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Standard Carb Settings ?

Submitted by mike_haworth on Thu, 25/10/2018
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Heavy Twins

My bike is a 1959 Dominator 99. It has had a major rebuild at some time and appears, in most respects, to be a very fine machine. I have a concern though with the carburation. Given the standard of work on the rest of the machine I am reluctant to just assume the builder got the carburettor set up wrong. However, I've already discovered the main jet size is 220 and I believe it should be 250. This makes me wonder if anything else has been changed.

The bike is not easy to start when cold and very difficult when hot. Should I change back to all ex factory settings to see if this improves matters ? Does the addition of electronic ignition (which it has) potentially warrant a change in carburettor settings/sizes ? Sorry if this is a stupid question but I fairly new to thsi stuff.

Mike

Submitted by philip_hannam on Thu, 25/10/2018

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Your jetting will depend on the model of carb that your bike is fitted with.

It ought to have a standard Monobloc 376 with a 250 main jet.

If it has a Concentric, then a 220 main jet would be near enough for the likes of a 928 carb.

Generally, the 99 engine was/is a very easy starter. especially with a Monobloc carb fitted. Perhaps you have a slide with the wrong cutaway or the needle position is off.

Checkout the attachment for the 376 Monobloc settings.

Attachments carb-settings-jpg

Submitted by john_holmes on Fri, 26/10/2018

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Concentric mains are typically 90% of the Monobloc size, so 250 becomes 225 so 220 or 230. Blocked pilot jet is the main cause of difficult starting, still out the pilot Bush with a 16 thou drill mounted in a red WD40 tube twisted between fingers. The pilot jet is too small to be cleaned out with air or carb cleaner alone.

Submitted by mike_haworth on Fri, 26/10/2018

Permalink

Previously philip_hannam wrote:

Your jetting will depend on the model of carb that your bike is fitted with.

It ought to have a standard Monobloc 376 with a 250 main jet.

If it has a Concentric, then a 220 main jet would be near enough for the likes of a 928 carb.

Generally, the 99 engine was/is a very easy starter. especially with a Monobloc carb fitted. Perhaps you have a slide with the wrong cutaway or the needle position is off.

Checkout the attachment for the 376 Monobloc settings.

Thanks for response and attachment. It is a Monobloc 376. I will reset everything to standard and go from there.

Submitted by mike_haworth on Fri, 26/10/2018

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Previously john_holmes wrote:

Concentric mains are typically 90% of the Monobloc size, so 250 becomes 225 so 220 or 230. Blocked pilot jet is the main cause of difficult starting, still out the pilot Bush with a 16 thou drill mounted in a red WD40 tube twisted between fingers. The pilot jet is too small to be cleaned out with air or carb cleaner alone.

John, Thanks for response. Will clean out as you suggest.

Mike

Submitted by john_holmes on Fri, 26/10/2018

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The Pilot Jet in the Monobloc is normally a 25, same size as the bush in the concentric, so the same drill will work, just much easier to get to.

Submitted by mike_haworth on Sun, 28/10/2018

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Previously john_holmes wrote:

The Pilot Jet in the Monobloc is normally a 25, same size as the bush in the concentric, so the same drill will work, just much easier to get to.

John,

Spot on ! The Pilot Jet was blocked. I had already stripped and cleaned the carb but clearly the solvent and high pressure air were insufficient for clearing this jet. I didn't have a drill that small but managed with a .016" piece of wire. Now it starts easily.

Many thanks,

Mike

Submitted by john_holmes on Mon, 29/10/2018

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Part of my spring refresh is the 16 thou drill, the ethanol in the fuel seems to make it an annual requirement, wire is fine on a monobloc, drill is best on a concentric or the wire pushes the dirt inside for it to reblock the jet.

 


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