Sorry this is a bit lengthy, but I need to tell you the story... I have a 1965 C-10 shortbed stepside with a 230 inline six. The engine was recently rebuilt (now has approx 2200 miles) - all original EXCEPT the block is a 250 (bored .30 over), and the cam is slightly more aggressive. From day one after break-in, the engine has always had a "stumble" upon acceleration which results in a backfire from the carb. I have completely rebuilt two of the carbs (original) and am confident they are not the problem. The distributer is new. Initally, the valves were adjusted too tight - specs in the manual call for a full turn after 0 lash, but this appears to be much too tight. With the valves adjusted the full turn, the timing had to be very far advanced for the truck to run its best (18 degrees BTC or so!). Going closer to spec (4 degrees) caused the backfire from the carb. After re-adjusting the valves - backing off to about 1/8 turn after 0 lash - the truck runs so-so at about 6-8 degrees BTC (much closer to 4 degrees specs). However, the backfire and stumble are still there - not at high RPMs - just at inital acceleration or just throttling up from idle. We suspect the vacuum advance is not working correctly. It IS better since the valve adjustment, but still not correct. The diaphragm unit is OK. Can anyone shed any light on this problem? Does anyone know how to increase vacuum advance that will help get rid of this hesitation issue? This truck was just driven across the country (see earlier posts - what an adventure). I was afraid the overtight valves may have caused damage, but I have taken a compression test and got: (1-6) 165, 165, 155, 160, 150, 165 - not perfect, but within specs. I am baffled and would greatly appreciate any help. FOLLOW-UP: Between rain showers, we have managed to do some more testing in trying to solve this problem. We have determined (with a vacuum gauge) that the intake manifold pressure is too low. Specs call for 17-21 in. - we are only getting 13-14 in. at idle. We don't believe there is any leaking from the intake. PCV valve tests correctly. The diaphragm unit on the distributer is fine. What the heck can cause this low intake vacuum? My only thought is that the hotter cam is affecting this? In looking at my engine rebuild paperwork, there is a circled number showing .474 Lift. Is this is the lift of the cam, then it is more than just "midly" more aggressive. It seems that the more we advance the timing (now trying it at 14 degrees), the better the engine runs and the less the hesitation and backfiring from the carb. ARGH! Any ideas? It seems scary to run around at 14 degrees advanced timing!!??
I am having the exact same problem with my 230 inline six with the same bore. It almost sounds like your were describing my engine. If you do come up with a solution, please let me know.
You need to go back square one. If you cant adjust your valves to one full turn after zero lash, then you have a problem. It sounds like your valve springs are weak. That would explain the low vacuum also.
With the oversize cam in there you need to put in heavyduty valve springs. Did you replace the valve springs with heavyduty ones when you rebuilt the motor? that could make a difference.
Got mine fixed! It was a problem with the carb. A slow leak around the inside of the carb gasket would let gas slowly leak into the intake manifold. When the gas would build up POOF!... intake backfire. The leak came due to the fact that the screws were to tight on the gasket. When I replaced the gasket and put the screws in right... the problem went away. A good side effect was that the engine now accelerates smoothly with no stalling. Hope this helps you. I know you tried two carbs but...maybe both carbs were bad...?
One full turn past 0 lash does sound like too much. Are you using hydraulic lifters? Do you have any other problems like dieseling when you turn it off? On my 350 (yeah okay, so it's a different motor) I got a little bit too happy with the preload on the lifters--I put on about 3/4 of a turn past 0--and it caused the thing to diesel and spit when I shut it off. I was told that when the hydraulic lifters "pump up" (on acceleration perhaps?)they can cause a higher lift and keep the valves open off their seats, which might cause a backfire and a rough run. I hope this helps! Good Luck. --TrustyRusty
