1974 Spider - Running, but w/ backfire (both ends) issues.

[HappyHog]

OK - so I finally got to spend a little quality time with my FIAT. Car sat for 20 years without even starting - for more details about all that you can read my recent new member post. Below is a partial list of all I have done since I got it back 2 years ago and results of some checks I have made in hopes that it will help you guys help me diagnose my issues.

New battery, gear reduction starter, new distributor (converted points to electronic ignition), new coil, new plugs and plug wires, new electronic voltage regulator, new brakes (calipers, rubber lines, master cylinder, rotors, pads), oil light sensor, removed and cleaned fuel tank, electric fuel pump, new fuel pickupand guage sender, rear end axle seals and pinion seal, new timing belt, rebuilt the carb (34 DMSA) and of course an oil change (used my conconction that I used in my dirt track race cars - Rotella 15-40, 1qt Lucas, and 8oz of redline additive.) I also redid the wood on the dash in some beautiful walnut and topped it off with a new dash skin - also disassembled the guages and cleaned the fogged up glass - will post more about that in another post in the future. Now back to the engine issues.....

She starts and runs good. Rich at idle, but ok. Problem is it backfires out the tailpipe AND blows fire out the carb when I get on accelerator half-way hard. Its almost like it happens when the secondaries kick in - but also does it sitting in the shop if I bounce the rpms up and let off quickly. (bounce the throttle)

Compression check:
1-120
2-105
3-100
4-120
Weak I know, but even being this tired, I don't think I should have this this problem. I was thinking maybe I had a valve leaking so I checked valve clearances and came up with this:
1 - I .016 E .017
2 - I .015 E .015
3 - I .022 E .019
4 - I .022 E .015
Tighter for the most part than what I understand is the recommended I .018 and E .020, but I didn't have enough of a variety of the disks to change it and felt like it should be close enough....afterall these are the same ones that were in it when it ran good in the 1990's before being put up in a garage (I have a history with this car).

I was thinking maybe I had a burnt valve - so on the middle two cylinders with the lower compression - I rigged up some low pressure to each of those spark plug holes with the piston at the top of the compression stroke to listen for air coming back up the intake. I got plenty coming through the crankcase, but none that I could tell comining back up through the carb - so I feel like my low compression is just rings.

A lot of you will point to the cam timing as the culprit - I wrestled with this a lot when I was replacing the timing belt and I am confident it is on with crank at 0 (this is #4 at TDC from what I understand), and the marks on the cam pulleys being on their marks on the valve covers.

I do have a few questions.....

Base ignition timing - books vary - some say 0 degrees and some say 10 degrees BTDC. The tag on the engine bay says 0 - but since I updated to a 79 style electronic igition system, I am thinking it needs to be at 10. It sure seems to run better at 10.

Vacuum - For the vacuum advance on the distributor - where I should I run the other end of the vaccum line? I currently have it run to the tube that is on the drivers side of the carb about half way up the main body of the carb. Should I pull vac off the intake instead?
Otherwise, I plan to do a close check for vaccuum leaks, but haven't gotten around to that yet.

Carb - I pulled it apart and cleaned it up with a standard rebuild kit before starting it for the first time. Had some pitting in the fuel bowl from sitting with old fuel in it - could I also have problems internally with the carb that could cause my issues? I am thinking about throwing even more $$ at this project and buying a new carb. I know the 34 DMSA's are hard to come by and from what I can tell are not made any more - what would be the best new replacement available at a decent price?

Any advice?

Happy

[HappyHog]

Guys - any help on this? What is the best carb to get for stock motor on a 74?

After playing around with it, I am thinking that is the cause of my issues...

I have it running ok, but still not as smooth as I'd like. It still has a little "spit" and hesitation at the 1-2 gear shift - still seems to run rich at idle, and seems like has a miss at all speeds. Doesn't seem to be way down on power, but it has been 30 years since I last drove this car, so maybe it had more power back then than I remember?????

I have base ignition timing set at 10 degrees BTDC using #4 - that gives me about 36ish degrees total advance when revved.

I have my issues narrowed to two possible culprits

Carb: I had to set idle mixture screws almost all the way in before it made a difference in idle. I do remember the spring loaded screw on the driver's side of the carb when I rebuilt it seemed "boogered up" on the needle end - maybe that's it (anybody know what this screw is? anybody know where I can get a new screw like this?). It is a 34 DMSA weber.

Vaccuum: The way I have my lines run now - the port on the drivers side body of the carb, I ended up running to the air cleaner connection where the crankcase vent comes in and I moved the vac line to the distributor advance further down on the intake manfold iteself.
Now I am only getting manifold vacuum when the engine rpms are increasing while revving the engine. Is this normal? My vac guage reads virtually nill at idle or a steady RPM of 2k or 3k. It does not seem erratic - so I am thinking I either have a major vacuum leak or I have a very worn engine across all 4 cylinders. Or possibly both. If my brake booster was bad, would it leak vaccuum? The brake system is all new except the booster and the brakes work, but still real spongy - like there is no vac assist. I just don't know if it's the cause or the symptom of my vac issues.

