I'm not sure what model honda you have, but I'm assuming a GX model 340 or 390. The "11hp" gx340 or "13hp" gx390 have identical measurements anyway.
You will have to see what shaft size your paint sprayer engine has - most likely it is a straight 1"x~3.5"L - then compare it to the I/D of the giant vac impeller. Many are 1", but 1-1/8" is also common on Giant Vacs. If it is 1-1/8" the Honda will not work out as the gx390s and smaller don't offer 1-1/8", but again you have to measure to confirm.
IF the impeller will work out - You may run into another issue when actually mounting the Honda on the Giant Vac frame, particularly if an older unit. When trying to use the stock engine mounting holes the Honda's muffler and/or muffler pipe/manifold may hit the truckloader's 'bulkhead'
Fuel tank clearance on front seam edge will also likely be tight
Re-drilling holes further back can solve this, but you need to mock it up with the impeller on (or measure impeller depth on crankshaft and make a mark on crank for reference) to make sure you keep adequate clearance. You probably will have less than 3/4" of room to work with here.
You may also have to swap to a smaller muffler in place of, or combo with re-drilling the holes. The *standard* muffler found on most gx340s and gx390s is big and protrudes far in front of the engine as the inlet is near the center of the muffler. There are smaller OE mufflers for 8/9hp gx240/270s, that use a much smaller oblong muffler shell with far forward inlet (pulling muffler tighter to engine) and bolt right on to the 340/390s. If it has the big square heat shield on the muffler it will likely have to be removed or just trim off the front portion. The small form-fitting shield will work as-is. The 'standard' gx240/270 muffler will provide almost no more clearance, it must be the small oblong shell type with offset inlet (e.g. Honda#18310-ZE2-W00)
I've done this repowering a couple older Salsco truckloaders with gx390s. A note, the smaller 8/9hp muffler on the 11/13 Honda will be a little louder and the muffler itself will get a bit hotter than the big fullsize 11/13 piece. We have had no issues, though.
Edit: One more thing, pay attention to the shaft height from engine base. The impeller should be pretty close to center of the housing/inlet. If it looks noticeably off and/or shaft won't even pass through bulkhead with engine base on frame, you will have to use some spacers to position the engine higher. If this is the case, it will probably be 1"
Last edited by BDLandscapes; 11-19-2012 at 11:43 PM.
Reason: More info