| Author | Message | ||
     Finnbiker |
Greetings from Finland! I'm looking for info on how to pressurize the Mikuni HSR carb in blow-through application. Current project is a -77 shovelhead (sorry, not a Buell...) with completely rebuilt engine. Last summer got about 6000 miles with it and now its time to make this American iron really fly. I have most of the stuff covered but the carb is the biggest problem. I'm planning to use Mikuni HSR42 and not a CV. Been in contact with Mikuni American but they could not help me. Ok, so far I know that I need the 786-2700-23 needle valve assembly since there will be a fuel pump pumping the gas into the carb (after a fuel pressure regulator controlled by boost pressure - actually it will be a malpassi bypass reg). I'm thinking of pressurizing the carb via the vent fitting and blocking the overflow fitting below the float bowl. A bit higher pressure in the float bowl than in the carb's venturi will enable the fuel flow from the float bowl to the venturi (through the needle of course). This pressure difference will stay pretty constant since when boost pressure increases in the venturi, the fuel pressure will always be a bit higher. But, is this info correct - meaning that other modifications are not needed besides enormous time spent in jetting and getting it tuned? I'm wondering what kind of effect plays the accelerator pump and choke, will they cause leaking and pressure drop when the carb is pressurized? I know that Buell's have used Aerocharger turbos with HSR carbs but I cannot find any info what is actually needed to get the carb functioning properly under pressure. The user 'buell_freak' asked about pressurizing instructions for a HSR carb in this section about two-three years ago but that thread died too early. | ||
| Sparky |
Get in touch with Firemanjim, a regular BadWeB poster and Land Speed Record racer. He and Terry Parsely at JT&S Performance have a good bit of turbo experience setting up and racing Bonneville bikes. | ||
| Finnbiker |
Thanks a lot Sparky, I will try to PM Firemanjim. | ||
| Firemanjim |
Finn, the Miks used the pressure rated needle and seat--I know Sudco had them last I looked. And need a good electric fuel pump that will put out 3-5 psi above your targeted boost,I used a MSN from Summit along with a boost referenced fuel pressure regulator. I used an Aeromotive 1-1 so it raised pressure in a 1-1 ratio with boost.Again from Summit. Need a return line to tank from regulator. Jetting is pretty standard for low speed stuff as off boost carb runs normal. Accel pump remains normal and the "choke" on a Mik is actually an enrichner and stays as stock. I ran boost signal to the bowl vent and the Aero kit had an extra fitting on Mik at accell pump to feed boost signal to carb. What turbo? | ||
| Finnbiker |
Thanks Firemanjim. Really appreciate you sharing your experience! The turbo I will use is the IHI RHF5 which I already have. It's a ball bearing version. It has integrated wastegate with pretty stiff spring in the actuator: opens above 15 psi which is too high to start with. So I need to maybe find a new actuator or open it up and modify the spring. I have never had an HSR carb in my hands so I just read the manual. Below the float bowl there is the overflow vent. Then about at the level of venturi there is another 'vent' fitting which has larger diameter in it. Is there a connection to the float bowl from the larger tubing inlet - meaning that this larger input is used to pressurize the float bowl? What is your opinion about the CV carbs used in blowthough turbos? These are also pretty easy to pressurize but I have no idea if the HSR is a better choice or not. Many japbikes have been succesfully turboed with CVs, also one friend of mine had a turbo in a Sporty with CV and it ran pretty well after it was properly tuned. The regulator and pump are as you described but from different manufacturer. Ignition is Dyna 2000i which will be driven with pressure switch connected to the manifold (ignition retard under boost). | ||
| Firemanjim |
I think Mik is better suited to blow through than the CV's,seems I remember them having issues with the change from vacuum to boost. And all the vents on any blow through need to "see" a boost signal to fuel properly. | ||
| Finnbiker |
A short followup on this issue. I did not use Mikuni for the turbo but instead got myself a HD 40mm CV carb. I succeeded well in setting up the carb, just one pressurizing line to the float bowl was required. This line was taken after the turbo charge pipe and before the plenum: A 5 mm pitot tube was welded and this was used to pressurize the carb. Now the float bowl always sees higher pressure than the carb's venturi. I dynoed the bike and A/F starts at 13.5 at 3000rpm, in 5500rmp the A/F is 11.5. A bit rich in the high end but this is easily fixed by putting a smaller main jet. Bike pulls very nicely and produces 100hp with 132Nm torq @5500rpm. Carb has 190 main and 48 slow jet. Needle is N65C and spring is original. No dynojet kit - carb is 100% original HD CV carb. 5000km's behind with the bike. So it really works. |
