CJ5 misfires at high RPM/hard acceleration

Gentlemen,

I'm having some problems tracking this down. When I accelerate hard or get up to a certain RPM (about 3000) at cruising, I get popping coming out of my exhaust & a huge lack in power. The engine has never stalled, but I can't go any faster in that gear due to the misfiring. I've done almost everything that I can think of short of checking the timing chain & replacing the carburetor. Here's what I've done so far:

Changed coil + ballast resistor + ignition wires/wire to distributor Changed fuel filter + replaced all fuel lines Changed fuel pump Changed points + condenser Changed plugs + plug wires (plugs gapped at .035) Changed cap + rotor Changed vacuum advance (this was confirmed bad & made a huge difference)

The initial timing is 6 degrees BTDC & the dwell angle is about 31. I get about 16hg of steady vacuum at idle & about 7psi of fuel pressure at idle. The idle & even light acceleration is perfect, which makes me think it's not a timing chain issue, but I don't know enough about what the symptoms of a slipped chain would be to make that call. When the engine is at TDC of piston 1's compression stroke, the rotor lines up perfectly with cylinder 1's plug wire, which also makes me thing it's not necessarily a timing chain issue. Since I have changed the plugs & tried to further diagnose the problem, my plugs have turned black. This along with the popping in the exhaust obviously mean my mixture is either too rich, or something is going haywire with the timing & causing the fuel to not burn at higher RPM. One thing worth noting, is that I have not changed the plugs since they have been fouled since I replaced the faulty vacuum advance. I didn't know if this would make a difference or not & didn't want to risk fouling a new set while troubleshooting. I can literally get flames to shoot out of my exhaust if I abuse it enough. This happens on both sides & I have dual exhaust, leading me to think it's not a valve issue, unless I'm really unlucky & have a problem on both sides of the engine.

The jeep is a recreational vehicle of sorts, used primarily to climb up & down sand dunes. The symptoms all occured when my fuel pump died about 2 years ago. After I replaced the pump, the engine started doing this. I've since replaced it again to rule that out with no change. I've also rebuilt the carburetor, but it was the first time I ever did such a thing, so it's quite possible I missed something. The accelerator pump works fine, the idle mixture screws are set very lean (since I think the mixture is too rich for some reason), the power valve has been replaced & I even checked it again last week & I can see spray coming out of the booster venturi when I rev the engine (it does this whether I'm moving or not). Another thing I noticed is that when the engine is cold, if I hold my hand in front of the exhaust, it sprays black fluid on me at idle. It's not a lot, but I noticed it when it was sprayed on my leg one day. It has no smell to it other than stinky exhaust, so I don't really know what this is. My coolant level has not changed noticeably, so I wouldn't think it to be a water leak, but I can't say with certainty.

Here are the basics: 1972 Jeep CJ5 AMC 304 Aftermarket parts: Edlebrock peformer intake Holley 600cfm 4-bbl carb Edlebrock shorty headers

Thanks for any help you guys can give on this. I'm at a loss & don't know what to do next.

-Matt

Reply to
matthew.nye
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If the plugs are just fouled, I would pull them out and clean them up with some light brushing and some B-12chemtool. Re-gap, re-install, and see if that helps.

Carl

Reply to
Carl S

How is the gas tank air vent filter?

It will kill the top end and act very strange when this gets blocked.

It is a $2.00 'emissions' filter on the bottom of the charcoal canister.

To test this, just drive with the gas cap loose or off and see what happens.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail > Gentlemen,

Reply to
Mike Romain

Try driving with the coil "+" terminal jumped straight to the battery to eliminate the possibility that there is an amperage loss through the ignition switch. Also, the old fuel pump may have send diaphragm chunks into the carb. How well did you blow out the passages?

I suppose it is possible that a pump seizing against the eccentric on a worn chain may have caused a tooth jump. If this is the original chain, it might be worth the time to change it anyway.

Reply to
CobraJet

Sounds like it might be runing rich. the 600 cfm Holly is a lot of carb for a 304 engine, at 3000 rpm you are only at about 1/3 of it's max flow. I'm no Holly expert so ask which jets the experts might suggest.

