Distributor problem? (backfire through carb)

[jovial_cynic]

Relevant info:

 

71 510

LZ hybrid (2.2 block, 1.8 head, high profile cam)

SUs

electronic matchbox distributor

 

I'm running through all the possible scenarios for this recent backfiring through the carb problem that's come up when accelerating from idle (and *only* when accelerating from idle), and the only thing I can think of is there I might have a problem within the distributor. All I know for certain is that if I step on the throttle, I get equal backfire through both of my SUs. And it didn't always do this; this is a rather recent problem (which rules out the cam being the issue).

 

I've adjusted the fuel mix, and I've played with the timing, knowing that too little fuel will cause backfire, as will too advance timing. So, I've gone rich with it until the engine bogs down from being so rich, and I've set the timing so far retarded trying to eliminate the backfire that my headers were glowing red. No good. No change.

 

The only thing I can think of is that when I step on the gas, something in the distributor is either shorting in such a way that a spark is jumping terminals (firing a cylinder that's got the intake valve still open), or the mechanical advance mechanism has gone wonky and is pegging full advance momentarily (so momentarily that I can't detect it on my timing light).

 

Does anybody have a spare electronic distributor in my area that would be willing to bring it down for testing, to see if I can rule that out as a potential problem?

[banzai510(hainz)]

try grounding the distributor better.

 

If the car runs good on the top end then How can it be the dizzy?

 

what is the timming?????????

 

your sure cam is timmed correctly? I will ASSUME yes.

ck the valve lash but highly dought it.

 

Try putting a weber DGV on there. and see if you have the same proplem.

 

 

personally most SUs should just go to the DUMP so people dont have these proplems.

[jovial_cynic]

I'll check the ground on the distributor...

 

As far as whether or not it could be the distributor, I dunno. I'm just running through my options, and I'm having a hard time thinking of what else it could be.

 

I've set the initial timing everywhere from 0 to 20 degrees advance, just to see if the ignition timing is having any impact on the backfire, and it's not. The distributor is giving me about 18 degrees (I need to reduce that a bit, since I'm getting a little bit of pinging towards the top).

 

I assume that the cam is timed correctly, on account of having done it according to the book, having done it according to your video, and having it running GREAT a little while ago. This is a new problem, which is also why I don't instinctively assume that it's the carbs -- they were running great a while ago, and I don't believe I've made any changes.

[banzai510(hainz)]

crap I wrote a big thing about this an if locked up on me.

 

I think the dizzy is fine but if you have one swap it out. get a spare anyways.

Why I say this is its OK past the bog.

 

Your timming is fine as you can dial it from 0-20.

 

You can ck the valve lash on the cam if you feel motivated as new cams or regrinds the lobes can dissapear if not broke in correctly. Maybe a lash pad fell out but that would give more a missing/loss of a cylinder sound.

 

SUs dont have accel pumps on the carbs so ck the vol stack oil.

 

 

I know SUs have some tubes running inbetween each other ck them out or maybe plug them and see if soemthing changes.

[DISLEXICDIME]

how's your fuel filter looking? Its a long shot but maybe it is partially plugged

my fuel filter was partially plugged and made my su's miss fire under hard load.

did you take the tops off your su's and mix them up?

if you pull the chokes on a little and drive it do the su's stop spitting?

[jovial_cynic]

I thought about fuel, so I checked the floats and the filter. The thing is, once I get off idle, I can drive it hard and there's no problems. It's *just* when I increase the RPMS off of idle.

 

No - choke or no choke makes no difference on the backfire, and neither does richening up the mix so much that it starts to stumble.

 

It's very odd. It doesn't seem like an adjustment issue... it seems like something is causing a spark to occur way too early when I initially jump off the idle... but I can't think of what would cause that to only occur at that time, and not continue to occur as the RPMS increase.

 

It doesn't happen if I slowly increase the RPMs. Very slowly. So this makes me think that it's possible that I'm having a fuel inrichment issue with the carbs (momentarily running lean on acceleration), but the 20wt dashpot oil levels are good, and the carb pistons are behaving the way they should when I watch them.

 

Anyhow, I'm on the lookout for another electronic distributor. If someone has one I can test nearby, I'd be grateful.

[DISLEXICDIME]

my su's like 30 weight.

 

did you make sure the manifold bolts that hold the header and the intake on are all tight?

I had a leaning out and spitting issue when i had my l 20 and it was because the angle was to harsh on my universal i used on my extension to get to the bolts. The angle was causing the u joint to bind and i thought they where all the way tight but they where not. It was causing the carbs to lean out from the vac leak and spit . after a couple days i figured out what i had dun. i tightened it up re tuned the carbs and it was good to go !

[datzenmike]

Where are you getting your vacuum for the distributor vacuum advance?? Direct from intake manifold or from a metered port on one of the SUs??

 

If from manifold are you disconnecting the hose before setting the advance?

