Hello! New on this side of the forum Been helping a guy I know with his 308. Rebuilt carbs, changed waterpump, cambelt, fixed the choke, changed points etc. We also balanced the carbs on idle and part throttle, and colortuned all plugs. After the car is back together, it runs much better when idling. But when driving it does not want to pull past 4000rpms, above 4000 it does also missfire a bit. Anyone experienced similar, good ideas on where to look? The marks on the flywheel where very faint and difficult to read properly, so guess that is a good place to start. Thanks!
I had this situation right up until one of the cam sensors went South. It was the last of the three that wasn't replaced. But check simple stuff first like Fuel filter, fuel flow, no vacuum leaks, etc.
Carbed car this, so no cam sensors. Fuel flow is a good idea, will check that. Vacuum leaks will be doublechecked as well. We will doublecheck cam timing as well.
I would start with ignition. I use a white and a yellow paint pen to mark TDC for each bank on the flywheel. Sometimes I will use the pick up on my timing gun going wire to wire to check for signal. Also check to make sure it's not a Fiero
This could be two things as I'm seeing them, ignition or fuel. Re ignition: When you set the points dwell, did you check them with a meter or just by feeler gauge? The feeler gauge method of setting the dwell I've found to be inaccurate as I've seen distributors with point gap measurements that vary while the dwell reads correctly. Be sure that is all 100% correct before touching the carburetors as most carburetor issues are ignition issues. If you don't have a dwell meter, a less accurate but usually effective method of checking the dwell I've found is checking with a large printed out protractor centered with the distributor rotor shaft. Mark the correct dwell on the protractor and mount the rotor arm and do your best to eyeball that it's doing what's it's supposed to. Less accurate but I've seen it work. Re fuel: Did you alter the float heights and/or check them before the carbs went back together? How about the needle valves and their opening/closing heights. Could be either a lean or rich misfire as about 4k rpm is when the main circuit fully takes over. I would really check the main jets thoroughly and be sure there are no blockages. What size mains and air correctors is this thing running? Modern fuel is a bit lighter than what these were originally designed to run so things are going to be lean if they're stock, but it will still run fine. I've found a 140 main with a 195 or 200 air corrector really wakes things up with these. Take your time, leave no stone unturned if the issue is not ignition. Carbs are a very simply creature and once things are set they will indeed remain set for a very long time. Good luck.
Three other ignition items to confirm/deny: 1. Are the mechanical advance systems working? 2. Are the physical relationships between the points and the dist rotors correct? Not only does the point dwell/gap need to be correct, the dwell for cyls #1 & 5 needs to end, i.e., points open, when the dist rotor is pointing at the red notch in the distributor housing rim. 3. Are the physical relationships between the dist rotors and the dist caps correct? With #2 correct, when the flywheel is at the idle firing location for cyls #1 & #5, the dist rotor needs to be pointing towards the red notch (and it helps here to use the wider tipped 308QV rotor).
Start simple and work your way out. I was having a misfire issue on my '75 and it is something in the electrics. Simple start before tearing stuff apart. Test the battery. Check the voltage with the car running. Go for a drive and test it again. Load test it with fans on and radio , etc. maybe you have a bad connection, short or an old wire/cable with too much resistance. That was causing my misfire.
Assuming the carbs are fine: It sounds like the fuel mix is leaning out at high RPMs, so first check the fuel pressure If the engine idles nicely without any backfires or drops then the voltage/current supply to the ignition seems to be fine (current load is highest at idle due to the longer dwell) A broken advance mechanism in the distributor which retards the timing at high RPMs would lead to a sluggish engine response but not necessarily to an 4000 RPM limit or backfires, with the exception of a two distributor setup where one broken advance mechanism would lead to two different advance curves so the two banks start to work against each other at high RPMs. That can be easily identified by the engine getting "gritty" at higher RPMS. Broken points/points wire can have the effect of loosing ignitions at high RPMs due to flutter, this will lead to backfires and reduction of the maximum RPM. Incorrect phasing (the position of the ignition finger blade to the terminal inside the cap, Steve mentioned this already) can extend the distance between the ignition finger blade and the cap terminal at high RPMS, leading to a weaker/dropped sparks at high RPMs/more advance. If you closely examine the ignition finger blade you can see some whitish erosion from the spark discharge. If those are all at the end of the blade the phasing is set up wrong. Switch on the fans/headlights while driving, does this lower the 4000 RPM limit? More backfires? Does the behavior change with more engine load (as in driving up a hill). I would start with the fuel pressure, then measure the current to each coil while idling with a current clamp (you should see around 2-4A per coil, depending on the meter type, if the reading is different between the two then either one of the coils or a points setup/wiring is broken), then check the points condition and ignition finger phasing. If nothing turns up, have the distributors checked on a distributor tester for correct dwell, flutter and advance timing. Good luck, Adrian
The points where set with feeler gauge, we didn't have a dwell meter. We will go back and doublecheck this. Perhaps buy a dwell meter as well. These distributors a bit finicky about this? Float heights and everything was done after specs, so should be correct. I am not sure about the needle heights, will need to check with my friend who did the work. The jets are standard types, so should be changed for power purposes I guess. Thanks for the tips, lot's to check. Thanks.
1. The ignition does move like it should when revved. 2. Good idea to check! 3. Will recheck! Thanks!
Yes, quite finicky. You guys are using new points right? Dump the feeler gauge method and check it at least with a large protractor and try and eyeball measure the dwells if you don't have a meter. Keep us posted
One thing to note is that if you have one side of the ignition system dead that the motor still runs fairly nicely. It will be down on power but still smooth, won't rev as fast, might not be able to pull the car at higher RPMs, etc. Do a quick check that you have spark both front and rear banks and then continue on with everyone else's suggestions.
Brian, we have checked all plugs with colortune. Confirmed spark and advance\retard on both banks. Today got a message from the owner, that the car after a few miles have loosened up a lot, and is now running good, apart from a steady hickup around 5000rpms second gear. 3rd, 4th it now revs nicely to 6000rpms. Fuel demand is highest around 2nd gear at WOT, so might be fuel related. Float levels perhaps?
Wow, fantastic responses - should almost be a sticky on how to fix .. I suspect the cause of the miss at 5000 rpm was nailed by Ferraripilot when he suggested richening the jetting. Smaller air correctors may be enough. This would be a safer place to start if the owner is performing WOT runs through the rev range without exhaust AFR and with the possibility of detonation (hiccup).
If you continue to have problems, check your spark plug wires (ohm). Also make sure the bend radius isn't too great (when they are connected to the spark plugs) and when you ohm out your wires, try to replicate the bend (near the plug extender) and see if the ohm reading changes between the wire position being bent or straight. If at any point you get an OL while bending the wire (and you get this reading on a consistent basis) it confirms your wire(s) are faulty.
It's very easily could be your distributor the distributors have an advance mechanism that is based on a mechanical device which is greased and will not work if it's not properly lubricated Sent from my iPhone using Tapatalk