Any thoughts?

[18Fiatsandcounting]

Hey Happy, it sounds like your engine is not very, errr, happy.... A few thoughts from me:

- The 34DMSA is a very good carb for these cars. A popular alternative (also a very good carb) is the 32/36 DFEV. Electric choke, although you can modify for a coolant choke. The DFEV is much easier to find, and there are many knock-offs available although sometimes the quality isn't there.

- 120 psi is pretty low on the compression, and 100 is definitely not good. I usually see worn engines in the 130 psi range, and the "nominal" for a stock spider is 150 psi. Compression measurements should be done with all plugs removed and with the throttle held wide open while cranking.

- Your timing sounds fine. 10 degrees BTDC at idle, smoothly advancing to 35 to 37 degrees max when revved, is what you want.

- Sounds like you might have a vacuum leak as well as issues with the carb. I'd start by plugging all vacuum lines to the intake manifold and carburetor. If you measure the intake manifold vacuum at idle, it should be around 13 psi (vacuum), or 20 something inches of mercury, whatever the scale on your gauge is. The vacuum should drop to zero at wide open throttle, but it should have some vacuum at part throttle and steady rpm.

- Any chance you have a leak at the base of the carburetor, where it attaches to the manifold? It's not unusual for the bottom flange of the carb to warp, and then it's hard to get a good seal. Intake leaks are the result.

- Sounds like you're getting too much fuel at idle and maybe all across the rpm range, perhaps related the idle mixture screw being buggered up. Places like Pierce Manifolds could probably get you a new idle mixture screw.

- Any chance that the electric fuel pump is too strong for the float needle valve? Excess fuel would get into the intake which causes all sorts of issues. How do your spark plugs look? If you look down the throat of the carb barrels with a flashlight with the engine idling, you should not see any fuel squirting or dribbling out. The idle orifice is below the throttle plate, where you can't see it.

- Finally, if you want me to take a look at your DMSA carb, let me know. Free except for needed parts and shipping. If you want to sell it, I'd be interested in buying it. Just for you to consider.

-Bryan

[Nut124]

Happy, sounds like you have done all the right things so far. Cylinder leak down test might tell something.

No manifold vac at idle is definitely not right. The engine should not run at all. What is the throttle plate position at idle?

Set ignition timing by 36 total. This should be in at around 3800, not much earlier. I use manifold vac for the dizzy. It adds about 10-15 deg of advance at idle.

Very lean can spit into the intake.

[HappyHog]

Thanks all - I was able to take a little time to mess with it today - checked plugs - they were sooty black - could be from running rich or oil leaking past rings - or both. While I had them all out, I double checked my compression. With all the plugs out, I got 120ish on 2 and 3 and 125ish on 1 and 4. Still not the "normal" 150 or even worn 130, but better. I even double checked this time with a second gauge.

Could it be because I didn't have them all out the first time or could it be because I have driven it a little and exercised rings that had not moved in 2 decades when I checked them the first time???? I have driven it a total of around 100 miles since getting it running. Odometer shows 78k now - but I dont trust that because it did not have the orginal speedometer that showed 33k miles in 1986 when I first got it. I could tell because the wall inside the glass was sliver and not black like the tach.

After I cleaned up the plugs and put them back in, I started it up and checked vac again - this time I used the brake booster line instead of one of the smaller ports I had been using. I got 11-12 at idle and 18 at 3000 rpm. Much more like what I expected first time I checked it.

I also sprayed carb cleaner around base of carb. Bryan, I think you are right - I may have a warped base becuase I could tell the RPM's dropped just a little. I am thinking I will pull the carb back off again and lay it on flat glass to see for sure.

Oh - yeah - I also pulled that spring loaded idle screw out of the base of the carb to check it out. It looked ok. Not sure it is orginal screw for my carb, but I put it back in and screwed it all the way in and backed it out 1/2 turn (battling rich at idle). I also looked down carb with engine idling and did not see evidence of fuel above the throttle plates, so I think I am ok with the throttle stop screw.

After all this, I took it for a spin for a few miles. Still stumbling and spitting a little at gear changes on acceleration unless I really focus on smooth, but otherwise it seemed to run pretty good. I'll drive it more when I get new tires.

I am getting closer. Will keep updating as I work through this.

[18Fiatsandcounting]

Sounds good, Happy. To be honest, what your car probably needs right now is just to be driven. Don't flog it like a rented mule, but just be mildly aggressive, rev it up, go through the gears, put some mileage on it. Fiats don't like to sit, and they can get mad as &%$* if they have sat for years.

Oily plugs will be, well, oily, whereas rich conditions will result in a matte-black, soot like appearance. Try a compression test with a teaspoon or so of engine oil dribbled into the plug hole before you crank the engine. If the compression improves quite a bit (more than 10 psi), then yes, your rings are going bad. Valve seats that are marginal can also lead to low compression.