Your vac advance was diagnosed as bad but how can you tell you fixed it correctly? If the total advance is not enough, you will get exactly the symptoms you mention. I'd try giving it another 5 degrees of intial advance, see if that changes anything. If it does, more distributor work is in order.

snipped-for-privacy@gmail.com wrote:

Reply to
RoyJ

A few things here. First 16 inch of vacum at a idle is LOW for a 304 with 6 BTDC timing. I have a old 79 J20 with a 360 that I have owned for 22 years now and it pulls about 20 inches at a idle or a bit more. Have you done a health check as far as a compression check and is vacum steady at a idle too? Another thing, as another poster said, a

600 is really too much carb for a 304 and it may be loading up over rich when you get going. You really want someting around a 450 is so tops and it would give you better responce and toruq throught usuable RPM range too. There is some mis conceptions on carbs here, bigger is not always better. The shorty headers are not a good match at all for that engine and its stock cam and might not be helping things either. You might try using a smaller power valve and one that come in later too with current carb say around 4 or 5 inches or maybe size main jets a few sizes smaller. My gut feeling is that it is going over rich and loading up when casrb starts to draw some. The reason it appeared after old pump failed is because it was likely weak on pressure side before failure which would have lowered fuel level in bowl some and in effect leaned it out. You could also install a external pressure regualtor and set it to 4.5 to 5 PSI as 7 PSI is a lot for a carb. You could also try lower fuel level in bowl about 1/4 iches and see if that help before you rejet because that will lean mixtures out. You need to focus on fuel issues here as the plugs are the sign post here, not the cause. One more question, what heat range plug are you running? with headers you usually need to go a bit hotter than stock.

----------------- TheSnoMan.com

Reply to
SnoMan

Thanks for everyone's replies. Here's what I've done from your suggestions:

- Having the gas cap off made no difference. I actually don't have any of the emissions stuff on it, so there's no canister, just the gas tank to the pump.

- Having the coil's + lead hooked directly to the battery made no difference, I tried switching the leads on the ballast resistor too first to see if I had it hooked up wrong. I may be wrong, but I think the ballast resistor leads can go either way?

- I tried what Roy suggested with interesting results. When I advanced the engine to 10 or 15 degrees, it would run noticeably better, but not perfect. I could actually get the secondaries to open up on the carb before it crapped out, which I haven't been able to do since the fuel pump change. I was grabbing at straws when I replaced the vacuum advance. I could suck air right through it with my mouth so I knew it was bad, that & having it plugged in at idle made no difference in timing advance. When I have the new one plugged in, it automatically advances it to about 10 BTDC at idle. I didn't see any adjustments on it, but maybe that's what is wrong? I couldn't get the engine started with it advanced so far when testing, I could only test it out by having it started first & then advancing it. The idle was terrible with a lot of misfiring at that point, if that means anything.

- I've had a few people also tell me it's over carbureted, but that's what has always been on it (my dad owned it for like about 15 years before me) & it seemed to have ran fine for the longest time. That is an interesting theory though that the old pump could have just been so tired that it didn't put enough pressure through to put it at it's full potential. I was considering a 500cfm 2-bbl holley if I had to purchase one, but it sounds like that might be too much as well? To clarify, the pump just started leaking when it went out, it didn't seize up or anything, in fact I didn't know it was going bad until I started getting sprayed with gas (there's no fenders, windshield, etc). I don't know if I cleaned the jets out good enough when I rebuilt the carb, so it's a possibility that part of the diaphragm got stuck in it. it does seem to flow OK though when I look at the spray coming out of the jets.

- The vacuum is very steady at idle, maybe 1hg of movement at most from side to side. I have not performed a compression check, what would that tell me if it were low? I don't have a compression tester available so I'm not going to be able to do that one right away.

- The plugs are bosch platinums with a heat rating of 9. I'm not sure what that equates to heat-wise though, is that a standard across all plugs?\

Any suggestions on the distributor? I checked the springs & weights of the centrifugal advance & they seem to be nice & responsive, but then again, I'm not sure what they're supposed to look like. Should I try replacing the entire distributor before I go crazy & take the entire front of my engine apart for the timing chain or start messing with the carb? Is there any way to test whether or not the timing chain has slipped a tooth without actually looking at it?

Thanks again guys,

-Matt

Reply to
matthew.nye

I'm a small block chevy fan with no hands on with the 304, but...........

