308 GTS Carb high speed miss

Ok guys.

I'm at my wits end on this.

1978 308 GTS North American with twin dizzy's.

This is the car that was sitting for 20 or so years and I resurrected over the last few months.

Right from the beginning after I got it running it had a bad high speed miss (4.5K plus).
After a adventure with bent exhaust valves on the rear head I have it all back together and it runs great except It still has the miss.

Starts instantly either hot or cold.

Virtually no popping back when starting cold (no choke).

As I'm currently running it
Empty cats, no air cleaner but velocity stacks in place, no smog pump but smog pump manifolds in place (has to pass Calif Smog which it did last year with flying colors and I expect won't be a problem), rear sniffer tubes blocked, timing set at 7 BTC with 34 at 5K popping and banging but timing rock solid on cyl 1 rear bank and 5 front bank.

Compression 150 + with all cylinders with 5 pounds. Which is as good as it was 20 years ago. Maybe a little low but may be my compression gauge.
Very little blow by and no oil residue dripping from blow by tube.

Full vacuum with vacuum reservoir, carbon canister and all one way valves.

So here's what I've done so far.

1. Changed out points for Pertronix Igniter II in rear dizzy. Lubed both advance mechanisms.
2.Changed all valves in rear head with new valve seals. Lapped valves, no leakage.
3.Changed engine related seals.
4.Changed outer cam drive bearings.
5.Changed to Hill tensioners.
6.Changed water pump.
7. Switched to Derek Whites's cam pulleys with round tooth belts.
8. Adjusted valves. In at 22-23 thou. Ex at 32-33 thou.
9. Degreed Cams Intake at 106 CL Exhaust at 109 CL (1 degree retarded.)
10. Synced Carbs a bunch of times.
11. Verified timing was advancing in both dizzys. (34 degree marks line up)
12.Changed to 6 ohm Pertronix coils. (It got a little better)
13.Changed plug wires to Accel SS per Birdman. (It got a little better)
14.Changed plug extenders. Also tried it without extenders, no change.
15. Changed from Champ N9YC (what I always ran in the old days) plugs to NGK BPR5IX Iridium then to NGK PBR6IX then back to Champ N9YC. (all new) (Virtually no change) Gaped at 24 thou, even tried 30thou.
16. Changed from 130 main jets back to 125 and accelerator nozzles back to 30 from 40 (both stock) got much worse.
17. Blew out all jet passages and put 130 jets and 40 accelerator nozzels back in. (much better)
18. Moved front Dizzy the full range available with no change (spark controlled by read dizzy but maybe set out of registration, not the case)
19. New fuel pump (cheap from ebay) then 2nd new Mr Gasket fuel pump as 1st cheap one stopped working.
20. Along the way I completely dissembled and cleaned all 4 carbs.
21. Although dizzy caps and rotor are old they appear to be in great shape.
22. Soldered fuse panels across top to eliminate intermittent electrical problems. (did this early on)
23. Verified TDC marks on flywheel.

It gradually got better with the new coils, wires and plug extenders but by no means right.

It got much worse with the 125 main jets and 30 accelerator nozzels.

The plugs (all 3 sets) look great. I've pulled them after long idle as well as after power run and I don't believe they could look any better. If any thing I would say they look a little lean but not much.

It misses under lite throttle as well as full. For example setting the timing or just running it up in 1st gear with lite throttle.

It idles great at 1K even with AC on with all carbs sensitive to idle screw adj.

It acts like bad plugs but absolutely wont rev over 6500 and takes forever to get there even in 1st.

So I'm thinking.

1. Maybe it's just lean due to empty cats and no air cleaner. Don't think that explains lite throttle miss. Could try 135 mains.
2. Broken valve spring. Think I would have noticed it when setting valves.
3. Bad Pertronix module. \ Trigger
4. Low voltage to coils. (Easy enough to check but wouldn't think that would be a problem under lite load)
5. Something wrong with ign advance even though it looks OK with timing light.

Thanks for any insight.

Barry

 

Yup got the clearances wrong. Should be mm not thou. Engine sounds great. Some valve lash noise when hot, none to speak of when cold. Nope standard rotor but it runs the same (firing on all eight with the front dizzy at either end of the adjustment. The way I set it initially was to rotate the engine manually to TDC (both banks) for 1 & 5, put the dizzys in then rotate it again with the ignition on and a timing light to see when it fired. Got it within a couple of degrees that way.

The fuel pump is rated at 3PSI so should be OK. In the old days I ran a Stewart Warner pump which was much higher pressure but it used the pressure regulator at the pump to control pressure. With the new low pressure high volume pump I just run the return line from the last carb in line. There are no flooding issues and I did put a pressure gauge on it at the last carb in line and it was right at 3psi. As I said this is the second pump and the problem is the same. I suppose its possible it could be running out of fuel at high RPM but the pump is rated 42 gph which should be more than enough.

Barry

 

I'm going to take a look at the fuel pump and plumbing 1st. In thinking about it I wonder if by using a low pressure pump (42gph 3psi) and the fuel return (with one way valve) from the last carb in line, stock Ferrari, is actually starving it for fuel at high rpm. Easy enough to block the return and see what happens. I've checked and cleaned the distributor caps multiple times and don't see anything wrong but that may be my next project. The Stewart Warner pump I used a couple of decades ago was much higher pressure and flow but the pressure regulator in the normal 308 carb plumbing took care of it.

