1982 Maxim 750 pilot screws

I have got my 750 running. I have replaced the float needles and float needle valves and I have adjusted the floats within specs. I have cleaned all the jets and needles.

One of the four carbs were replaced with one off another carb set. The origial carbs were all missing the tamper proof cap so someone has gone in before me. I do not have anything to adjust the pilot screws with except trial and error.

I started by taking one of the extra carbs that has not been touched and I drilled out the tamper proof cap. I then screwed it in all the way and counted the rotations. It was 1 rotation. I have not taken the tamper proof cap off the carb I replace in the set on the 750. I figure it is set properly although I don't know if it is clogged. I then screwed all of the 3 remaining pilot screws all the way in the backed them out i turn.

The bike would not crank so I backed them out 2 1/2 turns as a post here recommended. It would try to crank but would not. I continued to back it out 1 rotation at a time until I got it to run. I then would test ride it and come back and make an adjustment 1/2 turn at a time. I have backed it off maybe 10 rotations which seems like an awful lot. I got it performing the best I could and I started making adjustments 1/4 turn either side of the best running position. Either way made it run worse.

It runs poorly up to about 1/4 throttle then it takes off and runs fine. At its best performance It would run fine up to 1/8 throttle and run poorly from 1/8 - 1/4 throttle and then run fine. The rpm's at which it is struggling is about 3500 to 4000 rpm's.

Of the three pilot screws two feel loose and turn freely. The other one turns but is harder to turn in both directions. Do you feel as I do that I need to remove the tamper proof cap off the forth carb and clean and check it and then start all over again at 2 1/2 turns on all four carbs?

Thanks

 

If you lean it out too much you will hole out the piston running like that. Sounds like the carbs need cleaned bigtime. My book doesnt give a setting and says dont monkey with them. If the one you uncapped was 1 turn be sure it was off the same modle. If the experts say 2 - 2 1/2 I wouldnt go much further than that. If it doesnt run clean the carbs again. till ya gettin her running steady, than have the carbs sunk or build the tool to do it yourself.

 

Ten turns on the pilot screw will almost run you out of threads. That's too far. Well before this point it stopped making a difference. The idle mixture screws have very little affect except at idle which is why you're seeing it run better at 3000 RPM and above.

Read this if you haven't already.

http://www.xjbikes.com/Forums/viewtopic/t=14692.html

And yes, remove that other anti tamper plug. You need to in order to properly clean the carbs.

 

Also check the valve clearance. Without these being in spec you'll never get the carbs right.

 

The IDLE Screws are an Important Fine Tuning detail.

They must be adjusted to allow the Bike to IDLE.
They must also be adjusted to provide a quotient of Supplemental Richness to sustain Combustion(s) at the precise instant of transitioning from IDLE to Off Idle.

The Screw only regulates the AIR.
The Fuel is Siphoned when the Screw is Opened.
5 to 5-1/2 Turns is Fully-open.
10 Turns is doing nothing past 5.5

If the Screw at 5 Turns isn't supplying enough Fuel, ... rather than Turn the Screw Out anymore ... You have to "UP" the Pilot Jet.

 

Thanks biggs500. Yes you are right the pilot screws are almost to the top of the carb. I knew that was not right but that is where I could get it to idle without stalling. I have read that and that is why I started at 2 1/2 but it would not run. I will drill out the tamper proof cap and clean and inspect it and start over at 2 1/2 turns. I will also check the valve clearance.
Thanks maximdoug750. I think I need to open up the one carb that I did not want to open the tamper resistant cap. Like you say Yamaha says not to mess with it so that is why I did not. I think the carb is very clean but if I don't get it running right with cleaning the one tamper proof pilot screw I will take it apart again.
Thanks,

 

Thanks RickCoMatic. When you say "UP" the pilot jet does that mean put one in that is a little larger?

Thanks

 

Yes.

Up the pilot means a bigger size. how much is debatable, but they are cheap, so next 2 sizes gives you a choice. I'd pick the second next size if it were me and I had to choose one, but I'm no expert. Then the mix screw can fine tune from there.

 

Thank you pjk_xj700. Sounds like a plan. I will take the tamper proof cap off. Take out the pilot screw and clean it. I will close all the pilot screws back down and back them all out to the proper starting point. I f I can not get her to run at anything below 5 turns then I will get new larger pilot jets and start again.