I'm betting on a screwed up distributor. Several thoughts:

The vacumn advance works a bit counterintuitive: at full (high) manifold vacumn (idle or down hill cruising) the distributor advances your base setting around 15 degrees. I I read your post correctly you are seeing it advance from 6 degrees to 10 degrees when you plug in the advance vac line. Not anywhere near enough. Test: let it idle, get out the timing light, plug/unplug the vac line to the distributor. You should see about

15 degree jump. If not, pull the distributor cap and rotor, check to see that the the advance plate moves nicely when you really suck on it

The centrifical advance (weights and springs) puts in another 20 degrees or so, no advance at idle, full advance at 2500 to 3000 rpm (small block numbers here) Test by plugging the vac advance line, reving the engine as high as it will go (it usually stuggles to get to 3000 rpm), make sure you see the 20 degree advance come in with speed increase. It doesn't jump like the vac advance test. If not, make sure the springs and cam move easily.

The combination of these gives you advance numbers like this: Set basic advance to 6 BTDC with vac advance disconnneted Connect vac advance, should hop to 20 to 25 BTDC Rev slowly to 3000, no load, should go to 35-40 BTDC. Rev quickly or heavy load, to 3000, 6 + 20 or 26 BTDC.

I'd c> Thanks for everyone's replies. Here's what I've done from your

Reply to
RoyJ

Yes the ballast resistor can go either way. The gas you see squirting out when you move the linkage has nothing to do with the secondaries working or not. Although the 600 is "in theory" a little large for a stock 304, if it's tuned right there is no problem. I have a Holley 600 on a 289 that runs perfectly.

I agree that your total timing is on the conservative side, but I don't think it would cause your particular symptoms. I had a 390 in a station wagon that would eventually get up to 70mph, but only at light enough throttle where the vacuum stayed high. When I started to put my foot down, it would actually *decelerate*. Nothing I did with the distributor changed that. (I bought it in this condition, so I had no frame of reference). The previous owner had the timing apparently set right. But, just because the distributor is firing with the crank doesn't mean the cam is in the right place. Of course, it turned out to be the chain.

Reply to
Sea Jhett

I really think your problem is the timing, and could be caused by a few different things. Engines with aftermarket headers, cams, etc. usually like a little more timing. As long as you aren't experiencing ping and the starter doesn't kick back when the engine is hot, you're not hurting anything.

An engine with not enough timing advance will do exactly what yours is doing. It will also seem like it is running rich because the fuel mixture is exiting the cylinder before is has had time to completely burn. the popping noise you're hearing is actually from the exhaust valve opening while the mixture is still burning.

If you have access to a timing light with the dial adjustable advance on it, check the timing at idle with the vacuum advance disconnected, then rev the engine to about 3000-3500 RPM and check it there. It should have advance to about 30-34 degrees. If it is much less than this, then your mechanical advance is not functioning properly. My guess is that this is what is happening. IF you are not getting 30-34 degrees, you can test to see if the additional timing fixes the problem by advancing the timing to get about 30 degrees at 3000 RPM. One word of caution when doing this: you are compensating for a malfunctioning mechanical advance and will have a lot of timing at idle, so when you do your acceleration test like this, go light on the gas pedal to prevent a ton of pinging.

There is a slight possibility that the outer ring on your balancer has slipped on the rubber ring between the outer ring and the hub. This is one thing that would explain the timing issue. An

One thing that comes to mind that I have seen people forget during a Holley rebuild is the little needle check that goes in the fuel passage underneath the accelerator pump nozzle. If you leave that out, the engine vacuum at higher RPM can cause fuel to flow out the nozzle. It also causes other problems, which you don't seem to be experiencing, so it is probably in there.

A 600 may not be ideal for a 304, but it can be made to run just fine. There are literally thousands of 302 Fords, 305 Chevys and 318 Mopars running Holey 600 carbs on them. I am assuming you probably have the Holley model 1850 (probably with a dash number after it) The number will be on the front of the choke horn. The problem with these carbs is that they are set up to run on a wide range of engines, and are very rarely perfect out of the box for your particular application. they are normally tuned a bit rich as far as jetting goes, but nothing extreme.

If the engine runs the best with the mixture screws turned all the way in, or close to it, then obviously it is running too rich.

Hook your vacuum gage up to manifold vacuum, which is any point below the throttle plates. Going by your vacuum reading, you're in the right spot now. Set your idle speed to the proper RPM and then remove the sight plug for the float level and adjust it so that the fuel level is just below the hole. By rocking the vehicle slightly, you should see the gas flow out of the sight hole just a little bit.

