New , slight miss at very light throttle..... ethanol related ??

[Canadian1968]

When I got the charger out this year and took it for its first few drives I started to notice a slight miss fire at very light throttle. My first guess was some old gas in the tank. Then I noticed I had a cracked spark plug boot that was allowing spark to arc to the header. i thought for sure that was the problem. But those problems have now been eliminated. The miss is still there.

It is most noticeable when pulling away from a stop, at light throttle. The lighter, the throttle the more noticeable it is. The opposite, if I accell harder, its much less , or not there at all. Once at speed the car seems to cruise fine. Throttle response is still fine. The only problem seems to be the slow pull away from a dead stop.

When I have problems like that I always ask myself, " Ok what changed " in this case, there is nothing that I did. But there is something different with the gas! I am in Ontario Canada, I always get my gas from the same place. A Canadian tire around the corner from me , always had 91 octane with NO ethanol . This year I noticed a sticker on the pump " 10% Ethanol". A quick search shows that basically all pump gas in Ontario now has ethanol in it.

This is the only variable I can think of.

So my focus is on the carb. My wideband show lean blips in relation to the miss. The light throttle is pointing to the transition from idle into the main circuit. The miss is very subtle but still noticeable, so the change that needs to be made is going to be very minor I believe. The carb is an old school 3310 Holley. So it doesn't have near the tuning option (well without drilling ) as some of the new units.

What are some thoughts, suggestions???

 

[dadsbee]

Run 20 to 33% 100LL AV gas to boost the octane. Anything with ethanol has less BTU's in it.

 

[68 Sport Satellite]

Is your distributor vacuum advance hooked up, which applies extra timing at idle and part throttle? Couldn't a little too much timing from the vacuum advance timing cause this symptom? That, combined with the fuel?

 

[Geoff 2]

Check the plugs. If sooty replace them. Should be a 5 heat range, NGK.

 

[Billccm]

Check the carb nuts and make certain all vacuum lines are good.

 

[bandit67]

Ethanol added fuel is defiantly a lower performing fuel over non ethanol fuel, but to single it out as the cause of your stumble itself would be a stretch. It can cause rubber seals and gasket deterioration in the whole fuel/carb system if the components are not designed to handle the blend. Old gas station tanks had to have massive cleaning prior to switching over to E10 as the blend was so corrosive at cutting loose residue crud from years of storage of regular gas. It does the same when first admitted to our old fuel systems. JMHO of course.....

 

[Canadian1968]

I will do once over of the vacuum hoses .... the car does not have vacuum advance for the distributor.

I had pulled a couple of the plugs when fixing the boots. They look pretty good but I will pull them all on passenger side bank and see.

If everything still comes up fine. I guess will pull the carb and check it out

 

[Canadian1968]

All vacuum hose are good. They are actually all new.

Plugs are decent ... not ideal but the car isn't running ideal either.

Going to yank the carb see what the the bowls looks like and accel pump diaphragm

 

[AR67GTX]

A random miss at light throttle cruise or idle that didn’t seem to show up when accelerating was always a symptom of a bad plug wire(s) in my case. But yes, I have also experienced the excessive timing, constant miss/hiccup sensation at light throttle on the highway when all the centrifugal advance and manifold vacuum advance was pulled in and firing the cylinders too far in advance of TDC. I fixed it by building a stop on my vacuum advance can to limit it.

 

[Canadian1968]

well since my last post I have pulled the carb. It was more or less spotless inside. The accell pump shot seems find so I didn't pull that part. Reading up on ethanol fuel, I found out that E10 likes to burn at 14.0:1 as apposed to 14.7 for pure gasoline. I had my carb tuned to 14.7 for cruise conditions.

I bumped my main jets up one size (71-72) This puts me right at 14.0 at cruise/ light throttle . But that annyoing ,slight and inconsistent miss is still lingering.

