Backfiring at Speed; Timing Steady at First, then Jumping; but Steady at High RPM - JeepForum.com
 
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post #1 of 12 Old 09-24-2017, 09:15 AM Thread Starter
hkphelan
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Backfiring at Speed; Timing Steady at First, then Jumping; but Steady at High RPM

I recently purchased a 1981 Cj7 with a 258 and the original Carter BBD. I think the distributor and the ignition and electrical system are stock, but the PO removed the original air cleaner assembly and it currently has only a 4" open air filter on top of the carb. He also took the EGR out of the system; though the EGR valve is still present, there is no vacuum connected. From what I can tell, all of the associated hoses are plugged. There are also two vacuum ports on the back of the carb (facing the firewall) that are capped off. One cap was cracked and created a very distinct and steady hissing noise at all RPMs, even at idle. I re-capped it so at least it's quiet now. But I have no idea what those ports are for.

The real problem that I'm trying to chase down is a periodic backfire when going about 45-55 mph up hill. Sometimes it makes it up without the backfire, sometimes is doesn't. The sound seems to be coming from under the vehicle rather than under the hood. It is a simple pop, not crazy loud, but I think I feel the engine give out for just a flash. Sometimes it's a single, sometimes it's a series of three or so -- by that time I've backed off the gas enough to stop it.

Since timing is a common cause of backfire, I checked it a few times, always disconnecting and plugging the vacuum advance hose on the dizzy. The first time I checked it, it was straight on 8* BTDC. So I put away my timing light thinking all was well. Yesterday I thought I'd try it again a little more thoroughly. It gave me the same initial 8* response. I let if idle for a few more minutes and then gave it another look to admire how it was dead on, but this time I couldn't even find the rotating mark. Then is appeared again right at 8*, then it disappeared, then is came back. This cycle only about 4 seconds between changes. When I ran the RPMs up (I can't tell how high because the tach is on the dash, but I'd estimate 1800) the mark came back and held very steady at about 14* BTDC. When I dropped the RPMs it went right to the same jumping routine again.

I don't know if this makes sense but the vacuum at the distributor end of the hose was a steady 14 Hg at idle as well as at increased RPMs. FWIW, I tried to do the initial timeing at 800 RPMs, but again, I could only estimate because the tach is on the dash. Perhaps I need to get an under hood tach.

I've done some compression testing which I haven't been too happy with. I'm hoping either my technique or my gauge were unreliable. It was a rented gauge from AutoZone, but brand new. The readings were 110, 125, 112, 130, 130, 125 in that order. I tried the trick of putting a "couple of teaspoons" of oil in the cylinder per the Haynes manual. It definitely ran the compression way up to over 150. But then I realized that, despite my wife being an excellent baker and cook, I had done the old Teaspoon/Tablespoon error and had put 6 Teaspoons in rather than 2. So my results may have been invalidated, but, man, did it make some white smoke! I had to drive through the neighborhood a couple times to share the love!

Any thoughts on how to approach? Things that have been going through my mind are things like a Team Rush mod, an HEI dizzy, a carb rebuild, a new carb, piston rings (certainly not my preference!), etc. But I don't know which to address first. My biggest concern is how to get rid of the backfiring so I can drive it reliably on the back road highways. I thought timing was the answer to that, but how do I adjust timing that's all over the map?

Thanks in advance!

Howard

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post #2 of 12 Old 09-24-2017, 09:38 AM
jeepdaddy2000
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A misfire under load is usually caused by an issue in the secondary ignition system or a lean power circuit in the carb.

Start with a complete tune up. Plugs, wires, cap, and rotor. This will eliminate any intermittent problem in the secondary ignition system. Clean all contacts at the coil and inspect the connector at the ICU for any corrosion.

Recheck your timing. Be sure the vac advance is disconnected when you are timing it.

Insure your fuel filter is good and is clocked properly. Check you pump flow to insure the carb isn't starving under load.

Check the float level, and clean out the float bowl.

bazinga
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post #3 of 12 Old 09-24-2017, 01:13 PM Thread Starter
hkphelan
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Thanks, jeepdaddy. I will do and will be in touch. I did the plugs already, but the wires, cap and rotor will get changed hopefully today.
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post #4 of 12 Old 09-24-2017, 02:53 PM
bob4703
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Does the noise come during acceleration or deceleration? If it is under deceleration and goes pop, pop, pop that is exhaust rumble which indicates you may need a more restrictive muffler. Mine does that because the glass pack muffler a PO put on is shot and there is restriction. As Jeep Daddy said backfire come under load and be caused by a lack of fuel, spark, or air. You may want to replace the 4" air filter. They can clog up in a moment.