Now using your vacuum gage, set the idle mixture screws to achieve the highest vacuum reading at idle. If the idle speed rises when you make adjustments, then reset the RPM to the correct level. A well tuned engine with a stock camshaft should have about 20" of vacuum, but an engine with a lot of miles will read less. Worn rings and valve seats will reduce vacuum.

Try the timing test and see what happens. The carb tips I gave you will probably not affect your misfire condition, but were more for when you get to the point of tuning the carb.

Chris

Reply to
c

'All' the old Jeep engines 'I' personally have seen (and read about 'many') will 'NOT' run on Bosch platinum plugs.

They foul up almost immediately.

I would change those before doing anything else and put proper Champion truck plugs back into it.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's - Gone to the rust pile... Canadian Off Road Trips Photos: Non members can still view! Jan/06
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Reply to
Mike Romain

Without having experience on your engine / carb /distributor, I can tell you what is usually the case.

I am not familiar with the holley carbs, but most other carbs I have worked on, are designed to idle with the butterfly almost completely closed. The adjusting screw on the butterfly stop, is used just to adjust the buttefly to close almost 99%(enough to not bind on the throttle body) and must not be used for idle speed adjustment. Idle speed is either fixed, or adjusted by other adjusting screws (bypass screw / mixture screw) if available. If you can't have the engine idle this way, the carb idle ciruit and jet need cleaning. At the correct fully closed butterfly position, the vacum port that goes to the distributor's vacum canister, is blocked or above the butterfly. So the engine idles with only the static advance, no vacum advance signal should go to the canister on idle. If you have a canister with two vacum lines, for advance and retard, only the retard vacum signal should be active on idle. When you open the buterfly, the vacum port gets under the butterfly and receives vacum.

If your distributor has cetrifugal advance too, check that the centrifugal advance mechanism is working. Remove the cap and twist the rotor. It should be posible to be twisted about 10 degrees, and then spring back to the rest position. If it doesn't spring back, the internal springs are broken and you will need a new distributor. If it doesn't twist at all then you don't have cetrifugal advance.

Maybe you should investigate this. At least this and changing to Champion plugs would be the first things I would do.

Bill Spiliotopoulos, '96 XJ, 06' TJ.

Reply to
Bill Spiliotopoulos

A couple comments...

You are confusing a throttle body and a carburetor with your directions on closing the butterfly. Carbs adjust that for idle speed once the mix is set, TB's don't.

His engine could be set for 'either' a ported vacuum (vacuum only at throttle) or a manifold vacuum. (vacuum highest at idle)

Mine came stock with manifold vacuum, I have switched to ported after some modifications.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's - Gone to the rust pile... Canadian Off Road Trips Photos: Non members can still view! Jan/06
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Reply to
Mike Romain

Where do you get this BS at????? It is the wrong heat range that can cause them to foul not the brand if heat range is correct. (some brands to not cross reference properly) You have this guy chasing plugs when this is very likely a fuel issue as I explained above. I guess you just like to hear yourself talk so you can feel good about yourself rather than really help him fix problem. I have been around Jeep much much longer than you and owned AMC/Jeep V8's far longer than you have even owned a Jeep. Just watch and read and you might just learn something.

----------------- TheSnoMan.com

Reply to
SnoMan

Actually you kinda set idle speed and mixture togehter because when you change idle spped with a carb you change bleed air on plates and idle circuit some and you some alwaysm recheck settings of idle mixture after a idle speed change.

----------------- TheSnoMan.com

Reply to
SnoMan

'I' actually have worked on Jeep engines and 'fixed' a whole bunch with fouling Bosch plugs.

Lots of folks get suckered by the hype and change out the Champion plugs.

If you Google this group, you will find the the older Jeep engines will 'not' run on Bosch plugs, period.

It is well documented and a very common cause of fouling plugs.

It is also a very common topic on other groups that deal with American engines.

