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  #1  
Old 3 Weeks Ago
owlsplace owlsplace is offline
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Default Mikuni VM-38 bogs in cool weather

Finally got a couple rides in this year after putting on a lot of new parts: genset, rear wheel, reverted to stock sprocket ratio, shocks, swingarm bearings, exhaust system.
Last ride in cool temps (60s) the engine starting bogging a lot and the only way it would run was to leave the enricher on. It generally runs too rich as it is. The cams may not be stock on this engine.
I put the SUDCO Mikuni kit on last year. It did bog a bit on occasion but nothing like it does currently.
Any ideas? Float set too low? Plugged passage way?
I'm thinking the engine should operate down to freezing temps without bogging issues, especially after riding 20-miles. I need it to operate reliably into the 50-degree range.

Roger

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Old 3 Weeks Ago
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Put it back to the VM38-9 spec and see what that does.
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Quote:
Originally Posted by ryder rick View Post
Put it back to the VM38-9 spec and see what that does.
I'm presuming you mean this spec that IronMick posted previously. See photo:

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What spark plugs are you using now?
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Originally Posted by doodah man View Post
What spark plugs are you using now?
I think I put the 206 plug in last year with straight wires.

Drag Specialties Performance Spark Plugs - 2103-0206
Unique look plugs feature bold red logo/script and a black ceramic body
Available in the OEM heat ranges
Insulator is made of 95% pure alumina, features superior insulation and strong resistance to heat shock
Multi-rib insulator design eliminates flashover
Nickel-plated housing prevents oxidation and corrosion
Multi-layer gaskets and heat crimping process ensure a leak-free, gas-tight fit
Copper core electrode prevents oxidation and corrosion, has good electrical and heat conductivity and creates long spark plug service life
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I had the same symptom on my bike and it was resolved by replacing the ignition coil.

In cold weather would only run with the enrichment. I kept fouling plugs And would replace them about once a month with plugs 2 numbers hotter than stock. Finaly, bought a new coil and problem solved. Goes to show that just because doing something to the carb (using the enrichener) keeps it running, does not necesssrilly mean the carb is the source of the problem.
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Quote:
Originally Posted by billeuze View Post
I had the same symptom on my bike and it was resolved by replacing the ignition coil.

In cold weather would only run with the enrichment. I kept fouling plugs And would replace them about once a month with plugs 2 numbers hotter than stock. Finaly, bought a new coil and problem solved. Goes to show that just because doing something to the carb (using the enrichener) keeps it running, does not necesssrilly mean the carb is the source of the problem.
I can see that happening but it is probably the last thing you would get to. I will do this.
I did swap out the main jet with the two spares that came with the kit without much of any difference.
Didn't notice any float bowl contamination.
Reminds me of the starting problem last year that drove me crazy. It turned out to be that nefarious loose battery connection that everyone tells you to check but you don't because it is in the back and you know you tightened it... sort of, maybe!
Over hill, over dale...

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Those are non-resistor plugs, according to the Dennis Kirk site: https://www.denniskirk.com/drag-spec...rd/2001033.sku

What does "straight wires" mean? Non-resistor? If so, you want resistor wires and plugs with electronic ignition.
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Quote:
Originally Posted by GA_Ironhead View Post
Those are non-resistor plugs, according to the Dennis Kirk site: https://www.denniskirk.com/drag-spec...rd/2001033.sku

What does "straight wires" mean? Non-resistor? If so, you want resistor wires and plugs with electronic ignition.
Should have mentioned this engine was reverted to points.
The appropriate coil is 5 ohms.
Wires are:
Accel Custom Stainless/Copper 8.8mm Yellow Spark Plug Wire Set - 172074
8.8mm size is rugged enough for any application
Yellow or black heavy-duty silicone wire sets
Fits terminal nut spark plug
Included Items:
(1) 20 1/2" plug wire with (1) Straight coil boot and (1) 90 degree plug boot
(1) 8" plug wire with (1) Straight coil boot and (1) 90 degree plug boot
Specifications
ColorYellowDiameter8.8 mmLength20 in. / 7 in.Mfg #:172074Part #:20341Size8.8 mmTypeSpark Plug Wire - CustomUnitsPairWeight0.43 lbs
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Last edited by owlsplace; 3 Weeks Ago at 14:25..
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Default

This smells too much like an ignition problem. And (like thousands before you) you've found you can somewhat compensate for a f^#ked up ignition system by playing with the choke. In your case, enricher. Thus it has you thinking it's a carb problem. And maybe it is. But I doubt it. You know your ride. You're there. I'm not.

Often times there's more than 1 thing wrong. So where does one start ? Here's what I'd suggest:
  • stop messing with the carb
  • verify your ignition system is functioning correctly. If necessary, check all major components. At a minimum:
    • check / set your point gap. On both lobes. I suspect your point gap has closed up, but ??
    • when was the advance unit last serviced & lubricated ?
    • set your advance with a timing light
    • verify your advance is set at 40° BTDC.
  • test your ride again
  • what's your motor telling you now ?
Troubleshooting the next problem should go a lot easier knowing the ignition is properly set.
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