I know that surging is caused by a lean condition or when the timing's off....neither one of these reason is my situation. The engine surges only when it's hot outside (100*+) and I "open her up" (like getting on the interstate). When the surging happens, I ease up on the throttle and it seems to correct itself (doesn't surge anymore..as long as I keep it under 2900 rpms). I did notice the hot airflow from the fan (mechanical clutch) goes directly over the fuel line (see picture). The engine temps during this time is 205* and the oil pressure is 65 lbs.



Could it be the air that passes over the fuel line is SO hot, the fuel evaporates before it gets to the carb????? This is something I have never encountered.

 

i am no expert on this subject but sounds like possibly the DIZZY module or your COIL starting to crap out. My 77 was dieing at WOT and turns out my COIL was taking a roadtrip and wasn't producing a great spark at the higher RPM range.

 

 

It sounds more like a bad coil or a fuel line obstruction/filter problem to me.

 

205º is not "hot".
In fact, your '75 would have come from the factory with a 195º t-stat making 195º-220º a normal temp operating range so if you are thinking that 205º is "too hot" and the cause of your issues I'd look somewhere else for the cause of the problem.

 

I agree with you that 205* is within the normal operating temp. I was stating it to show that the engine was not running hot. The coil is a Petronix Flame Thrower that is less than 4 months old. And it's been doing perfectly sinceI installed it. My concern is where the fuel line runs (like the picture shows) right in back of the fan and a lot of HOT air passes around it. Could it be the gas is evaporating before it gets to the carb? Like I stated, I've only noticed the surging when the temps (outside) have hovered above 100* and I've been running around town for a while. Also, it has happened sporatically.


IF it is the coil, how do you check it?


If it's the coil, how do you check it?

 

I would replace the coil. They are short money and you will either fix the problem or eliminate it and have a spare for the future.

 

You have a good theory unfortunately I doubt very much that is the cause, I would start looking elsewhere, it could very well be going lean at WOT I know because I had that problem before but you need a A/F meter to figure that one out. Replace the fuel filter and coil first, they are cheap

 

if your gas was heating up, that would actually be a GREAT thing. the problem with gas as we buy it is that it is a liquid.. liquids and gases (air) dont mix great.. turn your gas into a "gas" and then mix with another gas (air) and you have better mixture. better mixture = better performance..
or these are just crazy ramblings of someone who has never really attempted to "preheat" their gas, as this sounds rather dangerous...
i dont see how the gas could evaporate.. your intake is producing a pretty good vacuum, and any "evaporated" gas would still be sucked down into the intake through the carb.. if you reallllllly wanted to chase this theory though, you could place a a/f meter above the carb and then drive the car, and see if the a/f meter picks up a lot of gas..

if you still feel your theory is correct, re-route your gas line, and see if that helps.. that and try to describe in more detail why you feel such is happening. diagnosing a problem over the internet becomes much easier when vivid picture is painted. "hot air over gas line" repeated paints the same picture. maybe present other symptoms that present themselves. you have said that this only happens at higher (above 2900 rpms), but then also state that it happens sporadically. if sporadic, is it sporadic as in, it happens at any rpm range? or is it by any temperature range? or sometimes at temp and above 2900 it acts up, and sometimes it doesnt?

i mean these suggestions not to prod.. just to help solve this. good luck with it!

 

Gas will percolate in the fuel bowls and cause problems, they have heat shields to fix that

 

I don't disagree with you. Whenever I suspect a coil issue I have one on hand to try and it only takes a few minutes. Then you can move on to other suspects.

I have also had high RPM stumbles be as simple as a float that is adjusted too low. Point bounce (if you still have points) can cause a stumble at high RPM's as well.