Triumphrat

BadMouth

Anyone with modified (higher HP) engines experience fuel starvation?

I was considering ditching the duckbill filter at the carbs & removing the filters from the petcock, then installing an inline filter.

Think it's worthwhile or a waste of time?

I'd like to make my next set of dyno runs my last and get the bike into it's final setup (enginewise anyway). I'm trying to eliminate any stumbling blocks ahead of time.

MickMaguire

Fitting an inline isn't a bad idea at all on these bikes. The worst issue is usually finding a good place for a sizeable (free flowing) filter. You don't need the extra flow unless you have done some MAJOR work (and even then you probably wouldnt), but the stock filters don't catch everything. I would not remove the petcock filters as they protect your petcock from rust particles.

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Mick...

jimmyj900

How did you determine it was low fuel flow? Misfire at high rpm, dyno results?

Jim

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BadMouth

I haven't determined anything yet.
I was just asking if anyone else had needed to make that type of modification. If the general consensus was that it has been an issue on other bikes with similar mods, I'd like to address it now, before I start the rejet shuffle.

Napalm

In my reasearch I was told the OEM carb performance could only be streached so far.

I think it was Wilkox engineering that told me but I can't remember. They said the carbs could only supply so much fuel. (my OEM's were Mikuini BST 36)

I hope I am wrong but I think the best solution (but unfortunately not the cheapest) is upgrading to the Keihin FCR 39's.

I know they are not cheap but life is unfair.(as they say on the mecatwin website)

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jimmyj900

Gotcha...

I checked the fuel flow one time and if I remember correctly it looked like about 1/4 gallon per minute -- way over what you'd need for sustained high-rpm operation.

I've run up to 150 main jets with no fuel starvation problems on the CVK's and the only thing that looks like an insufficient fuel supply is related to the carburetor airflow velocity, not the real fuel supply capacity of the system.

What can happen to drive the top end lean is insufficient air flow. With the stock exhaust system he instantaneous velocity with respect to rpm through the carb venturi actually decreases as rpm increases. That reduces the fuel 'draw' through the main jet and causes leaning.

The condition is due to a combination of back pressure (muffler, pipes, valves, cams, porting) and that the stock downpipes are not designed for efficient scavenging.

The stock 'green' cams are a low-overlap design and can't produce much scavenging effect so Triumph didn't need to put higher-perfomance pipes on the Clasics. If you look at dyno runs for these bikes you'll see that they have an air:fuel mixture transition at about 6000 - 6500 rpm where the restrictions of the exhaust system start to drive the mixture lean.

With the 'blue' cams installed you'll see the same effect with stock pipes -- a 'leaning' of the mixture at around 6000 - 6500 rpm but of a higher magnitude than with the 'green' cams. In this case the 'blue' cams have a good overlap but can't make use of it due to the restrictive exhaust system.

Make sense?

Jim

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