84 VF750F restoration - carb questions

I have finally gotten the carbs back together (they were apart when I got the bike) and have been trying to get it to run right. Perhaps someone can offer a suggestion or two on where to look next. A brief history of what I've done and where I'm at:

1st attempt after carbs back together - waaaay too rich, sucked back a litre of gas in about 30 seconds of running, plugs fouled and wet.

- took carbs apart again, cursed myself for not trying to figure out every single passage and port the first time, cleaned again, verified proper needles, jets, float levels, etc.

2nd attempt - better, but still similar. Noticed slides were not moving when the engine was reved. Plugs fouled and wet.

- took carbs apart again, beat myself with an old plug wire, and spent hours cleaning out the passages that go from the top of the diapram to the carb throat. Used an e string from a guitar. This was previously the only passage I was not able to verify with compressed air and wd-40.

3rd attempt - best yet, but after a few seconds of running started to spit back through the carbs. Havn't pulled the plugs yet but expect they are probably fouled. This time though the exhaust wasn't nearly as rich.

Mixture screws are at factory spec 2 1/2 turns out.
Slides are now functioning as I would expect.

What am I missing????
Thanks in advance for any suggestions.
Steve

 

Are all the plugs fouled equally? Are the jets stock? Have you checked each needle and seat to insure it is sealing?

 

Sounds like the floats may be to high. Get specs on their settings and adjust accordingly. Make sure the floats are not filling with fuel and sinking. Check the float needles to make sure the spring rod is working. The little rod that touches the float should be spring loaded and move in and out.

 

Hmmmm.
The needles and seats are new. I checked the float heights, all were good except for one that I adjusted slightly. All of the plugs are fouled the same - carbon sooty and wet with gas.

One thing that is bothering me is the air pipes: The air pipe between the front carbs is plumbed to the air pipe between the rear carbs, that makes sence to me. What doesn't make sense is that the second tube off the pipe between the rear carbs is then just left open and sticks up by the coils? There is no place on either the aluminum carb plate or the airbox to terminate this. It looks like the hose was terminated somewhere, but I can't seem to figure out where? I have searched and searched but can't seem to answer this question. I'm left wondering if there is somehow too much of a vent for the float bowls causing the fuel to run through it??? I'm grasping at straws here. I've been working on bikes and engines in general for more years than I would like to admit, but this is the first time working on a V-4. I have to say these are about the most complex set of carbs I've ever worked on.

 

I'm not sure exactly how this model 'plumbs' the float venting...but many models do just go to atmosphere. The Magna models around that era usually plugged into the plenum (aluminim carb mount). I kinda think as long as the vents are open it wouldn't cause the problem you're having anyway.

Didja check out the fuel enrichener plungers? They should be all be completely closed when the choke cable is off. They 'dump' alot of extra fuel and if one or two are hanging open even a little it's gonna run very rich after warmed up.

I'm always real nervous about 'box' carbs...I mean you don't know what somebody else has done to them. On occasion, I've come across 'would be' mechanics who think you clean jets by running a small drill through them. The orifices in the slow jets are REALLY tiny and any drill bit you commonly buy would make the bike run extremely rich. Don't know how you could tell without a very experienced eye unless you have known good ones to eyeball and compare.

 

The float bowls must be free to vent to atmosphere.What are the size of the jets and do the slide needles look like nails or do they have a sharp taper to them in the middle and have e-clips on top? If you jump the fuel pump to the battery (green wire is ground) and let it run, it should load up and stop when bowls are full, that would be an easy way to verify that the seats are sealing.Tough to diagnose from here, just throwing some thoughts out that may help.

 

TOE is suggesting that if your slide needles look stepped instead of a gradual taper and your needles have E-clips, that your bike has a jet kit and perhaps larger than standard idle jets.....which could cause the plug fouling problems.

Check to see if you have the standard slow jets as TOE says; you could also try turning in the mixture screws in about 1/2 turn and see if things improve.

It wouldn't really affect the idle mixture, but the emulsion tubes ( main jet holders) are different between front and rear cylinders and need to be installed accordingly.

 

Yahoo's and jet kits can be the worst thing to ever happen to a bike.Easy to unfuck just need to know if that is the issue.Jetting mods are handled by most with an iron fist and small changes are huge in the real world.

 

Thanks for the tips. The jets and needles are all stock, and by comparison to the ones in my spare carbs they seem to be in good shape. Based on the parts diagrams I'm pretty sure I have the emulsion tubes in the right carbs. I have double checked the float levels and the correct seating of the new float valves. I have also verified that the choke plungers are all the way in.

Upon reflection, I'm leaning towards the valve adjustment for a few reasons - the bike hadn't run for 6 years, so perhaps after running a short time some gunk has left the valve seats and they have tightened up. Also, when I adjusted the valves, the engine was probably at about 40 degrees F, much colder than normal for a valve adjustment. Any time in the past when i've seen spitting from the carbs it has been that the intake valves were not closing 100%.

I'm thinking that the richness problem was the ports that activate slides, and that the new spitting problem is the valves and is a seperate issue.
Hopefully I will get some time Sunday to play and see what's going on.

Thanks
Steve

 

I don't know if the airbox is intact or not but if you're running it without the filter and lid, or if you haven't had it on the road sometimes it's hard to judge whether it's running quite right or not. I can't tell from your post if the bike really runs crappy, or just kinda misses a bit.

