Trying to tune 304 V8..

I'm working on a '75 CJ5 with a 304V8. When I bought it a few months ago it was running very poorly. I took the carb apart and found that the float level was way too high, and the top carb gasket was misaligned. I corrected these two errors and it ran much better. The engine has an aftermarket air filter and headers. The stock #47 jets were in the carb. The exhaust was popping really bad on decel, and the engine surged on the hwy at high RPM. I bought #49 jets and after installing those the popping is gone, the engine doesn't buck like before when going from idle to throttle. However, it breaks up really bad above 1/2 throttle. It almost seems worse with the larger jets.. which doesn't make sense because i know it was lean before. When I first took the carb apart, I disassembled the powervalve. I did not document the position of the of the brass core. I have since read that you are not supposed to mess with the powervalve because it has been pre-calibrated. Do you think the powervalve could be my trouble? Or do you think I need to try even larger jets? I have checked the fuel pressure and it is within spec.
Thanks

 

Is your timing set right? Vehicles from this era needed high octane fuel, and to prevent knocking on today's gasoline, the timing may have been retarded. Just a thought.

 

I think I might have found the issue. I check the fuel pressure again and I only have 2 PSI. Going to change the filter and see if it goes up.

 

Update: I tracked the fuel pressure problem down to the filter after the pump. I got the filter swapped out and the pressure is back within spec. This however.. didn't help my problem at all. I started adjusting the timing and the thing died.. and won't start back up again no matter where I position the distibuter. I am am not using a timing light. My dad and I tried to set the timing with a light, and it had me put the distributer in a position that made it run awful. We are thinking the balancer is spun.. so the mark is no good. I'm not ruling out timing, but I wouldn't think it would idle as well as it does it the timing is off. I have advanced it so far that it puts a heavy load on the starter, then retarding it a little from there.

 

I got it started again. I advanced the timing a little further and it pulled hard for a couple seconds, then it was back to it's old tricks. I stopped several times advancing and retarding the timing to see if I could find a sweet spot, but no luck. I can't use a timing light until I replace the balancer.

 

I'd double check the timing chain to make sure it isn't worn and jumped on the sprockets. Also check the advance mechanisms in the distributor for operation and make sure it's hooked up to ported vacuum.


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You can bring #1 cyl to TDC on compression stroke and verify by taking dist cap off and making sure rotor is pointing to #1 plug wire, then mark your balancer at the TDC mark on your tab, this is your new TDC. I scribed my timing tab onto a piece of paper with a pencil and taped it to my balancer lining it up with my new mark. then you can use a timing light without buying a new balancer for now. Make sure you tape the timing marks on your balancer so you read the numbers before you reach your TDC mark or you will be setting the timing "after" TDC. Hope this helps you get it going.

 

Good idea, thanks for the tip.

 

I've had several 304's and have had to replace the timing chain on every one of them including the one I have now. Same symptoms as yours. The chains must have a tendency to stretch. When replacing research carefully as there is a specific washer that needs to be positioned correctly to get oil to the distributor gear. Good luck.

 

You could be onto something. That might account for why timing mark appears to be off. I think I could check the slack by pulling a valve cover, rotating the engine backwards, then forwards and watch the delay on the rockers. Right now I definitely have a fuel pressure problem. I'm down to 1.5 psi. Replacing the pump. If I still have issues I'll look at the chain next.

 

you also could check chain slack by pulling dist cap and doing the same but watching rotor movement.

 

I verified that the spark plugs are wired correctly. I will be posting a video in the next few minutes of me rotating the crank and watching the rotor. I would have to turn the crank 90 degrees before the rotor ever moved.

 

It's hard to see how far I'm moving the crank. I'm going from 12 o click to 3 o clock before the rotor moves any. I wouldn't think this is normal? If it's not the chain maybe it's a worn distributor gear?


https://www.dropbox.com/s/pei0qtm070qo943/20131008_212642.mp4

 

I couldn't see your link. The rotor should move with the crank, no delays. The dist gear mates to the cam gear and the cam and crank are mated with the timing gears and chain. Could be the dist gear is worn but I doubt it, should be easy to check, just lift it straight out of the motor, "look" and drop it straight back down. I replaced the cam and timing chain in my jeep about a year ago, I could rotate my crank a few inches before the rotor started to turn. After replacing the timing chain, there was no delay in rotor movement when I turned the crank.

 

Wow 90 degrees of crank rotation before it moves? That's way too much slack.

I wouldn't think a chain could even get enough slack to get that bad before failing catastrophically.

 

x2 - I would expect my rotor to move almost instantly as soon as I turned the crank. As I remember there is no slack in my 401 timing chain when it was built.

 

Remove the rotor cap and grab the top of the shaft and give it a twist both directions. Bet there is very little to none on slack because from your video, in order to get that much slack you would not have any teeth left on the dist gear.

 

I found a blown power valve and replaced it. I started out with #47 jets. It ran very cold nurtured, and lots of popping on deceleration. I went to #49 jets and things improved. I am now running the #51 jets and the throttle response is better, and the cold nurtured running is pretty much gone. I am still however, same as before having an issue above 1/2 throttle. The engine bangs out really bad as if it's not getting enough fuel. I don't know the history on this engine, it has been rebuilt, not sure if the cam in stock, it does have headers on it. Is it possible that I need even larger mains?
I have replaced the electric fuel pump and all the filters, it is not a fuel supply problem.

 

Did you fix the slop between crank and distributor? Your carb will not act right until the timing situation is proper because your timing will be changing. The timing will be trying to fire at different times causing a firing condition while a valve is partly open which causes pressure in the intake manifold instead of vacuum. This plays havoc with carburetion.

 

"It ran very cold nurtured, and lots of popping on deceleration. I went to #49 jets and things improved. I am now running the #51 jets and the throttle response is better, and the cold nurtured running is pretty much gone. I am still however, same as before having an issue above 1/2 throttle. The engine bangs out really bad as if it's not getting enough fuel."

That all could sound a lot like an intake vacuum leak. But Walt's point needs answering first