'84 VF750 Electrical Issue?

Hey all, so I've been out of commission for awhile but I finally got my bike rebuilt and running........sorta.

I feel like it's an ignition issue primarily so here's everything I've done:
-new plugs gapped to 0.032"
-new 7mm copper core plug wires and NGK resistor boots
-rebuilt carbs with BillyC's kits, only change was the slow jets up to 40 from 38, all other parts were ensured correct for the carb position (short springs in the backs, proper jet holders in the backs, 112 vs 110 etc)
-carbs were bench sync'd then installed, idle-dropped and vacuum sync'd.

Problem was on the first start, it fired right up and idled great, soon as I cracked the throttle, the tach fell off and it sounded like it was bogging down/falling on its face etc. and wouldn't rev at all. I played with the carbs before thinking it may be my 1-3 spark unit. Replaced that with a spare and things were better but not great. When I snap the throttle, it revs but if I hold it around 2000, it seems like a slight misfire but clears as I wind the throttle out further. Once I let go of the throttle, it mechanically snaps back quick but the idle tends to hang before settling back down around 1000.

For the spark boxes, I currently have a 1-3 unit from an '85 750 and a 2-4 unit from an '84 750. I found that if I put both '85 units on, the throttle response is much better but it won't idle at all unless I'm at full choke. Engine I should mention is out of the '85 as well, rest of the frame/wiring is '84. The other thing I noticed was with the '85 2-4 box, my fuel pump doesn't work. With the '84 2-4 box it does.

Coils were swapped out with the ones from the '85, both metered out at 3.0 ohms on the bench. Did this part only to rule out a bad coil but doesn't seem to have made any difference. When cranked with no fuel, each cylinder has good spark. Will recheck compression but my cold numbers were all between 170-180.

So to break it ALL down..........

both 84 boxes - good idle, bogs down off idle, fuel pump works as advertised
mix of 84/85 boxes - good idle, misfire at 2k, sluggish and underpowered, fuel pump works
both 85 boxes - no idle unless I hold the throttle open, better throttle throughout, fuel pump doesn't work

Any ideas? I'm doubtful it's a carb/fuelling issue as it seems like it runs like the champ it should except at idle, hence why I'm leaning more towards and ignition/timing issue somewhere.

Thanks again,
Tony

 

What do the spark plugs tell you? What is the condition of the carb boots?

 

Plugs, when I pulled them the other day were dry but sooty, I'd imagine thats a sign of being too rich? Currently gapped at 32 thou.

Carb boots were picked from the best that I had on hand, soaked in acetone/wintergreen oil so they're back to being soft and pliable, but I haven't checked for any air leaks around them yet - how tight is good enough for the screws??

 

Snug is good. They have grooves to seal.

 

be careful using "Acetone"& Wintergreen. Alcohol and Wintergreen is the usual combo. Acetone can dissolve some rubber and plastics.

 

Hmmm, I bought these for my VF1000f and they work fine....
https://www.ebay.com/itm/New-Honda-...820448?hash=item2897f09da0:g:a8cAAOSwmrlUyZdM

 

Evening' fellas,

So an odd one but it's sorted out now and I learned a few more things about these bikes. It makes sense to me now but didn't at the time but it turns out the starter clutch/pulse genny pickup is different between 84 and 85. The 85 has a longer raised surface than the 84 which I assume will affect the timing. Once I swapped the pulse gen harness and the rotor over from the 84 motor I had on the floor and got it back together, it runs like a whole new bike, no fuel pump issues, no misfire at 2000, no decel hang on the way back down.

I really appreciate the advice and suggestions from you guys though, I thought I ruined the boots with the acetone but they just needed a few weeks to evaporate and shrink back to normal size, all is good with them now and they're soft as they are when new.

Bike is at the shop getting a safety done on it and then........pictures for you guys and riding for me!

Cheers to you all
Tony

 

Great news Tony!!!!