The car was running fine. Then last week, it started to pop and backfire on the freeway at 70 in 5th. It did fine in 4th at 70. Later in the day going up a grade at 70, it started to backfire real bad and lost power. I pulled over and it wouldn't idle. It was hard to start and then died. Investigation found fuel on the air cleaner cover on the passenger side carb.

Click to view attachment

Is this an indication that this float is bad? Is there a way to test the float vs. the valve?

Click to view attachment

Does this look like too much fuel in the bowl?

Thanks,

Roger

I had the exact same issues a couple months ago with my carbs. It turned out that the idle circuit was plugged. Tomlinson's book suggest pulling the idle jets and the idle adjustment screws and then blowing compressed air through to clear the blockage. It worked for me.

You should certainly check your float adjustments and be sure the check valve in the bottom of the bowl is working correctly while you have everything apart.

One of the floats could have gotten stuck as well - especially if they are out of adjustment. Be sure they move freely up and down in bowl and aren't hanging up on the side.

Fuel level in the bowl is set by the floats - do you have the adjustment spec's?

Good luck!

Stop using mush fuel

Well I rebuilt the carb and put everything back together. It barely started and had the same poor running condition and backfiring.

I thought, maybe I've got an ignition problem. That checked out fine.

So, maybe a valve got out of adjustment? Holy crap. All the valves on 3 and 4 were very tight.

Adjusted ALL the valves. It started right up. Horrible idle. But, after going through the adjustment process got it to idle smoothly. I only adjusted the passenger side since that was the one I rebuilt. And everything had been working well with them beforehand.

Took it out for a test drive. Way down on power. Poor throttle response. After about a mile or two got a backfire. Took it home and put it away.

Concerned that while it was running with the valves tight might have done some major damage. How do I check? Any suggestions for what I should do next?

Usually, when I adjust the valves there is barely any adjustment needed. What would make it get thight so quickly?

Generally, major damage is indicated by LOUD NOISES. If I were you I would be erasing everything I think I know from my brain, and start gathering facts.

Adjust all the valves. Even the ones you just did. Make sure you're doing it right.

Check the compression on all four cylinders. Do this with all spark plugs out and with the throttle completely open. Make sure to disable your coil while doing this test.

In my opinion, to worst thing you can do is guess at what the problem is. Test, test, and more testing will lead you down the right path.

What carb linkage do you have?

When you did the rebuild did you soak the carb body to clean out the idle circuit and progression circuits? It's not that hard to pull the idle jets and idle mixture screws and then blow compressed air through there - both directions. I would do this on both sides, all four circuits.

Mark is right, adjust the valves and be sure you are doing it correctly. Test for compression. Once these things are done AND you are sure those idle and progression circuits are absolutely clear, then you should test again. You will need to adjust the mixture screws for best lean idle as well.

Good luck.

Thanks for the responses. Hopefully I'll have time next weekend to work on it. I need to find a compression tester and an air compressor. Good excuse to buy more tools.

I finally had a chance to start working though this today. I started with a valve adjustment. I pulled the plugs to make it easier and found this: Click to view attachment

The center electrode on 4 was burnt away. The body is discollored like from heat. There was carbon connecting the electrodes on 3. 1 and 2 looked perfect.

Adjusting the valves, I again found that even completely loose, the valves on 3 and 4 were barely in spec (.006 intake and .008 exhaust). I thought maybe the adjusters are worn, so I pulled off the rockers. They don't look bad: Click to view attachment

I have some new adjusters I ordered from the Bird. The say they are for 2.0, but are much larger than what I have. What gives? Click to view attachment

Also, when I put it back together I found that the nuts on the outside studs were not the correct thread. It didn't torque correctly until I replacedwith the correct size.

I checked the carb linkages. All fine.

I cleaned the carbs. Rebuilt and blew out all the passages on the 3/4 side carb.

I put everything back together and it barely started. It runs like crap. Won't idle. Barely any power. I'm at my wits end. Any idea where I should go next?

