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Discussion Starter #1
Hi;
Has anyone measured the vacuum that is present at the fuel petcock during engine cranking on a 1990-1994 MC22 CBR250RR? What is the idling engine vacuum measured at the fuel petcock?
With the fuel tank off the bike and an auxiliary fuel bottle supplying fuel this bike starts cold or hot even after sitting for a few days. The auxiliary fuel bottle was hung higher than the installed height of the fuel tank on the bike; this created a higher static fuel pressure than achieved by the lower fuel tank of the bike. However with the tank and petcock on the bike supplying fuel; reliable easy starting is hit or miss. The petcock is new and operates properly to flow fuel when a vacuum pump is used to apply a vacuum signal to it.
Reliable starting seems to be a rather common issue with the MC22 bikes . Do the MC19 bikes with electric fuel pumps and non vacuum-operated petcocks start more reliably assuming the pump is working properly?
I would certainly appreciate anyone’s thoughts.
 

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Well, you need to separate vacuum to open petcock from petrol-flow. They are independent and unrelated.

1. vacuum-diaphragm opens petcock. I measure around 7-10 in.Hg when petcock is fully open. If you've got worn engine with leaky rings & valves, it may not generate enough vacuum at low-revs to open petcock. Kinda catch-22

2. petrol-flow is based upon pressure-differential between supply and destination. So yes, bottle held high will have more pressure to push petrol through. Same with pump, if you've got kink in lines or low-level in tank, pump will supply more petrol than gravity-fed.

Test those independently. Use vacuum-gauge to see how much vacuum you have at petcock-nipple when cranking (use rubber T). Then disconnect fuel-hose from carb-inlet and aim into measuring cup. Open-petcock with vacuum-pump to vacuum-level you measured.

1. How much petrol do you collect in 15-sec?
2. How much petrol do you collect with maximum-vacuum pumped to petcock diagram?
3. Is there difference?
 

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Discussion Starter #3
Thanks Dannoxyz for your insight. I measure 5 in mercury at the petcock during cranking and 5 to 7 in mercury with the engine running. I will measure fuel flow rate from the pet cock with 5 in mercury applied with a vacuum pump and compare it to the flow rate at 28 in mercury. On your bike what vacuum levels do you see during cranking and while running? I am going to check the 4 carb rubber mounting tubes for deterioration that might be causing a vacuum leak as well as the integrity of 2 rubber caps on the middle carbs and the integrity of the rubber hose supplying vacuum to the petcock.
 

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I typically see between 7-10 in.Hg when cranking and petcock opens enough to generate sufficient petrol flow. When idling, there's 10-15 in.Hg.

Yours is on low side. Could be due to worn/unseated rings. Also valve-clearance/timing and seal very important as well.

Does vacuum increase when engine is warmed up?
 

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Discussion Starter #5
Vacuum does not increase appreciably once warmed up. When fully warmed up the idling vacuum is 4 to 6 in Hg. At 8000 RPM and normal operating temperature the manifold vacuum is 6 to 7 in Hg. I will have to diagnose why the manifold vacuum is low starting with identifying any leaks.
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Hmm, I had leaky petcock. Rebuilt with K&L kit from Canada. Now my petcock opens fully between 2-3 in.Hg.



Also good to do flow-test of petcock. Looks like mine has max-flow at fully-open position 2-3 in.Hg. So it would seem that your engine, while having low vacuum, should still have enough to open petcock fully.
 
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