Ignition test variation

·         Rod Ward

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I have a 2200 engine, SN: 3702, currently @ 37hrs.
The rev drop on the RHS system is around 20 RPM when tested at 1700RPM and the engine runs smoothly.
However the LHS shows a drop of around 50 RPM and a slight roughness in running and RPM variation of +/- 30-40RPM around the 50 rpm drop. There has since new always been a greater drop on the LHS system however the rough running over the last 10-15 hours has me a bit concerned.

I have checked all connections, replaced all plugs, removed and checked the distributor caps and checked the rotors ; the LHS rotor showed a minor increase in roughness that was able to be buffed back to acceptable.
Also checked the continuity of of the plug leads, all show a resistance of around 10Kohm.

On the subsequent ignition test the rev drop was still around 50rpm but the rough running had decreased.
After a 40 minute test flight the ignition test had returned to previous performance with the rpm variation occuring and rough running apparent.

So what can I do now? Is the observed performance within normal limits?
I would be prepared to accept the 50rpm drop but for the rough running.

Your thoughts.
Cheers
Rod

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·         Doug Smith

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Hi Rod,
I'd say 50 RPM is around the upper limit of acceptable drop and while a slightly different engine "tone" is normal rough running isn't. There are a lot of different things that can cause the symptoms you've described. In no particular order...

1. Spark plug gaps. The gaps need to be set before the plugs are installed; they're often close out of the box but rarely correct and sometimes vary quite a lot. Carbon deposits can cause spark inconsistency but if the plugs are new that should not be causing your issue.

2. Plug leads. Check the metal connectors in the lead are making good contact and that they're installed correctly at both ends. In particular it's very easy for the leads to start to slip off the posts of the distributor so it's worth checking that they're all on as far as they can go.

3. Ignition coil gap. The gap between the coils and the flywheel magnets needs to be correct for the coils to fire properly.

4. You've already checked the health of the distributor caps and rotors and the little roughness you found there might be a hint - if the system is having to throw a bigger spark than normal somewhere (think plug gaps, loose electrical connectors in the HT lead caps etc) it can stress the rotors. Depending what you find elsewhere it might be worth swapping the distributor caps and see if the issue moves with the cap or says put. Just make sure all the leads go on the correct distributor terminals: they don't run well with those mixed up!

5. Have a look and see if the two ignition coils are the same type. At various times we've tended to deliberately put 2 different brand coils on each engine (to reduce the chance of a total engine failure in the event of a bad batch of coils) and some need a little more RPM than others to perform at their best - 1700RPM should be enough but 2000RPM is the normal test RPM.

Best Regards,
Doug.

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·         Rod Ward

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Thanks Doug

All now functioning PURRfectly.

Plugs regapped, HT lead connectors now make a good fit, coil air gap reset, swapping caps made no difference, and both coils are Honda type.

I reckon the only significant change was the plug gaps as they were installed straight out of the box with the NGK gap setting of around 26-27 thou. However the general attention and cleanup to the ingition system may well have contributed as well.

Cheers
Rod

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·         Doug Smith

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Excellent news 🙂