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cj5huntingrig 01-25-2013 06:20 PM

1974 CJ5 258 with Weber 38 - backfiring issues
Okay so here's my setup:
Stock 258 w/ older Heddman Hedders,
New Weber 38 DGES w/ electric choke,
New HEI w/ new wires and new plugs opened ever so slightly
Emissions equipment is long gone
New Holley Red fuel pump @ 7psi max.
New Holley fuel regulator set @ 3psi w/ gauge.
Vacuum is steadily fluctuating from 15-17 psi ....?

My problem is as follows:
The engine idles perfectly and will do so indefinitely. I have the carburetor idle needles set @ approx. 1 3/4 turns out. The idle speed screw is just under 1/2 turn in past touching as directed. If I set my timing at approx. 12BDC the engine won't idle well and will backfire like crazy. It really likes to idle @ approx 17-20bdc which seems way early. I tried the vacuum advance straight to the manifold vacuum and the engine backfires under acceleration. With the ported vacuum coming from the carburetor, the engine runs fine @ idle and under load until the engine heats up in which case the engine starts to stutter and ultimately will backfire again.

Please, any guidance would be greatly appreciated.
Best Regards, Evan

vinnyabs 01-25-2013 06:45 PM

I have almost exactly the same set up. After about a year of adjustments and help from some well versed people on this, I have finally gotten mine to run better than some of my friends new vehicles. Yes it really is a CJ7. What size idle jets do you have installed? 1 3/4 is about right on turning out but if you are using the factory 45's I can assure you that is not enough fuel and will backfire. Also what main tubes are you using? Sean recommended the F7's and I couldn't agree more

cj5huntingrig 01-25-2013 06:53 PM

Vinny. Nice ride and thanks for the reply. I'm running stock everything on the Weber.
Sounds like I need to change out the jets. I will try to get a hold of some quickly and let you know how it goes.
Anyone with similar experiences?

vinnyabs 01-25-2013 07:23 PM

Thanks. Btw, my set up that has proven succesful is 50 idle jets, mix screws at 1 5/8, idle speed at 700-800, main Venturi F7, manifold vac. I pull about 25-27mmHg of vacuum at idle. You will see that there is not one perfect set up but I have found that the ones that are happy are all around these settings. Also don't discount the Viton tip needle in the bowl. I don't use a regulator. Just a mechanical pump and stock 2 outlet filter

Matt1981CJ7 01-25-2013 07:38 PM

I'd like to see more like 20" of manifold vac pressure at your elevation.

The fluctuating needle could be indicating low compression in one, or more, cylinders, or a vac leak.


swatson454 01-25-2013 08:08 PM

Vinny, You're making me proud :thumbsup:


I have a few questions that may help us determine where you're problem could be hiding.

*Are you using a bypass-style, oem fuel filter along with your fuel pressure regulator? If so, is it located before or after the regulator?

*How is the distributor cap oriented in relation to the vacuum canister. Funky question, I know. Hopefully like this:

See how the wire inputs on the cap are nearly on top of the vacuum canister?

*Do you know exactly what size your idle jets are?

*It sounds like you need to verify your exact TDC. Have you done so?

*Are you setting your base timing with the vacuum advance line disconnected and plugged?

A vacuum leak at the adaptor plates is also a possibility but I'd look at these other ones first.

These are the first things that come to mind.


cj5huntingrig 01-26-2013 08:25 AM

4 Attachment(s)
Matt, I think you may be on to something and I'm thinking I may need to rebuild the heads but I'm trying to eliminate all the possibilities. As far as I can tell, it's the original engine and has never been rebuilt.

Shawn, I think I'm attaching the pictures correctly. See Below:

I'm assuming the balancer may have slipped.
I'm not sure what size jets I have. I bought the K550-38 #14 kit and have not made any changes.
I have not verified TDC but will do so in the next couple of days. Today is my wife's birthday so no Jeep fun for me.
Stupidly, no I forgot to set base timing with the vacuum disconnected and plugged. I may try to do this properly today (if I can sneak out) and will let you know if it makes a difference.
I checked everywhere I could for vacuum leaks and could not find any. Next step is to replace the intake and exhaust gaskets and recheck.

Thanks guys. I really appreciate it

Joe73CJ5 01-26-2013 08:40 AM

I have heard that the holley pressure regulator flow does not keep up with the 38 consumption and need the Weber brand. No clue if its true, could be just brand preferences.

Matt1981CJ7 01-26-2013 01:12 PM

1 Attachment(s)
Something doesn't look right here....


vinnyabs 01-26-2013 01:32 PM

Not only what Joe said, but it doesn't worry you having fuel lines running underneath the frame to be grabbed and ripped off? Also another piece of personal experience, the headman headers can warp from heat and once you take them off, getting them to seal again despite torque can be almost impossible. Had to cut mine in the middle to allow enough flexibility to sit flat against the manifold. Went through 3 gasket sets before realizing it and trying to tighten up to stop the leak. Please do not re live my frustration. Start with the easy stuff first. Jets, timing properly adjusted, adapters sanded perfectly flat, etc.

Matt1981CJ7 01-26-2013 01:43 PM

1 Attachment(s)
The Holley fpr kept up with my Weber 38 just fine. I didn't really need it, as my ol' 258 ran fine without it, but I like to take a peak at fuel pressure once in while.


cj5huntingrig 01-26-2013 10:27 PM

Vinny, I am going to heed your advice. I think I do have more serious issues though and am considering just pulling the block and refreshing it altogether. By the way, I am fabrication a removable shroud to cover the pump and associated hoses. I mounted it there for ease of access.
I ran it today and although it backfired on accel and decel. It ran like a raped ape. Will do a compression test and check the valves and if they aren't as they should be, I will tear it down before I blow it up.
Has anyone tried the Offenhauser intake?

cj5huntingrig 01-28-2013 02:03 PM

I think I may finally have a good idea what is causing my timing/ backfiring issues. I went on the advice of a local Jeep guy and popped the top off the HEI to see if there was any play in the timing chain and yes there was between 1/8 and 1/4 turn of play when turning the crankshaft. Yikes...I ordered a new timing set and harmonic balancer today and will let you know if it makes a big difference or not.
Thanks all,

vinnyabs 01-29-2013 06:03 AM

Awesome. Not the problem but the fast find! Good luck and hope that's it!

cj5huntingrig 01-29-2013 08:38 AM

Yeah..I do also plan to re-jet but I want to get this fixed first so that I have a good base for reference.
Thanks again.

The time now is 04:11 PM.

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