Need help , Engine misses / stumbles

[Butchrecord]

Hi Guys,  I am new to this site and it's my 1st post.

I have a 49 bus coupe that I am working on getting it road worthy. Here is the story, motor is stock with a new carb ,  generator , voltage regulator , coil ,plugs , wires , Pertronics Ignitor ignition , electric fuel pump mounted by the tank (Pressure is 7.5 lbs. ), new exaust pipes with a Thrush Cherry Bomb . Timing is at TDC , Carb is set with a vacuum gauge witch reads a pretty steady 19 in. at idle. Engine accelerates strong but as soon as I let of the gas it stumbles at cruise. I have tried all I can think of . I thought fuel pressure might be to high, (manual says 4.5 to 5.5 lbs.. Also my distributor is maxed out against the stops to get TDC, no room for advance. HELP, I'm at my WALL !

Thanks in advance , Butch Record

 

[sser2]

11 minutes ago, Butchrecord said:

Also my distributor is maxed out against the stops to get TDC, no room for advance.

 

That means something is wrong with the distributor. Take it apart and check condition of centrifugal governor. Weights should move freely on their pivots, pivots properly lubricated, and weight springs in good condition. Check vacuum pot for tightness, complete return, and operation under vacuum. Replace shaft bushings if shaft free play is beyond specs. With distributor in place, spark advance at different engine speeds and different vacuum settings should be as described in shop manual.

[The Oil Soup]

There are two adjustments on the dizzy, one where it bolts to the block and another on the underside of the dizzy to confound twenty first century mechanics. Pics of the coupe?

[nolan in blaine]

I just dealt with the same thing for months on my 37. Drove me nuts. Turned out to be distributor bushings. Pull the cap and see how much play there is in the shaft. Doesnt take much to make them miss

 

[Butchrecord]

Thanks for the info , the distributor was repaired about 1 year ago , I did not feel hardly any play sideways on the shaft. I am pretty sure that I am 1 tooth off on the distributor as when I got it the engine was moved with the distributor out and the oil pump out of index. when I reinstalled them the dist. was almost to the stop , but my timing light showed TDC so I left as is. I just pulled the plugs with about 100 miles on them and they look great . My compression is 115 lbs. average. Can my electric fuel pump be forcing gas by the needle when I am just cruising and not using much gas? Or what ???  This is really baffling to me as the motor runs so much better with some new parts, but just NOT quite there. as for pic's here are a couple. Thanks , Butch

[Tom Skinner]

I thought (If a 250.6 cu. in. Engine) Timing should be  2 Degrees B.T.D.C.

Advanced somewhat for 87 Octane - about a 6 Degrees B.T.D.C. would be about right.

 

[Don Coatney]

Not sure what you are saying about the distributor being one tooth off. No teeth on the distributor just a tang so it only can be installed 2 ways. As mentioned there are two slots on the distributor to adjust timing. It is possible your spark plug wires are one hole off in the distributor cap and that may be why you cannot correctly adjust the timing.

 

 

[Adam H P15 D30]

Dump the Pertronix and retest...

and reduce the fuel pressure

Edited September 16, 2018 by Adam H P15 D30

[Plymouthy Adams]

look at your vacuum advance....at WOT it is not available...at cruise or decel it is high and pulling the points plate...check to see if when it is pulling your point plate that it is not changing the gap in the pertronix module..you could be just a tad of an angle on the module mount causing it to change the gap....have seen this before.

Edited September 16, 2018 by Plymouthy Adams

[Dartgame]

Be sure that the  fuel pressure is set to factory spec, maximum. If its 5 or 4.5 psi etc. You could be pushing more fuel past the needle seat, install a permanent mini fuel pressure gauge near the carb.

 

I’d check the dist. For free movement of the mechanical advance weights, and that the  vacuum advance is working and holds vacuum with NO leakage (very important). And as others have noted the plate has another set of slots to move around as well. I set mine at about this 7 degrees BTDC and it runs well.

[nolan in blaine]

That was another problem i found when i had my distributor out. a broke weight spring on one side and an tiny spring on the other that looked like it came from a pen.

[Dartgame]

Advance springs are important items. Common speed trick is to decrease the spring tension on the weights for a faster timing curve. I talked with George Asche about using lighter advance springs in a flat head dizzy, and he did not think it was something that helps the performance of these engines. I might try it someday once my car is streetable. Maybe next year.

[sser2]

22 hours ago, nolan in blaine said:

 tiny spring on the other that looked like it came from a pen.

The two springs are unequal. Weaker spring engages first to provide steeper advance at lower engine speeds under condition of lean mixture. Stronger spring engages afterwards for shallower advance at higher engine speeds. Lean mixture burns slower than rich one and requires more advance.  On the centrifugal advance graph, red is initial (fast) advance at lower speeds, blue is advance at higher speeds (slow), and green (no further advance) is when the mechanism maxes out.

[nolan in blaine]

I see.... That makes sense. might be something to play around with.

[sser2]

13 minutes ago, nolan in blaine said:

 might be something to play around with.

That's what they used to do tuning up performance engines. I remember seeing Lucas catalog offering a variety of weights and springs for the same model of distributor. They claimed that any kind of advance characteristic can be created using a combination of weights and springs.

[Fernando Mendes]

[Butchrecord]

Well ,I'm pretty sure I found a big portion of my issues. I pulled the distributor and vacuum unit , I can breath in and out with no restrictions.  I have been searching for a new vac unit but keep running into walls and cannot find one for the IGS distributor. Anyone have a leed for this part?

 

And THANKS to all that offered advice.  It was a real help !

 

Butch

[Merle Coggins]

Kanter has vac advance units for Chrysler engines. Pricy, but good. 

[kencombs]

Maybe time to consider the slant 6 electronic conversion.  Parts readily available for the advance and upgrade to electronic too boot.

[Adam H P15 D30]

2 hours ago, kencombs said:

Maybe time to consider the slant 6 electronic conversion.  Parts readily available for the advance and upgrade to electronic too boot.

Agreed. Did the slant 6 conversion and never looked back. Took about an hour or two. 

 

Adam

[Butchrecord]

What is the slant 6 conversion ? I already have the Pertronix and it works great . My brother in law did the distributor repair for me when i was in the hospital. What i thought was a new vac advance unit turned out to be spray painted grey ! I am trying to find one to buy now and not have to wait for my core to be rebuilt. I will put new bushings in and check the springs.

[maok]

Try it with the vacuum advance dis-connected. Block the hose coming from the manifold.

[Adam H P15 D30]

3 hours ago, Butchrecord said:

What is the slant 6 conversion ? I already have the Pertronix and it works great . My brother in law did the distributor repair for me when i was in the hospital. What i thought was a new vac advance unit turned out to be spray painted grey ! I am trying to find one to buy now and not have to wait for my core to be rebuilt. I will put new bushings in and check the springs.

Thrashingcows did an excellent write up here

 

I used a GM HEI module from a late 70’s car instead of the troublesome Mopar box and resistors.  Parts are super reliable and available at any parts store if you are traveling, DWELL control is much better with the HEI module than the Mopar one. Better than being stuck with a broken Pertronix module waiting for a replacement. 

 

Adam

 

Edited September 21, 2018 by Adam H P15 D30