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1974 Chrysler 120HP Conversion to MSD Ignition

3

32Tudor

New member
I am struggling to make this conversion work. The original CDI system had a weak spark under load and would load up at idle and not perform under load. I decided to convert the whole thing to automotive ignition, replacing the CDI and coil with the following:
1. Flame Thrower 1.5ohm coil
2. MSD Street Fire 5520 multispark box
3. Carbon core wires (required by MSD box)
4. smaller gap plug. (currently using NGK BP8H-N-10 plugs), but I think they are the problem.

Symptoms: Starts up with a weak idle. A little stumble, but seems to stay running for a minute or so. It has great performance at and above 2000 rpm. Below that it stumbles and gets progressively worse, until it won't idle and we paddle back to the boat ramp.

It has a new power head so we have to run extra rich oil mixture (25:1), which is probably contributing to the problem.

The cap looks good and the rotor to cap tolerance has been fine tuned to within .005. The current plugs are gapped at .030 and were originally at .035 with the same symptoms.

Carbs are refreshed and clean and mixture screws have been fiddled with to achieve highest RPM at idle.

I am looking for a small gap surface gap plug to try as I feel the traditional gap plugs are collecting raw fuel in the valley around the electrode and "loading up". Anyone have any success with an automotive ignition (with or without MSD) and want to recommend a plug to use?
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3

32Tudor

New member
flyingscott said:
Go on YouTube there is a guy there with a great video how to do that.
Click to expand...

Thanks for the suggestion. I have watched every video I could find on this topic. I never found one that used an MSD box. In fact one of the comments on a conversion to automotive coil video, is where a guy named "MVEVERETT" said without an MSD box it will load up at idle and lose some RPM of the top end. I have reached out to MVEVERETT and several other people. I did get a response from one guy, but he did not have any additional info for me.
If you have a particular video in mind, please post a link. Maybe I missed it somehow?
 
D

dspurgeon817

New member
Hey 32Tudor. I have a very similar motor in which I have successfully converted using a 6v oil filled coil with internal resistor and automotive spark plugs. It completely eliminates the cdi box and has been very reliable for me...Maybe an option for you. Ive done a lot of research and decided against the msd on that motor for several reasons
 
3

32Tudor

New member
Update: Using a surface gap plug with relatively small gap (.033) has helped a lot. I now have a stable idle and top end works good. I still have a stumble in the mid range (2000-3500rpm). Carbs are tuned, total timing is at 32 degrees. Base idle timing is at 8 BTDC. I would like the hear the reasons you do not like MSD for this. So far I feel it is helping on the low RPM side and giving me a more complete burn at idle. Plugs are "NGK 2522 Standard Spark Plug - BUHX".
 
3

32Tudor

New member
Update: I changed to a NGK BUHX (.030 surface gap plug). That helped a lot with idle load-up, but the problem is not completely cured. I still have a stumble on acceleration. It seems to be related to too much timing on acceleration, compared to the amount of throttle. The ratio of throttle to timing is fixed using a common rod that pushed the distributor and the throttle cam at the same time. Any thoughts on delaying the timing, relative to the throttle?
 
T

Tapphouse

New member
dspurgeon817 said:
Hey 32Tudor. I have a very similar motor in which I have successfully converted using a 6v oil filled coil with internal resistor and automotive spark plugs. It completely eliminates the cdi box and has been very reliable for me...Maybe an option for you. Ive done a lot of research and decided against the msd on that motor for several reasons
Click to expand...
I have a Chrysler 75 hp and very interested in cHanging over the ignition. WhT parts do I need, and where do I get them? Thanks
 
3

32Tudor

New member
Tapphouse said:
I have a Chrysler 75 hp and very interested in cHanging over the ignition. WhT parts do I need, and where do I get them? Thanks
Click to expand...
Believe it or not I am still struggling with this thing and considering switching back to Chrysler OEM ignition.
current setup:
BUHX surface gap plugs
carbon core wires (MSD does not recommend solid core wires because of EMI)
,6ohm flame thrower coil
5520 MSD box
OEM points, no condenser
Total timing at 32 degrees
Initial timing at 8 or 10
symptoms:
Starts pretty well cold but needs a little throttle for about a minute before it will idle independently.
Does not take the load of gear without stalling. I installed a momenta switch to override the neutral safety switch, so it can be put in gear and started. This allows a little throttle to be applied for starting in gear.
Stumbling or intermittent miss at idle and under load.
Intermittent miss at higher RPMs.
 
AllatSea

AllatSea

New member
I recently acquired a surprisingly well taken care of Chrysler 135 (1354hg) It's never seen salt, and shows signs of regular maintenance(nickel antiseize), a rarity. It didn't run, but what do your expect for $200. I quickly discovered an ignition problem. The optical sensor and the magnapower unit are shot and the spark plug wires are dated 1976 and have cracks in them. Since I just happen to have an old Allison Xr-700 laying about, I figure I will whip up a support bracket for the Xr700 optical sensor and swap systems. The Xr is capable of handling 8 cylinders at 7000 rpm stabily. I suspect this will be adequate for my purposes. I am going to swap in a 3 ohm coil(3 ohms is required for the Xr to function it technically would use a 1.5/1.5 ballast, but that wastes a couple of volts that make the unit less than 100% reliable) good to 11000 rpm. The Coil is Pertronix 60130, and I am going to use MSD Superconductor wires with NR14S Brisk plugs. I still have to check the bearings on it and do some porting work on the transfer ports so it might be a while before it goes out to play. The Xr-700 is simple as a bag of hammers and quite rugged. It should be an educational experience.
 
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