Light Throttle Cough

Vehicle Specs:
'69 CJ-5, 231 V6 Odd Fire, HEI, Holley 390 Cfm, K&N Flow Control air cleaner, NP 435, 4.88 Gears

Timing = 8* BTDC
Idle = 650 rpm

I tuned the idle mixture using a vacuum gauge for best lean idle. I also set the fuel bowl levels per carb. instructions.

The Problem:
Under light throttle conditions my Jeep coughs through the carburetor. As I take off from a stop under normal/slow acceleration I can actually feel the motor lose power or 'flatten out' and then cough. I think the losing power is the air/fuel mixture going to a lean condition and then the cough happens. I also noticed on the way home last night that on a slight grade close to my house that the carb. coughed/backfired several times while in 4th gear and running steady.

I have already increased the accelerator discharge nozzle to .031" diam. and am going to try increasing again to .035" diam.

Does anyone else have any suggestions to tuning the carb. to fix this cough/backfire situation?

 

What kind of intake are you running??

 

The only reason I changed the discharge nozzle was because that's what the tech support guy from Holley recommended. I questioned him on it but he didn't waver and told me to switch the discharge nozzle.

To change the main jet I just remove the fuel bowl and screw it out and screw in the new one, correct?

 

Offenhauser manifold.

 

is the power valve protected from backfire on this carb model ?
otherwise, once it backfires, your power valve may be bad

 

Its not protected on the 390. That last sneeze before the coughing started is what took out the valve.

 

IIRC there's a kit available from Holley that will upgrade any of the 4160-style carbs with backfire protection. Adds a check valve somewhere.

 

Are you sure Holley 390's power valve isn't protected? This carb. is less than 2 yrs. old.

 

Mine is 2 1/2 years old and it isn't protected. A bad or leaky power valve does just what you're describing. It creates a lean condition when blown since it can't open because the diaphpam can't read vacuum or actually lack of vacuum. Anyway they are easier to change than main jets just cause there is only one of them instead of two. The power valve sets right between the mains(main jets).There are two main jets not one, one for each primary barrel of the four barrel. They come in different values measured in inches of mercury. The lower the number, the earlier they open and let in more fuel. Works in conjunction with the acceleator pump.

 

So on the Holley 390 I just take off the front bowl and thats where the main jets and power valve are?

 

Just heard back from Holley Tech. support, the Holley 390 does have power valve protection.

Could me HEI distributor be advancing to quickly and causing this off idle cough/backfire?

Is there an easy way to check the mechanical and vacuum advancement of your distributor?

 

I'm also in contact with a Holley tech. guy by email. He is saying that I shouldn't need that big of an accelerator discharge nozzle and that if spray comes from the nozzle instantly when moving the throttle, I must have a timing or vacuum advance problem.

I do know that when I first put this engine in after the rebuild that the vacuum advance was being activated at idle. I tried the adjustable vacuum advance kit from Crane Cams suggested by somebody from this board but it didn't make a difference. I ended up buying a vacuum 'restrictor valve', not sure what to call it, used on some Ford vehicles. This stopped the problem with my timing advancing at idle. But I'm wondering if it is throwing off my timing advance and causing the backfire.

Anybody else experience this vacuum advance problem when converting to an HEI? Suggestions?? Should I keep looking at the carb. or switch to the timing?

 

This has sounded like a possible timing problem from the beginning. I'd do a real simple diagnostic and advance the initial timing (vac advance disconnected) to 12*BTDC and see if it makes any difference.
If someone has slipped in any kind of upgraded cam in the past, you'll find that you need more timing advance to start with. Worth five minutes to try.

 

bowtie48- are you saying advance the intial timing to 12*BTDC and leave the vacuum advance disconnected and test drive or advance to 12* and then hook vacuum back up?

 

Sorry. I meant set it at 12* while the hose is pulled off, then hook back up for the test drive. Sometimes people forget to disconnect it to set the initial advance, which can mess up your reading if there is some ported vacuum at idle.
I may be way off base here, but the two odd-fire Buicks I've owned in the past were happiest with higher initial timing, and had no issues with pinging. If set to 8* they would act much as yours is.

 

Be sure to plug the vacuum hose when you disconect it to set the timing. A golf tee works well for that.

 

So, what did you find out? Curious...

 

Bowtie- I haven't had a chance to re-time yet. I had a problem with my carb. with the rear float sticking open on my last test. Anyway I've got that handled and will try adjusting the timing and update here. The carb. issue is detailed here if you care http://www.earlycj5.com/forums/showthread.php?t=30317