swapping F motor into H

[bikerhawk]

Well, the F project bike is about to become a donor bike.

It was originally intended to be SWMBO's ride, but with 4 kids between 11 and 3, what little time she gets to ride, she would almost always be 2-up with me. Plus we still have the XS750 for those occasional whims.

So I am looking at swapping out my H's motor, which has only 3rd/4th/5th gear and gets mileage in the 20s, for the motor & carbs in the F that has 5 good gears and a bad clutch (no big deal at all...I even have a spare clutch somewhere). I've had the H motor out and completely torn apart twice, so the job doesn't spook me at all, except for finding the time to make it happen.

My question is: What do I need to change electrically? If I take the H's ignition stuff off that motor and put it on the F motor, will that work?

Am I missing anything else here when putting a 79 motor into an 81 bike?

[John]

Basically, just swap the ignition pick ups, vacuum advance, governor (counterweights) and associated items from the H onto the F motor. Basicall, everything under the the left side engine cover. Which carbs were you going to use? I had an XJ engine in my 80SG for many years , and all I had to change was the pick up coils and the alternator, and rotor assembly. I used the carbs from the SG.

[skids]

Lets see... I put a SG engine into an H frame. The good thing about the H components are the black box and the improved pick-ups and better plug coils with the lack of a ballast resistor. However, the wiring looms would be different to accomodate the differences. I think the engines will simply swap out and your old F pickup ignition components (and its associated black box) can be used for the H engine. Otherwise it should be plug and play.

[bikerhawk]

Originally posted by John
Which carbs were you going to use?

I think I'm going to try the F's carbs first, to see if that improves my mileage. The best I can get under the current setup is 27-30 mpg, and it gets as bad as 22 hammering down the interstate at 80+. I don't think there's anything wrong with the motor to cause that, and the only thing I can find with the H carbs is that the main jets are all 115, as opposed to stock 115 on the outers, 120 on the inners. Would that have that drastic of an impact on mileage all by itself?

Anyway, I really need to get my mileage up to the 35 range if I'm going to be able to afford this summer jaunt I'm planning...

I know I'll need to use the F airbox for the extra vents...no problem there. I already drilled out the holes from a later model box to replace the one on the F that was cracked...I'll just slip that one in.

[TopCatGr58]

Hey Tom,

You said that the F engine has good gears but bad clutch. A partially slipping clutch will also reduce your mileage, cause you're spinning the engine more than the wheels! After the swap with new plates and springs, you'll probably notice an improvement in mileage!
T.C.

[bikerhawk]

The H is the one w/bad mileage, and its clutch is fine. It's the F that has the bad clutch, and I've only been able to ride it once, with old, bad tires, a sticking front brake, and the slipping clutch. That ride didn't last long.

I didn't try to adjust the clutch on the F, but if I'm just going to pull the motor, it's not worth fixing all the other things (I didn't mention that in addition to the front brake sticking, there was NO rear brake ) to test it one more time. I was able to ride it just long enough to verify that all 5 gears were solid.

[bikerhawk]

re: bad mileage

Oh yeah, I did forget to mention...some PO put a 16" rear onto this H, I suppose to lower the rear end for a shorter rider. I know that will bring up the highway revs slightly, but enough to make the mileage suffer that badly?? I don't recall guys w/Specials complaining about their highway mileage that much...just that they can go barely 100 miles on that tiny tank. Good thing I have the 6.3 gallon tank, and they don't have my bad mileage...they would only be getting about 75 miles on a tank down the freeway.

I will save the 17" rear from the F and mount that with a new tire when my 16" Dunlop wears out, probably at the end of this year. I'm only 5-10, but I'll make it work...I like the idea of restoring the original spec and bring the RPMs down a bit for all this Kansas cruising around here.