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Discussion Starter #1
Hi guys
I’ve got a 302 sbf and it starts and runs absolutely fine but under load,say a hill it has a flat spot,feels like it’s holding back but if I continue pushing the throttle down it clears and suddenly springs to life?
Could the timing be out slightly?
Sorry to ask,I’m more of a bodywork guy!!
 

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Need to have more detail of parts, tune, gearing

Taking on a hill opens the throttle:

- In the carb the reduction in manifold vacuum may bring on the power circuit to riches the mixture. If not timed to the manifold vacuum present this could be too lean or too rich. Either would mess with the needed mixture ratio.

- Along with the opening of the throttle and resultant drop in manifold vacuum if this is using vacuum advance a situation can arise where without increasing the RPMs some amount of ignition timing will be lost, which can result in a power loss.

- Total gearing comes into this in that the higher the overall gearing which includes tire size results in more throttle opening to be used which directly impacts on mixture ratio and ignition timing. Here the term “higher” is referring to vehicle speed being faster as a ratio of engine RPM not numeric values of gear ratios, an example is a 2.73 rear axle ratio is referenced as high compared to 4.11 as being lower. Also, in this regard larger diameter tires contribute to a “higher” overall gearing where a smaller diameter tire contributes to “lower” overall gearing. This being that a larger diameter tire covers more distance in a rotation than the smaller tire, so the effect of the larger tire is to reduce engine speed for the amount of vehicle distance achieved per revolution.

So these are some of the reasons why we need a deeper level of the machine you are talking about inorder to understand the operating relationships.

Bogie
 

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Discussion Starter #4
Hi guys
It’s got a 600cfm vac sec Holley,edelbrock performer rpm,mild crane cam and headman long tube headers.
 

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Probably start with checking to see that choke is fully open when engine is warmed up, there's no slop in the accelerator pump linkage, pump moves/squirts as throttle moves, and then a timing light and check base timing and rate of timing advance as RPMs increase... say 6 - 10 degrees BTDC at idle and 36 - 38 BTDC by about 3000 RPMs... (with vacuum advance hose disconnected and plugged)
 

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Think the second thing I'd check, after the accel. Pump is the power valve From theatre I'd start looking at ignition ...
 

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Discussion Starter #8
Probably start with checking to see that choke is fully open when engine is warmed up, there's no slop in the accelerator pump linkage, pump moves/squirts as throttle moves, and then a timing light and check base timing and rate of timing advance as RPMs increase... say 6 - 10 degrees BTDC at idle and 36 - 38 BTDC by about 3000 RPMs... (with vacuum advance hose disconnected and plugged)
Hi guys
After doing the usual chores of being a parent I’ve managed to spend some time trying to sort this and have found that by altering the timing 8 degrees made a massive difference 👍
There’s still more that could be done I expect but it’s way better than it was.
Thanks for the help 👍
 

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Check your vacuum. It sounds like carb primary may be lean. Power valve in primary may be to light of spring. If single plane manifold you might need a riser(spacer). Or you could have crap in the metering block. Check float levels.
 

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Are you running EGR or is it blocked off?

If you mash the throttle does the thing bog?

If your vac lines are soft or more the 10 years old pull and replace them. The vac lines are cheap. But when they go they can cause all kinds of problems. Pull one at a time, cut the new line to the old length, and install. This way you never mess up or wonder where a line goes. If you have plastic lines toss that junk and replace it with rubber tubing.
 
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