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

[ThePaintedMan]

Hello all,
Tonight I'll be buttoning up the car for the first DE this weekend (if it passes tech this time), and I've got one last thing I'd like to track down before hitting the track. A few times I've ventured into the 4,000 RPM range and I get a lot of backfiring, primarily from the passenger side carb, I think. Otherwise it runs great at all of the other RPMs. Right now it's got a crappy 009 distributor with points, but seems to work well enough so I've decided to stay with it to keep the headaches down.

Would it be logical to assume that the high RPM misfire is because of spark scatter with the points setup? I ask because I have a Pertronix unit ready to go on a spare distributor that I'd like to try, but with only a few days until the DE, I'm not going to go poking and prodding unless there is a good chance this could fix the issue. Otherwise I'll just short-shift (IMG:style_emoticons/default/smile.gif)


Whatcha think?

[DBCooper]

Hello all,
Tonight I'll be buttoning up the car for the first DE this weekend (if it passes tech this time), and I've got one last thing I'd like to track down before hitting the track. A few times I've ventured into the 4,000 RPM range and I get a lot of backfiring, primarily from the passenger side carb, I think. Otherwise it runs great at all of the other RPMs. Right now it's got a crappy 009 distributor with points, but seems to work well enough so I've decided to stay with it to keep the headaches down.

Would it be logical to assume that the high RPM misfire is because of spark scatter with the points setup? I ask because I have a Pertronix unit ready to go on a spare distributor that I'd like to try, but with only a few days until the DE, I'm not going to go poking and prodding unless there is a good chance this could fix the issue. Otherwise I'll just short-shift (IMG:style_emoticons/default/smile.gif)


Whatcha think?

You've only ventured to 4000 rpm "a few times"? You'll spend a lot of time above that at the DE, so how does it behave past 4000? Get better, or worse? Two things, point float (or a small gap, or bad points), a weak coil or condenser on the ignition side, or plugged primary jets or a low float on the carburetor side. I'd bet on the carburetor since you say it's from one side, and it's the easiest to check/fix. so do that first. And you should put that Pertronix in already, it's doing no good in the box. It will clean up any points problem. Do that, see where you are. Get crackin, then go slay some V8's!!!

EDIT. I'm assuming you've adjusted the valves and they're good, carbs are well synced and no carb/manifold/head gasket leaks?

[ThePaintedMan]

You've only ventured to 4000 rpm "a few times"? You'll spend a lot of time above that at the DE, so how does it behave past 4000? Get better, or worse? Two things, point float (or a small gap, or bad points), a weak coil or condenser on the ignition side, or plugged primary jets or a low float on the carburetor side. I'd bet on the carburetor since you say it's from one side, and it's the easiest to check/fix. so do that first. And you should put that Pertronix in already, it's doing no good in the box. It will clean up any points problem. Do that, see where you are. Get crackin, then go slay some V8's!!!

EDIT. I'm assuming you've adjusted the valves and they're good, carbs are well synced and no carb/manifold/head gasket leaks?

Yeah... what can I say.... I'm a wuss. I'm always afraid I'm going to blow something up, so I rarely venture beyond 4,000. Guess it's time to take the skirt off (IMG:style_emoticons/default/poke.gif)

Everytime I've gone up that high and I hear those symptoms I've backed off, or shifted so I'm not sure what happens beyond it. Once I get the string alignment done, I'll go piss off the neighbors tonight to see if I can replicate it.

The low float makes sense, but I've set both to spec a few months ago when rebuilding them and they haven't come off since. It is possible I screwed one, or even both up though.

I'll check my points gap and/or install the Pertronix if I have time tonight. If not, maybe tomorrow after work. If that doesn't do it, I'll play with the carbs, but everything should be all set now that I've got the jetting figured out. Valves were just set as well and the carbs are synched, but those are a couple things I should check before this weekend.

DB, what is your name by the way? I've never caught it...

[DBCooper]

DB, what is your name by the way? I've never caught it...

Well, my ex-wife seemed pretty sure it was Dumb Bastard. That's not right, from my personal point of view, but it kind of stuck, so DB it is.

