Still a miss at 4,000 RPM..., Backfiring at WOT, even with all new ignition components Options

[yeahmag]

A cheat for the manifold to head gaskets is to either use a non-hardening sealant or some axle grease. I've had mine trued up on a mill and still used the sealer. Hylomar Blue is what I've been using for a few years now.

[ChrisFoley]

If it only happens under load, that's usually electrical, correct? Unloaded would point to fuel delivery?

No.
You can't get the carbs onto the main circuit without any load on the engine.

[r_towle]

If it only happens under load, that's usually electrical, correct? Unloaded would point to fuel delivery?

No.
You can't get the carbs onto the main circuit without any load on the engine.

Which may bring you back to ignition.
you are certain you have the firing order correct?
1432, clockwise.

Dont take this the wrong way....most of us have chased our tails and found it was that simple.
The car will run if you flip the wires...just not to well.

Wires also tend to arc alot at higher RPM's

Dark at night, run the motor...get it up to 3500 rpms.
Simple test is to touch everything bare handed.
touch each wire, run your hands down it from one end to the other.
Touch all around the distributor cap.

One shock will sting, but its harmless.

If you get frustrated, flip the carbs from one side to the other.
If it follows the carb, you know where to look for the problem.

Float levels, air bypass screws, cracked covers...its all possible to let in air leaks.

If it was me, I would touch the whole ignition system.
If nothing happens, flip the carbs, see if it follows you.
If it stays on the same side of the car, change both plugs and wires.
If is still stays, pop the valve cover off and rest the valves LOOSER on that side of the car.

If you are getting a rapid fire machine gun like backfire at the higher rpm,s its problably ignition related.

If you are getting random (no beat) back fires at the higher rpms, it may just be a vacuum leak you missed.



Rich

[messix]

I wouldn't go exposing your self to 40k of volts! it's not that I will kill you but that it can if you have the right circumstances kinda like Russian roulette!

I would run it up in rpm and grab a wire off the cap with some insulated pliers and see which wire has less affect on how the engine runs.

have you switched to electronic ignition yet?

I could be a bad cap or rotor... even if they are new.


when was you last valve adjustment?

[ThePaintedMan]

Played around some more last night. I can still somewhat replicate the backfiring even when there is no load on the engine (i.e. in the driveway). It doesn't backfire as hard or as repetitively, but revving open throttle at 3,500-4,000 does produce some backfiring.

I did not witness any arcing last night, either through touch or visually. Went ahead and cleaned the cap and rotor, with no difference. I did not try the "pull the plug" test, but I can try that tonight. But to answer your question Rich, it is indeed much more of a repetitive "machine-gun" like backfiring. Which is the only thing making me think it's ignition - otherwise, all signs point to fuel delivery.

My other thought is to ask Tim (one of the guys I'm rebuilding a set of carbs for) is if he'll let me rebuild them and then swap them out with these carbs just to try to rule them out.

Yes, it has the Pertronix installed now. It didn't make a difference unfortunately.

While I had it running, I went ahead and put the car up on ramps so it could cool down so that I can re-check the valves tonight as well.

[ThePaintedMan]

Checked the valves, everything was within spec.

Rechecked the wires tonight when it was much darker, and I could actually see an occasional arc from the coil wire and #3. It appears that where the wires clip into the holder on the tin has cause some very slight damage to the casing. I could produce some momentary arcing from that spot on each wire, which sucks because they were brand new 2 months ago. Took the wires out and taped them with some e-tape just to see if that fixed the problem. Went around the block and hammered it. No dice.

I'm going to order new carb to manifold gaskets, FI manifold to head gaskets, new wires and plugs. I'll try that next week and see what happens. If that doesn't do it, I'll see if Timothy will let me install the carbs I'm rebuilding for him as a last resort to see if it's really the carbs on this car giving me headaches.

[r_towle]

Use some light rtv gasket between the phenolic spacer and the head, and between the spacer and the manifold.

Rich

[r_towle]

So, you just replaced the wires, not too long ago.
Still have the old wires kicking around?

Rich

[jmill]

My SWAG is still the accelerator pumps. I'm not real familiar with the real old ones but did your rebuild kit come with new cams? The roller wears the cam out and you won't get the right volume. It's been quite a few years but I recall they came with different sized diaphragms and cams. Did you install the larger diaphragm and cam?

[jmill]

Just bombing around I see this fairly complete kit has three different sized diaphragms. No cams though.

[timothy_nd28]

I'm new to this thread, by all means use my carbs! What's mine is yours, I hope they work better. (IMG:style_emoticons/default/drunk.gif)

[r_towle]

Just bombing around I see this fairly complete kit has three different sized diaphragms. No cams though.

I forgot about those....

I have a bag full that I ordered up for mine.

He makes a good point above.
the main difference is the length of the piston on the accel pumps.
Materials seem to differ, but it should not matter.

You need to make sure you have the right pumps..

Rich

[rhodyguy]

so you noted arching on all 4 wires? unfortunately, with the pulley/cable linkage, the slave and master pulleys don't lend themselves to swapping the carbs left to right without reconfiguring the current setup. did you back the idle speed adj screw off of the slave side?

k

[stugray]

If the carbs have a vacuum leak from the shafts, could some heavy grease or even oil seal the leak long enough to determine if that is the problem?
Obviously not a long term solution, but an easy troubleshooting step.

Stu

[rhodyguy]

george, are the end washers, item #30 in the exploded diagram, in place? any chance you forgot one of the o-rings? one for each idle mixture screw and each idle jet.

[ThePaintedMan]

From all the diagrams and stuff I've poured over, I don't think the cams can be changed, or were not intended to be. The different accelerator pump diaprhagms you see in the picture are for different model carbs. The later style, adjustable accel pumps use a different diaphragm than the earlier style (non-adjustable, like I have).

I did try to throw some grease on the shafts, both when I assembled the carbs and recently to try to seal out any leaks. Unfortunately they're almost impossible to get to with the carbs bolted up on the car, but good call.

Kevin, all o-rings are still there in place, and all are new. Yes, idle screws were backed off when I synched them. The end washers are in place on the throttle shafts to help prevent air leaks (plus I installed the newer style sealed bearings which don't leak like the old style ones do).

[r_towle]

Early style carbs have three or four different accel pump dimensions...
I have the bag of parts to prove it...somewhere.

Took me several tries with several parts guys to get the right ones for the solid cam style carbs....the non-adjustable ones.

Rich

[URY914]

My car used to arc at the dizzy cap wires. One of the wires was arcing to the sheet metal. I could only see the arc at night. I cured it by making a dent in the sheetmetal with a hammer. (IMG:style_emoticons/default/smash.gif) .

I now have made several brackets to hold the wires off the sheetmetal.

[yeahmag]

Have you verified wirh a timing light that nothing funny is happening at that RPM?

[jmill]

From all the diagrams and stuff I've poured over, I don't think the cams can be changed, or were not intended to be.

I've replaced worn out cams in the past. You have to remove the shaft to do it. I've seen rollers freeze up and wear the heck out of them. I can't recall if there were different sizes. Might not be from the kits I've seen and the reading you've done. I've got triples now and haven't rebuilt a dual in a few years. Did you measure the pump volume and check that both sides have the same size pump by-pass valve?