Early S Motor ? - Suddenly Bucks & Snorts

[Kevin G.]

Literally overnight, my 2.2S motor (MFI intact) has become a bucking bronco between 3000 and 3400 rpms. I don't mean the normal contrary behavior these motors have at low rpms. I mean a real nasty crow-hopping, snorting and backfiring that is pretty violent. It behaves this way both cold and when warmed up, in 1st and 2nd gears - 3rd appeared to be ok, but I was only there briefly.

The wierd thing is that up to 3K, and once beyond 3400, the response is it's normal beautiful self.

Can anyone shed some light on this for me? I am totally clueless, although that state is pretty normal.

Thanks!

[Early_S_Man]

Kevin,

Is this buking and backfiring behavior under load and throttle-on ... or throttle backed off and on overrun?

[Kevin G.]

Hi, Warren

The aberrant behavior is throttle on only, working up to speed from a stop. Engine revs freely when not in gear, but that probably doesn't mean diddly. Normal backfiring when off throttle and clutch pedal on the floor (non-working rpm sensor).

Thanks for the help!

[Wayne 962]

You mean non-working throttle switch?

-Wayne

[NYSCAR]

Wierd that it's a repeatable symptom in the middle of the RPM range. If the car runs well at higher RPM's, I wouldn't think it to be a fuel problem. That leaves ignition - Check your ignition grounds, points, coil, cap, rotor, and your distributor timing/advance maybe?

Brad

[Kevin G.]

Wayne -

The thigamagig (technical term) with the rusted out transistors floating in the case. I tried replacing these and apparently failed.

I had heard it was an RPM transducer, speed sensor, et al. Is throttle switch the correct term?

[tmctguer]

my '73 with MFI started exhibiting similar behavior after going a long period between tune ups. i got out the "check, measure & adjust" book and replaced the plugs, set the timing & dwell, cleaned out the idle mixture screws, vacuum bores, then set up the part load adjustment and idle adjustment.

up until i adjusted part load, it continued to buck between 3K & 3.4K RPM. i'm running smoothly again through the entire power band.

i'd definitely start with the basics, assuming you have NOT done so already.

[Early_S_Man]

Kevin,

An ignition scope would be helpful, but even checking with a timing light on the (center) distributor lead ... might turn up an erratic spark at midrange rpm.

I suggest removing the distributor, cleaning, and lubricating the centrifugal advance mechanism; checking for bushing wear , and changing the points. The following thread should be helpful, though the advance curve data is not correct for your engine:

distributor lube...

The rattling Bosch box has been called Rpm Transducer, Speed Relay, and Speed Switch in various factory publications over the years! Speed Relay seems the most accurate description, as it IS sensitive to engine speed and it DOES contain a relay rather than a simple switch.

[tmctguer]

good point S Man.......gotta check the distributor advance, too.

[jkeyzer]

Points?

My points gave me fits before I ditched them.

[Kevin G.]

Thanks to you all for all the good points.

Further driving reveals that if I ease the load through the 3-3.4K range, there is little adverse behavior noted. That is with a very light foot. Any more normal application of throttle results in the bucking and backfiring. If the problem occurred through more of the rpm band, I would definitely suspect distributer timing, but the narrow band has got me perplexed.

I'll start looking at the distributer, since the change was rather sudden. Wierd though about the narrow range of the rpm band that is affected. Could be that the symptoms of a single plug or wire failure are hidden at higher rpms.

Regarding points, I'm running a Crane Fireball 700 optical ignition with an MSD 6AL if it makes any difference.

[NYSCAR]

A single plug wire failure won't make your car buck like that...I know I ran on 5 cylinders and you can hardly tell it other than the lower idle and loss of power. Perhaps your distributor cuve is way out of wack because one of your centrifugal advance weights are screwed up. I say looking at the distributor is a good idea. See if you can find someone local who'll swap one with ya for a test

Brad

[Wayne 962]

Check the mixture with partial load - that was (and still is) the only way to appropriately set the second mixture level on these cars...

This may require a dyno, or some creative use of your clutch...

-Wayne

[jkeyzer]

The mixture adjustments will not change overnight unless there is some catastrophic failure in the pump.

[A Quiet Boom]

Originally posted by Kevin G.
Regarding points, I'm running a Crane Fireball 700 optical ignition with an MSD 6AL if it makes any difference.

I had a Crane Optical on my 2.0, check that the vertical play in the distributer shaft doesn't allow the shaft ride up and let the trigger wheel bump the optical sensor. Also suspect is the 6AL itself as 3000rpms is the crossover point from multiple sparks to a single heavy spark. Is the MSD box mounted with the rubber vibration standoffs or just bolted to the body? Vibration kills these boxes and they do all sorts of weird things, you'll know it's dying for sure when it only works at very light throttle.

[Kevin G.]

Also suspect is the 6AL itself as 3000rpms is the crossover point from multiple sparks to a single heavy spark. Is the MSD box mounted with the rubber vibration standoffs or just bolted to the body? Vibration kills these boxes and they do all sorts of weird things, you'll know it's dying for sure when it only works at very light throttle.

I had forgotten about the crossover to single spark on the MSD unit - need to check those connections as well. But why would it work fine after 3400? BTW - the MSD unit is screwed to the firewall and uses the engine compartment insulation for vibration control .

Lot's of stuff to check on this weekend - thanks to all of you for your advice. I'll post findings as I come up with them.

[A Quiet Boom]

Kevin,

I've run a lot of MSD systems over the years as they are my prefered ignition so I've become quite familar with what happens when they fail. If these ignitions are vibration insulated they last a long time but if not they'll eventually "flake out" I wouldn't be surprised if the spark is erratic between 3000 and 3400 rpm, put a timing light on it and test. If it's erratic then there's a 90% chance the MSD is failing.