74 CJ5 Misfire-Backfire Help

My 74 CJ has a 304 V8. Last year I replaced the points and coil with a Pertronix electronic setup and a new petronix coil for no reason other than as an upgrade. Other than that,everything is stock.

This CJ has been running 100% great until last week. Last Friday on the way home it started to pop,skip, miss, and backfire. If I clutch it and rev the engine up it will not miss, or doesnt seem to. It was low on gas and I thought maybe the little bit I had left was bad or had some water in it. So the next day I filled it up on that Saturday with Premium and drove it to the local Hardware store(10 min ride). It drove fine and I figured that was the problem. When I came out to go to the house I was hard to start, would just turn and turn and finally started. Almost like it was vapor locked. Drove it home and it ran fine. Today 1 week later I drove it to my office, a 15-20 min ride, and it drove fine. Went inside for 10-15 mins, came out and once again it acted like it didn't want to start. Then on the way back to the house it started bucking,missing, and backfired so bad it BLEW the muffler apart....literally made it explode. I pulled over and looked under the hood, everything looked fine, other than the fuel filter looked very empty, but even then at idle and when I rev it by hand it didn't miss. I don't know where to start. It won't miss sitting in the garage, and it has to be well warmed up to start it seems. I don't want to risk having to drive it around and pull over when it starts because I am worried its going to break down on the side of road. Any Ideas what to check first.

It has ran perfect for the last year and this all started last weekend, and it only seems to do it after being driven 10-15 mins and or then being parked and restarted...when its cold it runs fine...firsts thoughts were a coil cause I have the other one I took off....any tips?



I will post a pic of the muffler later, it was demolished.

 

Is the distributor vacuum going through a thermal switch to engage the advance?

 

At the risk of sounding stupid.....What does that mean? As far as I know can tell all of that is set up like it was for the factory....only thing with the distributor that was changed was taking out the points and condenser for the petronix system.

 

There is a vacuum tube coming off the distributor and going to someplace on the manifold or carb. If it is going to the manifold then it is probably going to a thermal switch located near or on the location where the upper radiator hose connects to the intake manifold. When the engine reaches a certain temp then the switch opens and allows the distributor to advance.

I am no expert, but the way you described the issue is that once the Jeep was warm it started to mis-fire...which sounds like an ignition advance issue. If the thermal switch goes bad it could be the source of the problem.

I hope this description helps.

 

The only way the muffler will explode is if the engine dumps a lot of raw fuel into the muffler and it is ignited by a backfire.

I would first check the spark plugs to see what the mixture is like. Normal is a tan color, but I expect you will find the plugs are a sooty black. If not, then I would check the compression to see if one of more of the exhaust valves are stuck open.

Make sure that all your emissions equipment is connected properly. Do you have air injection? The purge valve (anti-backfire valve) could be bad. Read through the TSM to see what systems are there and how they should be connected.

 

I see at least two possibilities. A fuel pump going south and a partially restricted fuel filter.

 

Before we got the jeep in the late 90's all the emissions stuff was removed...we did a restore on it and the engine was completely rebuilt in 1998....it has about 20k since the rebuild, but we didn't source out new emissions stuff.

The air filter is less than 2 months old and still looks new. I will pull the plugs today and take a look at them. They were replaced maybe 1000 miles ago.

I thought fuel pump as well especially when I seen little to no fuel in the filter, but at the same time I thought that would cause alot of stumbling and misfire but no backfire, I dont know....I know they are cheap and I may change it just for the hell of it to make sure.



We bought this thing from the original owner in the mid 90's and it took us a few yrs but my Dad and I with help from my brothers did a full restore on it...It is a Renegade as well. Everything but the Tranny and TC was rebuilt. I drove it in High School for a 2 yrs and had to get a Grand Cherokee when I went off to College due to the long distance between the home and there so it pretty much reverted to my dad. When I went off the school it was driven very little, and last July when I got married my parents gave it to me and told me to come get it. Sitting and moving very little for the last 5-6 yrs did not do it well it seems.


Thanks for all the help, I will check all of this and get with yall.

 

gunk in the carb?

 

How old is your distributor cap - rotor button? These can crack or wear through or get full of moisture and cause missfire - crossfire - backfire - everything but correct fire. A bad coil can do this too - not sure if this type ignition has a conventional coil. You can take a lead (graphite) pencil and draw lines between the contacts inside the distibutor coil and get crosfire - backfire(if you want to mess with someone).

 

I didn't have much time this past weekend, but I think I have it narrowed down to the Pertronix coil that was purchased in Nov. They are sending me out a new one. I also went ahead and purchased one of those GM HEI style units off of Ebay, I am going to give it a try either way though.

 

Hei on my 304 made a huge difference. one of the best choices I have made.

 

all new when the pertronix was installed

 

just a little update.....I think we did too much at once.

