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CR750 Running (or not running!) issues

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philward:
I’m still trying to set up the CR750 to run correctly and was making reasonable progress. However, now the bike struggles to start, occasionally fires and runs (10 seconds) really rough before cutting out and won’t start again.

Reminder of initial set up – 836 Cruzinimage standard compression kit, standard carbs/jetting, foam pod filters, Boyer Micro Ignition/power coils, Moonpie race exhaust with K0 baffles (also, race alternator which has light weight rotor and works with starter motor clutch removed). Carbs bench set up (carb piston cut out height)

Here’s what I have done so far:-

On above set up and first start, started 6th kick and run as per video below. Fuel starvation in mid-range when riding.

https://www.youtube.com/watch?v=YmL8YYWeUMo

Put 135 mains in and raised needle to 2nd from bottom clip (2nd richest) – cured fuel starvation issue in mid range (not revved in higher range yet) – occasional back fire through carbs on start up and plugs black when run for a time at low revs (on a run were I was able to keep revs in mid range, they aren’t a bad colour) .

Rev counter sorted by Smiths so timed with strobe – no difference

Checked tappits and Balanced carbs on vac gauges using carb top adjusters (not air screw adjustment as in a Honda workshop manual I have?) – little change (obviously engine well warmed up at this time) – no change to above. At this point bike was running acceptably well except for the occasional back fire through carbs on start up and plugs black.

At this stage was when the bike struggled to start from cold and now only occasionally fires and runs really rough before cutting out and won’t start again.

Noticed that 2 of foam pods had holes in them and thought this might be a result of the back fires through carbs and bits might have ended up in carbs so stripped, blew all jets out and checked float heights. Repaired foam pods with some filter foam I had (will be changing to better smooth flow pods a bit later).

No change

Re-checked static timing – OK. Massive spark at all plugs but all plugs wet (when trying unsuccessfully start).

Can’t see plugs not sparking under compression suddenly?

Your thoughts and advice appreciated
Phil

Bryanj:
Phil, try a new set of plugs, sounds like they might be failing under compression mate.

philward:
That's my natural thought in the circumstances Bryan but thought why would all 4 fail at once! I have DR8ES and NGK Resister caps. Thinking whether double resister (plugs and caps would have any bearing on the running (or failure) when used with Boyer system?
Thinking of going for D8ES
Phil

Laverda Dave:
Phil, I understand you either have a resister cap or plug but never both? I would try the D8ES plugs as you mentioned and I think your problem will be cured.
Nice video and awesome looking (and sounding) replica.
Dave

philward:

--- Quote from: Laverda120 on November 24, 2018, 11:09:36 AM ---Phil, I understand you either have a resister cap or plug but never both? I would try the D8ES plugs as you mentioned and I think your problem will be cured.
Nice video and awesome looking (and sounding) replica.
Dave

--- End quote ---
Thanks Dave, just doing a bit of research and found on the .net site that some people have had trouble with the 8 plug in cold weather. Just thinking that this problem only occurred since the cold weather arrived, wondering whether a 7 rated plug Ward be better with this set up I have on the CR?
What do you think?

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