Engine won't run unless point gap set way too small

sreams

sreams

Well-Known Member
Messages
402
Reaction score
27
Location
Sacramento, CA
Update: Problem solved! See below.

I have a M30B35 with Weber DGEV carbs. It idles nicely and runs well up to about 4000 rpm (it stumbles above that)... but only if the point gap is set to about .007". Once I get much above that, the engine won't even try to start. I've got the gap set to .013" right now and the starter just spins the motor.

I can see that worn lobes could be a cause. Any other thoughts on what the problem might be?

-Scott
 
Last edited:
Dwell

not gap is what you seek. 38 degrees IIRC. Buy a cheap or expensive dwell or multimeter. That first and then report back if the rpms aren't where they need to be.
 
sreams said:
I have a M30B35 with Weber DGEV carbs. It idles nicely and runs well up to about 4000 rpm (it stumbles above that)... but only if the point gap is set to about .007". Once I get much above that, the engine won't even try to start. I've got the gap set to .013" right now and the starter just spins the motor.

I can see that worn lobes could be a cause. Any other thoughts on what the problem might be?

-Scott
Click to expand...

61P's advice is always sound. He notes dwell angle is important, and of course it is affected by the point gap. Typically reduced gap increases the dwell and increased gap decreases the dwell. As an adjunct to his suggestion, it probably bears noting that changing the gap/dwell can affect ignition timing. This is ordinarily why you set or at least check the ignition timing after setting the points. Have you checked or reset your timing? Just a thought.

11004004_z.jpg
 
I've been checking timing after any point gap changes.

I understand that dwell is everything... but wouldn't the fact that I can't even get spark at anywhere near the specified gap (.016") indicate that lobes are significantly worn? If a dwell of 38 is achieved with a gap of, say, .006"... might it be time to think about a new distributor?

-Scott
 
sreams said:
I've been checking timing after any point gap changes.
I understand that dwell is everything... but wouldn't the fact that I can't even get spark at anywhere near the specified gap (.016") indicate that lobes are significantly worn? If a dwell of 38 is achieved with a gap of, say, .006"... might it be time to think about a new distributor?
Click to expand...

Did not mean to be preachy about using a timing light. Sometimes, it is easy to overlook the obvious.

If the lobes are worn, you should be able to identify it fairly easily by examining it. Unless the distributor shaft is oscillating up and down (like a spindle sander) you would likely see grooves or worn areas in the distributor cam lobes made by the point's rubbing block. (Not unlike looking for obvious damage to the lobe on a camshaft where it contacts a valve or rocker -below)

Sorry if I missed it, but if your distributor has a vacuum advance (or even retard) function, the "breaker plate" upon which the points are mounted - rotates. It can get sloppy (laterally and longitudinally) over the years - enough so that setting the points becomes an exercise in futility. The same can be said for other wear areas in the distributor, if it results in the shaft upon which the cam is fixed having too much lateral movement. Although not essential, a dwell meter should tell you right away if the dwell is consistent/steady and the points are stable. Along those lines, are you able to get a steady timing light marks?

Secondarily, you could have issues with voltage to the coil, or has been suggested, the coil itself.

hth

WornCam01.jpg
before.jpg




autobooks_manual_051.jpg
 
Okay... so I picked up a multimeter that measures dwell today. With the point gap at about .009", dwell was 28 degrees. I was able to get to 36 degrees with the gap set to .007".

Unfortunately, the problem remains. I pulled my Flamethrower coil out of my Volvo 1800S in order to rule that out. No change with that either.

There does appear to be noticeable wear on the lobes, which would explain the tiny point gap required to get a reasonable dwell angle. I was actually planning on installing Pertronix at some point anyway... so I went ahead and ordered the 1867A kit today. Obviously, lobe wear will have no measurable effect on the performance of the Pertronix setup. I'll just need to make sure the magnet is seated securely.

-Scott
 
Problem solved!

My Pertronix kit came today. I installed it, and after resetting the timing, the idle was noticably smoother. I was able to drop idle speed a bit as a result. However, when accellerating at full throttle, the problem with stumbling actually became worse. It started now at 3000rpm as opposed to 4000rpm.

I pulled the Pertronix Flamethrower coil out of my '66 Volvo 1800S, and suddenly the car ran perfectly. So I picked up a new standard coil and installed it. Definitely a huge improvement over the obviously faulty older coil, but there was still some stumbling above 4000rpm. This got me thinking about plugs and plug gaps. The Flamethrower coils work well with larger gaps, but standard coils do not. I pulled the plugs (these were plugs that had been in this M30B35 motor from before I removed all of the fuel injection), and they were all gapped to about .035". Well more than the .024" specified for a carbureted M30 engine. I had new plugs among my parts. I gapped them to .026", and now the car revs freely to the redline.

From my experience, it looks like the plug gap, the faulty coil, and the worn distributor cam lobes were all contributing to the problem. So good to have a great running engine now.
 
Last edited:
Speaking of plugs, I tried a set of Bosch +4 plugs in the E3 (M30B34 w/ DGES) and it fell flat on its face. Could barely rev past 2500 in normal driving conditions, and would basically stumble and miss upon any application of throttle. Went back to the plugs that were in it (NGK BP5ES) and it ran just fine. I am wondering if the +4 runs much hotter such that it effectively leaned out the motor.

Any thoughts as to why the +4 plugs would cause such a significant difference? Would I need to put larger main jets in to compensate?
 
afeustel said:
Speaking of plugs, I tried a set of Bosch +4 plugs in the E3 (M30B34 w/ DEGS) and it fell flat on its face. Could barely rev past 2500 in normal driving conditions, and would basically stumble and miss upon any application of throttle. Went back to the plugs that were in it (NGK BP5ES) and it ran just fine. I am wondering if the +4 runs much hotter such that it effectively leaned out the motor.

Any thoughts as to why the +4 plugs would cause such a significant difference? Would I need to put larger main jets in to compensate?
Click to expand...


So far as I am aware most spark plugs are available in different heat ranges. You neglected to specify exactly which plugs you were using - other than the +4 style, so it is possible that the issue you encountered had more to do with the heat range rather than the plug style. To some degree, this topic has been discussed here: http://www.e9coupe.com/forum/showthread.php?t=4643

Plug3.jpg


Opinions vary, but conventional wisdom seems to strongly favor the more-traditional style single electrode plug, e.g., NGK BP5ES, BP6ES or Bosch equivalents (Used to be W7DC and W8DC). A few swear by the Bosch Silver plugs. This topic, along with plug gaps and ignition systems, has been discussed several times in the archives, too.

In short, a lean condition at higher rev range might benefit from changing mains, http://www.scuderiatopolino.com/TuningofWebercarburetorsrev2.pdf . Add to the symptoms, stumbling, missing or surging, and you might also consider the general health of your ignition system, as well as ignition timing and plug gaps.

hth
ps: Assume you meant Weber "DGES" 38/38.
Not that you really have any trouble, but here is a guide to that sort of thing: http://www.webercarbsdirect.com/v/vspfiles/images/Weber Trouble Shooting Guide.pdf
 
You must log in or register to reply here.
Back
Top