I notice it iis apparent once the car has warmed up , so I would say their is a heat relation to the issue. After checking out the carb and ,adding a bit more fuel with no change , I don't think it is carb, related any more. It would have to be strictly pin pointed to the transition slot / circuit at this point ??


I will continue my search, going to pull the distributor cap, see if everything looks good in there.

 

[Geoff 2]

What cam? Idle vacuum? Manual or auto trans?

If you want help, you will need to provide a LOT more info.

 

[furious70]

I would have thought 1 jet size would be enough but I'd play with 2-3 just to see. Your wb is a great asset. Richen the idle a bit, just to see what happens. If nothing happens go back to timing and spark, retard it 2* then and see what happens. Sounds like either a lean surge or still something arcing in the spark.

 

[Canadian1968]

What cam? Idle vacuum? Manual or auto trans?

If you want help, you will need to provide a LOT more info.

This is a brand new problem , nothing has changed on the car. I had the car running extremely well with the current setup. Something is either failing / or I thought it might be the new Ethanol fuel that i have to run. The fuel was the only thing that changed with the setup , from when it was running great for the past 4 years to now this odd miss.

THe blip is enough to show up on my Wide band, but not enough to show any fluctuation in vacuum. As posted before I pulled all the plugs from the side that the wide band is wired to , and all plugs look more or less even nothing really stood out as one cylinder firing off compared to the rest.

Other than doing a resistance check on my plug wires, is there anything else I could do to see if one is bad ?

 

[Geoff 2]

Yes, provide more info as requested.

Nothing has changed...but you are asking for help because it is not running properly.

 

[Canadian1968]

comp cams XE275
Vacuum at idle - 12" ( In Drive )
Timing 18* - no vacuum advance, recurved distributor
Mopar Orange ECU
factory 727 with Trans-go Stage 2 shift kit, SMR 3000 Stall
holley 3310 - 750 Vacuum, rear metering block added for adjustable rear jets, choke removed and main body blended
Stock 906 heads with mild porting
Headers, 2 1/2 exhaust with X-pipe
NGK Xr5 Plugs
FIrecore plug wires.
Street Fire (msd ) Coil


Going to try advance and retard timing to see if there is any difference

 

[oldkimmer]

So what’s the verdict? Did u recheck all the intake and carb bolts for proper torque? Kim

 

[Canadian1968]

Well the carb was pulled, so I can check that off the list. I went threw and rest timing / idle mixture and idle speed. I pulled the distributor cap and did noticed some carbon tracing or arcing on rotor cap?

 

[Geoff 2]

- A new rotor would be a good idea. Sometimes they track through to the shaft, under the blade.
- Congrats on the 5 heat range plugs. Many foolishly go to a colder plug with set ups like yours.
- The off idle transition is the most difficult to 'tune'....& stumbles/flat spots are often the symptom when it is not right. The stumble occurs because the engine is not making enough HP at that load setting. Try advancing initial timing to 25-30*; reset idle & mixture screws & test drive. Retarding ign will only make it worse.

Read about the importance of ENOUGH timing at idle....

 

[Canadian1968]

Little update .

I have not grabbed a new distributor rotor yet. But I did check the manifold bolts the driver side did take about 1/4 turn.

The way I have one of my vacuum lines is off front manifold vacuum port of the carb. Goes back to the fire wall and over near the brake booster. I have a T fitting that splits the hose to factory vacuum canister and the other end goes to an indash vacuum gauge . I tried plugging the hose to cansiter first, with no change and then plugged the entire carb port . Also basically with no change.

I have not bumped my timing way up to the 25 or 30 as suggested becuase that just doesn't make sense to me. Why would my motor randomly need such an advanced in timing when it ran perfectly as is last year? When I have not changed anything on the motor...... my search continues......

 

[RemCharger]

A miss will generally formulate from a lean condition.
You can tell on new vehicles when they need plugs , right in the leanest of cruise situation