Mik 42 or 45 on 80 incher

[Kippers99]

First off, let me appologize. I am sure this must be a repeat post. I have read for hours and hours and can not find information on my specific situation.

Please read the build below and let me know what the difference or trade off of is between the Mikuni HSR42 and the Mikuni HSR 45. Or just tell me that a Mik 45 should never be installed on an 80 inch motor.

I ride solo only. Most of my riding is around town with the occational 100 - 150 mile trip. I love low end torque and throttle response. Did that just make the decision for me?

Bike: 1999 EVO Night Train - 240 rear with chain final drive.
Motor:
Forcewinder XR2 intake - Breathers routed to catch can
SE Intake Manifold - 29636-99
Heads - Street Ported, 1.9" intake, Roller rockers, Woods Springs, comp releases, CC'd
Pistons - JE 10.5:1 domed +.005
Cam - Woods 6H
Crane HI4 ignition w/single fire coil
Hooker Step tuned 2 into 2 pipes with quiet baffle

Thank you for your patients as I learn

[TonyJ]

42 Will work just fine.

[Geezer-Glide]

42. I had one on my 80" Evo Roadking. I had a mild cam and mild head porting with a White Bro's 2:1 exhaust. Bolted the Mik on right out of the box and never looked back. Sweet carb. The 45 will be WAAYYYYY too big for your 80 inch'er.

[Harleyd]

I have been running a 42 on my 80" with an ev27 for about 7 years and its great. Around town low end you want to keep the velocity up. BTW, my bike is a 99 FXST.

[rcsoftail]

I would also use a 42 on an 80".

[Kippers99]

OK, as I thought, the Mik 42 seems to be the popular choice. I was thinking the smaller carb would produce a better velocity in the lower RPMs. I just did not want to leave anything on the table by not going to the 45mm Mikuni (the dif in cost is not too big). The tuner did say he would be tempted to try the 45; but, I do not want to lose ANYTHING from the bottom end.

Now, with that same thought of velocity; do I use the SE intake manifold or the stock EVO manifold?

I have both on hand.

[rcsoftail]

Either one should work fine. I am not very familiar with he SE one, but I think it has a little more volume with it. Some say the extra volume provides a little more mixture sitting there available for when you initially pop the throttle. I doubt you would see much of a difference with either one on an 80" motor. Since you already have the SE one, I would probably use it.

[dog155]

The best way to find what you really want is to try each one for a while.I would certainly try the 42 first,better for off idle.As often as most of us use wot it is not worth going too big.Good Luck

[Unca Mike]

The difference in diameter between the two is only 3mm or 1/8". Not much and because of their design you would think there wouldn't be much difference in responsiveness. The 45 should have more flow on the top end. However, I run a Mik 42 on my pumped and fast FXR4 80"er (92hp at the rear wheel) and I'm a happy camper. Would I get 94hp with a 45? Would responsiveness suffer with the bigger gulper? I don't know and I don't know anyone who has actually tested the two objectively on an 80"er but a 42 is pretty sweet.

[Hillside]

Have witnessed OUTSTANDING results on even 96" powerplants with a 42mm.
Scott

[tidesign]

42 on a PR 113

[blackjackcustms]

Both the 45 and the 42 are variable venturi. The only difference as far as airflow would be at the final bit of throttle opening. They are calibrated differently, and the 45 would be richer out of the box, but that can be changed Unless you open the throttle all the way, you wont see a diff as far as velocity. At any given opening point, you can match the airflow and velocity by opening less, until you get to the top. I'm running a 45 on my 103" EVO, and it works good, but I had to lean mine out as it was set up quite rich.

[Hillside]

I would imagine that they come with larger jetting, as the signal at the big carb will not be strong, unless it is installed on an engine that can utilize the flow characteristics of it.
Scott