[jmill]

What fuel pump do you have? Could also be you don't have adequate fuel pressure (volume). You'll notice it after a hard pull. Float level drops because pump can't keep up and you run lean.

Carb plumbed farthest from the pump will lean out first.

I'd look at stepping up on the main jet size if you have good fuel volume and it's not ignition related.

[1988Hawk]

Hello all,
Tonight I'll be buttoning up the car for the first DE this weekend (if it passes tech this time), and I've got one last thing I'd like to track down before hitting the track. A few times I've ventured into the 4,000 RPM range and I get a lot of backfiring, primarily from the passenger side carb, I think. Otherwise it runs great at all of the other RPMs. Right now it's got a crappy 009 distributor with points, but seems to work well enough so I've decided to stay with it to keep the headaches down.

Would it be logical to assume that the high RPM misfire is because of spark scatter with the points setup? I ask because I have a Pertronix unit ready to go on a spare distributor that I'd like to try, but with only a few days until the DE, I'm not going to go poking and prodding unless there is a good chance this could fix the issue. Otherwise I'll just short-shift (IMG:style_emoticons/default/smile.gif)


Whatcha think?

I have had issue with rev-limiting rotors over 4K, spitting and sputtering, not sure if your using one..........

[ThePaintedMan]

As far as I know, I've got plenty of fuel volume and pressure. This is a carter rotary style and I've got a quality inline regulator and gauge set to 3.5 PSI. No way to tell what's happening with the pressure when I'm driving, but when it's revved in the driveway I don't see any FP fluctuations. Even if that were the case, the passenger side carb is the one closer to the T (which is the one that I suspect is giving me issues). Would make much more sense for this to be a jetting/fuel issue than ignition if it's only one carb giving me trouble.

Think I might actually start with the mains. I've got 135s in there now and I have a set of 140s that I could try. Its possible my synch is off just enough at the higher revs and that I'm already borderline lean, so that at those RPMs it goes lean enough to start poppin.

No rev-limiting rotor on this distributor. Like I said, it might be easier to start with the jets since I know my way around carbs much more than ignition stuff. Eventually I'll probably look for a L-Jet dizzy and throw points on it just to cut down on the spark scatter if nothing else.

Thanks for the help and ideas!

[r_towle]

First off, you ARE a wuss.....at a de, redline is where you play.

Get a timing light and see what it says when you rev it to above 3500.

You may need to set the timing differently

[jmill]

Might be an ignition issue but if not and your looking for enrichment just towards the top end you can drop your AC jet size down below 2.00.

[ThePaintedMan]

Well I threw everything I had at it tonight. Did it in steps - first bumped up to 140 mains from 135. Then F7 emulsion jets, which I had been meaning to install for awhile to try to improve the transition from idle to main.

Then I sucked it up and installed the Compufire. I tested in between with some bursts around the block and no difference. However, I did get it up to 4,800 once when it didn't backfire as much. (IMG:style_emoticons/default/aktion035.gif)

Ran out of time as I have to drop it off at the shop tomorrow before work to get it aligned. But when I get it back I'll play with the synch again. It's the only thing I can think of that would cause one bank to act up and not the other.

I may try those A/C jets though. I've been told 180s can really improve the top end over the stock 200s.

[old dog]

Well I threw everything I had at it tonight. Did it in steps - first bumped up to 140 mains from 135. Then F7 emulsion jets, which I had been meaning to install for awhile to try to improve the transition from idle to main.

Then I sucked it up and installed the Compufire. I tested in between with some bursts around the block and no difference. However, I did get it up to 4,800 once when it didn't backfire as much. (IMG:style_emoticons/default/aktion035.gif)

Ran out of time as I have to drop it off at the shop tomorrow before work to get it aligned. But when I get it back I'll play with the synch again. It's the only thing I can think of that would cause one bank to act up and not the other.

I may try those A/C jets though. I've been told 180s can really improve the top end over the stock 200s.