Bosch might have the heat ranges totally screwed in their book for Jeep engines, who knows, but they don't work.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's - Gone to the rust pile... Canadian Off Road Trips Photos: Non members can still view! Jan/06
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Reply to
Mike Romain

I have another question if anyone is familiar with AMC engines. If the distributor & oil pump gears were removed at the same time, does it matter which way they go back in? I assume you can only be 180 degrees off, but the oil pump drive gear has turned relative to where it was when the distributor was removed, which in turn means that the distributor's actual drive gear will be in a different spot from when it was removed (if that makes any sense). Does this matter, or is irrelevant other than deciding the initial position of the distributor & cap? I assume that it wont matter, & can just be corrected by moving the distributor whichever way is required to get the timing back on track after the distributor is dropped back in, but it was one more thing that I remember doing after I had my symptoms show up, which may have been present merely from the bad vacuum advance. I'm just afraid I may have caused more problems by doing this.

I ordered a new carb & will pick up some new plugs to rule that out hopefully tomorrow. My timing light is just a basic light with no dials & I don't have access to anything else unfortunately. It's very hard to see the mark on the balancer after I get about 25 degrees advance with my current light. I think I may take a stab at looking to the timing chain too if the new carb & plugs don't make a difference unless you guys definitely do not think it's the chain. If the answer to my previous question about the distributor & oil pump drive gears is yes, how would I go about putting it back in correctly? I assume I put the engine at TDC on the compression stroke for cylinder 1, then make sure the cam & crank sprocket indicators line up vertically, but is there any specific way the distributor will need to go in relative to the oil pump drive gear or the cam shaft? It seems as if there should be some kind of line or marking on the distributor telling you which way it needs to be dropped in if that's the case.

Thanks again for everyones help, it's greatly appreciated.

-Matt

Reply to
matthew.nye

As I said I don't have any experience on Holley carburators or CJ5s. I have woked on Webers and Solexes, carbs with no separate throttle base, and with ported vacum for the advance.

Anyway, if he has a manifold vacuum and not ported vacum signal for the advance, then the centrifugal advance mechanism plays a more significant role and will cause more noticeable problems (similar to the ones the OP has) if it doesn't work. In the case of manifold vacum signal the distrubutor is timed with less static advance (the advance measured @idle with vacum disconected), so that the advance is at acceptable limits when the engine idles were it has full vacum advance. When the engine revs above 2800-3000 rpm, it needs about 28 degrees of advance at full throttle, provided by the centrifugal advance mechanism. If this is not working, and because there is no vacum advance due to high load (weak manifold vacum), ending-up with a total advance close to zero degrees, as much as the static advance causing incomplete combustion, loss of power and backfiring. The simple test for the centrifugal advance mechanism is twisting the rotor and expect it to spring back, but a timing light with the vacuum advance disconected will show for sure if the centrifugal advance is working.

About the idle speed setting using the butterfly, there are two cases:

1) Carb with only a mixture adjusting screw, where you fine-tune the idle speed by adjusting the butterfly opening, after having set the mixture screw to the point that the idle speed is at its highest, just before it begins to drop. 2) Carb with mixture and bypass screws, where the butterfly must be completelly closed and idle speed adjusted with the bypass screw, after the mixture screw is set correctly.

Bill spiliotopoulos, '96 XJ, '06 TJ.

? "Mike Roma>> Without having experience on your engine / carb /distributor, I can tell

Reply to
Bill Spiliotopoulos

Gotta agree with you here, Mike. I've fixed many a misfire by ditching the Bosch plugs. Not just in Jeeps, either, my '61 F-100 didn't like them, and neither did my bud's Harley.

-- Old Crow "Yol Bolsun!" '82 FLTC-P "Miss Pearl" '95 YJ Rio Grande BS#133, SENS, TOMKAT, MAMBM

Reply to
Old Crow

Oh oh....

You have low vacuum too right?

I don't know what you are describing as the 'oil pump' gear?

I am under the impression the 'distributor' drive gear that sits on the camshaft end 'must' be exact or your valve timing will not match the crank timing and no amount of fiddling with the distributor can 'fix' this.

This will give low compression, low vacuum and kill the top end power.

If the whole thing turned including the chain and bottom gear, then you can reclock the distributor using a timing light.

Setting up timing chains to their gears is 'not' forgiving, it must be exactly right. There are stamped dots on the gears. These dots must line up exact (pointing at each other) with the chain tight on the right hand side of the engine. (looking from the front, the left side)

To double check this, the stamped dot on the top gear must be turned to the 3:00 position, then you count the links downward to the dot on the bottom sprocket. There 'must' be 20 link 'pins' between the dots.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's - G> I have another question if anyone is familiar with AMC engines. If
Reply to
Mike Romain

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