V8 missfire

dmcsweeney

dmcsweeney

Active Member
Hi all, i have a serious missfire in my '72 3500s i've been trying to cure with weeks with no sucess. The car was running perfectly, then one day without warning i fired it up and it and when i hit the throttle it sounded like a 4 cylinder and blew black smoke. I tried driving and it had no power but lots of smoke. I checked that the chokes were not stuck on, checked the points and timing. I replaced the coil and the car ran perfectly again for about 200miles. Then after starting the car after being parked for a few hours the problem returned. Suspecting that the coil had burnt out due to a ballast problem i fitted a 12v coil and ran a new cable from the rev counter to replace the existing resistive cable. The car was still rough but cleared after a few miles and ran perfectly for a further 200miles before breaking down again. This time i replaced the coil with a sports coil but it made no difference. I then replaced the fuel pump as the filter was not filling fully. I also replaced the points and condensor. The car ran well for about 5miles and then went back to misfiring badly. I replaced the condensor again and checked the points, still nothing. The car does fire up on the first attempt and when it doesn't misfire it runs beautifully. The carbs were set up and balanced by a friend so i think the fuelling should be spot on. The fuel filter still doesn't fill to the top despite the new pump (i had also rebuilt the original pump and fitted it without results). The fact that the car is fine one minute and gone the next makes me think it's electrical. The only other problem i have noticed it the ignition switch sometimes fails to pop out when the key is removed. But as i said it never fails to crank or fire. I don't think this is the problem as i've had a switch fail before and it never caused a misfire. Any suggestions??
 
The existing resisted cable runs to the back of the fuse box, not the Tachometer.

Check to see if you have a voltage of 6 - 9V at the positive side of the coil.

It may be that the ballast resistor has failed, but to bypass it you need to run a new wire from the back of the fuse box to the +ve side of the coil to check.

The fuel filter is always half empty and is supposed to be.

It may be a carb problem with just one carb not working properly. As you have had some work done it recently, it may be prudent to have a look and check you havent got a bent/sticking needle, choke is not sticking on on one side, or nothing else obvious.
 
If you are using new condensers there are loads about that are either no good right from the start or fail within very few miles. If you think it's the condenser try swapping with a known good one, or replace it with capacitors of the correct rating. (Or go electronic.)

Other than that, start again from the beginning, checking everything again as you go through.
 
Take out the spark plugs when it's been misfiring and let us know what colour they are - you may find 4 are different which suggests one carb is playing up
 
Lads, thanks very much for your advice. On the subject of the ballast resistor cable i have double checked the wiring diagram in my manual and it shows the cable comming from the tacho on the 3500s, and the fusebox on the 3500. I had around 8V at the coil originally and now have 12V. I've carried out the same modification on my modified 3500S with sucess. Thanks for the fuel pump advice quottro, didn't fancy the thought of fitting an electric one to this almost totally standard car. I'll have a look at the plugs tonite, and will try swapping in a condensor off one of the other cars again (though i've tried a few already). The annoying fact is that the car had electronic ignition before i bought it and the owner changed it to points thinking it would be more reliable. I'm beginning to think it might be back to the carbs, i had an intermittant misfire before which was eventually cured by chucking out the entire fuel system (except the tank) and fitting a facet pump, new lines/hoses and a weber 500 (the pump and carb came in a job lot of parts i bought when i needed an SD1 box). The last owner set the carbs up to perfection, and i've checked the choke valves already, so i'll check for bent needles or sticking pistons as well as my electrical checks. Will keep you posted. Anymore advice would be welcome. One more question, which cyliners are fed from which carb?
 
dmcsweeney said:
One more question, which cyliners are fed from which carb?
Click to expand...

N/S carb ( passenger side RHD) supplies 1,4.6 & 7

O/S carb (drivers side RHD) supplies 2,3,5 & 8

In other words each carb supplies the middle 2 on the opposite bank, and the end 2 on its own bank.

(Incidently I've got some laminated data sheets,similar to the underbonnet stickers but with more detail, which show a lot of the answers to the most common questions that crop up on a regular basis, this included. Ideal for keeping grubby paw prints of your nice clean Workshop Manual)
 
Thanks Harvey, if the girlfriend stays away from me for the evening i'll try and stick my head under the bonnet! My weber equipped 3500s is also refusing to fire having been parked for a while in the workshop so it's going to be a busy one!! Will keep you posted on the result.
 
Hi all, she lives!! Was about to begin the checks suggested but decided to try firing her up first. She started and ran roughly for about a minute but then cleared and ran perfectly. The last thing i did when i worked on her last was to replace the condenser, but due to 'er indoors' getting impatient i only ran the engine for a short while and walked away when i noticed she still misfired. Drove her about a mile (before the wipers stopped in the middle of the screen) and all seems well. If i can get the wipers to work i might try taking her to work tomorrow see if she keeps running sweetly. Fingers crossed!! Thanks again to all of you that offered help.
 
You often find with ignition faults that it takes a few minutes to run right once you've fixed the problem as the plugs can be fouled by fuel / oil and don't clear properly until they heat up.
 