Derek, yes the Pertronix MR183 II ( I think) has two electronic modules in the one distributor, in my case the rear, and each module drives one coil. In my case the front distributor is used only to distribute the spark, it has no points or electronics which is why its difficult to insure its phased exactly right. Timing for both banks is controlled by the rear distributor only. The 6 ohm (low) coils I'm now using are rated at 40K. Also there are no ballast resistors.

I'll also check the voltage at the coils as Pertronix documentation indicates low voltage is the most common cause of miss fire.

Barry

 

I have the standard ground with the woven cable plus a separate ground from each distributor to the chassis. I had neglected to replace the two distributor grounds when I put it back together but installing them made no difference. Oh well.

Barry

 

I will find out today if its a fuel supply problem. I'm going to put the Iridium plugs back in it and block the return line from the last carb. If that clears it up I may just kill myself (just joking). According to the documentation I've found 3psi and 42 gal per hr is enough but I'm not sure what affect the return line has on quantity of fuel available to the carbs at high throttle settings. I'll also take a look at the one way valve in the return line.
If blocking the fuel return line does it I'll change the pump again for a larger one with the original pressure regulator and return line in place. I think the return line is important as it will prevent vapor lock.

 

OK I need some help from someone who really understands the 308 carb fuel system.

I teed a fuel pressure gauge in the return line coming from the last carb and got 2psi with just the pump running which is way low and hopefully the source of my problem. So I blocked the return line and my 3psi pump pegged my gauge at 6psi way to high. Don't know why its so high unless its just the head pressure in the tank with the bottom outlet. I tested it when I got it using a gas can and it showed 3psi as advertised. Anyway thats neither here nor there at this point. I also checked the one way valve and its functioning OK with virtually no restriction.

So I checked the manual, you know when all else fails read the instructions, and it shows the following. Page C13

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So 100 liters/per hr is 26 gal per hr.
m.H2O of 3 is 4.266 psi + .5 allowed tolerance is 4.97 psi
Calibrated fuel relief is 1.5 mm or .059 which is only 60 thou.

My question is where in the system is the "Calibrated hole for fuel relief". I'm thinking it must be in the tank return line off the rear carb and maybe it fell through the cracks when I changed the fuel lines.

Is the actual fuel pressure of a stock 308 with the original CORNA type fuel pump around 4.25 lbs at the carbs or is the regulator down by the pump reducing it?

At this point I'm planning on restricting the return line to the tank until I see 4.25 lbs at idle and give it a try. The pump I using is 42 gal per hr so volume shouldn't be a problem.

Thanks in advance.

Barry

 

Thanks

I'm going to measure it as it doesn't seem to be much of a restiction. I put a restrictor with a 90 thou hole in it in the return line and It raised the static fuel pressure from 2 to 3.5 lbs. I don't know if the fuel pressure will remain the same under power. The pump has a 42gph flow which is a bunch more the the stock one at 26 which, by the way, is marginal for a 200 - 225 hp engine. I will try a smaller restrictor to get it to 4.25 lbs. I search ferrarichat and common opinion seems to anything between 4 and 5 lbs is OK even though 5 is a bit above spec. I'm assuming that pressure is at the carbs.

Barry

 

Its not actually the banjo fitting its the bolt that holds it on. It has a nipple on it that the return line attaches to. It's original and the only one so I don't know whats wrong. I crimped the restrictor I put in the return line and now have 4.25 lbs of static fuel pressure. I hope to test it later this afternoon.

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Aaron
I just teed the pressure gauge in the return line. It's holding a steady 4.8 lbs pressure now while running at any reasonable RPM. Sounds better (haven't driven it yet) but still miss at high RPM.
Yes it was worse before all the mods including the Pertronix. I attributed it to point float as it had sat for 20 years and all I did was file the points ,get the dwell close and lube the advance mechanism. It may have been this way before I parked it I can't remember but didn't drive it hard the last few years.

When I first got it in 1980 (its a 78) and after getting it running correctly it would easily run to 7K.

Assuming its still not right when I drive it the next thing is rechecking the timing advance with timing light. (both distributors even though the front one doesn't have any points or electronics in it) Then pulling the distributor with the Pertronix modules in it and double check everything. Maybe the trigger ring is wobbling somehow as I wasn't impressed with the fit when I installed the Pertronix unit.

I will also clean the distributor caps with electronic component cleaner (again) checking for hair line cracks and check the resistance in the new plug wires. All I did when I installed them was check for continuity.

I checked the voltage to the coils this afternoon and its 12.2 with car turned on but not running and 13.02 while running so its fine. However I did find one of the coil nuts (positive for rear bank which is where the Pertronix is) barley finger tight and I was sure that had to be it but no love.

Everything I've done to it has made it a little better. When I first started it would hardly run past 4.5k and was missing before that. Now it runs really strong up to about 5K then starts missing and gets worse as RPM goes up.

I'm also beginning to wonder about the tach. I know its Ok at low rpm because idle RPM is about right. (1K) but the thing sounds like its running more RPM than the tach shows as it approaches 5500. I think I have an old Sears engine analyzer that has a pretty good tach on it. Maybe Ill try that to verify the car's tach.

I may find a shop that has a distributor machine and have them checked but that's really desperate for me. I can't count how may engines I've built and tuned both street and race and never ran into something like this. I'm sure when I finally figure it out it will be something simple and probably stupid.

Anyway I'll keep at it.