Lazy Throttle

[427yenko]

Not sure if this is the right place for this but I could not find a more suitable location, so here goes.
My bone stock '69 302 has been giving me fits on throttle response changes and I thought I would pose it to the crowd to see if I might find another avenue to search down.
I have played and played with squirters and cams and I have it pretty close now but still not as good as I feel it could be.
My problem is that when I start the car cold, I have the most wicked throttle response you could imagine. I have chambered exhaust and in neutral when I snap the throttle it reacts so quickly that it literally sounds like an explosion out of the tailpipes. The issue arises after the engine gets to full operating temp. As it gradually warms up the response gets lazier until at full temp it has lost its snap. Still good clean response but so different than when it is cold.
I am running full time vacuum at idle to the distributor. All in at about 6 inches. The can adds about 16 degrees. About 16 degrees of advance at the crank and about 23 degrees mechanical. The car runs great, but that cold engine response just has me wanting more at operating temps. Cooler weather helps as in the late fall it hangs on a little longer, but summer driving causes it to dissipate quickly.
My thinking is that the warmer air and engine is causing the mixture to get richer. I have tried to take out some pump shot by adjusting the spring on the pump lever and that has helped, but I feel it still could be better. Any thoughts and is anyone else experiencing this or is this just the way it goes?
Thanks for any ideas!
Bob

[John Brown]

No matter which vacuum advance can you use, the vacuum advance drops off the second you hit the throttle. It comes back at cruising, but it's gone the instant you jump on the gas. If there is no bushing (rubber or brass) on the pin that goes into the distributor advance cam slot, I suggest using the brass bushing from a MR GASKET advance curve kit as that limits mechanical advance and allows more initial timing to be used. The extra initial advance normally makes for improved throttle response.

[napa68]

Just some random thoughts......

You should establish a baseline with the harmonic balancer to ensure it has not slipped. If it has, most anything you try will be pointless unless you know where your timing REALLY is. I am a big advocate of having 36-38 degrees all in (initial and mechanical timing, don't worry about the vacuum can in this number) if your gasoline will tolerate it.

It could be carb related. Reading your description, it could be very rich. The cold engine hides that. These carbs are old and have been through more heat cycles that you can imagine. Eric Jackson just finished a carb for me where a float bowl was out .090. He did 2 others for me this year where the metering blocks were too warped to do anything with(and they were dead fat at an idle). Are the idle mixture screws responsive?

Does it smell rich? Does jacking the timing (beyond what your timing light is telling you) minimize the problem?

It can be frustrating, but can be diagnosed. It's a process. The more info you can provide, the better.

[Too Many Projects]

As said, lazy throttle response is, usually, not enough initial timing. Have you bumped it up 2 degrees at a time and see if that makes a difference ? Not unusual to have 18-20° initial. If your mechanical adds 23, you'll need the limiter bushing to get that back down.

[427yenko]

Thanks for the replies. My balancer has not slipped. On the dyno it made the most power at 39 degrees total. I checked the stats, and the initial is 18 and the mechanical is at 21. Again, I am running full time vacuum advance of 16 degrees all in at idle. I already am on the threshold of running too much initial as any more seems to become a problem for the starter.
The mixture screws are very responsive and are set about 5/8-3/4 turns out. That seems to be maximum vacuum (11 +/-) and rpm. I have the idle set at about 950-975.
Again, the car runs great, I just am so thrilled about the cold responsiveness that the letdown when warmed up is frustrating. If I had not ever experienced how crisp it is cold, I probably would not think anything of it. I just know that if it can be that crisp when it is cold, I should be able to get it that way when warm.
Could it be a manifold that is too warm? Perhaps open crossovers causing excess heat from the exhaust?
Thanks again!

[427yenko]

One more thing. I currently have a 31 squirter and the blue cam for the accelerator pump circuit. 30cc pump. I know that 28 is regarded as the typical squirter to use. I am of the thinking that a higher number squirter will limit the amount of time the shot moves through the cycle, But I have even gotten decent results with a 21 squirter. It just seems to have a small initial void with that and the 25. I have found that backing out the adjusting screw to limit the amount of fuel the pump can deliver has made some strides. I have been careful to be sure there is still 15 thousandths extra travel on the lever. I have also run the white cam with similar results. Always in the number one position. There are conflicting reports on the accuracy of the Holley pump cam charts particularly of the blue cam. I have seen the blue cam charted in two totally opposite curves.
It just seems to me to be a heat related issue. The car is not running hot, though.

[napa68]

If your combination is as stock as you indicate, I don't feel scrutinizing the crossover is necessary.

I admire your attention to detail with accelerator pump nozzle and cams, but don't know that is the root of your problem.

Have you had other 302's? IMO, their torque and consequently throttle response is not overwhelming.

You appear to be on top of the detail work. Having said that, soliciting a seasoned pro such as Eric Jackson would be in order. Eric has found more than his fair share of issues and smoking guns in a carb (and can rectify it). So many times the ignition side hold the problem and the carb gets the blame. Hence, my questioning about the ignition side.

[juliosz]

Is the heat shield on your intake intact? Also, don’t know if anyone else mentioned that the ignition coil could be breaking down when hot. A friend of mine chased a similar problem at the pure stock races a couple years ago and it ended up being the coil.

[427yenko]

This is the only 302 I have owned. I know they are low torque engines, but I am not referring to driving response. I am talking about sitting in the garage at idle in neutral. I have read articles from back in the day where some magazine article author was comparing the throttle response of the 302 with that of the hemi engines of the era, and was impressed that the 302 was comparable to the hemi in responsiveness. I guess I am not sure what they were referring to, but I cannot imagine it would have been torque.
The ignition system may perhaps have an issue at full operating temp such as the coil, I don't believe that is the problem however as there are no other symptoms to speak of. And when I say "problem" or "symptom", I am probably overstating it because the car drives out just fine. I just wonder if I am leaving something on the table because a throttle blip gets lazy at warm temps. I just cannot believe it should have to be that way. I know cars run better in cooler weather, but this seems more significant than that. Maybe I should post a couple of vids to better explain what I am talking about.

[67since67]

Originally Posted by napa68

So many times the ignition side hold the problem and the carb gets the blame. Hence, my questioning about the ignition side.

There's a saying amongst old mechanics. "80% of carburetor problems are electrical"...