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Unread 10-05-2013, 11:33 AM   #1
stephenspann27
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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

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2014 Overland JGC. Hemi/8 spd.
1975 CJ5 304 V8/3spd
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Unread 10-05-2013, 11:45 AM   #2
Blakescj7
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I hate dealing with carbs, usually if I'm in question ill restore it to factory specs and go from there tuning. No telling what was done to the carb especially if someone has been tinkering in it. Worst case scenario is to get a new one and that's not really that bad of a decision since edelbrock makes some very simple affordable units.
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Unread 10-05-2013, 11:47 AM   #3
Blakescj7
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Check your timing too btw.
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Unread 10-05-2013, 12:54 PM   #4
stephenspann27
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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.
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Unread 10-05-2013, 01:02 PM   #5
Blakescj7
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Yeah that's low. Should be around 5-6psi
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Unread 10-05-2013, 02:06 PM   #6
stephenspann27
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yea.. looks like the previous owner installed a filter before the pump, after the pump, and before the carb.. which tells me the tank is prolly full of crap.
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Unread 10-05-2013, 09:58 PM   #7
stephenspann27
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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.
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Unread 10-05-2013, 10:13 PM   #8
LumpyGrits
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Get a timing light.
Also get a vacuum gauge and check for leaks.
LG
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Unread 10-05-2013, 10:51 PM   #9
stephenspann27
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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.
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Unread 10-05-2013, 11:27 PM   #10
stephenspann27
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Thinking it could be the ICM.. or cross fire between the plug wires.
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Unread 10-06-2013, 06:40 AM   #11
Matt1981CJ7
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When you are tweaking timing, what are you doing with the vac advance?

What vac source are you using for the advance?

Do you have a bypass-type fuel filter (one inlet, two outlets) installed between the pump and carb, and is it installed in the proper orientation?

When was the last time the carb has been rebuilt?

You said the fuel pressure is "within spec". How did you check that and what were the numbers?

Matt
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Unread 10-06-2013, 07:12 AM   #12
stephenspann27
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Quote:
Originally Posted by Matt1981CJ7 View Post
When you are tweaking timing, what are you doing with the vac advance?

What vac source are you using for the advance?

Do you have a bypass-type fuel filter (one inlet, two outlets) installed between the pump and carb, and is it installed in the proper orientation?

When was the last time the carb has been rebuilt?

You said the fuel pressure is "within spec". How did you check that and what were the numbers?

Matt
The clogged filter that I removed did have two outlets, one of them was a vent. The filter I have in there temporarily does not have a vent. There is a second straight through filter just before the carb. I have been checking the pressure at several locations. I am getting 10lbs from the nozzle of the in line electric pump, and 7lbs at the carb. The difference in lbs is likely to do with them being two different brand (cheap) gauges, I have a 3rd gauge I can use as a tie breaker. I understand that the psi should be 5-7lbs.

The Vacuum advance is hooked to the proper port on the carb. I also checked the vacuum advance with a hand vacuum (good). The carb was rebuilt just before I got the jeep. I corrected a few mistakes the guy made, like float level, and a misaligned gasket. The carb has a lot of miles on it, you can see wear in the throttle bodies, and it was leaking fuel from the throttle plate shaft. I re-bushed the carb body to fix the leak. I noticed yesterday that when I sprayed the carb with cleaner, engine running, the idle changed. I torqued the nuts that hold the carb to the intake and the idle is smoother, and there is almost not change when I spray carb cleaner. There is still a little change though. Since this problem is at WOT, and not at idle.. a small vacuum leak should not be playing a role since vacuum at WOT is nil.
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1975 CJ5 304 V8/3spd
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Unread 10-06-2013, 07:24 AM   #13
stephenspann27
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This guy has the exact same symptoms: http://www.jeepscanada.com/jeep-mail...eration-47249/


Going to try to contact him..
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Unread 10-06-2013, 07:33 AM   #14
Matt1981CJ7
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A couple of things....

The "proper" port for the vac advance is a matter of considerable debate. Many of us prefer full-time manifold vac to the advance. Either way, you should be disconnecting the advance and plugging the line before setting initial timing.

The bypass-type filter acts as a pressure regulator, and also helps prevent vapor locks. The filter should sit level at roughly the same height, or slightly higher, than the carb inlet. And the return should be in the 12 o-clock position.

If the PO made mistakes on basic things like float level and gasket orientation, then I'd be highly suspect of the entire carb rebuild. What else did he screw up on? You may have to find out.

Tuning the engine with even the slightest vac leak is a mistake that will affect performance at all RPMs and load levels.

Matt
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Unread 10-06-2013, 09:24 AM   #15
stephenspann27
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The filter that that had the vent is this one:http://www.oreillyauto.com/site/c/de...2515&ppt=C0024

WIX 33040

It doesn't say anything about regulating pressure.. just water separation etc. The wix filter was installed after the pump, which is near the transfer case in the frame rail. The filter that is by the carb is a standard clear filter.
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