-----

Paved roads are prime examples of government over spending!
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post #5 of 12 Old 09-24-2017, 08:24 PM Thread Starter
hkphelan
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Well I just took it for a short road test and there is no improvement. Under acceleration at about 45 mph in third gear, it gave me the popping again. I wasn't going uphill but was definitely accelerating. The 4 inch air filter is new as are the spark plugs. Today I installed a new rotor, distributor cap, spark plug wires, and fuel filter.
After doing that I tried to relook at the timing. At idle, with the vacuum advance disconnected and plugged, it still did its disappearing trick. The line would be dead on 8* for a flash and then disappear completely for about 3 flashes. Then it would come back again. It was actually very steady in its routine. There for one and gone for three.
When I gave it a good amount of gas it settled in and stayed consistent at one spot somewhere around 12* or 14*. I don't remember where that spot was because I was more concerned with the different behaviors between idle and with gas.
I still need to do Jeepdaddy's recommendation of checking the float level and cleaning out the float bowl. First I have to figure out how to do that. I have a carb rebuild kit but haven't been gutsy enough to go for it yet.
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post #6 of 12 Old 09-25-2017, 08:40 AM
Matt1981CJ7
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The two vac ports on the rear of the carb are for the PCV (larger port) and TAC (smaller port) systems.

Since you have a aftermarket breather, the TAC port should be capped. But, the PCV port should absolutely be connected to the PCV valve. The canister purge is often T'd to the PCV line, as well.

The jumping timing could be a stretched timing chain. It's worth checking.

Matt


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post #7 of 12 Old 09-25-2017, 10:15 PM Thread Starter
hkphelan
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I went back and looked and I see that I misspoke. There are three vacuum ports on the back of the carb (see photos). One big one is connected to the PCV as Matt stated. The other two smaller ones are capped. The second photo shows a more overall view, but the front vent on the valve cover has only a foam filter that is hose-clamped in. Not the most attractive arrangement. The third is not really that helpful but it was meant to show that the hose to the PCV is T'd into a hose to what I guess is the canister purge as Matt described. And the fourth photo shows the EGR and the two plugged hoses that would otherwise be connected to each other.

Does that spark any thought?

Thanks,
Howard
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post #8 of 12 Old 09-26-2017, 11:23 PM Thread Starter
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Well I went into the carburetor today and tried to clean out the float bowl but it was perfectly clean. The idle tubes were clean as were all the gaskets and the . My guess is the PO rebuilt it not too long ago. However, I adjusted the float level a little. It was about 3/8" and I brought it to right at 1/4". I'm not sure what that will do for me and I didn't have a chance to test drive because it's raining in Texas and I have the top off. Looks like it'll be a while before I get a chance to drive it.

Referencing Matt's comment about the stretched timing chain, is there an easy way to check that?

Thanks,
Howard
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post #9 of 12 Old 09-27-2017, 02:05 AM
Matt1981CJ7
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Turn the crank manually to TDC. Then, while watching the rotor, slowly turn the crank the other direction. If the crank moves more than 7 degrees before the rotor moves, the timing chain is stretched or missing teeth and needs to be replaced.

Matt


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post #10 of 12 Old 09-27-2017, 02:38 AM
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When you start pushing down on the accelerator arm does the linkage and needles move at the same time or can you move the throttle arm some before the needles and step up piston move?

Check all your intake and exhaust manifold bolts for being tight.

If you held the throttle wide open for the compression testing she's a tired engine.

It sure sounds like a lean backfire to me......
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post #11 of 12 Old 09-27-2017, 06:33 AM
pedal2themetai
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Hi I see several problems..
1) the fuel filter needs to be re-orientated, the return line needs to be at the top or it can drain the carb of fuel when sitting..
2) The PCV valve needs hooked up?? the valve at the front should be the PCV valve (the hole at the back is for fresh air inlet and hooked to the air filter or have a filtered plug cap in it..) The hole at the front closedest to the oil cap is where the PCV is supposed to be. A hose should come from the canister to the PCV then to the port on the intake or carb base. With the PCV at the back of the valve cover can cause it to suck up oil when going up hill because the oil flows to the back of the motor.
3) if the timing jumps around or at times you can't see the mark (at idle) means the timing chain is shot and in need of replacement..
the ports on the canister should be marked to where they are supposed to go..
good luck
tim
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post #12 of 12 Old 09-27-2017, 06:51 PM Thread Starter
hkphelan
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Great comments and suggestions everyone. I'll get to them as soon as I can and will be in touch. Thanks for the help!
Howard
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