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Fuel starvation?

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Fuel starvation?
Started at 06:41pm on the 3rd June, 2018 by PO51UHD
PO51UHD Subject: Fuel starvation?
Hello All
Went out for a VMCC Run today on Connie (‘59 Constellation with single Concentric carb). For various reasons I haven’t ridden her for 2 years, and I was really pleased with the way she was running, mainly on ‘B’ roads and below 50mph. On the way back, and getting closer to home I opened her up on an ‘A’ road. Got up to 60 no trouble, then she started stuttering and slowing, eventually stopping completely. After a couple of minutes and a good tickle on the carb, she started again and ran well for the last mile home. To me, it felt like I was running out of petrol even though I had a half tank full left.
I wonder if it was one of two causes:
• fuel tap too small (it takes ages to tickle when cold so I’ve suspected insufficient flow for a while). The tap is one of our Host’s cheapies and probably designed for a Bullet...
• insufficient venting through the filler cap (when I stop and put her on the stand I sometimes hear a hiss from that area)
Has anyone experienced this, and where to start on diagnosis and solution please?!
Posted: 06:41pm 3rd June, 2018
Davedup Subject: Fuel starvation?
Wasn't just the carb getting too hot and the fuel evaporating was it? Dave
Posted: 08:53pm 3rd June, 2018
Valsp Subject: Fuel starvation?
Does the petrol tap have a built in filter? If so have you checked whether it is getting blocked by debris in the tank. It can get sucked up against the gauze when running hard and move away at lower speeds due to the fuel sloshing in the tank
Posted: 09:51pm 3rd June, 2018
Jamesy Subject: Fuel starvation?
Have you cleaned the carb and checked the float function?.It has happened to me recently and the first thing I did was visually check petrol in the fuel line to the was ok.It hasn't reoccurred so I am hoping it's a one off.Carb. was cleaned and checked recently.
Posted: 10:19pm 3rd June, 2018
Revband Subject: Fuel starvation?
"it takes ages to tickle when cold", a partial fuel blockage, possibly due to the cheap tap?, ethanol can expand the seals in the cheap taps and they block the flow.
Posted: 09:03am 4th June, 2018
Adrian Subject: Fuel starvation?
Following up on Revband's point, I notice that our hosts list a big-bore 1/4" BSP fuel tap intended for methanol use. If the thing's methanol-resistant it should also be able to cope with ethanol in pump petrol.

Part No. PTAP2.

"• insufficient venting through the filler cap (when I stop and put her on the stand I sometimes hear a hiss from that area)"

That could be fuel vapour exiting the tank, particularly if you shake it up on a warm day, but you can still check the vent holes in case of partial blockage

Posted: 10:04am 4th June, 2018
Alan R Subject: Fuel starvation?
Good point and well-spotted Adrian !!.......Back in the day when I was a "Methanol Jockey" on grass-tracking we used two taps on the fuel tank and Mk.1 Concentric fuel bowl to ensure max. fuel flow..........I can't remember the figures at the moment but as a rough guide Methanol mixes with air approx. twice as much as petrol you need to ensure a really good flow rate......I also think that "Hiss" might well be an indication that the fuel tank isn't venting properly...
Posted: 05:16pm 4th June, 2018
PO51UHD Subject: Fuel starvation?
Thanks for all those suggestions everyone. Methinks a Methanol tap and attention to the cap vent are on the horizon!
Posted: 08:17am 5th June, 2018
PO51UHD Subject: Fuel starvation?
Update: I checked the flow-rate of the existing tap. It took 2hrs 20mins to drain 5litres from the tank. Doing simple maths based on assumed mpg, it’s no wonder I experienced fuel starvation when I gave her some beans. I’m quite surprised she didn’t falter under gentle running!
High-flow tap is winging its way to me from our hosts as we speak!
Posted: 08:11am 8th June, 2018
ogri Subject: Fuel starvation?
Hi Stephen, I poster a near identical query (with the same title) just a couple of weeks back. Interesting you are going for the hi-flo tap and hope this will resolve the problem. Please can you let me know. Cheers.
Posted: 08:56pm 11th June, 2018
ogri Subject: Fuel starvation?
Hello Stephen, I posted the above reply. Today I got to open up the fuel tap and discovered what may have been my problem: the little rubber disc inside with the little holes has gotten compressed over the 14 years to the point that a couple of these had slightly closed over. A knowledgeable friend with me at the time pointed out this and said that these pieces, along with the small spring that maintains the pressure between the tap and the rubber are available as spares for his older Hondas. That idea, along with the countless on/off turns over the years of the tap rubbing against the rubber disc would again have the effect of slightly reducing the size of the little holes and, consequently, the flow of vital petrol. I wonder if our hosts sell these?
Posted: 09:57pm 26th June, 2018

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