'72 250 Zeniths 32/40 backfiring

[rgnprof]

Been a while since I've posted here with Zenith concerns, but my 250 is still running (although I've neglected the maintenance a bit).

I've had some backfiring on off-idle acceleration, typically worse when cold. Completed some maintenance today - new NGK plugs, Kingsborne wires, adjusted the valves, balanced the carbs, set the idle, checked the ignition timing. I didn't replace the cap and rotor as they looked fine - although I still may replace them. Car has a Crane ignition which seems to be functioning well although it is really hard to start when cold. I have adjusted the choke tie rods so that may help with the cold start. No problems starting warm or hot.

Car is running better than it has for years, BUT I'm still having some backfire and slight stumble under acceleration. I am not running any vacuum advance or retard (see my old posts...) and have set timing at 10 degrees BTDC. I've been running it that way for 15 years - or longer...no backfiring until the past year or so.

Possible causes of a backfire?

Thanks,
ryan

[rumb]

Timing is not set at idle, it is set at @3000 rpm. That way you know if the mech and vac advance are working.

Also when you press throttle can you see a spray of extra gas in the carburetor?

Check your timing chain stretch. A bad chain can be ~10 degrees off.

[Sugar Bear]

Backfiring/stumbling is usually a lean condition. Try using a high quality fuel cleaner such as Techron, BG or LiquiMoly. Rev the engine up to 3-3,500 and force the choke blade closed to create a fuel surge through the carb. Just before the car stalls release the choke blade and let it stabilize, repeat 5-6 times then do the other carb. Look at the top of the carb to find the air blades and clean them with carb cleaner and compressed air. Run the idle mixture toward the rich side.

Definitely check the timing chain stretch as rumb suggested.

Good luck!!!

[rumb]

Did backfires start after you adjusted the valves?

[rgnprof]

Ok - thanks for the suggestions. Let me provide a few updates.

Chain is fine and the backfiring was occurring before I adjusted the valves. Some of the valves were seriously off before and I'm pretty sure I got them adjusted properly.

I started tweaking carb settings and now I'm in more trouble than I was before! Not sure what happened, but I can't seem to get the car to idle now unless there is fuel dribbling from the primary nozzles in each of the carbs. And I'm experiencing some 'hair-raising' backfire as I rev the engine.

I should specify that it was idling fine before - it's almost like I have a sudden massive air/vacuum leak! I have checked the carbs for balance and they measure the same air flow - typically between 8-10 on my meter. Accelerator pumps are working...the only thing I did was remove the top section of the rear carb to replace the gasket (I noticed some fuel bubbling around the primary and secondary barrels).

I thought I might be having some trouble with my fuel return valve - mine is the '72 with the rubber, triangle shaped diaphragm. It has a history of leaking fuel (not good) so I think the proper procedure is to use 2 paper gaskets with the rubber membrane sandwiched between the 2. There is a small hose connected to the bottom of this triangle shaped plate that runs fuel back to the front carb and when I pulled that hose I saw no fuel, so I was thinking it was NOT operating properly. But, now I'm thinking fuel only gets routed back to the carb if the car is off idle (??)...I removed the valve and did notice that the new rubber membrane was torn around the edge - waiting on a replacement.

I have had a hard time finding information on this newer style of fuel return.

I'm at something of a loss - most obvious answer is vacuum/air leaks, but I'm not sure how they came up so quickly.

This is the gasket/rubber membrane set I ordered:
https://www.royzeinc.com/product/ze-60d/

[rumb]

Try spraying carb cleaner or brake cleaner around the carb and see if it changes the speed.

Perhaps do carb setup in order again per jamiecopp site? It's important they be done in order.

[Sugar Bear]

Closely compare every hole in the old gaskets to the new gaskets, they have to be a perfect match. Had one hole blocked because I had the correct gasket installed wrong, car would not idle at all.

Good luck!!!

[rgnprof]

Ok - couple of observations. After spraying a ton of brake cleaner around the carrbs, bases, gaskets etc., I notice no increase in engine speed so I don't think I have any leaks.

I had already removed the top carb section again just to check on the replacement gasket issue (there are so many different iterations of these things that kits come with a variety of gaskets), but I see no issues there - although I may triple check it shortly!

I have made some progress - can keep it idling now with no fuel dripping in front carb. But, when I cover up the rear carb the idle will slow and even stall if I keep it covered. No corresponding finding with the front carb. If I cover the small hole with a pencil eraser in the rear carb I notice a slight drop in idle but I don't see the same thing happening with the front carb.

3 other things quickly - if I remove the small hose at the bottom of the triangle shaped fuel return valve at idle - no fuel comes out. But if I remove at 2000 rpm (+/-) fuel flows pretty freely - which flows back to the front carb. I think the fuel return is working properly, if I can just keep it from leaking. Secondly, I forgot that I made a slight adjustment to the rear carb dashpot, moving the plunger further away from the throttle - I think I have moved it too far. And, I tested the idle control solenoids and they click and seem to be working correctly.

[rgnprof]

Now I have a new developing problem - gas in my oil. I noticed I couldn’t get the oil pressure gauge to come up fully while trying to rev the motor and I see smoke out the tailpipe. Checked the oil and it’s “overfill”…and when car is running I have oil leaking out the oil fill cap on the valve cover.

I’m assuming this thing is running really rich dumping fuel in to the pistons - at least I’m hoping it’s just related to my current carb problems.

[rumb]

Sounds like your floats are stuck open.

[rgnprof]

How big a deal is it that the center screw (the one hidden in the post) is stripped? I can't get it to tighten down.

All the others are screwed down tight...

[rumb]

Not the end of the world.

What did you find when you took the top off?

[rgnprof]

Haven't yet - I had noticed the center screw not tightening as I was double-checking everything a while ago.

[Sugar Bear]

If it has a mechanical fuel pump it can leak gasoline into the oil. If you remove the pump, look at the rocker arm and it's cavity, if it is spotless it was probably washed by gasoline. Since you are already having carb problems it could be floats getting heavy or a leaking needle and seat.

Good luck!!!

[dobrodan]

Sorry if you've already done all of this, so ignore the following if so.

The tops of these carbs need to be flat to completely seal to the center section. They are often higher in the middle from people overtightening the carb cover screw which pulls up on the center. This is also a common cause of the center screw stripping. If your center screw is not tightening that could be the main issue. These carbs, especially the later ones with the idle shutoff solenoids, are notorious for poor idling, siphoning gas and running on. Make sure that the center screw is the right length. There are shorter ones that look the same. If it is the screw that is stripped you're in luck. If it's the threads in the middle section that's another issue.

Here's what I would do:

1 - Take the tops off the carbs and confirm that they are flat. Easiest way is a piece of 200 grit paper on a hard flat surface. Run it across and keep checking the fresh surface. It can take a while to get it close. The tops are somewhat malleable so if you can tighten the center screw it should flatten out. When you put the top back on the carb get all of the screws started and tighten the center screw first and the outer screws last.
2 - confirm that the secondary throttle diaphragms are good. Easiest way is the open the primary all the way and operate the secondary throttle. If it returns slowly it is good. If it drops back quickly you'll need to change the diaphragm, which only takes a few minutes. If a diaphragm is bad you have a vacuum leak.
3 - Make sure that the venting valves are adjusted correctly. This screw is often mistaken for an idle screw, which it is not. A 1/8" drill bit is perfect to check the clearance.

While the top is off you might want to check all of the jets, nozzles and emulsion tubes to make sure they are in the right locations. The TDM has all of the specs (p. 206 in mine). Also make sure the nuts on the bottom of the carbs to the manifold are tight.