O.k. , just my 2 cents worth... lean everywhere, increase the size of the main jets. Lean on top , decrease the size of the air correction jets. If you really think it's one side over the other and they are synch'd , ( open as well as idle ) then make sure you don't have any vacuum leaks. Carb base gasket, intake manifold gskt ? Extra air in means lean on top ! Spray a little carb cleaner around the carb base at idle and see if anything changes. If it does, fix the leak. Good Luck !

[ChrisFoley]

Heck, I rev my street engine past 4K routinely - before upshifting, and once in a while in 5th as well. (IMG:style_emoticons/default/smile.gif)
I take it right to redline on a regular basis in the lower gears.
On the track you should be revving to 52-5500 rpm before every upshift.

[mepstein]

If I didn't rev my little 1.7 past 4K, I wouldn't make it out of the driveway. (IMG:style_emoticons/default/biggrin.gif)

Have fun and good luck at the Track

[ThePaintedMan]

(IMG:style_emoticons/default/idea.gif) Guess I've been babying her too much. I think it's my lack of funds and ability to rebuild an engine that causes me to be so careful with it. But if you guys say go for it, I don't need to be told twice (IMG:style_emoticons/default/smile.gif)

The car is at the shop getting aligned now. When I get it home, hopefully tonight, I'll play around with the synch some more and triple check for leaks. Chris, I have a quick question for you regarding your cable linkage. I'll send you a PM.

[rhodyguy]

back firing on accel or decel? exhaust only, not out of the top of a venturi? what about stability at 4k cruise mode?

[ThePaintedMan]

back firing on accel or decel? exhaust only, not out of the top of a venturi? what about stability at 4k cruise mode?

Acceleration and cruising, but for whatever reason its right at 4,000 RPM and up. Definitely coming from the top of the carb, which is what I've always called "backfiring" as opposed to what many call backfiring - burbles from the exhaust (which is actually fuel-overrun).

[ThePaintedMan]

Hi guys,
Running out of time, but at least I think this is a simple problem. "Old dog" had it right. Vacuum leak at the base of the carb. Gaskets are all new, but check the pictures below. Any of you familiar with Weber IDFs know if this passage is supposed to be open? Are the gaskets supposed to cover up these holes? I've noticed it before, but figured they were supposed to be open to the manifold. I can't quite tell if they are compressed all the way, or if there is really a vacuum in this location. You can see the indentations in the gasket below.

Leaving for the DE tomorrow, so I'm hoping to get it fixed tonight. If the leak isn't coming from here, I can't figure out where it would be coming from. (IMG:style_emoticons/default/confused24.gif) Nothing else stood out to me.

On that note, can anyone think of a quick fit that would get me through the weekend? I think Permatext grey is gasoline resistant. (IMG:style_emoticons/default/idea.gif)

[jmill]

Gasket is correct. Hole is correct. A vacuum leak would show up at idle and not 4K. I'd also take a close look at your linkage. If it's wonky you'll get pops. Show us a picture of your linkage.

[ThePaintedMan]

Gasket is correct. Hole is correct. A vacuum leak would show up at idle and not 4K. I'd also take a close look at your linkage. If it's wonky you'll get pops. Show us a picture of your linkage.

John,
Its pretty dark here, so pics would be difficult. But it's a Tangerine Racing cable setup that has worked great so far. Carbs were still synched, both at idle and at 3,000 RPM after checking with my snail gauge.

I though that port was correct on the bottom, but could never tell if a PO had messed with them. The reason I'm suspecting a leak between intake and carb is via the old carb cleaner trick. Sprayed the base of the carb with cleaner and the idle dropped noticably. I'm going to look at other areas of the carb and inspect for potential leaks, possibly the manifold vacuum tubes (these are an older set of carbs that actually have two brass tubes protruding from the base of the carb for manifold vac.) It's possible that the throttle shaft is leaking as well, since these are very, very old. One carb has already been re-bushed, but I can't remember which one.

[r_towle]

easy test

Switch carbs side to side and see if the popping follows the carb.

Still might want to consider checking the timing at full advance...
the 009 is not curved properly for our motors and it may affect your performance at full advance..
It may also not be delivering full advance.

rich