Need a little guidance with Engine stumble

Need some help. I have a 302 in my Ranger with GT40x heads and an F cam, dual plane intake and a 650dp. Distributor is a duraspark recurved by Scotty J. Ignition is a 6al2. fuel pressure is 5.5 with the engine off. I'll have to check it with the engine idling.

Sitting in the shop, it idles and runs good, and revs good with no stumble. When I take it out on the road it stumbles when I start accelerating and shifting gears. If I stand on the gas, it really stumbles and backfires.

Symptoms say it is going lean but I am not sure what to change. intake has great vacuum and I have the stock 8.5 pv in it. Also, the accelerator pump linkage is adjusted per Holley instructions.

Any ideas?

The accelerator pump linkage may be adjusted to the specs, but may need a different pump cam.

Holley's instructions are often tough to follow - quite a few people have adjusted the pump arm to have .015 clearance, which will cause a stumble. They specify .015 at W.O.T. which is to ensure the pump doesn't bottom out/bind. Look closely at the pump arm when JUST cracking the throttle - there needs to be NO delay in arm movement when actuating throttle from idle position. Positioning the pump cam differently can help, if needed. Sometimes, depending on which carb you have, a bigger squirter helps tremendously.

HorsinAround wrote:Need some help. I have a 302 in my Ranger with GT40x heads and an F cam, dual plane intake and a 650dp. Distributor is a duraspark recurved by Scotty J. Ignition is a 6al2. fuel pressure is 5.5 with the engine off. I'll have to check it with the engine idling.

Sitting in the shop, it idles and runs good, and revs good with no stumble. When I take it out on the road it stumbles when I start accelerating and shifting gears. If I stand on the gas, it really stumbles and backfires.

Symptoms say it is going lean but I am not sure what to change. intake has great vacuum and I have the stock 8.5 pv in it. Also, the accelerator pump linkage is adjusted per Holley instructions.

Any ideas?

Either something on the exhaust side is creating a restriction there or you need some better tuning ... or both.

rmcomprandy wrote:

HorsinAround wrote:Need some help. I have a 302 in my Ranger with GT40x heads and an F cam, dual plane intake and a 650dp. Distributor is a duraspark recurved by Scotty J. Ignition is a 6al2. fuel pressure is 5.5 with the engine off. I'll have to check it with the engine idling.

Sitting in the shop, it idles and runs good, and revs good with no stumble. When I take it out on the road it stumbles when I start accelerating and shifting gears. If I stand on the gas, it really stumbles and backfires.

Symptoms say it is going lean but I am not sure what to change. intake has great vacuum and I have the stock 8.5 pv in it. Also, the accelerator pump linkage is adjusted per Holley instructions.

Any ideas?

Either something on the exhaust side is creating a restriction there or you need some better tuning ... or both.

Its not the exhaust. I have been fighting this since before we added the exhaust. And I know I need some better tuning.. That is why I posted on here looking for suggestions. Tonight I plan to check the total timing at 3 grand as i recently read that total timing should be checked with the distributor vacuum disconnected. Scotty recommended a max of 28 degrees with his recurved distributor. If the issue persists after this, I will mess with accelerator pump cams.

HorsinAround wrote:

rmcomprandy wrote:

HorsinAround wrote:Need some help. I have a 302 in my Ranger with GT40x heads and an F cam, dual plane intake and a 650dp. Distributor is a duraspark recurved by Scotty J. Ignition is a 6al2. fuel pressure is 5.5 with the engine off. I'll have to check it with the engine idling.

Sitting in the shop, it idles and runs good, and revs good with no stumble. When I take it out on the road it stumbles when I start accelerating and shifting gears. If I stand on the gas, it really stumbles and backfires.

Symptoms say it is going lean but I am not sure what to change. intake has great vacuum and I have the stock 8.5 pv in it. Also, the accelerator pump linkage is adjusted per Holley instructions.

Any ideas?

Either something on the exhaust side is creating a restriction there or you need some better tuning ... or both.

Its not the exhaust. I have been fighting this since before we added the exhaust. And I know I need some better tuning.. That is why I posted on here looking for suggestions. Tonight I plan to check the total timing at 3 grand as i recently read that total timing should be checked with the distributor vacuum disconnected. Scotty recommended a max of 28 degrees with his recurved distributor. If the issue persists after this, I will mess with accelerator pump cams.

The exhaust side is more than just piping and manifolding, (everything from the cam lobe to lifter to pushrod to rocker arm, etc.), but, I am sure you do know that.

Sorry, I posted at all, certainly didn't mean to hurt your feelings.

Shoot, You certainly didn't hurt my feelings. I was just saying I knew it needed tuning.... all my tuning has been with race only vehicles, so I haven't really had to do this much carb tuning before.
To your point regarding the exhaust side, yes, everything inside the engine has been checked and rechecked, measured and remeasured. I did a good bit of work on it last night after shoveling snow and have it pretty close now. If I can get back in the shop tonight, I think I'll have it dialed in.

BBFTorino wrote:The accelerator pump linkage may be adjusted to the specs, but may need a different pump cam.

pmrphil wrote:Holley's instructions are often tough to follow - quite a few people have adjusted the pump arm to have .015 clearance, which will cause a stumble. They specify .015 at W.O.T. which is to ensure the pump doesn't bottom out/bind. Look closely at the pump arm when JUST cracking the throttle - there needs to be NO delay in arm movement when actuating throttle from idle position. Positioning the pump cam differently can help, if needed. Sometimes, depending on which carb you have, a bigger squirter helps tremendously.

Thanks for the tips guys. I made sure the arm was at .015 at WOT, then found some information from Holley about nozzles and pump cams, and by following this was able to solve the problem. Currently have 31 nozzles in it and the green cam and it is rev'ing great now. Now just reading the plugs to dial in the jetting.