Update on 750 motor build

seattle##gs · 2026-07-03T15:40:05-0600

I recently built a stock 750 motor for a guy and it came back a few months later for fouling plugs. The only non stock item are the JS lightweight pistons and the JS exhaust inserts (stainless shoes). The plugs were completely fouled so I put new ones in.
Single Mikuni.
This is a great running motor. The lightweight pistons really smooth it out, worth every penny. The rods are stock Norton. The motor still has the stock Norton sound which I like. But much smoother. If there is room in your rebuild budget I would highly recommend them.

Still working on the fouling problem. I blackened a set of iridium plugs in a couple of miles. Next, new plugs and open the pilot up. It was 1 1/4 turns. New plugs arriving this aft.

Torontonian · 2026-07-03T17:42:31-0600

Iridiums don't much like cleaning of the tips with a brush . The Iridium coating is very thin and delicate .
What are the needles and jet sizes and slide cutaway on the Mik . ?

cliffa · 2026-07-03T18:25:49-0600

Sounds as though the enricher isn’t closing or the seal is damaged.

Schwany · 2026-07-03T18:51:54-0600

I have a set of those exhaust inserts anyone is welcome to for the cost of shipping. No return though. I tried them and have no use for them.

Free unsolicited thoughts:
Have you tried removing the exhaust inserts? Using them might require some retuning of the Mikuni. Needle position main and so on. The Mikuni might magically run better without the inserts in the exhaust ports. Simple to try removing them. Well, maybe not so much on a Commando. Easy for my old Norton.

I use iridium plugs without issues. Not using a single Mikuni though.

If the engine actually does run good (skeptical if the plugs are fouling I am), could be a ring seating issue. Engine may need less mature adult care and some good old youthful thrashing to get those rings seated and cleaner running plugs. Over and out. Have fun.

Richard Tool · 2026-07-03T19:19:56-0600

cliffa said:
Sounds as though the enricher isn’t closing or the seal is damaged.

seattle##gs · 2026-07-03T20:46:46-0600

Schwany said:
I have a set of those exhaust inserts anyone is welcome to for the cost of shipping. No return though. I tried them and have no use for them.

Free unsolicited thoughts:
Have you tried removing the exhaust inserts? Using them might require some retuning of the Mikuni. Needle position main and so on. The Mikuni might magically run better without the inserts in the exhaust ports. Simple to try removing them. Well, maybe not so much on a Commando. Easy for my old Norton.

I use iridium plugs without issues. Not using a single Mikuni though.

If the engine actually does run good (skeptical if the plugs are fouling I am), could be a ring seating issue. Engine may need less mature adult care and some good old youthful thrashing to get those rings seated and cleaner running plugs. Over and out. Have fun.

I will take those inserts. I believe you are correct about the thrashing.

seattle##gs · 2026-07-03T20:54:51-0600

Update. I got a supply of regular Denso plugs, cheapies but same heat range. The air screw was at 1 1/4. So I opened it a whisker past 1.5.
In my short test ride in town (not nearly good enough) both plugs turned white though the left was slightly darker. The bike has very few miles since the rebuild and the Mikuni is brand new. 240 main, #3 slide, 6DH3 needle groove 2 from the bottom, P-0 needle jet, 40 pilot...the usual. I think it needs about 50 good freeway miles and fine tuning the pilot circuit.

dave M · 2026-07-03T21:24:35-0600

Are the Iridium plugs 'R' rated? I note that most plugs, plug caps and HT cable are now resistance spec. I recently worked on a 1929 Rolls-Royce where this was the case and it was hard to start and didn't run too well until I replaced all three components, after which I could easily start it on the crank handle. If you are using points then no resistance is best, if it has electronic ignition, I believe one of the three elements should be resistance spec, but check that this is the case with your particular ignition.

Torontonian · 2026-07-03T21:49:22-0600

seattle##gs said:
Update. I got a supply of regular Denso plugs, cheapies but same heat range. The air screw was at 1 1/4. So I opened it a whisker past 1.5.
In my short test ride in town (not nearly good enough) both plugs turned white though the left was slightly darker. The bike has very few miles since the rebuild and the Mikuni is brand new. 240 main, #3 slide, 6DH3 needle groove 2 from the bottom, P-0 needle jet, 40 pilot...the usual. I think it needs about 50 good freeway miles and fine tuning the pilot circuit.

Both plugs should pull and read even /equal on the pull . On a Mikuni single .

seattle##gs · 2026-07-04T00:00:39-0600

They should but quite often they aren't exactly the same. In this case I suspect the rings on one side have not completely broken in. They need a good long run in period.

Schwany · 2026-07-04T08:34:31-0600

seattle##gs said:
I will take those inserts. I believe you are correct about the thrashing.

PM me and you can stop by and get them in NE Seattle. A Pre-Paid Priority mail box is more than your round trip fuel cost across town. Especially if you ride a motorcycle over here. Have a happy 4th of July.

milfordite · 2026-07-04T10:24:01-0600

I'd try the 35 pilot jet.

Update on 750 motor build

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