[dgi]

It doesn't happen if I slowly increase the RPMs. Very slowly. So this makes me think that it's possible that I'm having a fuel inrichment issue with the carbs (momentarily running lean on acceleration), but the 20wt dashpot oil levels are good, and the carb pistons are behaving the way they should when I watch them.

 

Jovial... thought we discussed this throughly when you were installing them?

 

Reread your statement above. Odds are that is the correct diagnosis. 20wt dashpot oil is too thin in warm weather. It is going lean because the pistons are moving up immediately. That is not "fuel enrichment", it is fuel deprivation leading to backfire.

 

Dislexicdime is right on target. Go to at least 30wt. Betcha all will be well in SU world again.

[jovial_cynic]

I didn't mean "enrichment". I mean "not getting rich enough", or "going lean."

 

The thing is, I had 20wt oil prior, and it was running well... and I don't believe the weather was considerably colder. I'll grab some 30wt and give it a shot. Hopefully it does the trick. We'll see.

[tdaaj]

i think you need to update everyone

[jovial_cynic]

haha... I was waiting to take some pictures and show some video, to make it clear how ridiculous the problem was. :)

 

Ok - After swapping out distributors and seeing no change in behavior, and then swapping out vacuum advance canisters and still seeing no change in behavior, tdaaj noticed that I had a kink in my goofy hacked-together fuel line between my fuel filter and my fuel pump...

 

... and so it turns out that having a kinked fuel hose really does make all the difference in the world. It was never a timing issue - it was a fuel problem, but it didn't stem from the carbs as I feared.

 

:: sigh ::

 

At least this problem didn't cost me anything! (except maybe a hit on my pride...)

 

Thanks tdaaj! I still owe you a pack or two of beer, man.

[tdaaj]

 

Thanks tdaaj! I still owe you a pack or two of beer, man.

 

make it henry's root beer and we'll be ok

[jovial_cynic]

You've got a deal, man.

[jovial_cynic]

tdaaj... it looks like it might still be something else. the problem appears to persist.

 

when revving it in the driveway, it doesn't backfire as much, but it still does it a lot when i'm pulling away from a stop.

 

:: sigh ::

Edited July 20, 2009 by jovial_cynic

[DISLEXICDIME]

tdaaj... it looks like it might still be something else. the problem appears to persist.

 

when revving it in the driveway, it doesn't backfire as much, but it still does it a lot when i'm pulling away from a stop.

 

:: sigh ::

 

have you checked the floats on your su's?

http://www.jetlink.net/~okayfine/su/sumain.html#anchor108396

 

got a spare fuel pump ? if so swap it on and try that. or try bye passing the fuel filter and see if it helps. have you checked the fuel lines ? maybe you smashed one under the car some where .

[jovial_cynic]

I do have a spare fuel pump, but my O2 sensor is giving me good readings as I rev it, so I'm midly confident that this isn't a fuel issue.

 

I'm going to go through all the timing again and take photos so more eyeballs can view what I'm looking at to make sure that everything is set up correctly. I've ruled out the distributor as the source of the problem, and I appear to have fuel... so any timing issue is either at the oil spindle or the timing chain. And I'll go ahead and check valve lash and document everything.

 

This is all hurting my brain, though.

[DISLEXICDIME]

I do have a spare fuel pump, but my O2 sensor is giving me good readings as I rev it, so I'm midly confident that this isn't a fuel issue.

 

I'm going to go through all the timing again and take photos so more eyeballs can view what I'm looking at to make sure that everything is set up correctly. I've ruled out the distributor as the source of the problem, and I appear to have fuel... so any timing issue is either at the oil spindle or the timing chain. And I'll go ahead and check valve lash and document everything.

 

This is all hurting my brain, though.

 

did you ground the distributor ?

[jovial_cynic]

The distributor has two wires to the coil, and I believe is grounded straight to the front cover. I'm pretty sure that's not the problem.

 

I went through and checked all my timing again, as well as run through everything I can think of that I've tried:

 

1. oil spindle in the correct position

2. distributor rotor pointing at #1 when #1 is at top dead center

3. notch and groove properly aligned when #1 is at top dead center

4. engine idles at around 1000 rpms with around 18 degrees TDC (vac. plugged)

5. engine revving at 4000 rpms gives me a total advance of 35 degrees (vac. plugged)

6. ruled out faulty distributor by swapping one out

7. ruled out fuel-to-carbs by installing proper fuel line

8. ruled out lean condition, confirmed with O2 sensor installed

 

brain explodes.

 

Actually, I did just notice that my coil is apparently leaking oil. I yanked the plug wire off the oil and it was entirely wet. I'm aware that a failing coil can cause all kinds of random ignition problems, so I'm going to replace that before I try anything else.

 

Anything on the check-list that maybe I didn't catch? Anybody think of anything else I haven't tried yet?

[DISLEXICDIME]

bye grounded to the front cover you mean through the pedistal bolts ? that ground some times is not enough and you have to ground it to the chassis. but it sounds like you found the problem