1983 vf750f, flattens/sputters between 6k and 8k rpm

Hi all, first time posting.

I've had my '83 v45 for about a year, and it has always had a bit of a power loss between 6000 and 8000 rpm. Before I rebuilt the carbs it felt like the clutch was slipping between 6k and 8k, but then it roared back to life >8k. After a carb rebuild (cleaned, new jets + gaskets), in that same rpm range it sputters and sees a greater power loss. It's almost a total dead zone. The jets were all replaced as stock, but I moved the jet needle out by one or 2 ticks (http://www.dansmc.com/jet_needle1.jpg) trying to richen it up. This wasn't done willy-nilly; everything was put back together stock, and synched, but then I'd ride, come back to the shop, move the needle, ride, come back to the shop, move the needle, etc.

Since these carbs are a bitch to take apart, I figured I'd ask if anybody could help out before changing anything else or looking at timing etc.

Looking at some forums for other bikes, it also sounds like it could be a vacuum leak?

I have a buddy with an '84 750, and he's got the same problem, but much less aggressive.



Thanks!

Andrew

 

u didn't mention, stock exhaust and air filter ?? It's not a vaccum leak.

 

What makes you think it's carbs? Plugs, plug wires, resistor in wire cap end and coils will all create the same experience your having now. What are those pieces like and when were they changed last? You can not adjust timing, it's fixed. Possible spark generator but they normally don't go bad and when they do they just shut down. Slightly possible ign boxes.

With stock motor you don't need to raise the needles it will run to rich. How are you raising the needles seeing they don't have cut groove/circlip? Washers?

 

A few updates (bearing in mind that I'm not the first owner, obviously):

1. I just pulled the valve cover off the front bank, and one of the valve cover mounting bosses had the threads stripped off, meaning that the screw was just floating, not creating a seal. Drilled out the old helicoil and put a new one in, and that made performance in that 6-8k band 10 times better, it was like night and day. It's still a bit laggy in that mid range, but no longer undrivable...

2. Regarding the needles, someone must have rebuilt the carbs already, because the needles in there are the groove circlip-type. According to the light colouring of the plugs, it looked lean, so I moved the clips. Maybe I'll move them back to stock (or whatever the previous owner had them set at) and see if it further improves performance in that 6-8k range.

3. Everything is stock to my knowledge (except the jet needles...). The '84 (and maybe 83?) had a factory upgraded exhaust option, and my bike does NOT have that. It's got the black chrome exhaust, and a newish factory-type air filter in an unmolested air box.

4. Plugs are brand new.

5. I suspect the coils and wires are factory 1983, so I'll change those next.

What is it about the coils and wires that would make that motor perform great from idle-6k, and from 8k-11k, but not in between??


Thank you both for the comments.


Andrew

 

Why do you think that has changed the running behviour of the engine?

 

I rode it, and the flat spot at 6k was a ton less noticeable.

 

Wires and the resistor in the cap on Honda's can build resistance which may not let the plug fire correctly at the right time. There are several points to make contact and they could be corroded or what ever. They can also jump to ground then sometimes.recover. You need your plugs to look whiteish. Unleaded fuel doesn't really leave markings on porcelain like the old leaded gas did. Here's a great write up on how to read plugs with today's fuels. Ignore the race track only comment. Ken has to cover his ass on that. This is the best I've seen about plug reading today's fuel.

https://sites.google.com/site/ksmsinc/technical-info/title-3/reading-spark-plugs

Sounds like someone put a jet kit in it or at least changed the needles for some reason. Most guys who do that with carbs on a stock motor usually screw things up because they really don't know the principals behind what they are doing. The shape of the needles play a big part in A/F ratio at different rpms. Yours may be the cause of your problem. Could be to rich or lean. Plug checks are one way to find out but you'll need several sets of new plugs. Or find a place with gas analyzer equipment.

 

I think Grey is reading my diary again, my money is on drilled slides and overly aggressive after market needles from the Dynojet folks.

 

Ok. So my next steps then are:

- Advance the needles to where they were before I thought it was a fueling problem
- Resynch
- New coils/wires etc.
- Buy a set of stock slides/needless from an unmolested '83
- Verify that the new jets I got as part of the recent rebuild are in fact the stock setup. I bought a kit from v4market.com, so I'm assuming they sent me the right hardware.

I rode into work this morning though, after fixing that leak in the front-bank valve cover, and torque delivery was pretty awesome compared to before the fix from yesterday. Need to pull the tank this afternoon to check the rear bank cover and make sure something there isn't loose also...then check valve clearance for shits.

Thanks for the help everyone!

 

I must slightly--but most respectfully--quibble with Grey re plug color as i can see the optimal Cadbury light milk chocolate coloration on my great-running '86 700.

 

I'm sure that plug has a few miles and startups on it. That's most likely deposits of burned hydrocarbons, possible fuel additives etc. In other words used plugs. You can get a general consensus of what the combustion is like for running around the streets for awhile. You'll never be able to tune with a plug like that. Compared to my plugs after looking at them with 5k miles look whiter and cleaner than yours. If those are not used plugs then I'd say your A/F is rich and could be tuned better for more hp or responsiveness.

Yes your bike will run good with the A/F that way. It could run better though. Just a little fine tuning of the carbs, that's all. Bring it to Cleveland and I'll show you............................