that is convenient, between the manifold and the first restriction;be it cat resonator or muffler,and braze some brakeline into the hole. Leave at least a foot sticking out, then install a pressure gauge with a long enough hose that you can read it from inside the car. I route the hose up to the windshield area and clamp it there with the wiper blade. Then roadtrip! Run it up to the problem area and keep an eye on the gauge, If it any time the back-pressure exceeds 4 psi, there is your problem. You will have to figure out where the restriction is. That means getting under the car and looking for problems.If you don't see anything obvious, then you will have to move your pressure gauge to just aft of the first muffler, cat, or resonator and measure the back-pressure there. If it is still high, move to the next restriction. When the pressure finally falls to closer to zero bingo, you are right aft of the problem,cut it out! Then seal all the nipples you might have brazed in.
The second method is to just drop the headpipe and go for a ride. No more problems means something is in that exhaust system. If you use this method, try not to burn your car down; the exhaust coming out of the manifold can be 600 to 800 degrees F, depending on how hard that engine is working. It will set on fire anything it comes into contact with.Anything that burns that is, at under 600 to 800*F,lol.
I listed these in order of difficulty and probability of finding success.
>There is one wildcard that might be worth checking into first, and that is spark scatter. If the vacuum canister and the centrifugal advance system get to a place that is causing the spark inside the dizzy to spark to the wrong tower, well that could cause your symptoms too.The test for this is easy, just disconnect the vacuum line to the canister and plug the port on the carb.Then roadtest! No more problems means that this is a dizzy issue. Now you get to hunt it down. But no change means back to the top of the page.
And finally, there is always a chance that you have multiple problems. And there is also a chance that the engine just went into detonation.
And also that I am a complete and utter fool. I sincerely hope there is a solution in this post, cuz, I really hate saying sorry you did all these tests for nothing, cuz there was just something stuck in the air intake that was blocking the air from getting into the carb.Boy I would hate that...Or that the choke butterfly was slamming shut cuz somehow it got unhooked. I would hate to to put you thru all this work, and hear about a faulty choke.