Installed a new HEI and MSD 8mm wires, I figured what the hell go ahead and rebuild the carb to, we had them do that at the same time. The power valve that came with the kit wouldn't work so I got a holley one set for 6.5 hg( supposed to be a direct replacement on the MC2100). Then we decided why go ahead and do the Fuel pump and filter all at the same time as well.


We did all of that, it cranks up, we had to adjust the carb alittle but overall its good and gets up and goes.

The BAD- the thing has a obvious stumble right off of idle when you get into it to start off in first, rest of the gears are fine, and she winds up fast and seems to have more power. We cant get the stumble to go away though. Also what timing is everyone with HEI running, I was thinking it would be in the 5-10 deg range but its more than that to get it running right. Any idea's?

 

Timing set around 7 or 8 b.d.c. Plugs gap at .048 . My idle set at 700 r.p.m. May just be the carb isn't quite adj. right.

 

The 2100 has an adjustable pump shot. There are a few different settings for the linkage that will give you more or less shot volume as the linkage moves. You might also richen the idle mixture slightly, and see if that eases the transition from idle.

Note a larger pump shot and/or richer idle will both lower your mileage, but it sounds like you have a bit of a lean stumble.

Holley has instructions for tuning the power valve on their web site somewhere - you use a vacuum gauge. I think you go to a higher number valve (??) to make the valve open sooner? I recall the power valve is low-vacuum enrichment, so it should be closed at a high vacuum. The number is the transition from closed to open - so opening at a higher vacuum should mean that the enrichment happens sooner.

The power valves are only about $5 at your local speed shop, so going to a higher number might be worthwhile trying. If you get a cloud of black smoke on acceleration, then you've definitely gone too far! :driving:

 

I need help. We have fooled with this thing for the last 2 days and it still idles rough and has that hesitation when taking off in first. we adjusted the float, screws, the accelerator pump linkage, and now I wonder if I need to move up to a 8.5 Hg power valve vs the 6.5 I have now. I just think it runs rougher than it did before all this. I am to the point of going and buying a new carb from off the shelf and keeping this one for rebuilding.

 

Man, you pretty much jumped into the pool when you substituted the Holley power valve for your rebuild kit power valve. Now you have to swim.

Here's the Holley echnical note about picking a power valve: http://www.holley.com/data/TechService/Technical/power_valves.pdf You need to measure your vacuum at idle to pick the proper power valve.

Note that the idle circuit is entirely separate from the high-speed circuit. The idle screws should be both set to the same number of turns from seated, and to 'best lean idle.' Starting at about 2.5 turns out, turn both screws in a quarter turn at a time until the idle starts to stumble. Then back out a quarter turn. This is best lean idle.

You can also set the pump shot by adjusting the location of the linkage in the throttle arm and in the pump arm. Set to maximum and then reduce until you get a stumble, then go back a step.

Your rebuild kit will include instructions on which settings to choose. I don't quite understand why the power valve that came with the kit was not suitable - if you have the right kit for the carburetor, the power valve will be correct.

 

The PV that came with my kit was a dual stage(longer and requires a larger cap on the carb) while mine is a single stage. There was no way to make it work. at Idle I am running 15-16hg of vacuum so that means i need a 7.5 or 8hg rather than the 6.5 I have in there now. Everything else in the kit I got was for my carb other than the PV. I have tried the 3-4 settings on the arm to adjust the pump shot with no difference.

 

The 2-stage valve makes the kit you use way more expensive than a kit with the single stage valve, nearly twice the price, just FYI. You're right, the 2-stage valve requires the deep cover.

One way to get the right single stage valve would be to buy a kit for, say, a 1972 CJ-5 with a 304. I'm surprised that the kit you got has a 2-stage valve. You could try tag number 4RHD2 - that looks to be a carb for your year that has a single stage valve. Napa online shows a kit, CRB 25499A. One advantage of this kit over another Holley valve is that the instructions should have the proper adjustments listed. Plus you'll get a lot of extra carb parts, if that appeals to you.

Just a comment from an old parts department guy for anybody that may be reading - carb kits always come with an inner plastic bag, or a cardboard tray covered with shrink-wrap. The purpose of this inner packaging is to allow you to inspect the contents next to your carburetor, without actually opening the kit. Once you open the bag, it's yours! Be sure to inspect the parts for compatibility through the bag before you open it.

hth!

 

Thats what I couldn't figure out, we gave them the tag # off the carb and it still came with that 2 stage PV. Knowing even lesser about carbs the last 2 wks I didn't think to check the PV before I purchased it, other than that all the parts worked. There was even a note in the contents saying the PV would look different but function the same, we just didnt realize the cover wouldn't work.


before all of this I had one problem, a coil that would get hot and start to miss and misfire and backfire. Cold it ran 100% fine. Now I have a new GM HEI unit, New Fuel Pump and filter, New carb kit, and a jeep that runs worse than before we tore it apart.