Timing and backfire

[spinelll]

IDIOT, IDIOT ME just couldn't leave well enough alone.
I felt that my timing was a bit advanced so I retarded a little. Then a little more. Then I was way off (backfire, loss of power) so I started from scratch with the timing pin set etc.
That got me close but I still had lost some power and was still backfiring. Advanced it a bit, a little better. Advanced it a bit, a little better. So now I have decent power but I have occasional backfire upon deceleration when shifting. Since the only thing I messed with was the timing, should I keep advancing slightly until the backfire disappears? And while I'm at it, I think I'll reapply the "Don't fix it if it ain't broken" sticker to my forehead.

Leo

 

[Patrick L.]

Something just doesn't sound right.. Are you not correctly setting the base timing and then using the timing lever for driving conditions ?? If not, go the Marcos site for nice clear easy instructions..

 

[steve s]

Leo,

What's happening with backfiring is that unburned fuel is accumulating in the exhaust system, where occasionally the heat sets it off. There are MANY things that can be causing it and more than one may be operating in your case. There are two general conditions to be considered: car under load and car not under load.

Under load backfiring often indicates an electrical/ignition/timing problem boiling down to to too weak a spark to make the jump under the high pressure that exists with the throttle wide open at relatively low rpms. There's a world of possible culprits, including the timing that you've been working with. Stories abound, but the most intractable cases I've seen were due to bad distributor body (large plastic part) and intermittant bad ignition switch.

No-load backfiring, like when you're between gears while shifting or when you're coasting down hill, is, paradoxically, due to too little fuel. The problem is that under those conditions, with throttle closed, the motor is depending on the carb's idle circuit for gas, BUT it is being forced to turn at high rpms still trying to fire on every compression stroke. THere's simply not enough gas available to support combustion so the raw mix is swept out on the exhaust stroke into muffler etc where it can explode/backfire. Any thing that leans out the mixture will aggravate condition--too low carb float, intake system vacuum leaks, etc. SOmetimes, simply opening up gas adjusting valve (on choke rod) will fix problem. It might take more than one full turn open on a long down hill for an otherwise good-running car, especially if you're in 2nd gear! Avoid high-rpm gear shifting as much as possible.

Steve

 

[Chris in CT]

Leo, Just to add to the advice Steve gave above, try one very simple move. With the engine at idle. TURN IN the idle mixture screw on the carb. This will enrichen the idle fuel mix. Turn it in until the engine begins to falter, then turn it out until the engine just begins to run smooth again. Take you car for a test drive. This is a simple, easy move and has about a 90% chance of remedying your problem. Happy motoring!

 

[spinelll]

Steve

I have a Tilley. Is it the screw circled in the attached pic?

 

[steve s]

Steve

I have a Tilley. Is it the screw circled in the attached pic?

Yep, that's the adjustment screw that Chris was referring to. Backing it out allows more air to bleed into the idle circuit--too far makes for too lean, which I believe is what Chris suspects is going on.

Most folks do this adjustment by idling the motor down as slow as possible, then turning the adjustment in until motor starts to stumble, then--while counting turns--back it out until motor stumbles, then adjust to midway between those points. Erring a bit on the rich side may help you out. It's important to do this at low idle so motor is totally under control of that which you are trying to adjust, and not sneaking gas from the main jet.

Steve

 

[Purdy Swoft]

Steve

I have a Tilley. Is it the screw circled in the attached pic?

That is the screw , it adjusts the rich or lean for the idle curcuit, There is some possibility that turning it in slightly may help. I think that the main problem is a vacume leak caused by the Tillotson carb.

If you were running a Zenith I would suggest richening the gav . You could be running slightly retarded on the timing or improper use of the spark lever. I run mine fully advanced until the engine is in a strain or at low speed. If there is a leak at the manifold clamp or any leak between the engine and the muffler it will cause backfire on decelleration and backfire, a few seconds after shut down. I would explore all avenues.

 

[roccaas]

If it backfires after you shut the engine down, change out the condenser. Cheap and effective.

And Steve, thanks for a Graduate Level discourse on backfire. I copied and pasted it into my archives!

 

[steve s]

Leo,
Comments about your load and no-load backfiring may be getting jumbled up. IMHO, if you've got your timing problem squared away and are not backfiring under acceleration, you can pretty much quit worrying about electrical/spark issues. Time to look for vacuum leaks and such.

That is the screw , it adjusts the rich or lean for the idle curcuit, There is some possibility that turning it in slightly may help. I think that the main problem is a vacume leak caused by the Tillotson carb.

If you were running a Zenith I would suggest richening the gav . You could be running slightly retarded on the timing or improper use of the spark lever. I run mine fully advanced until the engine is in a strain or at low speed. If there is a leak at the manifold clamp or any leak between the engine and the muffler it will cause backfire on decelleration and backfire, a few seconds after shut down. I would explore all avenues.

I think the Tilly vacuum problem that is being suggested here is their tendency to warp, preventing a good seal where they attach to the manifold. Check with a straight edge, and grind or file surface flat, if needed. The iron Zeniths don't have that problem, but they do rust.

Although it doesn't have a secondary jet to feed, the gas adjusting valve (GAV) otherwise works the same (and I believe is interchangeable) as the Zenith's. Opening it up a bit is definitely an easy thing to try. The more vacuum leaks you have, the more you will need to open it up to get to the right fuel/air ratio. Also, ethanol mixed fuel probably requires a richer mix; Running at half-turn open rather than quarter-turn works for me--but I don't want to start a fight.

Steve

If it backfires after you shut the engine down, change out the condenser. Cheap and effective.

And Steve, thanks for a Graduate Level discourse on backfire. I copied and pasted it into my archives!

Glad to be helpful.

Steve

 

[spinelll]

Leo, Just to add to the advice Steve gave above, try one very simple move. With the engine at idle. TURN IN the idle mixture screw on the carb. This will enrichen the idle fuel mix. Turn it in until the engine begins to falter, then turn it out until the engine just begins to run smooth again. Take you car for a test drive. This is a simple, easy move and has about a 90% chance of remedying your problem. Happy motoring!

Turned it in, turned it out, backfire on acceleration gone!
I ordered a vacuum gauge and will check that just for fun.

Thanks all.
Leo