Thanks,

Roger

The larger lugs are correct for a 73+ motor. Looks like someone replaced your rocker arms with the earlier ones, and used elephant's foot adjusters. I'm not sure if those are genuine 911 adjusters; hopefully they are. Those are the best quality but unfortunately they are expensive.

Has any machining been done to the valve side of the rocker arm? The face of the hole that faces the valve stem needs to be machined a bit to make for clearance when using the 911 swivel-foot adjusters.

--DD

Those aren't swivels. There are two styles of the M8 rockers. That is just one of the stock styles.

Roger, while the rockers were off, you should put a straight edge across the valves. If one or more of them is higher or lower than the rest you have valve seat problems.

Also when you were adjusting the carbs were you using a sync snail?
The popping sounds a lot like slipped linkage

Popping is usually due to lack of fuel. A lean condition could explain the burnt away electrode as well. Your carb may be plugged.

I had similar problem once, backfiring, running badly & turned out the coil had vibrated in it's bracket & rotated around until it was grounding one of the connections on the sheet metal ! Hope that's it... check your coil... simple fix ! Best wishes... Terry

Thanks for the responses. I'll pull it apart again sometime and lay a straightedge across the valves.

How would I know if any machining took place on the rockers?

Start at the beginning and doubt everything you did.
Set the valves
Set the timing and dwell
Set the carb linkage to match using a tuning snail to sync the carbs so all four are sucking the same amount of air and fuel.

For gods sake, get a new set of plugs, and wires, and cap, and points.

Change you oil, it had fuel in it now, and Chang the oil filter.
Change your fuel filter
Check your fuel pressure, keep it at the right setting for your carbs.

Set the floats at 10mm closed, there are charts and pictures on redline webers carb site for 40mm IDF float settings.

This is a basic tuneup for the car, and looking at those plugs, you have been driving it without a tune up for at least two years, maybe longer...

Rich

Make sure the idle jets are clean. This might help setting up the carbs. http://www.carburetion.com/weber/adjust.htm

Take Drifter914's advice and check the coil. Even if it checks out good statically, it may be bad when under load. Only way to be sure is to use a known good or new coil in its place. I just went through a similar nightmare and it turned out to be the coil. Since they hardly ever fail, it was the last thing I tried after wasting a lot of time and money.

Well I just had a chance to investigate this some more.

I decided that before I tore anything apart, I'd check the compression. Results:

#1= 120 psi
#2= 130 psi
#3= 0 psi
#4= 110 psi

Obviously, #3 is the issue. I checked the valve adjustment on that cylinder and it is fine. What makes sense to me to do next is remove the head and see if there is a valve burnt/not seating. Am I going to have to drop the engine to do this? Or, can it be done in the car?

l'll re-evaluate the carb and change the oil with filter after I figure out the compression problem.

Brand new plugs installed after the carb rebuild.

Coil must be good since there is spark.

Rotor, cap and plug wires have only about 18,000 miles on them.

Thanks for the input.

-Roger

It’s been 5 years. But I finally got to drop the engine and remove the head.

Pretty ugly.

Repairable, or should I be looking for some new heads?

What causes this? Want to make sure it doesn’t happen again.

Thanks

Overheated engine,tight exhaust valve clearances,lean carb setting or lean FI,or timing wrong and a few others, surprized the valve seats didn't come loose...dodged a bullet there,lucky guy.
The heat gaskets don't look great and have been leaking.
Find a good used head...only solution but you need to find what caused it from the start,good luck.

Not really repairable.

Well . . . everything is repairable given lots of time and money.

Since heads are still readily available, you'll want to simply replace. There is no point in trying to repair these. These are very far gone and aren't worth repairing. HAM has new heads available.

As to cause. . . In my opinion it looks like damage from running lean and overheating and has eroded the head around the valve seat. Potentially had some detonation going on too to have caused that kind of massive errosion of the aluminum between the valves.

At this point, you need to take a very close look at your pistons and rings too. I'd be surprised if they were not in bad shape too given how bad these heads look. If there was detonation going on, it will have taken a toll on pistons, rings, and wrist pins. All of those can be inspected without splitting the case.