Hi Guys,

I just pulled the carbs off to install HRC replica carb springs from Rick Oliver, while I was there I gave them a quick clean, reset pilot screws, re-set the float heights to 12.5mm and made sure the emulsion tubes were in the correct carbs.

It's had a flat spot in the mid range since I got it but runs great otherwise, so I thought the springs would clear it up a bit and give me a bit better throttle response.

Now it really stumbles in the mid range and won't really rev past 6000-7000rpm. It revs ok when stationary sometimes (after I've tried a few times) but it still stumbles a little, definitely sounds like it's not getting enough fuel.

Current setup is pilot screws 1 5/8 turns out, 108 main jets, HRC springs, 1 shim under needle, float height 12.5mm, TSR 3/4 titanium exhaust.

The fuel lines aren't kinked, fuel tap works fine and vacuum line to the tap is fine, the carb slides move in and out smoothly by hand and there's no vacuum leaks at the carb boots. It does the same thing when running from fuel in a funnel.

I'm going to go back and double check everything, particularly the float heights. But I don't have much time to troubleshoot. Plus I'm suspecting it might be something stupidly simple that I've overlooked.

Thanks!

Does it use the standard honda air filter (honda part number will be cast into the plastic)?

Yeah, I recently replaced the air filter with a genuine Honda part.

Before I put it back together I ran it with the airbox off, just to check. It had the issue but I thought it was just because of the open carbs, chucked the airbox on and it got better but still stumbled occasionally, I felt it was good enough to warrant a test ride.

I'm beginning to suspect the float heights, even though they are set to stock now. They were all around 10.5-11mm before I changed them.

The previous owner said it was re-jetted for the exhaust but it has stock jets in there and pilot screws were set to stock settings too. So I'm suspecting maybe the float heights were changed and that was all. I have 115 jets but I checked the spark plugs and they looked like the fuel mixture was good, so I didn't install them.

EDIT: I also suspect the shorter carb springs will demand a little more fuel, as the needle will come out of the hole (sorry, I don't know the technical term) faster/easier. I couldn't find much info on float heights for RVFs but I guess that'll the first thing I do after I double check everything.

You'd certainly be wanting to fit bigger main jets with that exhaust system. 112 (aftermarket muffler) or 115 (muffler and system) is the usual upgrade.

There shouldn't be any reason to deviate from the factory float height setting, if you do need to it just covers up another issue that needs resolving.

Silly question perhaps, but have you checked that the pilot jets are all clear and unblocked? Any small fuelling issue can cause the type of symptoms that you describe, including blocked passages.

It could also be worth seeing that all carb slides are moving up and down correctly by running it without the airbox on.

I've just set the float heights back to what they were before, with the 108 jets. I'll give it a quick test run when I get a chance, just to make sure the springs aren't an issue. Then I'll go back to stock float heights with the 115s and see how it goes.

I pulled the pilot jets and they were all clear, sprayed carby cleaner through everything too, everything was clear.
The slides seemed to move ok but I wasn't paying full attention to them before.

Thanks for the help!

EDIT: Actually, I'll scrap that idea, set the floats back to stock and install the 115 jets and see where that takes me.

I was really surprised to find the stock jets in there with the exhaust.

Update:

I've set the float heights to stock, 115 jets in there, slides move freely when revving. It definitely seems better than before, but I won't have a chance to take it for a ride until tomorrow.

Hopefully it's all good... I'll eventually send it to RB Racing to get a dyno tune when I can save up some money.

Let us know how it is once you've ridden it. Make sure to rev it hard through at least the first few gears

It sounds like you already had a rich midrange and then made it worse by fitting the HRC springs

Drunkn Munky wrote:It sounds like you already had a rich midrange and then made it worse by fitting the HRC springs

Is the stock setup 1 shim under the needle?
That's what was there, though I'm not sure if it was a 0.5mm or 1mm shim. Maybe the needles aren't stock, I didn't look at them properly but they looked stock.
I'll check those things out too if it still has issues when I test it tomorrow.

Thanks for the info