Thanks for everyone's help again!

 

Replace the tiny o-rings on the pilot screws while you're at it. And thoroughly clean the passages.

 

Took the tamper proof cover off. It will run with just 2 complete turns now but that is too lean. Pops on deceleration. Thanks bigfitz52. I will have to buy the small o-rings. It is just a matter of adjusting and test ridding now. I am getting close. It flutters a little now between 3500 and 4000 rpm's but is getting better.

 

Was it IN SYNC before you started messing with the mixture screws?

Valves in spec and float levels accurately wet-set? No mention of valves in the above...

YICS blocked for the vac sync? (The one you did before messing with the mixtures.)

It sounds to me like you're twiddling the mixtures and haven't sync'ed the carbs yet (different set of screws.)

 

No I don't think it was in sync before it got all gummed up. When I got the bike you had to ride with the choke on because it took forever to get to a point where it would idle without the choke partially on. Plugs had heave black soot on plugs. My son quit riding and it set up about six months and would not crank. Then set another six months without trying to crank it.

I cleaned the carbs and put it back together. Since I took everything apart that is why I am trying to get it running. I don't think it was in sync when I got it, I doubt if valves were correctly set but may have been. It was hard to start when I got it. It now starts very easily.

I have not sync'ed the carbs as I don't have anything to sync them with. The new owner can take over where the valves and the carb sync is concerned. I have been up front with him. He knows I don't know anything about valves and carbs and I have told him that he will need to find someone that can work on this 30 year old technology. He understands that. I have two other Yamaha's that I ride. The plugs are a tanish brown color so I think it is closer than when I got the bike.

I am just trying to get it running which it is and pretty good but not perfect. It will need everything that all of you have mentioned. It is a very nice looking bike. Very clean and comes with two seats and extra rear tire, an extra petcock, and extra carb, a shop manual and a slew of other extra parts. I am selling it to him for $1500 with the understanding that he needs to take it to a mechanic that can work on these old bikes.

If anyone thinks I am asking too much please let me know as I do not want to rip this guy off. I don't care if it is worth more as I would be happy with the $1500 I just want to be fair with him.

Thanks

 

Did you make sure that the ENTIRE enriching circuit is clean? What about the pickup tube that extends down in the well in the carb bowl? Did you clean the jet at the bottom of the well?

I have to agree with the others......there's more to do than just putting new needles/valves in.

Read through and then follow the thread on, the Church of Clean. Do the valve shim clearance adjustments, too.

If you don't he's not gonna be happy til it gets done....

Dave F

 

Yes all the tubes, jets and needles are clean. I have read the Church of clean twice. The only thing that I cleaned that I did not know how to check is the tube that extends down into the carb bowl. I cleaned them but I don't know how to tell how clean they are. I sprayed them with crab cleaner and an air compressor about 3 or 4 times.

As for the valve shims I have not messed with. I am not a mechanic so I don't feel comfortable going into the engine. The guy that is buying it knows a mechanic that can shim the valves. I just read big fitz excellent how to post on shimming your valves. I know I would not feel comfortable attempting that not to mention buying the tools when I'm selling the bike. If I were keeping the bike it would be a different picture.

 

I just read some info that Chacal posted in his parts catalog. The previous owner put a pair of 2 into 1 exhaust tips on the bike. They are loud so there is not much baffling if any in there. Also there is a washable foam type air filter on the bike. The way I read Chacal's post is there should be slightly larger jets in the bike. All the jets are OEM. That may have a little to do with my fine tuning also.

 

Good Bet!

Nearly every Mod and Deviation from OEM Intake & Exhaust Systems requires a good deal ... of what is sometimes ... very-exasperating and frustrating: Experimental Special-Tuning.

 

Thanks RickCoMatic. I believe this is if not the cause of my frustrations is greatly contributing to it.

 

Ok seems to run better the more I turn the pilot screws. At 5 1/2 turns runs better than 3 turns. 2 turns leans it out and it starts popping on deceleration. Considering the after market pipes with hardly no baffles and the foam air filter I have decided to take RickCoMatic's and others suggestion to "UP" the Pilot jets.

Now my question is do I need to up both the pilot fuel jet and the pilot air jet or just the pilot fuel jet?

Thanks