Sounds like you're on the path to figuring out why it's running rich. By the way, the choke is opening all the way when warmed up, yes?

-Bryan

[HappyHog]

It has manual cable controlled choke - which I really like. It appears that I have it adjusted correctly too. All the way off and the choke plates are verticle (open) and when adjusted all the way on, they are shut nearly closed all the way and I can lock it anywhere in between. I also like the identical throttle position lock on these things. If you are struggling with a problem, you can keep the idle up so it don't die at every stop and it also helps when you are tuning by yourself without a throttle buddy.

My wife and I took a spin around the backroads where I live today. Climbed a steep grade with a couple of switchbacks - you know the sort of road these cars were made for. Still have that stumble and a slight spit at some gear changes, but nothing that makes it undriveable.

I ordered new tires tonight. When I get those puppies on, and the weather warms up, I will be driving it quite a bit more.

The violent backfires I was having at first up the intake seemed to have dissapated (on their own or with the 10,000 adjustments I have made - who knows), but I still get an occasional mild backfire out the tailpipe....from all that unburnt fuel from running so rich more than likely.

Happy

[HappyHog]

Update on my situation...

New tires on - rolls so much smoother now!

Now back to the tuning (or lack thereof). I robbed my old race car of its fuel pressure guage and rigged it up temporarily. Got 3 PSI of fuel pressure at the carb. I thought that might be a tad high, so I swapped our the fuel pump to the one that has been on it for year - same pressure. At least that's another thing ruled out.

Next I pulled the carb off (again). I think I found my problem. I cut a piece of glass to use on the base to clear linkages without breaking it down. I placed said glass on the bottom of the base of the carb. Could tell right away that the corners where it bolts down were pulled down. I used a feeler guage along the long side. Max gap appears to be 0.020". Not good.

What should I do next? Take it all apart and run the base down to a machine shop to have it trued? How much is too much to take off before I have more problems?

....or is there a compressable gasket available that will seal it to the manifold/spacer in spite of the warpage?

....or is there some other trick/solution to this problem?

Happy

[Nut124]

Happy, while I would not suggest trying to true up a head at home, this one I'd tackle myself with confidence. No need for a machine shop here.

There are ways to do it. You can carefully hit the flanges that are bent down. Or use a file on the high spots. If you have any flat surfaces, like granite countertops or such, you can lay a piece of sand paper and sand it down.

3 psig in my opinion is not too high. I run dual IDFs at about 4psig.

[18Fiatsandcounting]

Warpage of the carb base is pretty common, but I think 0.020" can be dealt with. Resist the temptation to use a pair of vise-grips to bend the corners back, as this usually just cracks the base and then the carb is junk.

You can take off the linkages and true it up on a piece of glass or granite with 600 grit paper or the like, as Nut suggests, or you can leave the linkages on, carefully hold the carb upside down in a vise (don't crack it), wrap some 600 grit paper around a straight piece of wood, and use that to carefully sand down the high spots. Or a file. You can see where you are removing metal because it will be much shinier, whereas the parts you aren't touching will remain grey.

-Bryan

[SpiderGuy]

You can see where you are removing metal because it will be much shinier, whereas the parts you aren't touching will remain grey.

-Bryan

In the Machine Shop we used Tool Makers Blue Ink to highlight low spots when machining a surface smooth. Heads, Seats, Valves, Decks, Discs, Drums etc.
Sprays on, drys real quick and shows up great to see the low spots. Won't gum up your Files, Grinding Stones, or Sandpaper. Very Handy stuff
Keith

[18Fiatsandcounting]

In the Machine Shop we used Tool Makers Blue Ink to highlight low spots when machining a surface smooth.

I use Dykem layout fluid, but the brush on type. Sometimes Prussian Blue. I'll have to look into the spray can version, sounds like it might make life easier.

-Bryan

[HappyHog]

Wooooh!

I got it dialed in now me thinks!

Sanded the carb base where the ears were pulled down. I used 420 grit sandpaper taped to a section of laminate countertop I had in the shop followed by 600 grit. Sanded till I got the perimeter of both barrels shiny, which was just this side of my arm falling off, I think. Took me about 2 hours of careful back and forth.
(Nut124 - I have granite countertops, but I sort of like being married, so I didn't go there!)

Put it all back together, and was able to adjust the carb. Before, when I had the bad vacuum leak, the idle mixture screw would not make a difference no matter what I did. This time I ended up with it at 1-1/2 turns out with it idling its best. The throttle stop screw was about a half a turn in from initial contact. She runs great now, and the black smoke in the idle exhaust is gone and the stumble at gear shifts is gone. I am ready to roll now!

Thanks for all ya'lls help on this. Wish I had thought about the blueing spray, Spider guy - as it turned out, really didn't need it, the shine was enough to reveal the progress. I also had my piece of glass nearby and used it to check with a feeler guage as I went.

I have pictures I took as I progressed on the sanding posted here https://imgur.com/a/hSzRjSd in case it helps someone.

Happy