If the bike runs generally OK but just kinda spits or misfires on one or two cylinders at idle and low RPM...you will want to inspect the carb synch. When it's way off, that's pretty exactly how they run.

For the best idle and low speed running, I usually end up more like 3 to 3.5 turns on mixture screws...sometimes half a turn can make a big difference. I usually adjust them when the bike is hot, while it's idling, by ear. You can also sometimes use the mixture screws to help diagnose which cylinder isn't quite right...that is, if turning one in has no effect on idle...you know something's wrong in that carb, or on that cylinder. If you turn one mixture screw all the way in...you should be able to hear that cylinder begin to misfire and nearly cut out...as you slowly turn it back out the cylinder will begin to come back and the idle speed will increase. The difference is subtle but it's definitely audible if you listen carefully.

 

If you're sure the carbs are good and the plugs still foul, you'd need to check the air filter for being blocked by dirt.

If you run the bike with filter removed and STILL have black plugs, then either the plugs are the wrong ones for the bike or something is seriously messed up in the carbs cuz you're getting too much fuel.

 

OK, trial run number 8 or so....
Checked the valves, indeed they had tightened up a bit so readjusted them. Fired right up, air box in place. Still ran rich, but better. Improved by backing out the mixture screws about a turn to 3 1/2. After a few minutes running fouled plugs again. All plugs black and sooty, and wet. Took the carbs off again, double checked emulsion tubes, correct. double checked float levels - correct I think. btw - carb rubber boots had wet gas in them after I pulled the carbs. Also checked each float valve by blowing in the fuel inlet tube while moving the floats - all ok (New float valves and seats)

Now, about this float level. I am going by the manual which says: " with the carb tilted 15 - 45 degrees from vertical, the float tank should just contact the valve" for a measurement of 7mm. I am assuming I am to measure from the float bowl split to the bottom of the float? Does anyone have a clearer explaination of adjusting the floats?

Any further suggestions are greatly appreciated, I'm running out of idea's.
Steve

 

you said you have a spare set of carbs? are they a spare set or a parts set? if they are complete, slap those on and see what happens.. If the carbs were apart when yo got them then maybe the right peices didn't get with the right carbs...

ok stupid question but is the choke stuck?

 

Try turning mixture screws in--not out-- with engine running until you cause each cylinder to stumble and loose power.....then back screw out 1 turn.

(Exactly what are the numbers on your idle jets ??)


Try that.

 

I checked all the jets and they are all the correct stock ones. The chokes are not stuck. In all the trial runs I have been stating it without any choke at all, despite the fact that it is only about 40 degrees F outside and the shop isn't much warmer. I have the carbs apart AGAIN!, and am going over them with carb cleaner and compressed air. I am still suspect of the vacuum port above the diaphram on the slides. My reasoning is that if the slides aren't rising correctly then it would be super rich as you twist the throttle. These had been blocked, and they are such tiny passages.

The spare carbs I have are in poor shape - not suitable to put on the bike without overhauling them.

Steve

 

I don't follow your reasoning cuz the slides, whether they rise or don't, should have NOTHING to do with a rich idle speed setting as long as they are in the fully down position

Might try lowering the fuel level ( raising the float, going to 8mm from 7) as an experiment, but you NEED to try the screw adjustments i suggested above.

As mentioned, some goofball could have drilled the slowjets or otherwise enlarged them.

On an older bike, it's risky to assume that the sparkplugs you find installed are the correct ones.

Float needles and seats could be incorrect for that bike but still fit; your supplier could have sent you the wrong parts.

After all your starting and idling, make sure your battery is fully charged.

 

A few comments...at that temp, if the bike was running normally you would need choke for it to start and it would take a few minutes to warm up and idle smoothly. It probably is so rich it starts and runs OK cold but as it warms up and fuel requirements are much less it all turns to crap.

Sounds like you're checking the float level correctly. Maybe the problem isn't related to float level. You could run an auxiliary tank to the carbs on your bench and see if they hold that way.

It's looking more and more like some 'ham handed' individual drilled your slows...can you clean one pilot jet out of the spare carbs and use it to visually compare hole size? Or have the spares been in the same 'hands' as the ones you are using?

One other thing I can think of...seems to me the carbs on this bike have needle jets mounted on top of emulsion tubes but as separate pieces (Squirrel might know off hand for sure). If that's the case they can disappear quite easily and leak ALOT of extra fuel into the venturi under all settings if not reinstalled. If you look into the carb venturi the jets I speak of should protrude into it. Does this make sense to you, Steve or anyone else for that matter?

 

Thanks for the input guys.
I can't keep the bike running long enough to mess too much with adjustment, and the only way to keep it running is with the throttle, it won't idle. After a few minutes of this the plugs foul up and it's all over. I have to the best of my ability verified that the jets are all stock and ok. The spare carbs are out of a '83, so they are slightly differant. The plugs are (were) new, spec per the manual.
I have bit the bullet and spent the hundred bucks for a gallon of carb soak, I am soaking them one at a time now and will try again.
I will let you know what I find out in a couple of days.
Steve

 

ALSO......there should be an aluminum washer under each float needle seat or fuel level could be changed.