2 issues I can't quite track down......any thoughts welcome....

Donald Wetzel

Donald Wetzel

Daily Driver
Running dual DCNF's, Vick's electronic distributor, their recommended coil, original Marelli alternator (just trying to include what might be relevant)......

Issue 1: A miss at idle and high rev-zero throttle coast. While tuning with a Gunson (AWESOME instrument), I found that #2 is a very bad miss at idle. No, I didn't run a compression test yet.....but it is weird, runs and fires fine above 2000 RPM.....the miss is only at idle. Tried swapping plugs, wires, inspected the cap for cracks, made sure it's getting fuel.....it is only at idle. Idle voltage at idle is a hair below 13 at the alternator.

The throttle "coast" is hard to explain.....watching the Gunson, snapping the throttle from closed to open to closed I'm going "missing but blue-barely yellow on throttle opening-back to missing on coast".
I should have taken a video with the Gunson.....it showed a lot clearer what Im trying to say here.....

Issue 2: A "weak" accelerator pump shot on one of the Webers. One acts properly, a nice clear shot, no drips....the other throws me a raspberry and dribbles slightly. Changed the accelerator diaphragm, new pump jets on both carbs, made sure both inlet from the fuel bowl and outlet to the jets is clear, and the demand valve is not plugged.....

The worst/best part is......it runs and pulls like a BEAR, pulls hard til 6500 (as far as I'll risk it), sings that lovely Weber song getting there.....a (very) informal 0-60 of maybe 8.5 or so........it's these 2 little issues that are stumping me....
 
I would guess you have an idle circuit issue with #2, maybe the idle jet is clogged. As to the accel pump, make sure the spigot into the venturi is not plugged. They have a really tiny orifice and I have had many Webers with this port totally clogged.
 
carl said:
I would guess you have an idle circuit issue with #2, maybe the idle jet is clogged. As to the accel pump, make sure the spigot into the venturi is not plugged. They have a really tiny orifice and I have had many Webers with this port totally clogged.
Click to expand...
I had so many issues with the choke circuit (or what ever it is called) causing idle issues I finally ended up closing it up all together and just dealt with having to pump the pedal for cold (winter) starts.

Odie
 
Odie.....that's already out of the mix, one of the first things was to do the choke delete....

Carl....for the idle circuit, that's why I asked for ALL thoughts, that's what I'm going to hit tonite.
For the accel...I did blow canned air thru the port (diaphragm cover off, accel jet installed), it seemed to flow well......but worth another look
 
Doing some digging (easier at work waiting for code to run than at home keeping a 7 yr old occupied lol)......I have a feeling the issue might be here.......
weber(1).JPG
 
I just read this, and my first thought was "pump check valve... the little ball inside gets stuck (sometimes goes missing completely) ... pretty common and I keep these as a stock item because it's common.

and / or the pump jet itself can block up... and no you can't really poke something down there to try unblock it... it's a metered orifice.

SteveC
 
But isn't that circuit common to both #1 and #2 cylinder and would cause a miss in both? I'm talking about the discharge port into the venturi which is separate for each of the two venturis and could thus effect only one of the cylinders. DCOE, IDFs and DCNFs each have a lot of circuits common to both venturies but if you have a problem in only one, you have to look at parts of the circuits that are separate for each cylinder.
 
I think Steve is talking the accel pump issue....and he might be seeing an order soon, depending on if it's stuck or missing... ;)

The jet itself works fine.....it's new, and does squirt putting canned air on the diaphragm side.....seems like it isn't getting enough oomph, like it's bypassing that ball check...
 
they are available new from all over... 64290.014 is the Weber part number, same for 36/40/42/44 DCNF.

I've never used a Gunson, so is that indicating a rich or lean condition that is causing the miss?

Even with the "choke" actuation mechanism removed... there are still the enrichment passages and plungers fitted, fuel could still be spilling from these at idle conditions.

Other baffling scenario's I've run into have been throttle actuation linkages causing a very very very slight throttle shaft movement at idle... causing very very slight fuel spillage from the pump jet constantly (very difficult to see except for rapidly vaporising spots on the throttle plate) which went away once engine speed increased...

SteveC
 
Last edited:
My experience is with IDFs on spiders but I have had both leaking cold starts and the self actuating accel pump that Steve mentions. Both issues are mention in John Passani's book. I tap the base of the cold start passage and plug it with grub screws, you can also use epoxy. Just using the factory blank plate shuts off the air leak from the cold start but not the fuel. The self actuating accel pump, I think, is usually caused by mistuned carbs that cause a rocking of the motor and with too little clearance between the pump arm and pump cam causes the pumps to self actuate. A third thing to check is to make sure you are not idling on the man circuit rather than the idle circuit. This causes fuel to dribble out the main jet nozzle and you can see the fuel splashing on the closed throttle plate.
 
Not ignoring anybody, just very little time this week to get in the garage......damn "real life" lol

I appreciate all the insight, hopefully can get a wrench in my hands Sunday....
 