Cheers Richard, all seems well so far, touch wood. Drove her to work this morning without as much as a cough, even when cold. Hope it gets me home! Brought a few ignition spares and tools just in case. The wipers are sticking so just need to sort those out during the day.
 
Better get a work permit off "er indoors" first

See if the wipers park properly with the blades off the screen

I supspect a clean up of the motor and the parking switch will sort it out - not difficult
 
Thanks dave, the wipers were slowing down and stopping on the all speeds, by giving them a gentle push they finished the sweep and parked properly. I removed the connector to the motor and found corrosion. I gave it a very quick clean and they improved dramatically. I'll clean them up properly this evening, along with the intermittant/parking connections. The previous owner never drove in the rain, and it's been broken down most of the time i've had it, so who knows when they were used last. Other half away today so i'm going to concentrate on trying to get my weber equipped car running today. She hasn't run in 6 months and refused to fire so i'll give her a full check today and tackle wipers later. THanks again.
 
Hi All, 20 miles later and the misfire has returned. The car fired up instantly yesterday morning and drove to work without problems, however about 2 minutes from work on the way home it started to misfire, getting worse and worse until it sounded like it was only running on 4 cylinders. Will check the plugs today to try and establish if it's related to one of the carbs. Failing that i'll replace the condensor again.
 
Not mentioned to date that I could see is the distributor cap. I recall it being a very regular failure when we had an early 3500S in the 70's. Tracking between pillars was the main problem, visible to the eye when running the engine after dark!

Chris
 
chrisyork said:
Not mentioned to date that I could see is the distributor cap. I recall it being a very regular failure when we had an early 3500S in the 70's. Tracking between pillars was the main problem, visible to the eye when running the engine after dark!

Chris
Click to expand...


Chris has made a very valid point regarding the distributor cap. Inspect in the dark with the engine running. Any tracking at all either coming from the cap or from the leads will need attention. Replace all if necessary.

It would seem unlikely that having changed the capacitor within the distributor for another, that the replacement should also fail within only a matter of minutes.

Ron.
 
dmcsweeney said:
Hi All, 20 miles later and the misfire has returned. The car fired up instantly yesterday morning and drove to work without problems, however about 2 minutes from work on the way home it started to misfire, getting worse and worse until it sounded like it was only running on 4 cylinders. Will check the plugs today to try and establish if it's related to one of the carbs. Failing that i'll replace the condensor again.
Click to expand...

This may sound silly, but you have the positive wire connected to the positive side of the coil don't you?

I had an old Bedford CF once which gave the previous owner the same problem to such an extent, he sold it to my for £75.00

The condensor would only last a few miles. The coil was wired back to front.

Just a thought.

8)
 
Cheers lads, cap and leads had already been replaced along with all other components. Even in torrential rain it never failed to start. Having run a pinto engined sierra when in college i know all about cap and lead problems!! You could tell if rain was on the was with that car!! I think i've finally figured the problem out, it's fuel starvation. I fired the car up saturday and it ran like a dog so i pulled the reverve level and it cleared within seconds. I then pushed it back and the car ran on four cylinders. I pulled it out again and it and the car stalled and wouldn't start, filter empty. I noticed once before that it would run ok on the main line and missfire on reserve. The misfire used also begin shortly after getting petrol. The fuel guage doesn't work (failed sender unit, have a replacement which i haven't fitted it yet) so i just fill her and set the trip and work out my fuel levels based on a 17mpg average. I think dirt has found it's way into the tap or else there is a problem with the tap. Will blow back to the tank through the fuel lines and hopefully this will cure the problem. Will also check the cable is operating the tap correctly.
 
Quottro, check the connnections, they're ok. Double checked this because there are 2 positive connections on the coil (one from the starter and one from the tacho, i connected one to the negative before and melted the points!! Easy to make a stupid mistake
 
Just an update on this problem. I managed to draw fuel through the system using an electric pump attached to the fuel filter outlet, however, everytime i stopped the pump the fuel level dropped. I removed to pump and found the valves jammed open by what looks like some kind of sealer. I rebuilt the pump with new valves, diaphram and seals. I checked it's operation and refitted it. Pumped the fuel up with the electric pump and then reconnected the line to the carbs. Still no joy, i can only assume the pump is blocked again!!
 
Looks like you're on the home straight there! There aren't too many places for "sealer type contamination to come from. The tank itself is extremely unlikely to rust because of its location. Possibly the most likely suspect would be the O ring in the main reserve changeover valve breaking up. Have you got a very slight petrol leak from there? Its down on the bulkhead on the drivers side.

Otherwise there are the two hose to pipe joints on the way from the filler to the tank (likely to be red goo). The sealant on the flange for the level float in the tank, the seal on the flange to the tank take off (carries a mesh filter and both the main and reserve take off's. Or could someone have used some PTFE tape on some of the screw and olive couplers along the pipe runs (not least the main reserve changeover again).

There is a main reserve changeover tap on Ebay at the moment: http://cgi.ebay.co.uk/ws/eBayISAPI....ksid=p3907.m29&_trkparms=algo=LVI&its=I&otn=1

Chris
 
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