340-4 - Carter AFB. I went though the carb with a new kit and it was pretty easy to work on. I have changed the fuel filter already. I have the initial timing at 12 but that's as far as I've gone with that. I know that vacuum and mechanical advance work, but don't know what the total timing is yet. My suspicion is lack of fuel, or weak valve springs and I am leaning toward the valves for some reason (old). I run ethanol free 89 octane in it. The only engine modification was the porting of the heads. Engine runs great at all rpms under 4000 and bolts right away off a dead stop. Good power and I can tell that the secondaries are opening fine. Compression was 175 in all cylinders. It does seem to do fine if you creep up to the higher rpms also. Once I get some decent gears in the car I will be trouble shooting the higher rpm stumble, unless the remedy pops up here! Thoughts on the valve springs?????
I have not, but have been reading on here about a good way to do that. I do have a new fuel pump but I know it would be a good to check in case there's some restriction.
Make sure the float level is correct. At that RPM the float chamber may be partially emptying out which takes head pressure away from jets. Fuel pressure also enters into this equation as mentioned above. They also sell different springs for the needle valves that bring the needles up at different vacuum levels. Check to be sure the secondary choke plate (I know there's another name for it) isn't opening prematurely, at 4K you're into your secondary's. If that plate opens too soon you will get a bog when the secondary's open. I've seen some people take them out not understanding what they do and I've seen them stick open.
Looks like I will be pulling the carb and verify the float level. When I went through the carb I do not remember verifying that. I do not get a bog when the secondaries open, only when at WOT for a sustained period of time. Float level and fuel pressure make the most sense so far. Thanks guys!
A 1967 car is not very new,even though the outside of the gas tank looks good and rust free,the inside can have a lot of rust build-up inside that can flake off and cause a restriction at the pickup. At WOT if your pickup in the tank has rust flakes surrounding it,it will act like it is running out of gas at higher RPMs as the suction from the fuel pump increases,it can pull the rust flake towards the pickup(if that is the problem)slowing down the fuel flow. Had this happen on my 65 Impala,thought I was having carb problems,but it turned out to be the gas tank(gas tank on the out side looked great,the inside,not so much).I changed the tank and pickup,and no more problem. A lesser costly,but more labor intensive alternative is a gas tank clean and reseal kit,Eastwood sells such a kit among others.GL Derek
That's where I was going next. Sounds like the sock on the pick-up in the tank is contaminated or breaking down and sucking shut. If fuel pressure can be monitored during the problem it would confirm this.
I finally found a little time to delve into the stumbling at high rpm. I read a lot of posts for similar issues and one came up with more information on the dwell setting. It was the dwell! I must have messed around with the timing a bit after setting the dwell and it was at about .32. I readjusted the idle screws a bit, reset the dwell and timing twice and all is great. .29 on the dwell and went from 12 degrees of advance to 10. I run 89 ethanol free and had just a touch of pinging at 12, 10 is perfect. I can finally tromp on it full throttle. I can be rolling at 20mph and put it down and bust the rear loose. Now if I could just get my rear gears done, I could finally run the quarter a couple times.
I'm glad it's running well, but I wouldn't be surprised if the issue returns in the future. Three degrees of dwell shouldn't make that much of a difference, and neither would two degrees of timing (as far as a high RPM stumbling is concerned). Might you have had a bad tank of gas or something?
I will try to go and find the thread on here that gave me the idea to try the dwell setting for my issue and see what it said exactly. I don't think it was bad gas as I've run a number of tanks through it and the issue stayed the same. I am still thinking I need to verify my fuel delivery though and make sure everything is okay there. I have a new gas tank and sending unit on the project list anyway so sometime this winter I will have a look in there. Maybe something slightly plugged in the carb or not set right there. Now that I think of it, I adjusted the choke slightly for smoother warm up in the morning but not sure how that would affect anything on the higher rpms (the electric choke pulls off fully). I know I won't mess with anything and enjoy it as is!
http://www.v8buick.com/showthread.php?283500-High-speed-bogging&highlight=dwell+stumble There's the link I found with someone having similar issues.