Pump check valve for issue 2...

enrichment circuit spilling fuel would explain both scenarios of issue 1... at idle the throttle plate is closed and below the throttle plate there is a pressure less than atmospheric... this is all that's needed to pull the fuel past a poorly seated enrichment plunger... the high revs and no throttle input is sort of the same, except the manifold vacuum is going to be even higher until the engines cycling speed decreases.

it's very common for the plungers and their holes to be full of crusty white crud ... removing the plungers is easy.. just flick the little brass retaining disc out (don't lose them, replacements are a PITA to find) you may find the plungers quite stuck / difficult to lift... maybe... but persist and carefully work them free... a soft wire brush usually cleans them nicely... soft gun brushes (brass bristles) are great for cleaning the hole in general... but it's the tapered seat that is going to need to be in good condition, or else it won't seal so be careful at the bottom of the hole to not dig into the tapered seat. If they are damaged then the best way to move forward is to tap the enrichment holes on the underside of the carb and fit a small grub screw... it's a really small size ... like M4 x 0.75 or something similar (been a while since I've done one) ... the other option is to recut the tapered seat... and there is actually a special tool for this! looks a bit like the tool you would use to recut the sealing face on a regular water faucet (but who really does that anymore?) and I have only ever seen one (and no I don't own one)

that's what I would look at first anyway.

SteveC
 
Finally got some garage time, and here's where I'm at.....

The accel pump issue....nothing is missing, got a rattle of the check ball in the pump demand valve as well as the check valve at the inlet itself. Cleaned all passages again, it has new accel jets.......it now works, but definitely not as "strong" of a shot as the other carb......maybe 75%.
Really didn't narrow it down to a specific problem....maybe just the general cleaning was needed more than anything....

The second issue.....I narrowed it down to the starting enrichment, had to be the slightest leak there...the miss was missing too rich. Now...I do have "blank" starter jets....so it had to be a seating issue with both the jet seat and the starter valve seat. Cleaned as best I could, still could not get rid of the rich at idle condition. Considered the grub screw solution......

.....and it hit me....

because my "jet" is not a jet, it's just a solid piece of brass.....it doesn't matter if there's a leak at the starter valve if there ISN'T one at the jet. If there's no physical way for air or fuel to even get to the starter valve......so my easiest solution was to make sure the jet itself was sealing.

The tiniest, thinnest layer of blue permatex gasket maker on the tip of the jet......problem solved.....and it's not a fix I can't undo when I have the ability to get everything sealing correctly....

Ran out of time today, but going to do the same for all 4 starter jets......then re-tune both carbs with the Gunson......then reload and see what other issues pop up.....that, and figure out how I'm going to mount the heat shielding....
 
I think you will find blue permatex isn't all that good at holding up to being exposed to straight fuel like that... it will probably deteriorate...but at least it let you nail the issue down for certain.

SteveC
 
Donald Wetzel said:
Finally got some garage time, and here's where I'm at.....

The accel pump issue....nothing is missing, got a rattle of the check ball in the pump demand valve as well as the check valve at the inlet itself. Cleaned all passages again, it has new accel jets.......it now works, but definitely not as "strong" of a shot as the other carb......maybe 75%.
Really didn't narrow it down to a specific problem....maybe just the general cleaning was needed more than anything....

The second issue.....I narrowed it down to the starting enrichment, had to be the slightest leak there...the miss was missing too rich. Now...I do have "blank" starter jets....so it had to be a seating issue with both the jet seat and the starter valve seat. Cleaned as best I could, still could not get rid of the rich at idle condition. Considered the grub screw solution......

.....and it hit me....

because my "jet" is not a jet, it's just a solid piece of brass.....it doesn't matter if there's a leak at the starter valve if there ISN'T one at the jet. If there's no physical way for air or fuel to even get to the starter valve......so my easiest solution was to make sure the jet itself was sealing.

The tiniest, thinnest layer of blue permatex gasket maker on the tip of the jet......problem solved.....and it's not a fix I can't undo when I have the ability to get everything sealing correctly....

Ran out of time today, but going to do the same for all 4 starter jets......then re-tune both carbs with the Gunson......then reload and see what other issues pop up.....that, and figure out how I'm going to mount the heat shielding....
Click to expand...
Have you checked to see that both carbs are using the same accelerator pump cams? If they were not originally purchased together as a set, there is a chance they could be different.
 
Steve....yep, need something more permanent.....but will take a variable out of the mix enough for testing....

Don.....they were purchased as a set. Its acting like the ball valve is not fully seating, and letting too much flow back into the bowl.
 
fiatfactory said:
I think you will find blue permatex isn't all that good at holding up to being exposed to straight fuel like that... it will probably deteriorate...but at least it let you nail the issue down for certain.

SteveC
Click to expand...

And once it gets free inside a carb it goes looking for main jets...
 
You must log in or register to reply here.
Back
Top