Thread: 2.25 petrol - poor running with Zenith

Thanks for the input guys. Regarding purchasing another carburettor - it's an option, but it's several hundred dollars that I don't want to spend until I have some reasonable evidence that my current carburettor is actually the problem.

Anyway, Roger visited yesterday, so a big thank-you is in order there. If you want me to shout you a tank of fuel, that offer is still open! We were able to try a whole pile of stuff that's difficult (or impossible) working solo, here's a summary of what happened.

• Valve timing: couldn't find EP mark, but sticking a screwdriver in the spark plug hole and watching the exhaust valve, it looked reasonable. It might be out, but probably not by enough to stop it from running.
• Vacuum advance: there might be an issue with the mechanism at the distributor, since sucking on the vacuum hose at the distributor wouldn't advance it. Still, it wasn't even idling right, so while it's a problem, it's probably not the problem.
• Fuel pump was delivering a healthy quantity/pressure of fuel.
• Spark strength: pulled a plug and held it against the block, spark was strong and a good blue colour.
• Ignition timing: about as good as it was going to get, bang on 6 degrees advanced.
• Pulled carburettor apart. Nothing to see, but it was a quick job so why not check it. No blockages or anything else obvious, the float height was out by a few millimetres but that's all.
• It idled best when holding a hand three-quarters over the carburettor inlet. This might indicate a lean condition, rather than the rich condition I initially suspected. This throws a vacuum leak right back into the picture.

Given the above, I want to try two things:

1. Testing for a vacuum leak. If anyone has a smoke machine or knows a way to test for leaks without the engine running, advice is welcome.
2. A known-good carburettor. If anyone in the Sydney area (or visiting) would be willing to swing by and let me temporarily install a carburettor off a currently-running engine, that would be great. Shouldn't take more than an hour, and I promise not to tweak any settings.

As always, any advice is welcome, especially around DIY vacuum leak testing. Thanks once again to Roger, too!

G'day Will,

The realization is you have more than one problem/fault with your poor running.
You have found a major problem with the vacuum advance not working. You need that to work!!!.
(I suspect your garage mechanic compensated for this fault by advancing the timing at idle however, causing other problems with normal driving.)
The other mechanism in the distributor is centrifugal advance. To test if it is working turn the rotor anti-clockwise and you will feel spring tension as rotor moves anti-clockwise. (The 25D distributor does not have very much centrifugal advance.)

Pointless proceeding until you replace the vacuum advance and make sure the centrifugal advance is working.

Let us know what model of distributor is fitted and what carburetor is currently fitted on the engine.
Note. The EP mark has to be there you just haven't found it yet, maybe covered in oil/grease or wrong stroke on No1.

You are one step closer to fixing your problems.

Chris

Originally Posted by Busted Syncro

G'day Will,

You have found a major problem with the vacuum advance not working. You need that to work!!!.
(I suspect your garage mechanic compensated for this fault by advancing the timing at idle however, causing other problems with normal driving.)
The other mechanism in the distributor is centrifugal advance. To test if it is working turn the rotor anti-clockwise and you will feel spring tension as rotor moves anti-clockwise. (The 25D distributor does not have very much centrifugal advance.)

Pointless proceeding until you replace the vacuum advance and make sure the centrifugal advance is working.

Let us know what model of distributor is fitted and what carburetor is currently fitted on the engine.
Note. The EP mark has to be there you just haven't found it yet, maybe covered in oil/grease or wrong stroke on No1.

You are one step closer to fixing your problems.

Chris

Thanks Chris.

I didn't realise the vacuum advance was quite that important - I figured I'd at least be able to get it idling well. My method for testing the advance was to suck on the end of the vacuum hose and watch the mechanism - I'll have to check the hose isn't blocked or collapsed first, but it does sound like the distributor needs to come out, at least for a check. I seem to remember centrifugal advance being fine, but I'll double-check that too.

I have a Lucas 25D (positive earth) and a repro Zenith.

The EP mark is definitely there somewhere, it's just that the surface rust on the outer edge of the flywheel is making it really hard to see.

G'day Will,

I note the vacuum advance pipe is now vacuum hose as being a thicker and reinforced wall tubing and must be clamped properly.
Yes by sucking hard on the hose the contact breaker plate will move easily. You can replace the vacuum advance mechanism with the 25D distributor fitted on the engine. (It also has the knurled wheel to make small changes to timing when running 91/95.)

Probably no need to remove distributor at this stage as the timing is setup correctly.
I mentioned replacing the capacitor as well as back firing is a classic symptom of faulty cap. If you have a capacitance tester it should read 0.5uf and have no leakage resistance.

(I fitted a Ducellier distributor used on the S3 2.25 engine. This design fixed the problems with the Lucas 45D and you can adjust vacuum advance rate, dwell independent of gap and wider centrifugal advance.)

I know nothing about the Zenith except it does have flat spots during acceleration and others have mentioned warped flanges. Like all aftermarket parts I would check the jets are the right sizes as per the manual particularly the main jet.

The problem you mentioned with the Solex of leaking fuel from the float bowl cover is classic sign of worn needle & seat or leaking seat or jammed needle causing high fuel level in float bowl. That can be fixed as parts are available.

Just a few thoughts on a rainy afternoon.
Chris

Finally made some time to pull things apart and take a look.

With the distributor off the motor, sucking on the vacuum advance (no pipe/hose involved) does nothing, so I've ordered a vacuum advance mechanism. Holding the driving dog at the bottom of the distributor while rotating the arm backwards also doesn't feel "springy" so there could be an issue there too, although I'm pretty sure I tested the centrifugal advance the last time I had the distributor out. I didn't quite get time to test capacitance, although the condenser is pretty new so I'm not holding my breath there.

No luck on borrowing a smoke machine, so I'll have to make one myself. I'm hoping to get some time for it this weekend.

I also got an endoscope, so I'm going to take another shot at locating the "EP" mark on the flywheel and checking valve timing.

While the distributor is out, I've also got an electronic ignition kit that I may as well fit. I doubt it'll make a difference, but it's unlikely to make the situation worse.

I'd still appreciate anyone with a running 2.25 petrol + Zenith combo stopping by.

I know you're looking at fuel issues and you said you've checked the spark plugs and got a good spark, but have you checked the coil?

I'm just thinking back to your statement of " I'm trying to find out why throttle kills it".
The 2.25 petrol in my Lightweight had exactly this problem. Ran like a dream, smooth-as at idle, but any whiff of throttle and load and it'd stutter and die.

I finally traced this to a severely overheating coil which in turn was caused by a dodgy set of points.
Might be worth checking?

On that subject - make sure your coil is designed for 12v not 8v; i.e. not designed for a ballast resistor.

Yep! From memory I think that means you need 3ohm resistance, not 1.5.