Forum

[Gerald]

Hi Stephen,
I would love to know what carb settings and jets you have ended up with as I am doing an identical tune-up on my 1993 350cc Bullet. I was planning to us a 500cc Micarb as I already have one in new condition and funds are limited! I had already anticipated that the whole range of the carburation would have to be considerably enriched.
Do let us know how you get on.
Thanks, Gerald

[simon]

Ive got an 1 1/8" 389 Monobloc on my 350 with an 8:1 piston and i found that it would pink after a fast run. I went for a cooler plug although I started with a B6HS and went to a B7HS so not as cool as yours. It largely cured the pinking although I also fitted an aluminium barrel so that probably helped too.

[PO51UHD]

Gerald

Glad I'm not the only fool on this mission...!

I tried a succession of richer jets on my Mikcarb 28, finally ending up with the following setup:

- standard (for a 500) pilot and slide

- needle jet P4 with standard needle

- main jet 130

- MrH's bellmouth-with-gauze on the inlet

- Indian shorty silencer that's reasonably open without being ridiculously loud!

I'm not sure if I'm there yet with the main jet as I haven't done any full-throttle trials to date - still running-in a rebuilt engine. As I said in the original post, BW ported the standard Indian head for me and I have MrH's alloy barrel and 8:1 piston. I could go further with compression ratio, but on a limited budget and with no desire for an all-out racer I felt this would be OK. And it goes ok too!

I also have Boyer mk4 ignition and it starts first kick.

The one problem you may have is getting a suitable manifold. The OEM 500 one has the wrong stud centres. I bodged one of these with a round file and Stanley knife to get it running, but it didn't look pretty... Finally bought one for a Mikuni with the right centres from Motocarb in Liverpool. Have a look at http://www.motocarb.com.

Good luck!

Stephen

[jefrs]

NGK spark plugs https://www.ngksparkplugs.com/about-ngk ... lug-basics - a lower number indicates a hotter plug.



A spark plug is intended to "burn" at some arbitrary temperature to keep itself clean. This does not affect the combustion temperature (unless it is so hot it turns into a glow-plug). The old pictures of tobacco-brown plugs being correct no longer holds with unleaded petrol. So it becomes trial and error, if the plug keeps fouling use a hotter plug, if it looks too white use a cooler one. Imo it should look dirty grey not ash-white, with some hard carbon on the metal.



Running on indicates it is running very weak as well as the plug being too hot.

[Gerald]

Thanks Stephen, that's a big help!
I found that our good host's 8 to 1 piston only gave 7 to 1 on my 350 but that's still an improvement on the original compression, which I think is more like 6 to 1... Some years ago I bought a Meteor Minor sports piston which has a much higher dome, so using this I am hoping to get something like 8.5 to 1. I will also be fitting some sports camshafts as well as a five speed gearbox which I was lucky enough to pick up secondhand so I am hoping for a much livelier machine.
I think your 130 main jet is a good starting point; of course these machines were set up lean when sold so it will be very interesting to hear how you get on.
Thanks for the tip re the manifold, I was wondering how to get round that.
Well done!
Gerald

[Tim NZ]

A 130 main jet for an otherwise mostly stock 350 will be on the too-rich side...


H High comp piston is more likely to be UNDER 8-1 (measure the combustion chamber to know, not presume...)




Dennis C:
A head that has been 'ported' and breathes freer needs a cooler plug for precisely the same reason why a 'race' motor runs a cooler plug: Increased performance potential; it runs hotter at higher rpm. (Power ~ Heat)

eg: 'Strangled' Trihard 750cc T140 = N5 Freer breathing 650cc T120 = N3 (B6ES - B8ES)






Scared of overheating/seizing from running too lean?


With an Exhaust gas temp sensor you will see that Leaning-off a mixture once max power has been reached, temps drop, and the onset of misfire fire potential commences. (Too-lean will ALWAYS misfire)


The reason for 'lean seizure' is more to do with an engines running clearances and whether or not it is able to absorb/dissipate the additional heat? NOT because it is 'too-lean' as such.

[Dennis C]

Yes Tim I understand that, I started race tuning Velocette's around 1962,(now retired) but a plug getting that hot on a bike which is still running in? or just finished, needs in my opinion further investigation.

[jefrs]

Maybe something maybe not. The motor is air-cooled and running them stationary can have them over-heating in 10 minutes or so. I picked up a little infrared thermometer in Maplins for about £10, the sort you just point at the thing to be measured. Useful tool tells you when you are cooking the motor in the workshop.

[Presto]

There is nothing arbitrary about the heat at which a plug operates. The ideal operating temperature of the central electrode is between 400° and 800°c. If the plug tip temperature rises too high, say above 950°c, the tip becomes incandescent. The glowing plug may then ignite the mixture before the spark occurs. This effect is referred to as ‘pre-ignition’. Where pre-ignition occurs the fuel charge does not burn progressively and the combination of the extreme heat, typical of pre-ignition, and the shock wave from the uncontrolled explosion of the fuel charge may result in serve damage to the piston crown. Plugs are designed to run at high temperatures. Failure to reach these temperatures often results in the plug ‘oiling-up’. A plug tip temperature below the optimum, below 400°c, will not allow the plug to self-clean, to burn off carbon and fuel deposits. Allowing an engine to idle for long periods may also result in plug oiling, for the level of heat generated, though comparatively high, will not be high in relation to the plug’s ideal working temperature.

[jefrs]

Arbitrary means undecided and in this sense undetermined, some temperature at which the plug cleans itself, we don't really need to know the numbers. For the spark plug it is a rather broad temperature range. Steel softens around 1000°C and yes it would be glowing then, far too hot. Aluminium melts at 660°C, we don't want that. Can't remember the numbers but my IR thermometer shows the outside of the head on and by the plug cooks at relatively low temperature; liquid cooled motors run around 95-98°C. The plug electrodes do need to "burn" at a temperature sufficient to keep the plug clean (electric spark ionisation is a form of "burning"), this is managed by the amount of porcelain around the electrode to insulate or conduct heat away (same thing really). The electrode tip runs at a much higher temperature than the head, it is intended to do that; the fuel combustion heats things up, the metalwork cools it down, but the spark heats the electrodes up further. It requires something else besides, in addition to, having a plug that is too hot to cause pre-ignition; usually but not exclusively mixture; other possible factors lack of cooling/overheating, friction heat/lack of lubrication, timing/valves - most of which can be quickly excluded.